Soil Testing

How it's done and what's important to know
10
Jun

Soil testing

Soil testing is a key tool in fertilizer management.  It reduces guesswork and helps in minimizing environmental pollution. When done and used properly, soil testing can save the grower money and increase crop yields.

Successful soil testing requires:

  • Proper sampling
  • Using the adequate extraction methods by the laboratory
  • Correct interpretation of the results

Soil test can determine parameters such as soil texture, pH, available nutrients, organic matter and salinity-related parameters.

 

COLLECTING THE SOIL SAMPLES

The reliability of the soil test depends first and foremost on the sample taken. Since only a very small part of the field is being tested, the soil sample must represent the field or the plot from which it was taken.

  1. The best soil sampling practice is to take several sub-samples from the field and mix them together to form one representative sample. Depending on the uniformity of the field and crop, it is recommended to sample at least each 10 hectares (about 20 acres). Take the samples before applying any  organic fertilizer to the soil. Use clean tools and a clean container.
  2. From each plot, take 10 to 20 sub-samples and mix them.
  3. Soil samples should be taken from the depth where the active root system is , usually  to  a depth of 15-20cm.
  4. Place the mixed sub-samples in a plastic bag. About 0.5 – 1 Kg of soil is required for soil testing.
  5. Before sending the bag to the lab, identify the sample properly, by indicating on the bag the plot name, crop, date of sampling and type of analysis needed

 

BASIC PRINCIPLES OF SOIL TESTING

Soil parameters tested in soil analysis for agriculture can be classified into:

Structural propertiesSoil fertility and related parametersSoil salinity
Soil texture and typeAvailable nutrients  (Common nutrients tested in fertility test: phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese, zinc and copper. )Electrical conductivity (EC)
Soil bulk densityCation 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 multiple extraction methods. It is important 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. 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 you see in the soil test report, but not all of it is available for uptake by plants.

To estimate the amounts of nutrients available to plants, different extraction methods were developed. The extraction method is a procedure that includes adding an extractant (a chemical) to the soil sample. The extraction releases nutrients that are 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.

In order to be able to interpret the test results, we must determine which  extraction method was used for each nutrient. Otherwise, the numbers in 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 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:

Soil test

 

Fertilization and irrigation ebook

 

 

 

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