How to Conduct Effective Soil Testing
Soil testing is a key tool in fertilizer management. It minimizes guesswork and helps reduce environmental pollution. When done and utilized properly, soil testing can save growers money and increase crop yields.
Key questions for effective soil testing:
- How to take soil samples properly ?
- Does the lab use adequate extraction methods?
- How to interpret soil test results?
Soil test can determine parameters such as soil texture, pH, available nutrients, organic matter and salinity-related parameters.
HOW TO COLLECT THE SOIL SAMPLES?
The reliability of the soil test depends first and foremost on the sample taken. However, since only a very small part of the field is being tested, the soil sample you take must represent the field or the plot from which it was taken.
- Take several sub-samples from the field and mix them together to form one representative sample.
- Sample at least each 10 hectares (about 20 acres), depending on the field’s uniformity and crop.
- Take the samples before applying any organic fertilizer to the soil.
- Use clean tools and a clean container.
- From each plot, take 10 to 20 sub-samples and mix them.
- Soil samples should be taken from the depth where the active root system is , usually to a depth of 15-20cm (6-8 inches)
- Place the mixed sub-samples in a plastic bag. About 0.5 – 1 kg (about 1-2 lbs) of soil is required for soil testing.
- Properly identify the sample on the bag with the plot name, crop, date of sampling, and type of analysis needed before sending it to the lab.
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BASIC PRINCIPLES OF SOIL TESTING
Soil parameters tested in soil analysis for agriculture can be classified into:
| Structural properties | Soil fertility and related parameters | Soil salinity |
|---|---|---|
| Soil texture and type | Available nutrients (Common nutrients tested in fertility test: phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese, zinc and copper. ) | Electrical conductivity (EC) |
| Soil bulk density | Cation Exchange Capacity (CEC) | Sodium Adsorption Ratio (SAR) – a parameter used to evaluate the effect of sodium on soil structure. |
| Soil pH, soil acidity
Percent organic (OM) matter or organic carbon |
Soils can be tested for multiple parameters, using various extraction methods. However, it is essential to guide the lab which tests you want it to perform.
Soil structural properties are usually tested once, before starting the agricultural activity on that soil.
Soil fertility and salinity are tested more frequently. Labs will usually not indicate whether they tested the soil for salinity or fertility and growers have to figure that out from the extraction methods used. In most cases, the soil analysis report includes parameters related to salinity together with parameters related to fertility.
Salinity test can be used for giving fertilizer recommendations for soils with low CEC only. However, in medium-high CEC, nutrient levels in the salinity test cannot be used for that purpose.
SOIL FERTILITY TEST
Soil test results provide information on the nutrients available to plants. Results are given in units such as ppm or meq/100g, where ppm in this case refers to mg/kg.
Soils usually contain much larger amounts of nutrients than what is reflected the soil test report. However, not all of it is available for uptake by plants.
Different extraction methods have been developed to estimate the amounts of nutrients available to plants. An extraction method involves adding an extractant (a chemical) to the soil sample, which releases nutrients attached to soil particles. Some extraction methods are more suitable to specific soil conditions. Therefore, different labs may provide different results for the same soil sample.
To accurately interpret the test results, it is essential to know which extraction method was used for each nutrient. Otherwise, the numerical values themselves do not have much significance.
For example, a result of 17 ppm phosphorus is considered high if tested with the Olsen method, low if tested with Bray-1 and medium if tested with Mehlich 3.
SOIL SALINITY TEST
Salinity test uses only water for the extraction, without the use of chemicals. Soil is mixed with a specific amount of water and the solution is then tested. Common methods include the saturated paste extract, 1:2 extract (1 volume soil: 2 volumes of water)and 1:5 extract.
The purpose of the soil salinity test is to provide a measure of the concentration soluble salts in the soil. It helps to evaluate the conditions in which plants grow – salinity and specific toxicities.
The level of certain elements that might be toxic to the plants if their threshold is exceeded. Usually Boron, chlorides and sodium.
Calcium and magnesium are also tested in the salinity test, as they are used for calculating the SAR.
Results are reported in units of ppm, mmol/L or mg/L.
Note that while in the fertility soil test ppm refers to mg/kg, in salinity test it refers to mg/L. This is because in the fertility test measures the nutrients adsorbed to the soil particles, while the salinity test refers to nutrients in the soil solution.
The following is an example of soil test results of the same soil sample:
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