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Basics of Soil Chemistry

Basics of Soil Chemistry

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Basics of Soil Chemistry

Soil chemistry:

• “Soil fertility” is defined as the quality that enables a soil to provide the proper nutrient compounds in proper amounts.

• “Soil productivity” is defined as the capability of soil for producing a specified plant. • ’Criteria of essentially of elements were proposed by Arnon in 1954..

• “D.J. Nicholas” advanced the term “Functional (or) metabolic nutrient”.

1. Deficient: when an essential element is at lower concentration that severely limits yields and produces deficient symptoms.

2. Insufficient: When the level of an essential element is below that required for optimum yields (or) when there is an imbalance with another nutrient.

3. Toxic: When the conc. of either essential (or) other elements is sufficiently high to reduce plant growth severely.

4. Excessive : When the concentration of an essential plant nutrient is sufficiently high to result in corresponding shortage of another nutrient.

Establishment of essentiality of elements:

 Nitrogen – Theodore de Saussure;

Mo – Arnon and stout,

Na – Brownell and wood; Co-Ahmed and evans.

• Some elements like Ca and Mg counteract toxic effects of other elements – balancing function.

• Ionic forms of nutrients absorption:

Nitrogen – NO3 ; NH4+; Mo – MoO4–; P – H2PO4-, 1; C – CO3–; HCO3-; S – SO4-2, B – BO3-3, HB4O7-.

• Absorbed nutrients (exchangeable ions) are always in equilibrium with dissoloved fraction.

• Hydrogen ions are leased to the medium in exchange for metal cations and OH- (or) HCO3- are released in exchange for anions.

• In passive absorption ions move into a cell along their electrochemical potential gradient without expenditure of energy by the cell.

1. Mass flow hypothesis – Eg: Cl-, SO4-2, NO3-, Ca and Mg which occur in sufficient quantity in soil solution.

2. Diffusion: Along concentration gradient – Mulder. Eg: P, K, Zn, Cu, the concentration of which is low in soil solution.

3. Ion exchange:

a. Carbonic acid theory: Na+, K+, Ca++.

b. Contact exchange hypothesis – Jenny and Over Street. It takes place between plant root and clay surface due to overlapping of oscillation volumes around the adsorption site.

4. Donnon equilibrium theory: When a different ion of similar charge is present in the external medium there will be no exchange.

Active absorption mechanisms:

1. Lundegardh’s hypothesis: Uptake of salt dependent of “anion” respiration. Transport of anions occurs through “cytochome system”.

2. Carrier hypothesis: Requires ATP. kinase carrier + ATP Carrier Pi + ADP Carrier Pi + ion Carrier Pi ion Phosphotase Carrier Pi ion Carrier + Pi + ion


• Soil organic matter = organic carbon X 1.724.

• Surface soil has higher ‘N’ content than deeper soils.

• Organic fraction of soil nitrogen is 98%.

Mineralisation : Organic pool Inorganic pool  

1. Aminisation: Hydrolytic decomposition of proteins results in the release of amines and amino acids.

2. Ammonification: The amines and amino acids released are coverted to ammonium compounds.

3. Nitrification : Biological oxidation of ammonium ions to NO3 is known as nitrification.

Aminisation, ammonification – Heterotropic bacteria. Nitrification – autotrophic bacteria; NH4 NO2- – Nitrosomonas, nitrococcus NO2 NO3- Nitro bacter.

• The nitrosomonas, nitrobacter are usually refered to as the “nitrobacteria”.

• High C:N ratio prevents the release of ammonia. If the ammonia is present in too high conentration, it constrains the nitrification.

• Nitrification will be appreciable at filed capacity of soil moisture content and takes place even at (or) below the wilting coefficient.

• Nitrification takes place at a pH 5.5 to 10. • If C:N ratio is more than 30:1 favours immobolisation and less than 30:1 favours mineralization.

• Several clay minerals with 2:1 type structure have the capacity to fix ammonium and K ions. “Vermiculite” has the highest CEC and hence high fixing capacity.

• Ammonium fixation by clay mineral is greatest in subsoil than the top soil because of higher clay content of subsoil.

• Leaching losses of NO3 is low in unirrigated, arid, and semi – arid region and high in humid areas where irrigation is practiced.

• NO3 ion are reduced to N2 in poorly drained and low in aeration soils by denitrification. Eg: Thiobacillus denitrificans; Pseudomonas denitrificant.

• Volatilization: Application of ammonium containing fertilizers (or) urea which hydrolyses to NH3 gas. The escape of ammonia gas into atmosphere is called volatilization. .

