soil part 4

Chapter 1 CLASSIFICATION

SOIL Introduction
It is natural for people to sort out and classify objects of their environment. Because of the many uses of soils and being objects of common experience and observation, people classify soils. With increasing sophistication of agriculture, greater knowledge about soils, and greater complexity and diversity of soil uses – classification of soils has become more scientific and organized.
Purpose of Classification
The purpose of classification is to establish groups, subdivisions, or classes of the objects under study in a manner useful for practical and applied purposes. We classify soils to predict their behavior, identify their best uses, and estimate their productivity.
Modern Classification Systems
Soil Map Units for the FAO/UNESCO Soil Map of the World and the US Comprehensive Soil Classification System are the two widely used systems. In the US system, differentiating characteristics selected are the properties of the soils including soil temperature and moisture. A formative element from each of the higher categories, the family, and the series form the soil name. With a little experience, one can make several statements about soil properties simply from analyzing the name of the soil.
Structure of the US System
The system contains six categories from the highest to the lowest level of generalization. The categories include the order, suborder, great group, subgroup, family, and series - which (most of the time) retain its geographic place name. It is important to understand the tables and the general operating procedure in application of the differentiating characteristics* of the comprehensive system to the soil population.

In soil classification practice, one tends to look at the entire soil population at the highest category and place them in any one of the ten soil orders. Once accomplished, the practice is to consider the nature and properties of only the soils within a given order and determine the suborder. The great group of the soil within the suborder is determined in the same manner.

Malaysian Soil Taxonomy
The Malaysian soil taxonomy refers to the US system. It places soils into the six categories based on their differentiating characteristics (Table 1.1).

Table 1.1: Differentiating characteristics of the categories of the Malaysian Soil Taxonomy

Category No. of Taxa Differentiating characteristics
Order 11 Soil-forming process as indicated by the presence or absence of major diagnostic horizons
Sub-Order 28 Genetic homogeneity – according to presence or absence of properties associated with wetness; major parent materials; nature and origin of organic materials
Great Group 87 According to profile morphology with emphasis on upper sequum; degree of weathering; presence or absence of diagnostic layers, e.g. plinthite
Sub-Group 279 Central concept for great group
Soil Family 325 Properties important for plant growth - particle size classes; mineralogical classes; temperature; and color
Soil Series 527 Kind and arrangement of horizons, color, texture, structure, consistence, and major differences in chemical and mineralogical properties of the horizons

The US System of Soil Taxonomy
The current system of soil taxonomy has worldwide application. Because of the necessity of selecting and classifying individual soils, soil scientists choose some minimum soil volume to represent an individual soil. Such a minimum volume, called pedon, is about 1.0 to 10.0 m2 and as deep as roots grow. Most soil individuals (each a uniform body of soil) are present in larger volumes, referred to as polypedons (many pedons). The six categories in the US system of soil taxonomy are, in decreasing rank: order, suborder, great group, subgroup, family, and series.


A given soil, Kuantan series for example, would have the following classification:
• Full name: Kuantan series of the very fine, oxidic, isohyperthermic family of Typic Hapludoxs
• Order name: Oxisol – highly oxidized soil
• Suborder name: Udox– adequate moisture
• Great group name: Hapludox – minimum horizon development
• Subgroup name: Typic Hapludox – central concept
• Family name: Typic Hapludox, very fine, oxidic, isohyperthermic
• Series name: Kuantan – a town in Pahang, Malaysia
Classifying soils
Through observation of the soil landscape and the land surface features, we have some ideas about soil formation. A thorough examination of the soil profile is required to identify the diagnostic horizon to determine the order of the soil being classified (Table 1.2). Table 1.3 describes the 11 soil orders. It is interesting to note that Malaysian soil orders have alternative names derived from the national language, however; not many people in the field of soil science know them. For that matter and for communication purpose, we still maintain the names of the soil orders classified based on the US system. Table 1.4 shows the Malaysian soil orders and the corresponding US soil orders.

Table 1.2: Simplified key to soil orders
Soil Characteristics Order
1. If the soil has more than 30% organic matter to a depth of 40 cm Histosol
2. Other soils with a Spodic horizon within 2 m Spodosols
3. Other soils with an Oxic horizon within 2 m and no Argillic horizon Oxisols
4. Other soils with more than 30% clay in all horizons – some cracks when dry at 50 cm Vertisols
5. Other soils that are dry more than 50% of the year and no Mollic epipedon Aridisols
6. Other soils that have an Argillic horizon but a BS at pH 8.2 less than 35% at a depth of 1.8 m Ultisols
7. Other soils that have a Mollic epipedon Mollisols
8. Other soils that have an Argillic horizon Alfisols
9. Other soils that have an Umbric, Mollic, or Plaggen epipedon, or a cambic horizon Inceptisols
10. Other soils Entisols

Table 1.3: Soil orders and their description
ORDER DESCRIPTION
HISTOSOLS Tissue or Organic Soils
ENTISOLS Origin: From recent soils without pedogenic developed horizons
INCEPTISOLS Beginning, inception, and incipient pedogenic horizons
ANDISOLS High volcanic ash content
ARIDISOLS Dry more than 6 months of the year (not applicable to Malaysian soil)
MOLLISOLS Soft, organic-rich surface horizons
VERTISOLS Turn self-swallowing clays
ALFISOLS Movement of Al, Fe, and clay into B
SPODOSOLS Wood ash, gray color of E horizon
ULTISOLS Highly leached, clay accumulation in B
OXISOLS Very highly oxidized throughout the profile
Note: Letters in bold are the formative elements to denote order in a soil name


Table1.4: Malaysian soil orders and the corresponding orders of the US system

Order
Malaysian US System
Organah Histosol
Dankanah Lithosol (shallow soil over hard rock)
Hitanah Spodosol
Abuanah Andisol (volcanic ash)
Oksanah Oxisol
Balanah Vertisol
Kutanah Ultisol
Butanah Mollisol
Jenanah Alfisol
Lahanah Inceptisol
Baranah Entisol

