EPISODE 1: Cation Exchange Capacity, Organic Matter, and soil pH.

November 22, 2019

11.21.19 | Soil Testing Part 1 from Central Valley Ag on Vimeo.

CVA’s Agronomy Focus Video will be a three-part series on how to interpret the soil test report from a soil lab. In episode one, we cover some of the primary soil readings that help us understand soil texture and a soil’s ability to supply nutrients and a good environment for growing the crop. In Episode 2, we will focus on Phosphorus and Potassium. In Episode 3, we will look at Sulfur, nitrate, zinc, and other micronutrients.

Episode 1 | Cation Exchange Capacity, Organic Matter, and soil pH.  

Cation Exchange Capacity: CEC is a measurement of the estimated exchange sites available in the soil. The exchange sites are negatively charged and attract positive ions. CEC is strongly related to the texture of a soil due to clay particles having a much higher cation exchange capacity than sand particles. The coarser the soil (higher sand content), generally, the lower the CEC. Organic matter also influences CEC since organic matter has a high cation exchange capacity. CECs below 10 indicate a high percentage of sand in the soil. CECs from 10-15 have higher silt content; CECs from 15-20 are a more balanced mixture of sand, silt, and clay; and CECs above 20 have increasing amounts of clay in them. As the CEC increases, the amount of nutrients and water the soil is able to hold also increases, but so do some of the management challenges associated with high clay percentages. Knowing the cation exchange capacity helps you develop a plan to manage the nutrients in that soil and how to best apply additional nutrients.
Organic Matter: OM measures the amount of plant and animal residue in the soil. OM is a source of many nutrients essential for plant growth. Soils generally range from less than 1% OM to over 5% OM. Soils low in organic matter are less able to hold and mineralize nutrients and will depend more on applied nutrients than soils with higher organic matter levels. A soil’s organic matter gives us an idea how much that field will depend on us to supply the nutrients for the crop and how much help we can expect from mother nature. High OM levels are not always good. Sometimes a soil will be high in organic matter because poor drainage causes a buildup of crop stover that cannot breakdown into humus in the soil. 

Soil pH: pH is the measurement of hydrogen ion concentration and acidity. Soil pH can have a major impact on crop production and should be one of the first areas addressed if amendments are needed. Soil pH in a range from 6.3-7.0 is best for corn, soybean, and wheat production. Alfalfa often produces better yields with pH ranges from 6.8 to 7.5. Soils with a pH under 6.0 should be limed to reduce their acidity. Soils with a pH higher than 7.5 can be challenging due to their ability to tie up nutrients in forms unavailable to crops. It is usually not economically feasible to lower a high soil pH. Normally we would recommend a grower try to manage around a high pH with appropriate herbicide choices, variety choices, and possibly banding nutrients to create lower pH in a microenvironment around those nutrients.
Buffer Index: The Buffer Index (or Buffer pH) is used to determine the amount of lime needed to correct an acidic soil. In general, the higher the soil’s CEC the more lime it will take to correct the pH. The soil’s buffer pH is really meaningless until the grower makes a decision based on soil pH that he or she needs to lime. Then the buffer pH is needed to decide how much lime to apply.  

Posted: 11/22/2019 3:41:56 PM by Kelli Reznicek | with 0 comments

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