LECTURE 17

SOIL TESTING

Soil Testing, STL Functions

Soil Testing

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

24represented
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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.
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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.

Common Nutrient source

Plant nutrient
extractant extracted
NO3- KCl, CaCl2 Solution
NH4+ KCl Solution -
Exchangeable
Available N KMnO4 - NaOH Mineralizable
Organic N

H2PO4-/ HPO42- NH4F - HCl (Bray- Fe/ Al mineral

(Available P) p) solubility
NaHCO3 - P Ca mineral solubility
(Olsen-P)
K+(Available K) NH4OAc-K Exchangeable
Ca2+, Mg2+ EDTA Exchangeable

SO42- CaCl2 Solution AEC

Zn2+, Fe3+, Mn2+, DTPA Chelation
Cu2+
H3BO30 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
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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.

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