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Posted 5 April 2018. PMN Crop News.

Reading a Soil Test Report

Source: Article.

By Dan Davidson


Bloomington, Illinois (February 21, 2018)--Reading a soil test report is a good skill to have, even if someone else is writing recommendations.

When a soil sample is submitted to a commercial laboratory they usually analyze pH (water and buffer), organic matter, phosphorus, potassium, calcium and magnesium. And, if requested, they will run sulfur, nitrate and micronutrients including boron, copper, iron, manganese and zinc.

Water pH: This is a measurement of pH in the soil solution. A pH of 7 is neutral. As the pH drops, soil becomes more acidic, whereas if it goes up it becomes more alkaline. When pH drops below 6 to 6.2, we think about applying lime. And for each pH unit dropped, like from 7 to 6, the hydrogen-ion concentration—which determines acidity and alkalinity—increases 10-fold. If the pH drops to 5, we’ve dropped 100-fold in hydrogen-ion concentration, which means it needs to be limed as soon as possible.

Buffer pH: Buffer pH is measured because it estimates lime requirements. The difference between the buffer pH and the water pH is the buffer pH measures the total acidity in the soil, while soil pH gives us the active acidity. Buffer pH is used to determine how much lime to apply. That recommendation is what is needed for 100% effective lime. Most ag lime is 50, 60 or 70% effective lime, so you must adjust lime rate to come up with the amount of ag lime you need to apply.

Soluble Salts or Electroconductivity: Electrical conductivity (EC) reading is a measure of salinity. Measuring EC is not common in Illinois or the central Corn Belt, but as you go west salinity can become a problem and impact both plant and soil health. If the soluble salt or EC reading is above 1.5, we have a saline condition; if too low, <0.1, it indicates a lack of nutrients and poor biology. We like to have an EC reading around 0.3 to maintain a biologically active soil. If above 0.6 or 0.7, then determine what nutrient salts such as nitrate and sulfates may be too high.

Excess Lime: This is a way of determining if you have a calcareous soil that contains free lime and is associated with a high pH (>7). Across the Western Plains, Mountains and West Coast there are calcareous soils, which can create some micronutrient deficiencies. You shouldn’t have any free lime if the pH is below 7.

Organic Matter by LOI: Most organic matt is measured by LOI (loss on ignition). A soil sample is dried and weighed. Then it is placed in a furnace at 680° F and the organic matter carbon is burned off. Soil is cooled and then reweighed. The difference between the two weights is the amount of organic matter that was present.

Phosphorus ppm: Phosphorus needs to be extracted from the soil and three extracts are most common, Bray, Olsen and Mehlich. Laboratories often provide all three ways, but Mehlich is becoming the most common extract. If Bray is used, then the laboratory will run Olsen if soil pH is above 7.2. Olsen is better at extracting P in high pH soils. Your soil test will report either ppm or lbs. phosphorus if very low, low, medium, high or very high. Note that lbs. P is double ppm.

Mehlich or Ammonium Acetate: Either Mehlich or ammonium acetate are used to extract potassium, calcium, magnesium and sodium or exchangeable cations. A soil test will report ppm or lbs. of each nutrients and whether very low, low, medium, high or very high.

Mehlich or DTPA Micronutrients: Either Mehlich or DTPA (a chelate) are used to extract zinc (Zn), iron (Fe), manganese (Mn) and copper (Cu). If you’re above 1 ppm zinc, you’re probably OK. Some agronomists like to go to 1.5 instead of 1. Iron is more soluble as the pH becomes more acidic. If you have a low pH, the iron test will be high. Iron should be above 4.5 ppm. If it’s below 4.5, and pH above 7, and high excess lime, iron deficiency chlorosis (IDC) may appear. For manganese, it should be above 3 ppm and copper should be above 0.20 ppm. Rarely is copper deficient.

Hot Water Boron: Boron (B) is becoming more recognized as an important nutrient in soybeans. On corn and soybeans, it should be above 0.25 ppm. For alfalfa, it should be above 0.5 ppm. While B may become phytotoxic at high rates, research from the University of Arkansas shows that we would never apply the rates of boron necessary to create toxicity.

Percent Base Saturation: Since labs are measuring all the cations they calculate and report base saturation. There are a lot of theories on how to balance nutrients based on Base Saturation and the Albrecht method adheres or promotes balancing nutrients. Most of the time Base Saturation only reports K, Ca, Mg and sometimes Na. If you subtract the total from 100, that would be hydrogen (H+). For K it should range from 3 – 5%, Ca should range from 60 – 65%, Mg should range from 11 – 13% and Na should be less than 1 – 2%. If Na is zero, great but if about 4-5%, then apply gypsum to reduce it. Calcium replaces sodium on the exchange complex. Desired base saturation ranges are variable depending on one’s belief and philosophy.

Soybean agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics.

Dan Davidson