Diagnostic Horizons for Classification
Epipedons are the uppermost horizons and include the followings:
Albic – is a strongly leached E horizon, white layer near the surface of Spodosals, or in upper Alfisols profiles. Eluviation (leaching out) of coatings of clay and free iron oxides left the remaining sand and silt to cause the white layer.
Anthropic – a surface horizon like the mollic epipedon but contains over 250 ppm of citric acid soluble P2O5.
Histic – is an organic surface horizon underlain by mineral soil. It that contains more than 30% organic matter depending on clay content and is water saturated for 30 days at some season of the year unless artificially drained. It is thinner than 30 cm if drained or 45 cm if not drained.
Mollic – is a dark, friable surface horizon, and not strongly acidic. When mixed to a depth of 17.5 cm, it contains 1% or more organic matter. Color values – darker than 5.5 (dry) and 3.5 (moist). Structure cannot be massive and hard. Base saturation is over 50% and the epipedon is not naturally dry more than 3 months per year.
Ochric – a surface horizon that is light in color - color values  5.5 (dry) and  3.5 (moist). It contains less than 1% organic matter, or is hard or very hard and massive when dry or dry more than 3 months per year. It is common in Aridisols, Entisols, and Inceptisols.
Plaggen – a man made surface horizon  50 cm thick – created by years of manure addition
Umbric – is an acidic dark surface horizon like the mollic, but is less than 50% base saturated and not naturally dry more than 3 months per year.
*Arenic – a loamy fine sand or coarser horizon, that is more than 50 cm thick, over an argillic horizon.
*Grossarenic – a loamy fine sand or coarser horizon, that is thicker than 100 cm, over an argillic horizon.
* used for thick, sandy surface horizon

Subsurface Diagnostic Horizons (Endopedon ‘B’ Horizons)
Agric – formed directly under plow layer. Has silt and clay. Humus accumulated as thick dark lamellae.
Albic – light colored E horizon with color values  4 (dry) or  5 (moist)
Argillic – is a clay accumulation horizon or a B-horizon that has at least 1.2 times amounts of clay as some horizons above or 3% more clay content if the eluvial layer has  15% clay or 8% more clay if eluvial layer has  40% clay.
Calcic – at least 15 cm thick and has secondary accumulation of carbonates, usually Ca or Mg, in excess of 15% calcium carbonate equivalent and contains at least 5% more carbonate than underlying layer.
Cambic – is a horizon of very fine sand, loamy fine sand or finer texture with some weak indication of either an argillic or spodic horizon, but not enough to qualify as either. It is a weakly developed B-horizon and is common in Inceptisols.
Kandic – is an Argillic horizon of kaolinite-like clays.
Gypsic – a horizon of calcium sulfate enrichment. It contains at least 5% more calcium sulfate than underlying material and is more than 15 cm thick.
Natric – it meets the requirements of an argillic horizon, but also has prismatic or columnar structure. Either Na+ saturates over 15% of the CEC or it has exchangeable Mg++ and Na+ than Ca++. Exchange acidity is at pH 8.2.
Oxic – is a highly weathered B-horizon at least 30 cm thick and contains more than 15% clay. It has a high content of low charge 1:1 clays and sesquoxides retaining 10 meq of cations / 100 g clay from unbuffered NH4Cl and 16 meq/100 g clay by the NH4OAc pH 7 methods.
Petrocalcic – is an indurated calcic horizon and has hardness 3 or more. At least half of the dry fragment breaks down when immersed in acid, but does not breakdown when immersed in water.
Petrogypsic – is a strongly cemented gypsic horizon. Dry fragments will not slake in H2O.
Placic – is a single, thin (2mm to 10 mm thick), dark reddish brown to black iron or manganese pan that lies within 50 cm of the soil surface. It is wavy, involuted, and slowly permeable.
Salic – is at least 15 cm thick. It contains secondary soluble salt enrichment (saline).
Sombric – is a free draining horizon. It has the darkness and base saturation status of an umbric epipedon. Illuviation of humus, but not Al or Na, formed it.
Spodic – has an illuvial accumulation of free sesquoxides and organic matter.
Sulfuric – is a mineral or organic soil horizon with high sulfides and a pH  3.5 and toxic to plant roots. It has yellow mottles of jarosite.
Soil Moisture and Temperature regimes
Each soil has certain temperature and moisture characteristics. We measure and use them to classify a particular soil. The various levels of soil taxonomy use the following groupings of moisture and temperature regimes (Tables 1.5 and 1.6)


Table 1.5: Soil temperature categories
Terms Meaning
Pergelic Mean annual soil temperature  0º C
Cryic Mean annual soil temperature 0 - 8º C with summer temperatures less than 15º C
Iso When used as a prefix on the temperature regimes below, iso refers to soils with the average temperature for the 3 warmest months differs by less than 5º C
Frigid and Isofrigid 0º C to  8º C
Mesic and Isomesic 8º C to  15º C
Thermic and Isothermic 15º C to  22º C
Hyperthermic and Isohyperthermic  22º C












Table 1.6: Soil moisture categories

Terms Meaning
Aquic Water saturated for at least enough time (several days) so that reducing condition exist
Aridic or Torric Dry more than half the time when not frozen and never moist more than 90 consecutive days when soil temperatures are above 8º C at 50 cm depth
Perudic In most years precipitation exceeds evapotranspiration every month of the year
Udic In most years, these soils are not dry as long as 90 cumulative days
Ustic In most years, these soils are dry for more than 90 cumulative days but less than 180 days
Xeric Only in the non-iso temperature areas with dry summers and moist winters






Table 1.8: Abbreviated Soil Family Minerology Classes

Class Definition
I. Used if applicable in any textural family
Carbonatic  40% carbonate
Ferritic  40% reduceable Fe2O3
Gibbsitic  40% gibbsite plus bohemite
Oxidic % ext. Fe2O3 + % gibbsite/% clay  0.2 and  40% any other mineral;  90% quartz in 0.02 – 2
Serpentinitic  40% serpentine minerals
Gypsic Carbonatic with  35% gypsum
Glauconitic  40% glauconite
II. Used only in sandy or loamy textured families
Micaceous  40% mica
Siliceous  90% silica minerals
Mixed None of the above group I or II families
III. Used only in clayey textured families
Halloysitic  50% halloysite
Kaolinitic  50% 1:1 clays and  10% expanding 2:1 clays
Montmorrilonitic  50% montmorrilonite
Illitic  50% mica (illite)
Vermicullitic  50% vermicullite
Chlorolitic  50% chlorite
Mixed None of the above mineralogical properties

Example of Soil Classification
The criteria used to classify Oxisol in Malaysia are as follows:
Category Criteria

Suborder Moisture regime - aquic or udic
Great group Nature of oxic horizon – aeric characteristics and base saturation
Sub group Central concept and intergrades
Soil family Particle size class, mineralogy class, temperature regime, and color class
Soil series Parent material

Each soil great group falls into three kinds of subgroup:
1. the group representing the central (typic) segment of the soil group
2. the group which has properties that tend toward other orders, suborders, or great group (intergrade)
3. the group which has the properties that prevent its classification as typic or an intergrade to other soil category (extragrade)

The Kuantan series of the very fine, oxidic, isohyperthermic family of Typic Hapludox is an example of a soil name based on the US classification system. Hapludox is the combination of the formative elements of the order, suborder, and great-group names (Tables 1.9 and 1.10). We determine the family textural class name by the textural triangle and Table 1.8 is a guide to determine the family mineralogical class.