• The BNF in different organisms is brought about by enzyme “nitrogenase” enzyme.

Group Rhizopium sps Alfalfa Rhizobium militoti Clover R. trifoli Soyabean R. japanicum Lupini R. lupine Bean R. phaseoli Peans & vetch R. leguminoserum

• N2 fixation by symbiosis with non – legumes. Casuarinaceae – Casuarina; Betulaceae – Alnus Actinomycetes – Frankia.

• Rhizobium – symbiont, Azactobacter – free living Azospirillum – associate symbiont in maize, sorghum, bajra, ragi crops. BGA – Irrigated rice – Freeliving Azolla fern BGA – Rice fields. Mycorrhizae (symbiosis) – Plant root fungus – all crops.

• The major loss of N from most soils is that removed by “crop plants”.

• In plants the total N content ranges from 0.2 to 4.0%,

 • Nitrogen imparts dark colour to plants.

Phosphorus: –

• Phosphorus has been considered “key to life”.

• P contaning Ca minjerals in soils are mostly apatites – Ca10(PO4)6.

• Flour apatite – 3Ca3(PO4)2.CaF2, Carbonate apatite – 3Ca(PO4)2.CaCO3. Hydroxy apatite – 3Ca3(PO4).Ca(OH)2, Oxy apatite – 3Ca3(PO4)2.CaO. Tricalcium phosphate (TCP)– Ca3(PO4)2 OCP – Ca8H2(PO4)65H2O. Dicalcium phosphate – CaHPO4. 2H2O MCP – Ca(H2PO4)2. H2O • Of all the above, the readily available for plant are mono calcium phosphates.

• The apatite minerals are the most insoluble and unavailable of the group.

• Phytin is a calcium – magnesium salt of “phytic acid”. Phytin is the most abundant of the known organic ‘P’ compound.

• Monovalent forms are preferred by plant than divalent (or) trivalent.

• H2PO4- proportion is high at pH 5.0, HPO4-2 is high at pH 9.0 (very alkaline soil).

• 6.5 to 7.5 pH is considered to be best for optimum availability of ‘P’.

• Phosphate fixation is more in 1:1 type of clay minerals.

• Gibbisite – Al2O3 3H2O, Goethite – Fe2O3 3H2O.

• The concentration of ‘P’ in the soil solution is the measure of “intensity factor” (I) of ‘P’ nutrition.

• The source of soil solution P replenishment is known as the quantity factor (Q) of P nutrition.

• ‘P’ compounds in soils are categorised in three major groups

1. Readily available – soil solution P

2. Slowly available – labile P

3. Very slowly available – non – labile P.

• Tropical clay soils high in Fe and Al need a high level of Q to assure a given I level. Potential buffering capacity (PB C) = Q/I.

• Total P content in plant ranges from 0.03 to 0.3%; In seeds upto – 1.5%.

• Excess of P causes trace element deficiency particularly Fe and Zn.

• Dificiency of P causes bronzing of leaf margins in cereals.

Orthoclase – KAlSiO3O8 – K Feldspar.
• 1:1 type minerals like ‘kaolinite’ do not fix ‘potassium’.

• Illite fixes – K ions. Al+3 cations will occupy the K selective binding sites.

• All K above the optimum level is considered as a luxury . Luxury consumption becomes particulary wasteful.

• K regulates water movement within plant cell.

• ‘K’ deficiency leads to “leaf scorching” (or) popularly known as “tip burning”.

• Reduced crop yields without the appearance of definite symptoms is called “hidden hunger”.

• The function of K is catalytic in nature.

Secondary nutrients include Ca, Mg & S.

Ca: –
• Ca is mostly present as primary minerals such as “basic plagioclase”.

• Ca is the most dominant cation in all soils. Calcite – CaCO3.

• Light textured soils suffer greater loss of Ca than heavy soils.

• Ca is essential for formation of cell wall.

• Deficiency symptoms:

1. Failure of terminal bud development.

2. In fruit trees “dieback”.

3. In guava cold leaves are chlorotic with red brown spots.

4. Severe loss of colour in young leaves of “brassica sps”.
Mg: If large qualitative of Mg salts are added to soils containing montmorillonite, the minerals are altered to vermiculites.

• It is low in acid soils.

• Dolamite limesote is the richest source of Mg.

• Mg is constituent of chlorophyll and maintains dark green colour.

• In sandy soils too much ‘Mg’ may create ‘K’ deficiency.

S :-
• ‘S’ behaves like nitrogen in many respects

• Sulphates are present in subsoils that contain 1:1 clays.

• ‘S’ is present in the oils of plants of mustard and onion families.

