Soil Test Correlation and Calibration
The purpose of soil test correlation is to develop an extraction procedure for a particular nutrient. Therefore, we estimate our results and try to correlate the extractable form of the nutrient with the amount extracted chemically. This procedure is known as soil test correlation.
The area to be sampled depends on the area for which the test is done. But there must be a minimum of 20 to 30 samples for the area. Suppose we are extracting for P.
We grow the crop in the greenhouse supplying other nutrients except for P. We may grow maize or fast-growing grass which is grown for about 6-8 weeks.
The crop is harvested, and the dry weight of plant material and the concentration of P. in the tissue are analyzed. From dry weight and concentration of P, the uptake of the nutrient is calculated.
Uptake of P = Yield X concentration P in tissue. The uptake = Available P in soil
The next steps are to find a reagent in the laboratory that would extract from the soil the same amount of P. that was available to the plant.
This is an ideal condition. But we must be able to find an extractant that would extract an approximate amount as the plant uptake of the “available” form of the nutrient.
(1) NH4 CAC (2) IN HCL (3) 0. INH CL
(4) Bray test (P-1) 0.03N – NH4 F in 0.5 N HCl – This measures P bounded to A1 – and Fe –compounds by forming (AIF6) – with AL and Fe particles thus releasing H204.
We calculate the correlation coefficient for each of the reagents that are selected and choose the reagent that gives the highest correlation coefficient.
Through statistics, we use multiple regression
Y = a + b (O.M) + c (P –test)
r2 = % of availability that can be explained by O.M. and P-test. The more the factors used the better the correlation test.
Y = a + b in (Zn test) + CpH+ d (silt + clay) + E (Fe & A1 oxides) + f (O.M).
The study is done in a Greenhouse rather than in the field because
It is easy to control many variable factors: Temperature, moisture, light, rooting volume, aeration, other nutrients, insects, and disease in the greenhouse.
Sample-taken to the greenhouse more nearly represent the soil in which plants are growing.
We measure uptake of P rather than yield because (1) uptake has a more nearly linear curve since uptake will increase continuously.
Read Also: The Interpretation of Soil Test Data
Soil Test Calibration
After the greenhouse experiment, we have to calibrate this value further under field conditions.
Information Needed
The shape of the yield curve for a particular soil to know the point of maximum point of excess.
Amount of nutrient soil
Amount of available nutrients in the soil. That is done by soil analysis.
Calibration of soil test methods can be carried out in the greenhouse but must be confirmed in the field. Critical levels of available nutrients and optimum fertilizer rates are determined by testing several rates of each nutrient and obtaining a yield curve.
Local experience is employed to determine a base rate considered to be optimum and economically feasible. Treatment levels are then spaced below and above the base rate.
Experiment treatments must be adequately randomized and replicated to facilitate statistical analysis of the yield and soil test data. The best statistical design is the simplest that will produce the desired result.
Each soil should be properly characterized physically, chemically, and taxonomically in order to facilitate the extrapolation of soil test information to similar soils in similar environments.
Each essential economic crop of the area and essential nutrient elements should be calibrated because crops have different nutrient requirements.
In some developing countries like Nigeria for instance, mixed or multiple cropping should be considered in soil calibrating work in addition to monocropping.
Read Also: Soil Sampling Procedures
