🧫 SOIL FERTILITY

LECTURE 13

. SOIL FERTILITY EVALUATION

Calibration, Correlation of crop responses, yield prediction,

fertilizer recommendation

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SOIL TESTING RESEARCH IN INDIA

Farming is a business and, like all businesspersons, the farmer operates

farming business for profit. Obviously, to reach a decision before planting a crop,

a farmer needs all of the reliable information from the country’s soil evaluation

programme. In our country, system of fertility evaluation is being modified from

time to time in various stages incorporating crop response data.

Some Important landmarks in soil testing research in India:

Year Landmark

1953 Soil fertility and fertilizer use project

1955 Establishment of Soil Testing Laboratories

1956 All India Coordinated Agronomic Research

Project

1960s Establishment of Agricultural Universities

23 (1/16)

1967 All India Coordinated Research Project on Soil

Test Crop Response (STCR) correlation

1967 All India Coordinated Research Project on

Micronutrients in Soils and Plants

22 (6/6) 8 (6/6) 1970 All India Coordinated Research Project on Long

term Fertilizer Experiments

1980s Emphasis on fertilizer prescription for whole

cropping system based on initial soil tests.

Since Liebig’s time around 1840 many methods and approaches have

been tried to get a precise or workable basis for predicting the fertilizer

requirements of crops. The fertilizer use project initiated in 1953 following a

study by Stewart (1947) was the first systematic attempt in the whole of the

country to relate the knowledge of soils to the judicious use of fertilizers.

The establishment of soil testing laboratories was initiated in 1955-56.

The project on Model Agronomic Experiments on Experimental Farms

and Simple fertilizer trials on cultivators’ Fields was started in 1957. The Soil

Test Crop Response (STCR) correlation work carried out at Indian

Agricultural Research Institute, New Delhi had resulted in the selection of

soil test methods and categorizing the tests into low, medium, and high soil

fertility classes.

Soil fertility evaluation

Soils are very heterogeneous in respect to forms of nutrients they

contain, which greatly complicates the interpretation of Soil Test for

assessing fertilizer needs. The total amount of a nutrient present in a soil

offers little information relative to the amount of that nutrient which is

available. Numerous extracting solutions and procedures have been used to

remove nutrient elements from the soil, but none remove exactly the amount

that plant roots obtain.

This means that, in order to interpret the data, the results from each

analytical procedure must be correlated with the plant response obtained in

field experiments by applying that fertilizer nutrient.

By this calibration, the requirement of fertilizer is calculated for achieving

specific yield target. Soil fertility evaluation preferably employs a particular

method of calibration as per the utility of the outcome.

Crop Response Data

In nutrient rating experiments, soil test data is correlated with the

response of crop. The response is measured in terms of ‘percent yield’ or

‘percent yield increase’. Both represent the ratio of the yield obtained in

unfertilized soil (nutrient limiting deficient soil) to the yield in fertilizer nutrient

applied soil (non-limiting or nutrient sufficient soil). The yield in non-limiting

soil is otherwise known as maximum attainable yield or yield of standard

treatment with all nutrients applied.

While comparing native fertility, in experiments that are conducted at

different locations, percent yield is used.

Yield without fertilizer nutrient Percent yield =

Yield in standard treatment

X 100

While conducting multi location trials, or a pot experiment in a single

location with soils brought from different locations, percent yield increase is

worked out for every level of fertilizer application. The highest yield obtained

in the experiment with all fertilizer nutrient applied, is taken as Maximum

attainable yield

23 (2/16)

Yield at fertilizer

Yield without

Percent yield nutrient level fertilizer nutrient

X 100

increase =

nutrient level

Maximum attainable yield

The soils analyzed by standard nutrient extraction method are

grouped. The soil tests are calibrated into different fertility groups by various

procedures.

Present approaches in formulation of fertilizer recommendations extensively

used in India:

Soil testing programme was initially started adopting the International

soil fertility evaluation and improvement programme (Fitts, 1956). In this

approach much stress is laid on laboratory characterization, followed by

potted-plant studies ultimately leading to field - verification trials. These two

initial steps help in eliminating arbitrariness of field trials. However, this

approach had low adoption as it arrived one critical value below which

economic responses are possible. Later, many improvements were

suggested.

AGRONOMIC APPROACH

This is based on fertilizer rate experiments (recording yield at

increasing nutrient levels) conducted at many locations. The level at which

yields are high are recorded and averaged. From these results, the optimum

dose of fertilizer is recommended for a crop at given agro-climatic region.

Eg. A blanket dose of 120-50-50 kg/ha of N, P2O5, K2O respectively is

recommended for rice.

CRITICAL LIMIT APPROACH

Waugh and Fitts (1965) developed this technique which is largely

meant for less mobile nutrients like P, K and micronutrients. Soils vary

considerably in fixing capacities. Due to it a part of applied P, which is fixed

in soil, is not readily available to plants.

The method includes incubation study. For P, Soil is incubated for 72

hours with graded doses of soluble P in the form of monocalcium phosphate.

Then, the amount of phosphates released (extracted) with an extracting

reagent (Olsen or Bray) will be determined. The extracted P versus the

amount of P applied is plotted. If the relationship is unique, then for high P

fixing soils a larger amount of fertilizer P application is needed. This amount

of P fixed is estimated as X-value.

Critical limit

% Yield

0 2 4 6 8 10 12 14 16 18 20 22

Soil Test P

23 (6/16)

Then, in potted experiment fertilizer P is added at 0, 0.5X, X, 2X levels,

and the test crop was grown to find out the soil critical value. The percentage

yield obtained is plotted against the soil-test value for different soils. By using

a plastic overlay, these data are grouped into two populations as described by

Cate and Nelson (1965). The soil-test value where the vertical line crosses

the x-axis is designated as the soil critical limit.

Critical limit for the soil test value is the limit below which a positive

or economic response to added fertilizer is possible and above which the

response diminishes at a faster rate or vanishes .

The validity of critical value is verified by conducting verification trials

in the field locations where from the soil samples have been collected for pot

studies.

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Summary Cheat Sheet

Key Recall Points

• SOIL FERTILITY is exam-relevant for SSAC122 and objective questions in soil science.
• Use soil-test based interpretation with focus on pH, CEC, and nutrient availability.
• Apply the 4R principle: right source, right rate, right time, and right method.

Exam Traps

• Do not mix up soil fertility concepts with fertilizer quantity alone.
• Numerical and term-based questions often test definitions, units, and threshold values.
• In problem-solving, interpretation must follow soil reaction, crop stage, and management context.

References

3 sources • [1] [2] [3]

[1]

ICAR e-Course: Soil Chemistry, Soil Fertility and Nutrient Management

Official

[2]

Brady and Weil, The Nature and Properties of Soils

Book

[3]

Indian Council of Agricultural Research

Official