Table 1.9: Suborder names in the US Comprehensive System – formative elements and meaning
Formative
Element Derivation Mnemonicon Meaning or Connotation
alb albus – white albino Presence of albic horizon (a bleached eluvial horizon)
and Modified from Ando Ando Andolike
aqu aqua – water aquarium Characteristics associated with wetness
ar arare – to plow arable Mixed horizons
arg Modified from argillic horizon – argilla – white clay argillite Presence of argillic horizon ( a horizon with illuvial clay)
bor boreas – northern boreal Cool
ferr ferrum – iron ferruginous Presence of iron
fibr fibra - fiber fibrous Least decomposed stage
fluv fluvus - river fluvial Flood plains
fol folia - leaf foliage Mass of leaves
hem hemi - half hemisphere Intermediate stage of decomposition
hum humus - earth humus Presence of organic matter
lept leptos - thin leptometer Thin horizon
ochr base of ochros - pale ocher Presence of ochric epipedon (a light-colored surface)
orth orthos - true orthophonic The common ones
plagg plaggen - sod Presence of plaggen epipedon
psamm psammos - sand psammite Sand texture
rend Modified from Rendzina Rendzina Rendzinalike
sapr sapros - rotten saprophyte Most decomposed stage
torr torridus - hot, dry torrid Usually dry
trop tropikos – of the solstice tropical Continually warm
ud udus - humid udometer Of humid climates
umbr umbra - shade umbrella Presence of umbric epipedon ( a dark-colored surface)
ust ustus - burnt combustion Of dry climates –usually hot in summer
xer xerox - dry xerophyte Annual dry season





Table 1.10: Great Group names in the US Comprehensive System – formative elements and meaning
Formative
Element Derivation Mnemonicon Meaning or Connotation
acr akros – at the end acrolith Extreme weathering
agr ager - field agriculture An agric horizon
alb albus - white albino An albic horizon
and Modified from Ando Ando Andolike
arg Modified from argillic horizon – argilla – white clay argillite An argillic horizon
bor boreas - northern boreal Cool
calc calcis - lime calcium A calcic horizon
camb cambiare – to exchange change A cambic horizon
chrom chroma - color chroma High chroma
cry kryos - cold crystal Cold
dur durus - hard durable A duripan
dystr; dy dystrophic – infertile; dys -ill dystrophic Low base saturation
eutr; eu eutrophic – fertile; eu - good eutrophic High base saturation
ferr ferrum - iron ferric Presence of iron
fluv fluvus - river fluvial Flood plains
frag fragillus - brittle fragile Presense of fragipan
fragloss Compound of fra(g) and gloss See formative elements of frag and gloss
gibbs Modified from gibbsite gibbsite Presence of gibbsite
gloss glossa - tongue glossary Tongued
gyps gypsum -gypsum gypsum Gypsic horizon
hal hals -salt halophyte Salty
hapl haplous - simple haploid Minimum horizon
hum humus - earth humus Presence of humus
hydr hydro - water hydrophobia Presence of water
hyp hypnon - moss hypnum Presence of hypnum moss
luv; lu louo - to wash ablution Illuvial
med meda - middle medium Temperate climate
nadur Compound of nat(r) and dur
natr natrium - sodium Presence of nitric horizon
ochr base of ochros - pale ocher Presence of ochric epipedon
pale paleos - old paleosol Old development
pell pellos - dusky Low chroma
plac base of plax –flat stone Presence of a thin pan
plagg plaggen - sod Presence of plaggen horizon
plinth plinthos - brick Presence of plinthite
psamm psammos - sand psammite Sand texture
quartz quarz - quartz quartz High quartz content
rend Modified from Rendzina Rendzina Rendzinalike
rhod base of rhodon - rose rhododendron Dark red colors
sal base of sal - salt saline Presence of salic horizon
sider sideros - iron siderite Presence of free iron oxide
sombr sombre - dark somber A dark horizon
sphagno sphagnos - bog Sphagnum moss Presence of sphagnum moss
sulf sulfur - sulfur sulfur Presence of sulfites
torr torridus - hot, dry torrid Usually dry
trop tropikos – of the solstice tropical Continually warm
ud udus - humid udometer Of humid climates
umbr umbra - shade umbrella Presence of umbric epipedon
ust ustus - burnt combustion Dry climates –usually hot in summer
verm vermes - worms vermiform Wormy or mixed by animals
vitr vitrum - glass vitreous Presence of glass
xer xerox - dry xerophyte Annual dry season




Table 1.11 lists some of the soils already recognized. The list contains some of those soils thought to have might be or might not be present in Malaysia. Obviously, we exclude Aridisol.

Table1.11: Some soils recognized
Order Classified Soils
Alfisols Plinthaqualf, Natraqualf, Duraqualf, Tropoaqualf, Glossaqualf, Albaqualf, Umbraqualf, Ochraqualf
Paleboralf, Fragiboralf, Natriboralf, Cryoboralf, Eutroboralf, Glossoboralf
Agrudalf, Natrudalf, Ferrudalf, Glossudalf, Fraglossudalf, Fragiudalf, Paleudalf, Rhodudalf, Tropudalf, Hapludalf
Durustalf, Plinthustalf, Natrustalf, Paleustalf, Rhodustalf, Haplustalf
Durixeralf, Plinthoxeralf, Natrixeralf, Rhodoxeralf, Palexeralf, Haploxeralf
Entisols Sulfaquent, Hydraquent, Cryaquent, Fluvaquent, Tropaquent, Psammaquent, Haplaquent
Cryofluvent, Xerofluvent, Ustifluvent, Torrifluvent, Tropofluvent, Udifluvent
Cryorthent, Torriorthent, Xerorthent, Troporthent, Udorthent, Ustorthent
Cryopsamment, Torripsamment, Quartzipsamment, Udipsamment, Tropopsamment, Xeropsamment, Ustipsamment

Histosols Sphagnofibrist, Cryofibrist, Borofibrist, Tropofibrist, Medifibrist, Luvifibrist
Cryofolist, Tropofolist, Borofolist
Sulfohemist, Sulfihemist, Luvihemist, Cryohemist, Borohemist, Tropohemist, Medihemist
Cryosaprist, Borosaprist, Troposaprist, Medisaprist
Inceptisols Cryandept, Durandept, Hydrandept, Placandept, Vitrandept, Eutrandept, Dystrandept
Sulfaquept, Placaquept, Halaquept, Fragiaquept, Cryaquept, Plinthaquept, Andaquept, Tropaquept, Humaquept, Haplaquept
Fragiochrept, Durochrept, Cryochrept, Ustochrept, Xerochrept, Eutrochrept, Dystrochrept
Humitropept, Sombritroept, Ustroept, Eutroept, Dystroept
Fragiumbrept,Cryumbrept, Xerumbrept, Haplumbrept, Plaggept (sub-order)