• Sulphide production in paddy soils causes a disease known as “Akhiochi” (or) H2S injury.

• Due to H2S injury, bubbles will be coming out from the affected areas. Soil colour will be black.

• “Akhiochi” disease occurs in soils that are lacking ‘iron’.

Micronutrients: Viets classification (1962).

Pool A : Water soluble ions. It is usually non existent for Cu and Zn, very small for Fe & Mn. Low redox potential and low pH increase Mn & Fe but not Cu and Zn.

Pool B: Cations exchangeable by NH4. It is small for Cu and Zn.

Pool C: Cations exchangeable only by strong chelating agents like EDTA, DTPA.

Pool D: Micro – nutrient cations held in secondary clay minerals.

Pool E: Cations held in primary minerals.

• Chlorides occur mostly in Pool A and Pool E. Borates – Pool E.

Molybdates – Occur in all the pools.

Mo – Ultra basic rocks;

Cu – basic rocks.

• Micronutrients are more concentrated on the surface soil.

• Presence of moisture reduces availability of Cu & Zn and increases Fe & Mn.

Low Si/Mg ratio – Leads to fixation of Zn in soils.

• More of CaCO3 percent in soil, leads to the less availability of Fe, Mn, Zn, Cu and increases the Mo availability.

• Boron is available in more amounts in coarse soils.

• ‘B’ deficiency is commonly associated with dry areas.

• Antagonism effect: Most of the micro nutrients are not available due to the antagonism effect shown by them. Mn vs Fe, Fe vs Zn, Cu vs Mo.


It is the only non – metal among the micronutrients.

• ‘Tourmaline’ is the main boron containing mineral found in most soils.

• Boron fixation is more in fine textured soils. • Borax – Na2B4O7 . 10H2O.

• ‘B’ indicator plants – sunflower. cabbage, cauliflower, sugarbeet.

• B deficiency causes heart rot/crown rot of sugarbeet. topsickness of tobacco.


Chalcopyrite – CuFeS2 Chalcocite – Cu2S

• ‘Cu’ deficiency leads to ‘exaunthema’, die back in citurs.

• In wheat, glumes are empty.

Fe :-

Pyrite –FeS2, Siderite – FeCO3, Hematite – Fe2O3.

• It is structural constituents of pigments in micro – organism

• Black pigment – Clalics – Contain iron.

• Fe – EDTA – Ethylene diamine tetra acetate – for acid soils.

Fe – HEDTA – Hydroxy ethylene diamine tetra acetate – for alkali soils.

Fe – DTPA – Diethylene triamine pentaacetate – for alkaline soils.

Fe – EDDHA – Ethylene diamine dihydroxy acetate – for calcareous soils.

• Deficiency causes white foliage in nurseries.


Legumes show ‘Mn’ deficiency.

• Red soils contain more ‘Mn’.

• It is constituent of nitrite – reductase and hydroxylamine reductase.

• It involves in ‘Hill reaction and Calvin cycle’.

• Deficiency symptom:

“Pahala blight” of sugarcane. “Grey speck” of oat

• Paddy is more susceptible for Mn deficiency. Symptoms develop early on 3rd emerging leaf.

• Mn toxicity is very common on acid soils.


Molybdenite – MoS2 Wulfenite – PbMoO4 Powellite – CaMoO4.

• It is a constituent of “nitrate reductase”.

• Deficiency causes “whip tail” in cauliflower.

• The toxicity of molybdenum on animal health was “teartness” (molybdenosis).

Zn :-

the retention of Zn in soils to other cations H>Zn> Cu>Mg>K. Sphaelarite – Zns  
• It requires for the synthesis of ‘tryptophan’.

• It is a constituent of alcohol dehydrogenase and galactose oxidase enzymes.

• Deficiency in rice – ‘khaira’.

• ‘Zn’ deficiency is cotton – little leaf:

Maize – white bud.

• Critical pH range for availability of Zn is 5.5 to 6.5.

Problem soils:

• Leaching is problem in humid regions and salt accumulation is a problem in arid and semi – arid regions.

• Soil becomes “Saline” in areas where annual evaporation exceeds annual precipitation.

• Beans are damaged by chloride in the soil solution at equal ostmatic pressure, which sulphates do not harm the crops.

• Grasses are more tolerant to chlorides than sulphates at equal O.P.

• At very high O.P, magnesium is more toxic than ‘Ca’ than Na.

• A soil which does not permit the normal growth of plant is a “Problem soil”.

1. Acid soils: Two types in AP..

a. Laterite soils – These are open textured with “massive structure”.

b. Ferrugenous red soils – derived from crystalline metamorphic rocks.

