Soil testing is the base for management decisions about fertilizer requirements. It involves the estimation and evaluation of the available nutrient status and acidic reaction of a sample of soil. After testing, a fertility map is prepared where the available nitrogen, phosphorous and potassium is marked as low, medium or high. Areas of sufficient and insufficient nutrients are marked out and nutritional requirements are determined. Fertilizers such as NPK, lime or gypsum are recommended to improve soil fertility. Fertilizer addition, which is based on soil testing, usually leads to an increase in yields and profits by providing the correct amounts of needed nutrients. It also leads to uniform application of nutrients in a field. As nutrient availability becomes less variable, the crop growth is more uniform. Regular soil testing also contributes to environmental sustainability as the use of excess fertilizers can be avoided.
Conception of Soil Testing
In most of the soil testing laboratories in India, the soil pH, electrical conductivity, oxidizable organic carbon, available nitrogen, available phosphorous and available potassium are determined by chemical analytical methods within a short period. Hence, Soil testing is the rapid chemical analysis of a soil to estimate the available nutrient status, reaction and salinity of the soil.
Objectives of Soil Testing – The objectives of soil testing area as follows:
- To estimate the available nutrient status, reaction (acidic/alkaline) of a soil.
- To evaluate the fertility status of soils of a country or a state or a district.
By soil test summaries the fertility status i.e., available nitrogen status or available phosphorous status or available potassium status expressed as HIGH, MEDIUM or LOW. A soil fertility map showing such fertility status can be prepared. The soil fertility map can be used for –
- Delineating areas of nutrient (e.g.,N, P, K) sufficiency or areas of nutrient (e.g.,N, P, K) deficiency,
- Studying soil fertility changing pattern due to crop cultivation over a period of years,
- Determining nutrient (e.g.,N, P, K) requirement for the deficient areas etc.
- To prepare a basis for fertilizer recommendation, lime recommendation or gypsum recommendation.
Soil Testing Programme – A soil testing programme has four phases as follows:
- Collection of soil samples.
- Chemical analysis of soil samples.
- Calibration and interpretation of the results of chemical analysis.
Before giving the soil samples to a soil testing laboratory for chemical analysis, collection and preparation of soil sample should be done with perfection.
Method of Collection of Soil Samples – Collection for field crops
- Polythene bucket
- 12 inches scale
- Ball point pen/Lead pencil
- A sheet of thick paper
- Polythene sheet (2ft x 2ft)
- Determine the soil unit (or plot).
- Make a traverse over the soil unit (or plot).
- Clean the site (with spade) from where soil sample is to be collected.
- Insert the spade into soil.
- Standing on opposite side, again insert the spade into soil.
- A lump of soil is removed.
- A pit of vee (V) shape is formed. Its depth should be 0-6″ or 0-9″ or 0-12″. (i.e., depth of tillage).
- Take out the soil-slice (like bread-slice) of ½ inch thick from both the exposed surface of the pit from top to bottom. This slice is also termed furrow-slice. To collect the soil-slice spade may be used. Collect the soil samples in a polythene bucket.
- Collect furrow-slices from 8-10 or sometimes 20-30 sites. Select the sites at random in a zigzag (or criss-cross) manner. Distribute the sites throughout the entire soil unit (plot). In lieu of spade auger may be used. Do not take the prohibited samples and local problem soils.
- Furnish the following information in two sheets of thick paper with the sample. One sheet is folded and kept inside the bag. Another sheet is folded and attached with the bag.
- Name and address of the farmer (or farm owner).
- Name of the block.
- Plot number or any other number that identifies the plot (or Soil unit).
- Soil texture (sandy/clay/loam).
- Availability of irrigation facilities.
- Availability of drainage system.
- Depth of soil sample.
- Information of the previous crop.
- Name and variety of the crop.
- Dose of organic manure, if applied.
- Dose of fertilizers, if applied.
- Informations of the crop that will be grown.
- Name and variety of the crop.
- Season (pre Kharif/Kharif/rabi).
- Problem, if any.
- Date of sample collection.
- Signature of the farmer (or farm owner).
Collection for plantation crop
- Dig a well (pit) of 1.8 meter depth. (Depth may vary depending on root-depth).
- Collect the soil-slice of ½ inch thick from the exposed surface of pit at different depths as follows: 0-15, 15-30, 30-60, 60-90, 90-120, 120-150 and 150-180 cm.
Collection for local problem soils – Local problem soils are treated as separate soil units (plots). Hence, separate composite samples are collected from problem soils. The problem soil samples are not mixed with normal soils (i.e., non problem soils). Both surface soil and subsoil samples are collected.
Collection of surface soil sample-Take 10-30 furrow-slices or cores that extend through A1 horizon.
Collection of Subsoil sample Dig a well (i.e. pit) of 1 meter depth. Take soil-slices of ½ inch depth below A1 horizon from different depths as follows: 0-15, 15-30, 30-60, 60-100 cm
Rating of Soil Test Results: On the basis of soil test results, the soils are grouped into different categories. The categories with respect to organic carbon, available PO, KO and N are a follows:
|Categories||Organic Carbon(%)||Available N (kg ha-)||Available PO (kg ha–)||Available O (kg ha–)|
|High||Above 1.5||Above 450||Above 90||Above 340|
|Low||Up to 0.75||Below 280||Below 45||Below 150|
The categories of soils with respect to soil pH are as follows:
|7.5-8.5||Tending to become alkali|
The categories of soils with respect to conductivity (total soluble salts) in mmhos/cm (dSm-1) followed are as follows:
|1 – 2||Critical for germination|
|2 -.3||Critical for growth of salt-sensitive crops|
|Above 3||Injurious to most crops|
Finding the Rate of N, P2 and KO Application from Recommendation Table
Some soil testing laboratories in India use a table that contains the rate of N, PO and KO application on the basis of soil test results as a tool for fertilizer calculation. One example of such table is given bellow.
Soil Testing Labs
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