Table1.11: Some soils recognized continued
Order Classified Soils
Mollisols Natralboll, Argialboll
Cryaquoll, Duraquoll, Natraquoll, Calciaquoll, Argiaquoll, Haplaquoll
Paleboroll, Cryoboroll, Natriboroll, Argiboroll, Vermiboroll, Calciboroll, Haploboroll
Paleudoll, Argiudoll, Vermiudoll, Hapludoll,
Durustoll, Natrustoll, Paleustoll, Calciustoll, Argiustoll, Vermiustoll, Haplustoll
Durixeroll, Natrixeroll, Palexeroll, Calcixeroll, Argixeroll, Haploxeroll
Rendoll (sub-order)
Oxisols Gibbsiaquox, Plinthaquox, Ochraquox, Umbraquox
Sombrihumox, Gibbsihumox, Haplohumox, Acrohumox
Sombriorthox, Gibbsiorthox, Acrorthox, Eutrorthox, Umbriorthox, Haplorthox
Sombriustox, Acrustox, Eutrustox, Haplustox
Torrox (sub-order)
Ultisols Plinthaquult, Fragiaquult, Albaquult, Paleaquult, Tropaquult, Ochraquult, Umbraquult
Sombrihumult, Palehumult, Plinthohumult, Tropohumult, Haplohumult
Fragiudult, Plinthudult, Paleudult, Rhodudult, Tropudult, Hapludult
Plinthustult, Paleustult, Rhodustult, Haplustult
Palexerult, Haploxerult
Spodosols Fragiaquod, Cryaquod, Duraquod, Placaquod, Topaquod, Haplaquod, Sideraquod
Placohumod, Tropohumod, Fragihumod, Cryohumod, Haplohumod
Placorthod, Fragiorthod, Cryorthod, Troporthod, Haplorthod
Ferrod (suborder)
Vertisol Chromudert, Pelludert
Chromustert, Pellustert
Chromxerert, Pellxerert
Torrert (sub-order)


Chapter 2 SOIL SURVEY, INTERPRETATION, AND LAND USE PLANNING

Earlier, the principle objectives of soil survey were to predict the possibility of growing crop on new soils and to learn enough about certain soils to predict their respond when irrigated with a known quality and quantity of irrigation water.

Later, soil surveyors expanded the objectives to include the rational means of transferring technology from one soil to another and the interpretations for predicting land use for every soil mapped. A soil survey is the process of studying and mapping the earth’s surface in terms of units called soil types (series and phase [soil surface texture, slope, erosion, and depth]). It uses the soil taxonomy system.

First surveys were simple and limited - they answered practical agronomic questions of soil differences and limitations important in improving and expanding crop production. Examples of those questions include:

1. Was a new soil area suitable for crop production?
2. How much fertilizer did it need?
3. What were the problems of water, salts, or acidity?
4. Could other crops be grown more profitably?

With the increase in scientific knowledge and demand for more useful information, soil surveys have expanded. Today’s soil surveys include information to make scientific interpretations about using each soil-mapping unit.

The soil survey is useful for the following purposes:

• Engineering construction
• Locating sources of sand and gravel
• Forestry management
• Urban development
• Game management
• Recreation development
• Predicting erosion hazards
• Irrigation and drainage
• Land use planning
Making a Soil Survey
A soil surveyor works alone (in Malaysia soil surveyors work in groups). In the survey process, he/she carries the following materials and equipment:
• An aerial photograph or a topographic map of the area
• A geologic map of the area
• A digging tool or soil auger
• A hand level (Abney level or clinometer) for measuring slope
• A pH kit
• A Munsell color book
• 10% HCL to identify the presence of lime

The surveyor has a ’legend’ describing the profile characteristics of the most extensive soils of the area. He or she walks over the land area at regular interval and by frequent borings with an auger and digging pits using a shovel, he or she will take notes of the soil differences and related surface features. His or her observation and notation of soil color, horizon thickness, texture (by feel), pH, soil structure, and other features such as slope (see Table 2.1 below for symbol used to denote erosion for mapping unit), evidence of erosion, land use, vegetative cover, and cultural features will allow him to determine the soil mapping units and the soil boundaries. The soil surveyor sketch boundaries directly on the aerial photograph to represent changes from one soil type to another and each mapping unit area is labeled with a code symbol from a prepared legend.




Table 2.1: Slope characteristics and symbols for soil survey
Symbol Slope Description
º %
1 0 – 2 0 – 4 Level
2 2 – 6 4 – 12 undulating
3 6 – 12 12 – 23 rolling
4 12 – 20 23 – 38 Hilly
5  20  38 steep land







Soil Survey Reports
The soil survey reports provide a permanent record about the soils of the survey area. In addition to a map showing the distribution of the different kind of soils in the area, the publication describes the soils and summarizes research findings on the effects of soil on plants and engineering practices.

The text provides descriptions, laboratory data, and other information about the properties of the soils. From these basic data, soil scientists/agronomists interpret the potentials, suitability, and limitations of the soils for crops, pastures, forest, wildlife habitat, recreation, engineering, and any other uses known to be important at the time of the survey. Based on our current understanding about soils, we can make interpretations and predictions of their uses. The text also discusses on land use and management to bring out specific relationships to individual soils or group of soils shown on the map.

The properties, responses to management, and suitability and limitations of each kind of soil are given to enable the public to make full use of the soil map, whether for producing crops or for locating building sites or sources of construction material. We make predictions based on the behavior of each kind of soil under specified uses and management systems.

A published soil survey contains instructions for its use, information about the survey process, an account on the general nature of the area, a description of the general soil map, a classification of the soils, a discussion of soil formation, references, and a glossary.
Types of Soil Survey in Malaysia
Malaysia conducts three types of soil survey - reconnaissance, semi-detailed, and detailed surveys.
Reconnaissance Soil Survey
In this survey, traverses are spaced at 4 km apart while the examination points along the traverse are 500 m apart giving coverage of 200 ha for every examination point. The scales of the published maps can range from 1:50,000 to 1:250,000.
Semi-detailed Soil Survey
Semi-detailed survey assists in planning land use for agriculture. In this survey, soil surveyors examine soils at much closer intervals. Traverses are spaced 1 km apart and the examination points are spaced 200 m apart giving a mapping intensity of 20 ha per examination point. The scale of the map can range from 1:25,000 to 1:50,000.
Detailed Soil Survey
This survey is for specific purposes such as land leveling, irrigation designs, general farm planning, and fertilizer recommendations. Traverses are spaced at 200 m x 200 m giving a mapping intensity of four (4) ha per examination point or at 100 m x 100 m giving a mapping intensity of one (1) ha per examination point. The scale of the published map is 1:10,000.
Users of Soil Survey
The soil survey is for

