The atmosphere contains 70% N2 gas, required in large quantities by crops, but N2 gas is not normally available except to some leguminous plants by virtue of their nitrogen-fixing apparatus (root nodules).
This element is the most likely to be deficient in crop production. Nitrogen is a major soil nutrient needed in large quantities. It undergoes different reactions in the soil depending on soil conditions.
Phosphorus is less abundant in plants compared to nitrogen and is taken up by plants as H2PO4- or HPO42-. Phosphorus is involved in energy transfer reactions, cell division, conversion of sugar to starch, flowering, and fruiting.
It provides resistance to certain diseases and assists in root development, especially lateral and fibrous roots. This article discusses the various reactions of nitrogen and phosphorus in the soil system and their importance in plant nutrition.
Nitrogen Dynamics in Soil Fertility
The major form of nitrogen in the soil is organic, with about 95% of total nitrogen in organic combinations as amino acids, nucleic acids, and nucleoproteins. Inorganic nitrogen constitutes only about 5-6% of total soil nitrogen, present in forms such as NO3-, NH4+, and NO2-.
These inorganic forms could also be present in exchangeable form, fixed form, or in solution. Nitrogen stimulates leaf and stem growth. Nitrogen deficiency causes reduced growth and pale yellowish-green leaves.
The older leaves turn yellowish first since nitrogen is readily moved from older leaves to new growth. If the soil is cold and wet, nitrogen in the soil is less available to plants. Excess nitrogen may cause potassium deficiency.
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Symbiotic Nitrogen Fixation in Leguminous Crops
Legumes and free-living microbes in the soil can fix atmospheric nitrogen in the root nodules of most legumes. Rhizobium and Bradyrhizobium are the two genera responsible for the fixation of nitrogen.
Ammonification in Soil Nitrogen Cycling
There is a need to convert the organic form of nitrogen to inorganic form through a series of biochemical processes known as nitrogen mineralization.
Ammonification is the first step, involving the conversion of organic matter, amines, and amino acids to ammonium. This is affected by heterotrophic general-purpose organisms, usually facultative anaerobes. Ammonification also includes the hydrolysis of urea-nitrogen.
Nitrification in Soil Nutrient Transformation
After the conversion of the amino (NH2) group to NH4+ form, there is a conversion of NH4+-N to NO3-N by soil bacteria. The organisms involved are nitrifying bacteria, usually autotrophic aerobes.
Denitrification and Nitrogen Loss in Soils
This is an avenue for nitrogen loss, usually in poorly aerated soils. NO3-N is converted to atmospheric nitrogen and lost to the atmosphere.
Immobilization of Nitrogen in Crop Residue Decomposition
When crop residue is relatively high in carbon relative to nitrogen, available NH4+ or NO3-N is temporarily tied up or immobilized by bacteria that decompose crop residues.
Other Nitrogen Reactions in Soil Systems
Nitrogen fixation involves NH4+, which could react with 2:1 clay and become trapped inside layers, e.g., chlorites, illite, vermiculite. Other processes include volatilization of NH3 and leaching or erosion losses.
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Phosphorus Dynamics in Plant Nutrition
Phosphorus is important in the germination and growth of seeds, the production of flowers and fruit, and the growth of roots. Phosphorus deficiency causes reduced growth and small leaves that drop early, starting with the oldest leaves.
Leaf color is a dull, bluish-green that becomes purplish or bronzy. Leaf edges often turn scorched brown. Excess phosphorus may cause potassium deficiency. About 50% of soil phosphorus is in organic form, including phospholipids, nucleoproteins, inositols, and nucleic acids. Inorganic phosphorus is found as definite PO32- compounds and surface films of PO32- held on inorganic particles.
Definite phosphates include monocalcium phosphate, dicalcium phosphate, tricalcium phosphate, hydroxylapatite (Ca10(PO4)6OH2), fluoroapatite (Ca10(PO4)6F2), Ca3(HPO4)2, Fe and Al phosphates, occluded PO32-, and solution PO32-.
Phosphorus Reactions in Soil Chemistry
Acid soils favor the accumulation of Fe, Al, and Mn. In strongly acidic soils, phosphorus is most abundant as H2PO4-. This reacts with Fe, Al, and Mn to form insoluble phosphates, making phosphorus unavailable in acidic soils.
In alkaline soils, phosphorus is in the form HPO42- (pH 6.5-8.5), where Ca and Mg dominate the exchange sites, reacting with PO32- to form insoluble compounds. Phosphorus is maximally available at near-neutral pH. Phosphorus deficiency is noted by purplish coloration of leaves (phosphorus is immobile in soil but mobile in plants), with effects more pronounced in older leaves.
Other symptoms include stunted growth and delayed maturity. Practical control of phosphorus availability includes (1) controlling pH by liming, (2) band application (2.5 cm to plant), and (3) addition of organic matter to disperse PO32- since it is negatively charged.
Nitrogen and phosphorus are not stable in the soil. They undergo myriad conversions in the soil, depending on soil conditions.
Nitrogen undergoes fixation, ammonification, nitrification, denitrification, immobilization, and other reactions, while phosphorus mainly undergoes fixation reactions in the soil.
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