Soil Nutrients Point Wise Notes For Competitive Exam

Soil Nutrients

• Higher concentration of phosphorus causes deposition of iron on root surfaces or just inside the root, possibly as iron phosphate.

• Iron is oxidized in the rhizosphere.

• Grasses have more oxidizing power than legumes.

• Iron availability is reduced with oxidation.

Iron availability is higher in submerged soils due to reduction of insoluble ferric compounds to soluble ferrous compounds.

• Organic matter improves availability of iron.

Zinc in earth surface is mainly found as sphalerite a sulphide mineral.

• Zinc also exists as a constituent of organic matter.

• Zinc transformations are mainly influenced by pH.

• Within 24 hours after application of fertilizers only 2% soluble zinc is present in soil solution in soils with pH above 7.0.

• Adsorption of zinc on clay complex and organic matter is high at higher pH.

• Zinc availability is more in acid soils.

• Zinc availability reduced with higher level of phosphates due to low solubility of zinc phosphate.

• Zinc availability reduced with high organic matter as zinc is strongly adsorbed on humus.

Zinc deficiency is common in calcareous soils due to adsorption of zinc or carbonates of Ca & Mg and formation of insoluble carbonates.

• The primary minerals of manganese are present chiefly as oxides and to some extent as carbonates and silicates.

• Secondary minerals of manganese are pyrolusite and manganite.

Manganese in the soil is generally considered to exist in divalent (Mn2+), trivalent (Mn2O3) and tetravalent (MnO2) forms in equilibrium.

• Availability of manganese increases in acid soils as in the case of iron.

• In acid soils, the available manganese is up to 16 per cent of total manganese content of soils, while in alkaline soils it seldom exceeds one percent.

• Manganese availability is more in submerged soil due to reduction of insoluble manganic compounds to soluble manganese compounds.

Tri and tetra-valent forms of Mn are present under high pH and oxidizing conditions.

Calcium carbonate decreases manganese availability due to strong binding of Mn2+ on calcium carbonate.

Organic matter decreases the availability of manganese.

Copper in earth crust occurs mainly as sulphides and most abundant mineral of copper is chalcopyrite (CuFeS2).

• Solubility of copper fertilizers decreases with a rise in soil pH. It is due to adsorption of Cu2+ on soil colloids and due to precipitation as copper hydroxides.

Copper is fixed strongly on organic matter.

• Molybdenum is present in soil as MoO42-.

• Unlike other micronutrients, molybdenum availability is high at high pH.

• Oxides of Fe and Al increase the adsorption of molybdenum.

• Higher level of P fertilizers increases availability of molybdenum, but sulphur reduces it.

• Higher soil moisture increases molybdenum availability due to decrease in ferric iron which is responsible for adsorption.

• Organic matter increases availability of Fe and B, but reduces availability of Mn, Zn and Cu.

• Acidity increases the availability of Fe, Mn and Cu.

• Alkalinity increases the availability of Mo.

• In calcareous soils the availability of Zn and Mn decreases.

• Submergence increases the availability of Fe, Mn and Mo.

• Based on the temperature requirement, microorganisms are grouped into psychrophiles (< 10 °C), mesophiles (20-40 °C) and thermophiles (>40 °C).

Actinomycetes dies under anaerobic conditions.

• The optimum moisture for aerobic bacteria is 50 to 75 per cent water holding capacity.

Actinomycetes predominates in dry regions.

• Most microflora of the soil are mesophiles and grow well at 15 to 45 °C with optimum being 37 °C.

• Direct sunlight is injurious to most microflora.

Bacteria grows well within pH range of 6.5 to 8.0.

Fungi prefer acidic pH range (4.5 to 6.5).

Actinomycetes prefer slightly alkaline conditions.

• Fungi involved in decomposition of cellulose are Penicillium, Trichoderma, Aspergillus and Fusarium.

Lignin is decomposed by Basidiomycetes fungi.

• Protein is decomposed by fungal species like Fusaria, Aspergillus, Mucor and Citromycetes.

• Mycorrhizae that live inside roots (endotrophic) are Phoma and Rhizoctonia.

• The ectotrophic mycorrhizae which develop on root surfaces are Amanita and Boletus.

• Rhizobium is a heterotrophic, aerobic bacteria.

• The Rhizobium present in the root nodules are known as bacteroids and each host cell contains several bacteroids.

• Though Rhizobium is aerobic, nitrogenase enzyme functions only in the absence of oxygen.

• The host cells that contain bacteroides produce a reddish protein called leghaemoglobin.

Leghaemoglobin has high affinity for oxygen and even the low level of oxygen present around the host cell is absorbed.

• There are some actinomycetes which fix nitrogen on non leguminous plants.

Casuarina alder lives in association with Casuarina and fixes atmospheric nitrogen.

Free living bacteria capable of fixing atmospheric nitrogen are Azotobacter chroococcum, A.vinelandii and Clostridium pasteurianum.

• Azotobacter is a heterotrophic bacteria.

Clostridium is an anaerobic bacteria.

Azospirillum fixes atmospheric nitrogen in grasses.

• Nitrification ceases below pH 5.0, the optimum being 6.5 to 7.5.

• The optimum temperature for nitrification is 30 °C to 35 °C.

• The important denitrifying bacteria are Pseudomonas and Bacillus.

• The optimu C:N:P ratio is 100:10:1 and if the C:P ratio is more than 100:1, immobilization of phosphorus occurs.

• The C:S ratio of 50:1 is critical.

• The pH of most acid and alkaline soils converges between 6 and 7 within 2 to 3 weeks after flooding.

• Hydrates of ferric oxide are reduced to ferrous compounds within two to three weeks after submergence. As a result, the soil colour changes from brown to grey.

Manganese is more rapidly reduced and rendered soluble than iron compounds.

Actinomycetes and fungi are absent in submerged soils.

• Availability of zinc is reduced under flooded conditions due to precipitation as zinc hydroxide (Zn(OH)2 or as zinc carbonate (ZnCO3) due to high concentration of carbon dioxide.

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