• Farmers, foresters, and agronomists to evaluate the potentials of the soils and the management needed for maximum food and fiber production.
• Planners, community officials, engineers, developers, builders, and home buyers – they can use the survey to plan land use, select sites for constructions, and identify special practices needed to ensure proper performance
• Conservationists, teachers, students, and specialists (in [1] recreation and wildlife management [2] waste disposal and pollution control) – the survey help them understand, protect, and enhance the environment
Land Suitability Classification
In the soil capability classification, soil surveyors grouped together mapping units that have comparable potential productivity and need similar management to maintain or improve their level of productivity. The purpose of classification is to serve as a guide of the capability of land for agriculture uses and growing crops. We divided soils into the following five classes based on their limitations for crop growth (Tables 2.2 and 2.3).
Class 1 – Soils with no limitations or minor limitations
They are suitable for the widest range of crops and require moderate level of management. These soils exist on flat to undulating terrain on the landscape. They are deep and well structured soils with good water and nutrient-holding capacities.
Class 2 – Soils with moderate limitations
These soils are suitable for growing narrower range of crops than Class 1 soils and require a moderate level of management for satisfactory yields. The soil-water management may include minor erosion control measures and minor drainage-irrigation works.
Class 3 – Soils with one serious limitation
These soils are suited for narrow range of crops. We must be conserved them for long- term crop production. We must control soil erosion - an intensive fertilizer or amendment application is required and/or the soils need irrigation and drainage.
Class 4 – Soils with more than one serious limitation
These soils are limited to a very narrow range of crops and often only to specific crops. A very high level of management is required to maintain a moderate level of continuing productivity. Major soil-water conservation measures are required for cultivation of crops on a long-term basis. Class 4 soils generally exist on hilly terrain.
Class 5 – Soils with at least one very serious limitation
These soils are least suitable for growing crops and left as primary forest or regenerating forest.
Suitability/Capability Class Symbol
S1 - Highly suitable
S2 - Moderately suitable
S3 - Marginally suitable
N1 - Currently not suitable
N2 - Permanently not suitable
Table 2.2: Soil limitations to crop growth
Symbol Type of Limitation Description
a acid sulfate layer pH  3.5 and SO4  0.1%
c depth to compacted layer Laterite layer, compact subsoil
D or d drainage Excessive to very poorly drained
E or e erodibility Slope and texture
H disturbed land (human influence) Mining and urban land
N or n nutrient imbalance Toxicity and deficiency
o organic horizon Organic clay, muck, and peat
R or r stoniness Stones/gravel/boulders on and in soil
s salinity 4 classes of salinity
T or t texture and structure Workability
















Table 2.3: Soil limitations for crop growth
Symbol TYPES OF
LIMITATIONS CATEGORY
VERY SERIOUS SERIOUS MODERATE MINOR
a Depth to acid
Sulfate layer - 0 – 25 cm from the surface 25 – 50 cm from the surface 50 – 100 cm from the surface
c Depth to
compacted layer 0 – 25 cm from the surface 25 – 50 cm from the surface 50 – 75 cm from the surface 75 – 1000 cm from the surface
D Drainage Excessively drained Somewhat excessively drained -
d Very poorly to poorly drained Imperfectly drained Moderately well drained
G Gradient  38% slope 23 – 38% slope 12 – 23% slope 4 – 12% slope
N Nutrient imbalance Toxicity caused extremely by high contents of certain elements - - -
n - CEC  5cmol kg-1 CEC 5 - 10cmol kg-1 CEC 10 - 15cmol kg-1
o Organic horizon thickness -  125 cm thick from the surface 50 - 125 cm thick from the surface 25 - 50cm thick from the surface
R % stoniness to 100 cm depth  75% with 0-25cm stone-free soil 50-75% with 0-25cm stone-free soil 25-50% with 0-25cm stone-free soil 10-25% with 0-25cm stone-free soil
r - - 50-75% with 0-25cm stone-free soil 25-50% with 0-25cm stone-free soil
s Salinity  4 dS m-1 2-4 dS m-1 1-2 dS m-1 0.1-1dS m-1
T Texture and structure - Coarse textured and structureless Coarse to moderately coarse textured and weakly structured -
t - Fine textured and structureless to strongly coarse structured Fine to moderately fine textured and weakly structured
H Human Disturbed land: urbanization (u) and mining (m)
Chapter 3 PLANT NUTRITION AND THE CARBON CYCLE

Introduction
Low crop yields are often primarily due to lack of plant nutrients. Higher plants can synthesize all the substances they require from 13 essential elements along with carbon dioxide, water, and energy from solar radiation. They are autotrophic or photolithotrophic. Sixteen elements were classified as essential for all crops: C, H, O, P, K, N, S, Ca, Fe, Mg, B, Mn, Cu, Zn, Mo, Cl.
We use two criteria to establish essentiality of an element:
1. An element is essential if the plant fails to grow and complete its life cycle in a medium devoid of the element, compared with normal growth and reproduction in a medium containing the element.
2. An element is essential if it is a constituent of a necessary metabolite, such as sulfur in the amino acid methionine and Mg in chlorophyll.
Sources of Plant Nutrients
Natural organic and inorganic substances are the primary sources of plant nutrients in agricultural and natural ecosystems. Supplementation of the natural fertility with commercial fertilizers is a modern agricultural practice. All the chemical elements in plants come from soil, water, and atmosphere [the biosphere]. The atmosphere is the sole source of carbon. Nutrients of the biosphere recharge continuously by recycling – otherwise they would eventually become exhausted. Plant nutrient movement is a two-way street – nutrients enter plants as elements or ions and eventually return to the environment as elements through biodegradation by microorganisms [Fig 3.1].


Figure 3.1: The nutrient cycle

Microbial Transformation of Carbon in the Environment – The Carbon Cycle


Carbon is one of the major components of living organisms - approximately 50% of the dry weight of organic matter is carbon. The major energy stores of organisms are reduced carbon compounds derived from photosynthetic fixation of atmospheric carbon. The plants that fix carbon and the animals that consume carbon will eventually die. Microorganisms, particularly bacteria and fungi, decompose the dead plants and animals and return carbon to the environment as carbon dioxide. It is essential for microbes to decompose carbonaceous material and return the by-products to the atmosphere for higher organisms to continue to thrive. The supply of carbon, the major plant nutrient, is likely to decrease in the absence of microbial transformation.

The primary producers of carbon compounds are mainly higher plants. Other producers include cynobacteria, algae, and photosynthetic bacteria. Among the cynobacteria, chlorococcum and palmogloea are the carbon fixers. Nostoc, gloeocapsa, and anabaena are algae that fix carbon. Photosynthetic bacteria that fix carbon include thiocapsa and rhodopseudomonas.

Organic matter reaches the soil from the higher plant primary producers in the form of dead leaves, roots, and stems (known as litter) or root exudates. The addition of litter may be direct from leaf fall and root death or via the bodies and faeces of consumer- animals.

