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21. Determine the Organic-matter of Your Soil

applying pesticide

Organic-matter content is one of the most important but often overlooked soil properties. Pesticides typically attach to organic matter, reducing the chances of leaching. Therefore, the lower the organic matter content in a soil, the more likely a pesticide will leach and threaten groundwater.

When the organic-matter level is low, there is also an increased risk of applying excessive amounts of pesticides and possibly damaging the crop. Therefore, some pesticide labels will list lower application rates for soils with a lower organic-matter content. When you know your soil's organic-matter content, you know when you can lower rates, which means savings in dollars.

county soil survey One way to estimate organic-matter content is to determine the soil type and look up its properties in the county soil survey.
color chips You can also obtain color chips from your local University of Illinois Extension office. Because organic matter is linked to soil color, you can use these color chips to visually estimate the soil's organic-matter content. To be most accurate, however, the next time you have your soil tested for fertility, check the organic-matter content as well.
ariel photo of farmland When a lab tests for organic-matter content, it usually measures the level of organic carbon in the soil. If you decide to have your field tested for organic-carbon content, be aware that levels can vary within the same field. If one soil is clearly dominant in a field, test that one and gear your application rate to it. If several soils are dominant, test all of them. Then gear your application rate to the one with the lowest organic-matter content.
soil texture chart Organic matter is one of the most important soil properties that affect pesticide leaching, but not the only one. Texture and permeability are two more. Texture refers to the relative proportion of sand, silt, and clay in the soil. Leaching is more likely in sandy, coarse, or light-textured soils. Pesticides are less likely to leach in fine- or heavy-textured soils, such as those with a high percentage of clay.
field with standing water Permeability refers to the ability of water to move through a soil. If a soil has low permeability, water from a heavy rain may pond on the surface. If a soil has high permeability, water flows through the soil more freely and may cause chemicals to leach.
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