Soil Testing Methods For Agriculture

In addition to each element level that will be reported back, you will also receive a few other important soil characteristics as well:

1. Soil pH

This is a measure of acidity or alkalinity of your soil on a scale of 0 to 14, with 7.0 soil pH being neutral. Soils with a pH below 7.0 are acidic, while soils with a pH above 7.0 are alkaline. The pH is important because nutrient uptake can be affected when pH is to high or low. Row crops are typically most efficient with pH between 6.2 and 7.2. Outside of these pH parameters, certain crops can have a difficult time absorbing essential nutrients. With a low pH (less than 6.0), it may be necessary to apply calcium to adjust pH upward. Lowering the soil pH is a more difficult and expensive task—sometimes, an application of elemental sulfur can be used to lower soil pH.

2. Electrical Conductivity

Electrical conductivity can be measured using a meter and probe as well. The probe consists of two metal electrodes spaced 1 cm apart (thus the unit of measurement is microSeimens or milliSeimens per centimeter). A constant voltage is applied across the electrodes resulting in an electrical current flowing through the aqueous sample. Since the current flowing through the water is proportional to the concentration of dissolved ions in the water, the electrical conductivity can be measured. The higher the dissolved salt/ion concentration, the more conductive the sample and hence the higher the conductivity reading.

3. Water Holding Capacity

Firstly, establish the depth of the root zone, either by observing the depth to which roots from the previous crop have extended, or by noting the depth to a restrictive layer. The roots of most annual field crops occur in the top 120cm of soil, if there are no restrictive layers. Some perennial species may extend roots to 600cm or more if soil conditions are ideal and moisture is present.

Secondly, use Figure 2 to calculate the water holding capacity of each soil layer in the root zone. For example, 25cm of clay loam with an available water of 1.8mm water per cm of soil, can store 45mm of available water.

The water holding capacity of a soil is calculated by summing the capacity of each layer in the root zone.

4. Calcium Carbonate

Soil carbonates are typically measured by dissolving carbonates in acid solution (equations 1 and 2) and then determining either H+ consumption, Ca and Mg production, or CO2 production. This method estimates the amount of soil carbonate by digestion with excess acid.

5. Organic carbon

The determination of soil organic carbon is based on the Walkley-Black chromic acid wet oxidation method. Oxidisable matter in the soil is oxidised by 1 N K2Cr2O7 solution. The reaction is assisted by the heat generated when two volumes of H2SO4 are mixed with one volume of the dichromate.

Macro and micro Nurtrients

1. Nitrogen

Nitrogen is tested as nitrate (NO3) form only unless requested otherwise. By multiplying the analysis ppm number by 0.3 per inch of soil sample depth, you can determine the total pounds of nitrogen in the soil.

For example: 8 inch soil sample depth equals 0.3 X 8 = 2.4

If 12 ppm of NO3 is reported, then multiplied by 2.4 equates to 28.8 pounds of nitrate-nitrogen in the soil, which means your nitrogen applied should be effectively be reduced by 28.8 pounds from the total N needed to grow your next crop.

2. Phosphorus (P)

Soils with 25 to 35 ppm P are typically adequate on most soils. There are three common analysis methods to evaluate the presence of soil phosphorus.

• Bray test is best with neutral and low pH soils
• Olsen test is used on high pH soils (this test generally reports phosphorus at lower levels)
• Mehlich III test can be used on most pH values of cropping soils

3. Sulfur (S)

Sulfur is measured as sulfate, which is the available form of sulfur the plant can use. Sulfate is also subject to leaching. For most common soil types, soils with a range of 7-15 ppm S are considered adequate.

4. Zinc (Zn)

Soil tests can also predict if adding zinc will impact your plant health and crop yields. The desired ppm for zinc ranges from 1.0 to 3.0.

5. Iron (Fe)

Iron ppm of 10-20 is typically common on most soils. Iron chlorosis is a problem with iron shortage and high pH issues, so applying additional iron could potentially help to alleviate any iron chlorosis problems you might see.

6. Manganese (Mn)

Manganese at 8-11 ppm is typically sufficient. Mn availability is influenced by soil pH, and low pH can increase Mn availability, while high pH can lessen it.

7. Copper (Cu)

Only small amounts are needed by plants. Copper at 0.8-1.0 is adequate for most crops. The majority of the copper deficiencies occur in highly acidic soils.

8. Potassium (K)

The soil test measures the exchangeable potassium in the soil. Look for a minimum of 165-220 ppm to supply the needed amounts of potassium to maximize production.

9. Calcium (Ca)

Calcium is typically plentiful in soils with a pH of 6.0 and higher; however, calcium can be applied as gypsum and not affect soil pH. Calcium ppm of 1400 or higher is generally right for most crops.

10. Magnesium (Mg)

Magnesium is often adequate in soils with a pH of 6.5 and higher, though magnesium at 100 ppm or more is acceptable.

11. Sodium (Na)

This part of the analysis is primarily for use in repairing saline or alkali soils. Sodium is not a soil nutrient—adding other elements, such as gypsum or elemental sulfur, will help with water infiltration to flush away the sodium you have present. The range for sodium in most common soil types is typically 80-120 ppm.

To make the best fertilizer applications in your fields, consult with your independent agronomist or fertility specialist.

Although no soil analysis is perfect, the information and insight from a soil test can help you to improve your nutrient efficiency, diagnose in-season plant deficiencies, and ultimately prevent unnecessary yield loss.