How to Read a Soil Analysis Report
You know that a healthy, rich soil will give you the best opportunity to achieve high yields. After you’ve collected a quality soil sample and delivered it to a soils laboratory of your choice, what comes next? Reviewing your soil analysis report!
The soil analysis report that you will receive back from the lab should consist of all the attributes of the soil test you selected when you sent off your samples.
This can include a fertilizer recommendation for achieving the yield goal of the commodity to be grown, which are usually based on local university research for your particular area. But each laboratory can adjust these recommendations based on your specific needs and their expertise. The quantity and quality of information you receive for the cost of the analysis makes soil testing a terrific value.
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 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. Buffer pH
This is not a characteristic of the soil. Instead, when a soil’s pH is below 5.8, this characteristic is used to estimate the lime (Ca) required to correct the soil pH to around 7.0. That is the only reason it is reported on your soil test analysis, and only on soils with a pH below 5.8.
3. Soluble Salts
This measures the electrical conductivity of the soil solution to determine the risk of salt injury to plants. Soluble salts are largely affected by environmental conditions—soils that contain high salt content are called saline soils (NaCl). Soils high in sodium (Na) are referred to as sodic soils. Salts can accumulate from excessive fertilizer applications and poor quality irrigation water, and where rainfall is limited. With proper soil drainage accompanied by rainfall or irrigation, the salt can sometimes be flushed out of the root zone to correct the problem. Don’t be too concerned about correcting soluble salts unless they are reported over .75 mmhos/cm (millimhos per centimeter, which is the basic unit of measure of electrical conductivity in soil) on your analysis.
4. Excess Lime
This a measurement of the amount of free lime in the soil. The reading can be important in your herbicide selection and fertilizer applications, so that you can avoid product tie-ups with the calcium present—which would render it ineffective and unavailable to your plants.
5. Organic Matter (OM)
Generally speaking, the higher the organic matter, the healthier the soil. This is reported as a percent, and it measures the the ability of the soil to supply nutrients, water and other physical wellbeing to growing plants. Organic matter accumulation is a slow process. Reduced tillage has been shown to have a positive impact on organic matter and soil tilth. Row crops should be at around 2.5% OM or higher, though it is not uncommon for sandy soils to be lower.
6. Cation Exchange Capacity (CEC)
This measures the ability of the soil to store and release nutrients. This number also helps to define the soil’s texture and composition. Sandy soil to loam soil CEC will vary from 1 to 40, but the most common range is from 13-25 CEC.
7. Percent Base Saturation
Percent base saturation is closely related to CEC and pH. This measurement indicates the nutrient supply and balance of cations for K, Mg, Ca, H and Na. Soils with a high percent base saturation can be more fertile because they often have a higher pH, and can contain greater amounts of these nutrients for use by plants.
Elements reported on a standard soil test include both macronutrients and micronutrients.
Their saturations are reported in parts per million (ppm). With the exception of nitrogen, simply multiplying ppm by two will equal pounds per acre of each nutrient.
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 total N needed to grow your next crop.
2. Phosphorus (P)
Soils with 25 to 35 ppm P is 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 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 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 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 on 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.
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