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objectives
A soil analysis is a process by which elements such as P, K, Ca, Mg, Na, S, Mn, Cu and Zn are chemically extracted from the soil and measured for their “plant available” content within the soil sample.
Nitrogen is one of the major elements required for life. It will stimulate above ground growth, and produces the rich green colour that is the characteristic of healthy plants, because of this Nitrogen is essential for plant life. 78% of the atmosphere is covered by molecular Nitrogen (N2); this form of Nitrogen cannot be used by animals. This molecular Nitrogen must first combine with Oxygen or Hydrogen to produce compounds such as Ammonia or Nitrate, or some other organic form of Nitrogen. This is called Nitrogen Fixation. Some Nitrogen Fixation occurs by lightning and some other by blue green algae. However, the bulk of Nitrogen Fixation is preferred by bacteria living in the soil. Some of the Nitrogen Fixation bacteria were living free in the soil, while the others were living within the root nodules of some plants such as soya bean, peanut, beans, clover, alfalfa, etc. Because of Ammonia or Ammonium is produced by the decomposition process, the decomposition of materials in the forest is also a source of Nitrogen. The movement of Nitrogen from the atmosphere into inorganic forms, followed by the incorporation of Nitrogen into plant matter is represented as the Nitrogen Cycle, which is shown in the figure given below.
The rate of plant growth is proportional to the rate of nitrogen supply. If the soil is deficient in Nitrogen, the plants become stunted and pale. However, an excess of Nitrogen can damage the plants just as over-fertilizing the lawn can burn and damage the grass.
Kjeldahl Digestion Assembly, Ammonia Distillation Assembly.
The Kjeldahl method permits the available nitrogen to be precisely determined in the plant and in the soil. The method of determination involves three successive phases which are,
Digestion of the organic material is carried out by digesting the sample with Con. H2SO4 in the presence of CuSO4.H2O as a catalyst and K2SO4which raise the digestion temperature. The organic material decomposes into several components i.e.,
C → CO2, O → H2O and N → NH3
In the organic matter, some nitrates are present, most of which are lost during the digestion. The loss may be disregarded for most soils. Since the amount of NO3- - N is far lesser than the Organic Nitrogen.
2 C6H3 (OH) NH2COO + 26 H2SO4 → (NH4) 2SO4 + 25 SO2 + 14 CO2 + 28H2O
The Ammonia content of the digest is determined by distillation with excess NaOH and absorption of the evolved NH3 is in standard HCl.
(NH4)2SO4 + 2 NaOH → Na2SO4 + 2 NH3 + 2 H2O
NH3 +HCl → NH4Cl
The excess of standard HCl is titrated against standard NaOH using Methyl Red as an indicator. The decrease in the multi equivalence of acid as determined by acid-base titration, which gives a measure of the N content of the sample. The end point is determined by a change of colourfrom pink to yellow.
2 HCl + 2 NaOH → 2 NaCl + H2O
The chemical analysis of the soil for nitrogen is less precise when the requirement for this element needs to be forecast over a longer period of time, as they vary not only with species, but with the phase of growth and season as well. Therefore the chemical test for NO3- and NH4+signifies the momentary status when the sample is taken and measures must be taken instantaneously. The analysis of the extractableNitrogen content of the soil using a given extractable method.
In reaction to crop response study provides a basis of Nitrogen fertility levels, which will rationalize the use efficiency of Nitrogen fertilizer content of the soil are also needed for the evaluation of C-N ratios of soils which give an indication of the process of transformation of organic Nitrogen to available Nitrogen like ammoniated nitrate Nitrogen.
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