🧫 SOIL TESTING

This lesson explains key concepts in a structured way and connects them to practical agricultural applications and exam-oriented understanding.

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SOIL TESTING

LECTURE 16

SOIL TESTING AND CORRELATION

The composite samples obtained from fields are used in pot

culture plant studies to assess the crop responses (in terms of dry matter) to

added fertilizers. Calibration of the responses to soil - test values was

obtained by using different methods. Fixation studies are conducted to

ascertain the capacity of soil to fix nutrient to be applied, finally, the results

are interpreted to confirm nutrient deficiencies, index soil fertility, and

establish “critical level” for each nutrient. The results are verified in the next

stage, in field experiments.

Fertility group/ Index

The International soil fertility evaluation and improvement programme

(Waugh and Fitts, 1965) advocated grouping of soils into low-medium-high

categories.

The method:

• Collection of representative soil samples and analysis by using

different extractants for the ‘available’ nutrient

• Conducting of potted-plant studies with graded doses of added

nutrient supplying all other nutrients to meet the crop requirement

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• Computing of the percentage yield responses

• Plotting of the soil-test data obtained with different methods and the

percent yield response; and to find the scatter distribution for the best

correlating soils test methods. The scatter diagram will be generally

curvilinear. The response is less, as the soil test value increases.

• The general groups are low, medium and high

The fertility groups followed in Tamil Nadu

Fertility group	Available nutrients (kg/ ha)	Col3	Col4
Fertility group	KMnO4-N	NaHCO3-P	NH4OAc-K
Low	<280	< 11	< 118
Medium	280-450	11- 22	118-280
High	>450	> 22	>280

This classification indicates that low classes of soils would markedly respond

to added fertilizers and high status of soils does not respond to them. In the

medium range, nothing could be predicted. In addition, by this grouping, it

was not possible to indicate how much fertilizer was to be added to get

economic yields. Thus, this grouping is qualitative. Arbitrarily, the

recommended dose of fertilizer for a crop is increased by 25% in low status

and reduced by 25 % in high status.

% Yield increase

Col1	V. LOW MEDIUM HIGH V. HIGH	Col3	Col4	Col5	Col6	Col7	Col8	Col9	Col10	Col11	Col12	Col13

Soil Test P

Fertility index

Fertility index expresses the relative sufficiency as a percentage of soil

nutrient amount adequate for optimum yields . The probability of a

response to fertilizer application increases with decreasing soil test level.

More than 85% of soils testing very low may give greater response and

profit. About 60-85% of soils, having medium soil test value may give little

profit. Around 15% soils with very high soil test may have little response.

In general, the very low to very high classification is easily understood by the

grower. However, separate group classification need to be done accounting

for greater variability associated with crops and soils.

Soil Test Rating	Probability of Crop Response (% Yield increase)	Fertility Index
Very low Low Medium High Very High	95–100 70-95 40-70 10-40 0-10	0-10 10-25 25-50 50-100 100+

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Summary Cheat Sheet

Key Recall Points

• SOIL TESTING 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