Organic matter in the soil exists in three forms: the insoluble soil carbon, the soluble soil carbon, and the microbial carbon. Cellulose and lignin of plant cell wall, chitin in exoskeleton of arthropod and walls of some fungi – make up the insoluble soil carbon. Under condition of acidity or poor aeration, insoluble carbon might be stored as peat – which is mostly lignin, cellulose, and hemicellulose. Humus is a part of this insoluble material and it occurs in almost all soils - except most barren sands.


Soluble carbon in the soil is immediately available to other organisms. Living organisms (e.g., plant roots) and decaying bodies of primary producers, consumers, and microbes release soluble carbon and it is immediately available to other organisms.

Prior to redistribution to other pools, all forms of organic matter pass through microbes during the decomposition process - known as microbial carbon.

The environmental factors determine the direction and the rate of flow of the carbon cycle, which in turn affect the number and activities of the microbes. Light, a key factor in photosynthesis, is the major route for carbon fixation. Temperature affects carbon fixation and decomposition. Low or high water level reduces microbial activity and anaerobic condition slows down the rate of decomposition. High soil acidity inhibits organic matter decomposition. Minerals, especially nitrogen and phosphorus, are usually short in supply during later stages of plant decay.
Microbial Physiology

Bacteria, fungi, and actinomycetes are the major groups of microorganisms that transform carbonaceous materials into various products. The capacity of these organisms to grow in a given habitat depends on their ability to utilize the nutrients in their surroundings.
Nutrition

Nutrients serve three functions to the organisms:

1. Providing materials required for protoplasmic synthesis

2. Supply energy necessary for cell growth and biosynthetic reaction

3. Serving as acceptors for the electron released in the reactions that yields energy to the organism (in aerobes oxygen is the acceptor; in anaerobes either an organic product or some inorganics replace the oxygen)

Energy sources for microbes include the following substances: cellulose, hemicelluloses, lignin, starch, peptic substances, inulin, chitin, hydrocarbons, sugars, proteins, amino acids, and organic acids.
Elemental Composition of Microbes
Bacteria, fungi, actinomycetes, algae, and protozoa generally contain nitrogen, phosphorus, potassium, magnesium, sulfur, iron, and probably calcium, manganese, copper, zinc, cobalt, and molybdenum as integral parts of protoplasmic structure. These nutrients together with carbon, hydrogen, and oxygen build the microbial cell. The molecules containing carbon and nitrogen in microbes account for the differences in their nutrition. Organic fraction (carbohydrates, proteins, amino acids, vitamins, nucleic acids, purines, and pyrimidines) make up a large fraction of protoplasmic material.

Nutrients provide energy, carbon, and mineral; serve as electron acceptors; and supply the growth factors (amino acids and vitamins) to the microbes.
Growth of microbes requires an energy input:

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (1)

Microbes use energy to drive a second reaction, for example, protein synthesis for cell build up.
Organic Matter Decomposition
Organic matter subjected to microbial decay comes from several sources. Plant remains or forest litter, animal tissues, and excretory products are food for the microbes. Organic constituents of plants include: cellulose, hemicelluloses, lignin, water soluble fraction-simple sugars, amino acids, and aliphatic acids, alcohol soluble fraction-fats, waxes, resins, and some pigments, and proteins (contain nitrogen and sulfur). As plant ages, its water-soluble constituents, proteins, and minerals decrease, while cellulose, hemicelluloses, and lignin content increase.
Carbon Assimilation
Organic matter decomposition serves two functions for microbes: provides energy for growth and supplies carbon for the formation of new cell material. Carbon dioxide, methane, organic acids, and alcohol are merely waste products as far as microbial development is concerned. These materials are metabolic wastes released in the microbial acquisition of energy. The essential feature of soil inhabitants is the capture of energy and carbon for cell synthesis.

The cells of most microorganisms commonly contain approximately 50% carbon. The source of carbon is the substance utilized. Assimilation is the process of converting substrate to protoplasmic carbon. Under aerobic conditions, frequently, microbes assimilate from 20% - 40% of the substrate carbon - the remainder is released as CO2 or accumulate as waste products.
As microbes assimilate carbon for the generation of new protoplasm, there is a concomitant uptake of nitrogen, phosphorus, potassium, and sulfur. Assimilation of minerals is an important mean of immobilization i.e. Microbes reduce the quantity of available nutrients in the soil.
Decomposition and CO2 Evolution
The most important function of the microbes is the breakdown of organic materials, a process where microbes replenish the limited supply of CO2 available for photosynthesis.

The number and diversity of compounds suitable for microbial decay are enormous. A host of organic acids, polysaccharides, lignins, aromatic and aliphatic hydrocarbons, sugars, alcohols, amino acids, purines, pyrimidines, proteins, lipids, and nucleic acids undergo attack by one or more population. Soil inhabitants destroy any compound synthesized biologically - otherwise these compounds would have accumulated in vast amounts on the earth’s surface. In addition, many of the compounds synthesized by the organic chemist are readily decomposed. Measurement of CO2 evolution determines the decomposition rate.
Breakdown of Carbonaceous Materials
A number of factors affect the mineralization of added organic materials. The rapidity of oxidation of a given substrate depends on:

• Its chemical composition

• The physical and chemical conditions in the surrounding environment – temperature, oxygen supply, pH, inorganic nutrients, and the C/N ratio of the plant residue
The maximum rate of decay of carbonaceous materials occurs at optimum temperature between 300 and 400 C. The diminishing supply of O2 suppresses the decay process of plant constituents. Respiration of microbes is greatest at about 60% - 80% of the water holding capacity of the soil and the decomposition proceeds more readily in neutral than acid soils.

Nitrogen is a key nutrient for microbial growth and organic matter decomposition. If the N content in plant and the animal tissue is high and the microbes can readily utilize the element, the microbes satisfy their needs from the source - additional quantities are unnecessary. If the substrate is poor in the element and decomposition is slow, supplemental N will stimulate C mineralization. Nitrogenous amendments cause an increase in CO2 evolution and a greater loss of cellulose, hemicelluloses, and other plant polysaccharides.

Soil inhabitants decompose nitrogen rich materials such as legume tissues very rapidly. Addition of ammonium or ammonia to straw or other N-deficient substrates greatly enhances decomposition. The rate of decomposition of plant materials depends on the N content of the tissue.

Crop plants generally contain about the same amount of carbon, approximately 40% of their dry weight - the C/N ratio compares their N contents. Thus, a low N content or a wide C/N ratio is associated with slow decay due to the insufficient supply of N to the microbes. The C/N ratio of the microbial population is an important factor that determines the availability of minerals to plants (Table 3.1 and Problem 3.1).

