🔬 Soil Testing

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.

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
• 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

• 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.

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 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 from 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 country soil area or a statewide basis by the use of the soil test summaries.

👉🏻 Phases of Soil Testing programme

• Collecting the soil samples
• Extraction and determining the available nutrients
• Calibrating and interpreting the analytical results
• 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.
• 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.
  • 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.

    

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 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.

Permanent Manurial Experiments

• Permanent manurial experiments are conducted to study the long term effect of continuous application of plant nutrients either singly or in combination and with or without organic manure on crop yield, nutrient uptake, and physico-chemical and biological properties of soil.
• The first one started was the classical field experiment at Rothamsted Experimental Station, Harpenden in England in 1854 by Lawes and Gilbert.
• Most of PMEs test common treatment combinations like: Fertilizer N or P or K alone, Fertilizer N and K, Fertilizer N and P, Fertilizer NPK, Farmyard Manure (FYM), Residual effect of FYM, Hand weeding, Zn or S addition, Control (no manure), tillage, irrigation, etc.
• Similar to Rothamsted experiment, in India PME was started at Coimbatore in 1909 and this was the first of its kind in our country. This called as Old Permanent Manurial experiment (OPM) is being conducted in red soil (Alfisol) with cereal-cotton crop rotation under rainfed conditions.
• Subsequently in 1925, a second experiment was started with the same treatments and called as New Permanent Manurial (NPM) experiment to test the effect under irrigated conditions.
• In OPM and NPM a uniform fertilizer dose of 25-60-75 kg N, P₂O₅ and K₂O/ha is being applied in all these years. As these are designed with very low dose of fertilizers and manures without any replication and randomization, they do not match to the present day use pattern of fertilizer/ manure and statistical analysis.
• Therefore, to study the effect of intensive cropping and manuring new set of experimental design was implemented all over India by ICAR by a coordinated scheme on Long Term Fertilizer Experiment (LTFE). During 1972, LTFEs were started at 11 centres and later further expanded at seven more centres.
• Under this scheme, a third experiment was started in 1972 at Coimbatore called as Long Term Fertilizer Experiment (LTFE) in medium black soil (Inceptisol) to test intensive cropping system with Ragi-maize cropping system.
• The major findings in these experiments are:
  • Application of single nutrient (N or P or K) alone resulted in lower crop yield. Combination of NP and NPK gave higher yield.
  • Phosphorus became a limiting nutrient when it was not applied and it reduced the yield to the extent of 50-60 per cent. Application of P along with N, K, and FYM raised the available soil P from low to medium status
  • Potassium was depleted even when it is applied continuously
  • Continuous use of N fertilizer alone reduced the soil productivity.
  • Addition of FYM with NPK significantly increased the crop yield to the tune of 15-20 per cent over NPK alone. Addition of organic manure improved pore space and water holding capacity of soil. Combined application of organic manure and inorganic fertilizer not only increased the yield of crops but also improved the soil productivity.
  • Population of bacteria, fungi, Actinomycetes and Azotobacter were increased due to organic manure incorporation. The activities of enzyme urease, dehydrogenase, cellulase, and amylase were favored by organic manure application.