• Humus is a final product of organic matter decomposition.

• Lime as a reclaiming agent: calcite limestome – CaCO3, dolamite limestone – CaCO3 MgCO3. Quick lime – CaO, Blast furnace slage – CaSiO3 (by product of iron industry).

• Due to high soil acidity, Al, Mn, Fe, become highly solubulized and available in toxic amounts.

• In acid soils activity of bacteria and actinomycetes is adversely affected. Fungi are active in acid pH.

• All the micronutrients (Fe, Mn, Cu, Zn, Co) except Mo are available in the acid pH. Mo deficiency has been identified in leguminous crops.

• In saline soils increasing osmotic pressure result in decreasing water availability to plants.

• In alkali soils high exchangeable Na depress the availability of Ca & Mg.

• Reclamation of saline soils – By using good quality irrigation water

Depth of drainage water Ddw leaching requirement = = Depth of irrigation water Diw

• Reclamation of alkali soils- use of gypsum; use of pyrites and molasses.

• Reclamation of saline alkali soils – use of good quality irrigation water.

• Salt tolerant varities. Rice – CSR 1, 2, 3, Wheat : Kalyana sona, Groundnut – TMV – 10, Kadiri. Ragi – Godavari, Sarada, Pea – P163, Sunflower – EC 68413, 68414, 68415.

• High seed rate and closer spacing have been tried under saline conditions as compared to normal conditions. • Ammoium sulphate and CAN are superior to urea in alkali soils.

• Confusion of leaf hopper damage with boron deficiency in alfalfa.

• Yellowing (or) chlorosis is caused by deficiency of [N, Mg, Fe & Mn]. Deficiency of N & Mg are seen in the lolder leaves while Fe, Mn deficiency are seen in the younger leaves. Mo deficiency often look like mild N deficiency in legumes.

• Phosphate – Bray’s modified test K – Sodium cobltinitrite test Indicator plants:- N – Cauliflower, cabbage K- Potato Mg – Sorghum (CSH – 1) Mn – Rice (IR – 8) Mo – Tomato, Cauliflower P – Rape seed Ca – Cauliflower Zn – Hybrid maize Cu – Wheat B – Hybrid maize, sunflower.

• Sunflower pot cultural technique for boron.

• Sackett and Stewart technique (Azatobactor test for P & K).

• The Mulder’s Aspergillus niger test for Cu & Mg.

• Mehlich technique for available K2O.

• Mehlich – Cunninghamella plague mentod for ‘P’.

• The fertility gradient approach of “Rammoorthy”.

Nutrient estimated Method Available N Alkaline KMnO4 Available P2O5 Olsen’s method – 0.5M NaHCO3 Available K2O Neutral – N ammonium acetate Readily oxidizable organic carbon Walkley and Black’s rapid titration method.

• In case of typical black soils % extractable P = 100 – Clay %.

• “Gross tetany in cattle” is caused by eating forages deficient in Mg.

• Summer drought aggravates Fe deficiency in many plants.

• Salt tolerant fruit crop – Datepalm; coconut.

• Number of salinity classes in the USDA classification of irrigation water are 4.

• Cell sap producing dark blue colour with “diphenylamine” indicates “High NO3- N”.

• The number of sites on soil colloids at which exchangeable “K” held are 3.

• Use of fertilizer containing high sulphates adversely effect utilization of micronutrient Mo.

• Water containing B more than 2.0 ppm is unsuitable for irrigation.

• Ideal Ca & Mg ratio for maximum availability of Mg in the soil is 7:1.

• The less soluble compound of ‘B’ good for application on sandy soils to correct ‘B’ deficiency is colomonite.

• Soil dispersion is caused due to presence Na of in excess amount.

• Crop which needs more Ca – Soyabean.

• Elements required for nitrifying bacteria are Ca & P.

• Ferns require ‘Al’ in their nutrition; green algae – Scenedesmus requires vanadium.

• Anion fixation is more in kaolinite.

• Phosphorus mineralization occurs when CP ratio of organic residue is < 200:1.

• Nutrient which impart vigour and disease resistance in plant is K.

• ‘A-value’ technique is used to study availability of nutrients in soils.

• In case of immobile nutrients, the deficiency symptoms first appear on young leaves.

• Application of O.M. to soil reduces ‘P’ fixation.

• In Jenny’s pot culture test, the crop grown for assessing the fertility status of soils is lettuce.

• Divalent form of Mn exists in acid soils.

• Metallic ion capable of forming into an anion in alkaline pH is Zn.

• Zn application particularly increase crop yields in saline sodic soils

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