Table 3.1: C/N Ratio of the Microbial Population

Microbes C/N Ratio


Fungi 10:1
Bacteria 5:1
Actinomycetes 5:1
Mixed population 8:1
















Problem 3.1: In a lab experiment, 250 mg of corn stalk residue added to each of the three soils containing three different microbial populations (see table below). Which soils need supplementary N? (C content of residue is 40%, C/N ratio 57:1)


Soil 1 Soil 2 Soil 3
Aerobic bacteria Fungi Mixed population


Cell C 7 mg 35 mg 25 mg

Cell N 1.4 mg (C/N 5:1) 3.5 mg (C/N 10:1) 3.1 mg (C/N 8:1)




Solution:

Carbon content in residue: 0.4 x 250 mg = 100 mg

Nitrogen content in residue: = 1.75 mg

The shortage of N is 1.3 mg for soil 3 and 1.7 mg for soil 2.


In the decomposition of the entire plant residue, decomposers may utilize the extracted tissue constituent or pure organic compound. The water-soluble fraction contains the least resistant plant components and is the first to metabolize. Decomposers absorb amino acids, peptides, and low molecular weight carbohydrates and recycle them.

Microbes do not readily utilize materials rich in lignin compared to lignin poor products. The resistance of wood and saw dust to microbial attack is probably linked to the abundance of lignin in such materials. Microbes metabolize young, succulent tissues more readily than residues of mature plants. As plant ages its chemical composition changes—the content of N, proteins, and water-soluble substances falls - the proportion of cellulose, lignin, and hemicelluloses rises.

The size of organic particles subject to attack, govern the rate of decomposition. As a rule, the small particulate materials are more readily degraded than are the larger particles.
Chapter 4 THE NITROGEN CYCLE
Introduction
Nitrogen is one of the major plant nutrients derived from the soil and most susceptible to microbial transformation. Nitrogen, the building block of protein molecule (the base of all life – for example, the smell of protein in voodoo plant attracts insects) is an indispensable component of plant, animal, and microbial protoplasm. Its deficiency reduces the yield and quality of crops. Since nitrogen is lost by volatilization and leaching, it requires continued conservation and maintenance. In the soil, nitrogen undergoes a number of transformations that involve organic, inorganic, and volatile compounds.

Free-living microbes convert atmospheric nitrogen to organic compounds or the same conversion by microbial-plant association - in this case, it makes nitrogen directly available to the plant. Animals use the nitrogen present in the proteins or nucleic acids of the plant tissues. In animals, the conversion of nitrogen to other simple and complex compounds takes place. When the animals and plants die and decay, the decay process releases organic nitrogen as ammonium. Vegetation uses the ammonium or microbes oxidize it to nitrate. Nitrate may be lost through leaching, taken by plants, and reduced to ammonium or elemental nitrogen and escape to the atmosphere.

The portions of nitrogen cycle governed by microbial metabolism are composed of several individual transformations. In nitrogen mineralization, microbes decompose and convert part of the large reservoir of the organic complexes to the inorganic ions - ammonium and nitrate, used by plants. Microbial mineralization results in the degradation of proteins, polypeptides, amino acids, nucleic acids, and other organic compounds. In nitrogen immobilization or assimilation, conversion of simple substances to complex substances (organic N) takes place. Microbial immobilization leads to the biosynthesis of the complex molecules of microbial protoplasm from ammonium and nitrate.

Nitrate in the soil may be lost in several ways:
• It moves downward out of the zone of root penetration due to its solubility in the soil solution
• Plants utilize it
• It escapes into the atmosphere as N gas through denitrification
Figures 4.1 and 4.2 summarize the fixation and losses of N.








Figure 4.2: Illustration of the N cycle







Fixation of nitrogen made available to the plants occurs in different ways. Table 4.1 summarizes some of the fixation processes.

Table 4.1: Nitrogen fixation processes

NON BIOLOGICAL
Industrial – production of fertilizers
Lightning

BIOLOGICAL
Asymbiotic
Bacteria
Aerobic (Azotobacter)
Anaerobic (Clostridium)
Anaerobic-photosynthetic (Chromatium, Rhodospirillum)
Blue Green Algae (Nostoc)

Symbiotic
N-fixing organism Host plant
Rhizobium Legumes
Actinomycetes Non-legumes
Blue Green Algae Fern

Nitrogen Fixation
Symbiotic Nitrogen Fixation
Each type of host plant has its own specific symbiotic rhizobium.
Rhizobium Host plant
Rhizobium phaseoli Phaseolus vulgaris
Rhizobium japonicum Arachis hypogea
Pueraria sp.

Infection Process
The association of bacteria and the host plant starts with an infection process.

• Free bacteria in the soil are attracted to the young root hairs. A particular protein binds the surface of the root hair cells to the bacterium.
• Roots produce hormone - indoleacetic acid - stimulated by the bacterium. The root hair curls and the bacterium invades it.
• The bacterium migrates to the root cortex via infection thread.
• Cortical cells divide and form nodules.
• Bacterium increases in size and forms bacteroid (nodule bacteria).
The nodule is the intimate association of bacteroid with the vascular tissue. In this association, four conditions favor the reduction of nitrogen to ammonia.

1. The enzyme nitrogenase with magnesium as the cofactor for the reaction
2. Adenosine Triphosphate, ATP – provided by the host plant
3. Reducing agent, ferredoxin
4. Anaerobic condition – accomplished by leghemoglobin (LHb) through its association with oxygen: LHb + O2  LHb . O2
This association effectively removes the oxygen from the vicinity of the nitrogenase and ensures optimal rate of nitrogen fixation. The vascular system transports sugars, water, and minerals to bacteroids and removes fixed N2 as amino acids, amides, and ureids. The reduction of atmospheric N2 to ammonia (NH3), mediated by the enzyme nitrogenase, can be summarized by the following reaction.
N2 + 6H+ + 6e- + nMgATP 2NH3 + nMgADP + Pi (1)
Non-symbiotic Nitrogen Fixation

Nitrogen fixation is through bacterial, lightning, and industrial processes.
Aerobic Bacteria
The organisms involved in the fixation of atmospheric nitrogen include blue-green algae and certain free-living bacteria such as Azotobacter, Rhodospirillum, and Clostridium. Rhodospirillum is a photosynthetic organism, whereas Clostridium and Azotobacter are saprophytic organisms of aerobic and anaerobic characteristics, respectively.
Atmospheric Sources of Fixed Inorganic Nitrogen
Electrical discharges in the atmosphere (lightning) result in the conversion of some elemental nitrogen to oxides of nitrogen such as nitric oxide (NO) and nitrogen dioxide (NO2). The electrical arc of lightning produces extremely high temperatures, which in the presence of oxygen and elemental nitrogen result in the production of nitric oxide. This gas further combines with oxygen to form nitrogen dioxide, which upon being absorbed in water produces nitric acid.

