Nitrogen deposited in the soil may undergo mineralization; this is the conversion of organic nitrogen to inorganic plant available forms (nitrate (NO3-) and ammonium (NH4+)).1 Nitrate is a highly mobile nutrient in the soil. It is negatively charged and so cannot be held on to by negatively charged soil (clay and silt) particles. This is why it is vulnerable to being leached down the soil profile. Nitrogen is a necessity in crop growth as shortage of this nutrient lead to poor crop growth, reduced yield and economic loss to the grower. A grower can avoid these disasters by employing good management practices such as; long term fallow treatment, leguminous cover cropping and regular manure application which help to maintain balanced nitrogen level in soil.
Growers are often advised to know the nitrogen status of their soils so they can be accurate in their application of additional nitrogen. Knowledge of soil nitrogen level also helps reduce the amount of environmental pollution that occurs from runoff of excess nitrogen in the soil. Soil nitrogen level is mostly determined by doing a soil test analysis. In addition to this, a grower can observe for deficiency symptoms in plants or do a plant analysis to know if his soil is nitrogen deficient.
Nutrient deficiency symptoms on the crop
Deficiency symptoms on crops usually occur when there is a severe shortage of plant available nitrogen. Visual symptoms of nitrogen deficiencies include:
• Pale green to yellow leaves: This is a consequence of insufficient production of chlorophyll in leaves. Nitrogen is a key component of chlorophyll in plants therefore its deficiency reflects in chlorophyll production. Chlorophyll is the substance responsible for the green pigmentation in leaves and stems.
• Stunted plant growth: A visible sign in a nitrogen deficient soil is the stunted growth of plants. Nitrogen is necessary for cell division and enlargement. Shortage of this nutrient slows growth and development.
• Reduced crop yield and protein content: This is a consequence of poor growth and development during the plants life cycle. Low protein is especially devastating in grain crops such as bread wheat; where the grain quality is judged by the protein content.
This approach is used by scientists and farmers to confirm deficiency in plants. Sometimes judging a crop’s deficiency merely by looking out for symptoms can be inconclusive. Plant analysis confirms suspected nitrogen deficiency. It also gives specific results on which sections of the field are nitrogen deficient. Plant samples are usually collected from the field and analysed in a soil or plant testing laboratory. Some of the methods used include:
1. Total nitrogen determination using a combustion analyser. The LECO FP‐428 combustion analyser is used in this process
2. Total kjeldahl nitrogen (TKN) digestion procedure
3. Determination of extractable nitrate in plant tissue: ion-selective electrode method
4. Determination of ammonium-nitrogen in plant tissue
Soil test analysis
Some methods for soil test analysis include:
1. Predicting the soil’s nitrogen mineralizable potential: This approach determines the soils potential to convert organic nitrogen to the inorganic plant available form. It can be determined using biological or chemical methods.5,6 In the biological method, a sample of the soil to be tested is incubated in a laboratory. During this process the soil is exposed to conducive conditions (warm temperature and increased moisture) for nitrogen mineralization. The NH4+ and N03- produced during the incubation is then measured.4,5 The drawback to the biological method is that laboratory conditions are hardly ever proper representations of field conditions. It is difficult (if not impossible) to fully simulate the constant fluctuations in temperature or moisture that occur in field conditions.4 The chemical method as the name implies uses chemical extractants to determine the soils nitrogen mineralizable potential through the growing season.6,7 A commonly used chemical method is the Illinois Soil Nitrogen Test (ISNT). This technique uses alkaline hydrolysis to measure the labile nitrogen forms in the soil during the growing season.8
2. Soil nitrate testing: The soil nitrate test determines the actual concentration of nitrate in the soil. It can be carried out before planting (pre-plant soil nitrate test) or during the growing season (pre-side dress soil nitrate test (PSNT). Soil samples are collected from the field and analysed in a soil testing laboratory. During transportation to the laboratory care is taken to store soil samples in cloth bags as this discourages microbial activity leading to mineralization or denitrification.9
The methods listed for soil nitrogen analysis in this article are not exhaustive (please see bibliography for further reading). The choice of which soil nitrogen test to use is often influenced by cost and efficiency. An ideal method for analysis should be accurate and sustainable.
Bibliography and further reading
1. Hofman G, Van Cleemput O (2004) Soil and plant nitrogen. Pp 1-48 International Fertilizer Industry Association, Paris.
2. Lee et al (1996) Comparison of methods for determination of nitrogen levels in soil, plant and body tissues, and water. Communications in Soil Science and Plant Analysis 27: 783-793.
3. Yash PK (1998) Handbook of reference methods for plant analysis, CRC Press Taylor & Francis Group, Florida.
4. Luce et al (2011) Predict soil nitrogen supply in humid temperate soils. Advances in Agronomy 112: 55-102.
5. Stanford G, Smith SJ (1972) Nitrogen mineralization potentials of soils. Soil Science Society of America Journal 36: 465-472.
6. Cordovil et al (2007) Comparison of chemical methods of assessing potentially available organic nitrogen from organic residues applied to a sandy soil. Communications in Soil Science and Plant Analysis 38: 989-1006.
7. Sharifi et al (2007) Evaluation of some indices of potentially mineralizable nitrogen in soil. Soil Science Society of America Journal 71: 1233-1239.
8. Khan et al (2001) A simple soil test for detecting sites that are nonresponsive to nitrogen fertilization. Soil Science Society of America Journal 65: 1751-1760.
9. Griffin et al (2009) Recommended Soil Nitrate Tests. Retrieved from: https://udextension.s3.amazonaws.com/lawngarden/files/2012/10/CHAP4.pdf.