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🧫 SOIL TESTING

SOIL TESTING.

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

SOIL TESTING

LECTURE 17

Soil Testing, STL Functions

Soil testing is defined as a ‘programme for procedural evaluation of

soil fertility by Rapid chemical analysis particularly to assess the available

nutrient status and reaction of a soil’.

A soil test is a chemical method for estimating nutrient supplying

power of a soil. Compared to plant analysis, the primary advantage of soil

testing is its ability to determine the nutrient status of the soil before the crop

is planted.

The result of a soil test is known as soil test value . A soil test value

measures a part of the total nutrient supply in the soil and represents only as

an index of nutrient ability.

Soil test do not measure the exact quantity of a nutrient potentially

taken up by a crop. To predict nutrient needs of crops a soil test must be

calibrated against the response of crops in nutrient rate experiments in

greenhouse and fields. Thereafter, interpretation and evaluation of the soil

test values primarily form the basis for fertilizer recommendation.

Soil test programmes have the following objectives:

  • To provide an index of nutrient availability

  • To predict the probability of profitable response to fertilizer

  • To provide a basis for fertilizer recommendation

  • To evaluate the soil fertility status and a county soil area or a

statewide basis by the use of the soil test summaries.

Phases of Soil Testing programme

  1. Collecting the soil samples

  2. Extraction and determining the available nutrients

  3. Calibrating and interpreting the analytical results

  4. Making the fertilizer recommendation and management

Soil sampling

The most critical aspect of soil testing is obtaining a soil sample that is

representative of the field . There is always a considerable opportunity for

sampling error. If a sample does not represent a field, it is impossible to

provide a reliable fertilizer recommendation.

The soils are normally heterogeneous, and wide variability can occur

even in a uniform fields. Intensive soil sampling is the most efficient way to

evaluate variability. The sampling error in a field is generally greater than

the error in the laboratory analysis.

Soil Unit : It is an area of soil to be represented by a composite sample. 24 (1/4)

After the soil unit is determined, the soil samples are collected throughout

the area. The number of samples for combining into each representative

composite sample varies from 5-20 samples in an area of an acre. Usually a

composite sample of one kg of soil is taken from a field.

The size of the area may be sometimes even less for areas that vary

in appearance, slope, drainage, soil types, past treatment These areas are

to be sampled for separately. The purpose of the procedure of making a

composite sample is to minimize the influence of any local non-uniformity in

the soil.

Normally for all field crops, sampling soil up to 15 cm depth is

practiced. For deep-rooted crops and tree crops samples up to 1-2 m

may be necessary. While sampling, first a uniform portion is taken from the

surface to the depth desired. Second, the same volume is obtained from

each area.

Preparation of the composite soil sample in the laboratory

It involves the following steps: Drying, grinding, sieving, mixing,

partitioning, weighing, and storing. Uniform mixing and sampling is done by

Quartering Technique :

The soil sample is coned in the center of the mixing sheet. Cone is

flattened and divided through the center with a flat wooden sheet. One half

is moved to the side quantitatively. Then each half is further divided into

half, the four quarters being separated into separate ‘quarters’. Two

diagonally ‘opposite quarters’ are discarded quantitatively. The two other

are mixed by rolling. This process is repeated, until 250-500 g composite

soil material is obtained.

24 ( 23 /4)

For micronutrient analysis – sampling and processing of samples

should alone be done only with stainless steel materials, plastic, or wood to

avoid contamination.

The soil test values calibrated nutrient functions are advocated to the

farmers as a package of nutrient management that aims at judicious use of

fertilizers . Ultimately any soil testing and interpretation must involve

‘ economics’ because it is used to make a fertilizer recommendation to

achieve an economic goal that would give maximum profit per hectare of

land.

Extraction and determining the available nutrients

Many chemical extractants have been developed for use in soil

testing. The ability of an extractant to extract a plant nutrient in quantities

related to plant requirements depends on the reactions that control nutrient

supply and availability . The extractants commonly used in soil testing

programmes are given below.

Plant nutrient Common
extractant		 Nutrient source
extracted
NO3
-		 KCl, CaCl2 Solution
NH4
+		 KCl Solution -
Exchangeable
Available N KMnO4 - NaOH Mineralizable
Organic N
H2PO4
-/ HPO4
2-
(Available P)		 NH4F - HCl (Bray-
p)		 Fe/ Al mineral
solubility
H2PO4
-/ HPO4
2-
(Available P)		 NaHCO3 - P
(Olsen-P)		 Ca mineral solubility
K+(Available K) NH4OAc-K Exchangeable
Ca2+, Mg2+ EDTA Exchangeable
SO4
2-		 CaCl2 Solution AEC
Zn2+, Fe3+, Mn2+,
Cu2+		 DTPA Chelation
H BO 0
3 3		 Hot water Solution
Organic C Chromic acid Oxidizable C

SOIL TESTING LABORATORY

Soil Testing Laboratories of the Department of Agriculture funded by

State Government are functioning at identified centres in each district. Soil

testing services are also extended to the farming community in the Soil

Laboratories operated by Central government and Agricultural Universities.

The major functions of State Soil Testing Laboratory are:

  • Analysis of soil samples which are collected from the farmers by the

Assistant Agricultural Officers for texture (by feel method), lime status,

Electrical conductivity, pH and available N, P and K status at lower

charges/ sample; and advocating fertilizer recommendation for different

crops. Available micronutrients will be analyzed on request.

  • Analyzing irrigation water samples for EC, pH, cations, and anions;

Assessing their quality based on different parameters; and suggesting

suitable ameliorative measures for different soil condition and crops.

  • Adopting two villages for a particular period by each soil testing

laboratory; collecting and analyzing the soil and irrigation water samples 10 (4/8)

at free of cost and advocating the recommendations.

  • Collection of benchmark water samples from the wells marked in a

particular block/ water shed/ taluk. After analyzing the water samples for

different properties, water quality map will be prepared.

  • Based on the soil test value for the soil samples collected during the

particular year they are rated as low, medium, and high; and village

fertility indices will be prepared.

  • Conducting trials related to soil fertility to solve the site-specific problems.

Functions of mobile soil testing laboratory

  • The staff of the Mobile STL will visit the villages, collecting and analyzing

the soil and irrigation water samples in the village itself and giving

recommendations immediately.

  • Showing the audio-visual programmes through projectors in the villages

educating the importance of soil testing, plant protection measures and

other practices related to crop production.

  • In addition, Mobile STL is carrying out other regular functions of

stationary soil-testing laboratory.

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

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