Equations for these reactions are:

N2 + O2 2NO (2)

NO + O2 → NO2 (3)

3 NO2 + H2O→ 2 HNO3 + NO (4)
The nitrogen eventually enters the soil as nitric acid in rainfall.
Ammonification
Whenever organic residues return to the soil, the activity of heterotrophic organisms slowly returns the nitrogen in these plant and animal wastes to the inorganic form. These organisms utilize carbon compounds as energy source and are involved in the decay and simplification of organic wastes. In the process of decomposing organic residues in the soil, hydrolytic, reductive, and oxidative microbial activities simplify the nitrogenous materials such as protein and amino acids.

Hydrolytic activities involve enzyme systems from the heterotrophic bacteria and saprophytic organisms of all types – the addition of water to the compound separate a complex molecule into simpler forms. Oxidative reactions result in the production of carbon dioxide. Both reactions continue to simplify the nitrogenous material until in the end, they produce a mixture of organic acids, carbon dioxide, and ammonium ions (see reaction below).

RCHNH2COOH R – COOH + CO2 + NH4+ (5)
H2O

Another example of ammonification is the hydrolysis of urea represented in the following reactions:

CO(NH2)2 + 2H2O (NH4)2CO3 (6)

(NH4)2CO3 2NH4+ + H2O + CO2 (7)

In the first reaction, urea in the presence of urease and possibly other enzymes combines with water to form ammonium carbonate. Ammonium carbonate further hydrolyzes to form the ammonium ion - it immediately becomes chemically attached to the soil colloids (second reaction).

If we measure the pH of the soil near a hydrolyzing urea prill, the pH would rise due to the formation of ammonium and hydroxyl ion. Temperature, numbers of soil organisms, water, and amount of plant residue present in the soil govern the rate of ammonification reaction.
Ammonium Fixation
Another possible fate of ammonium in the soil is reaction with certain silicate clays, particularly those with expanding lattice such as montmorillonite, vermiculite, and illite. This reaction, commonly known as fixation, traps ammonium ions between the layers of clay platelets and occurs in subsoils. The ammonium ion is slowly available to nitrification and to plant absorption. Ammonium fixed in this mechanism can be replaced by large amounts of cations (Ca++, Mg++, H+), which tend to expand the lattice of the clay. Addition of cesium, rubidium, and potassium contract the clay lattice and tend to render the fixed ammonium even more unavailable.
Nitrification
Ammonium ions produced by ammonification process in the soil immediately enter other reactions. One common fate of ammonium produced is the biological process of nitrification.

Other fates include:
• Direct absorption by higher plants
• Possible fixation by certain clay minerals
• Direct utilization by heterotrophic organisms engaged in decomposition of carbonaceous residues

In nitrification ammonium is converted to nitrite (NO2-) and finally to nitrate (NO3-). There is a net loss of eight electrons in this conversion process – autotrophic organisms, Nitrosomonas and Nitrobacter use the energy generated.

NH4+ + O2 NO2- + H2O + 2H+ (8)

2NO2- + O2 2NO3- (9)
The production of nitrite in the first reaction can be toxic to some plants. The generation of two protons (hydrogen ions) results in acidification of soils upon continued application of ammonia or ammonium sources of nitrogen.
The environmental factors that control the rate of nitrification include:
• Oxygen supply – Bacteria requires the oxygen. Good soil structure or good tilth facilitates the exchange of gases between the soil and the above ground atmosphere promotes nitrification.
• Population of nitrifying organisms - An increase in the number of nitrifying organisms increases nitrate production.
• Soil pH - A very acid soil depresses the number of nitrifying organisms present. An optimum rate of nitrification occurs around pH 8.3 to 8.5 due to the availability of calcium and phosphorus. Liming markedly enhance nitrification in acid soils.
• Temperature - Below 50 C and above 400 C, nitrification is very slow. The optimum temperature is between 300 C and 350 C.
• Soil moisture - Nitrifying bacteria are more sensitive to excess soil water than they are to extremely dry conditions-excess water reduces the soil oxygen supply. As soil moisture tension declines to zero, nitrate accumulation drops to zero.
• Concentration of ammonium ions - Numbers of bacteria tend to increase with the presence of large amounts of ammonium ions. When the C/N ratio is extremely wide (60:1), the heterotrophic bacteria population decomposing the carbonaceous residue, absorb any ammonium released from the organic matter. When the C/N ratio drops to 20 or 25:1, a net release of nitrogen occurs from ammonification and ammonium ions are available for nitrification. The rate of conversion of nitrogen to nitrate in soil is dependent on the carbon: nitrogen ratio of the readily oxidizable carbonaceous residue present in the soil.
• Nitrification inhibitors - They are toxic to nitrifying bacteria and nitrification slows down. One compound, 2-chloro-6-(trichloromethyl)-pyridine marketed as N-SERVE (nitrapyrin) is toxic to Nitrosomonas The compound when applied to the soil slows the conversion of ammonium to nitrate and improves the recovery of ammonium nitrogen under conditions where leaching or denitrification occur. Another example of nitrification inhibitor is potassium azide (KN3). The effectiveness of nitrification inhibitors is dependent on conditions conducive to nitrogen losses, leaching, and denitrificcation. Its primary use will be to areas receiving large amounts of rainfall or supplemental irrigation water and having a coarse textured soil such as irrigated sands.
Dentrification
A series of denitrification reactions represent a net reduction in the oxidation state of nitrogen from the nitrate or nitrite forms to more reduced compounds – nitric oxide (NO), nitrous oxide (N2O), and elemental nitrogen (N2). The final products are volatile gases and unavailable to plants. Bacteria that carry out this process are heterotrophic in nature – they decompose carbonaceous residue and obtain the energy and carbon supply from organic compound oxidation. Denitrifying bacteria: Bacillus, Psuedomonas and Clostridium, are aerobic, but in the absence of oxygen, they are able to utilize the nitrogen atom in nitrate as terminal electron acceptor. A proposed scheme for denitrification by microorganisms follows the path below.




Ammonia Volatilization
When we add ammonia or ammonium salts to a soil, there is a possibility of volatilization of ammonia to the atmosphere. The following reaction represents the equilibrium between these two forms of nitrogen and water:
NH3 + H2O  NH4+ + OH- (10)

The factors discussed below affect the equilibrium:

• In alkaline soils, containing high concentration of hydroxyl ions, the equilibrium shifts to the left and ammonia gas forms at the expense of ammonium loss.
• Loss of water from the system tends to shift the equilibrium to the left.
• Around hydrolyzing urea particle, the soil pH rises rapidly due to the formation of OH-. This will drive the reaction to the left forming free ammonia and ammonia volatilization may occur if the reaction is taking near the soil surface.

Water is important in converting ammonia to ammonium. Subsequently, soil colloids adsorb ammmonium and hold it tightly against leaching. Low cation exchange capacity can cause ammonia volatilization due to insufficient exchange capacity to adsorb ammonium ions from the ammonia-water-ammonium system.