Characteristics of Soil Analytical Methods for Practical Advisory Work
The methods must be precise, reliable, simple, and cheap. Accuracy refers to the correctness of the method while precision refers to the reproducibility of an analytical result.
Methods for the determination of available nutrients take into recognition that the available fraction of the total amount of nutrients in the soil is not uniformly available, viz:
The method for the determination of available nutrients is designed to estimate either the total available fraction or only part of it. The different forms are interrelated.
Plants take up nutrients from the soil solution (water-soluble form or intensity factor).
The maintenance of a high-intensity factor depends (via the exchangeable form) on a high content of easily mobilizable reserves (capacity factor) and on the rate of transfer from one fraction to the other.
Choice of method depends on the special behavior of the nutrient in the soil and the production level, e.g.
Cationic nutrients have a high exchangeable fraction which should therefore give a fair estimate of the available fraction, especially if the reserves are relatively low.
At low production and fertilization level, plants are forced to use largely mobilizable soil nutrients and the methods must place emphasis on the reserves.
Read Also: General Principles of Soil Analysis and Soil Testing
High-yielding plants at high production levels should have a good supply of easily available nutrients in the soil since these crops are selected for high production and not for high utilization of nutrient reserves.
For several micronutrients (Fe, Mn, Zn, Cu.) the chelating reagent DTPA (Diethylene Triamine Pentaacetic Acid) seems to be quite useful and it is applied in several countries.
Chemical soil analysis can be done at any time of the year, unlike field tests which must be done during the growing season.
Evaluation (Calibration) of soil Methods. Calibration: is necessary in order to select the best analytical method for a given soil and production condition.
The main standards, for calibration, are the relative yield and the plant nutrient uptake, of (as a substitute) the nutrient contents.
The Relative Yield is obtained from fertilizer experiments where the nutrient tested is the main factor and solely responsible for the yield obtained.
The Nutrient Uptake of Contents of plants can be used if fertilizer experiments are difficult to carry out (there is no uniform field of deficiency, especially for trace elements). In the range of latent deficiency and optimum supply, the contents of nutrients in plants reflect the contents of available nutrients in the soil and can therefore be used as an index of calibration.
The Final Evaluation i.e the comparison of soil extraction as r2– value (%). The r-value often used is much less adequate and less intelligible, soil methods should have an x2-value of at least 50-70% in order to be useful e.g. for manganese Mn.
The method with the highest r – value is the best method (especially if the regression is rather steep).
The correlation could be improved by using several soil factors in addition to the available fraction (Multi-functional correlation). The calibration of soil methods should not be done within one country but also on a regional basis such as in the uniform area as the arid zone of the Middle East. Good cooperation can save a lot of individual calibration work.
Read Also: Plant Nutrient Essentiality for Proper Growth
