Soil organic matter can decrease leaching potential of pesticides.

Every chemical has a risk of moving through the soil profile, leaching and getting into groundwater. One variable that can minimize the risk of leaching is organic matter.

Spring is coming and with that means pesticide applications. Selection of pesticides is very important. Farmers want to use the chemical that will be the most effective while ensuring minimal financial loss. Leaching potential of the chemical is rarely considered. Every chemical has a risk of moving through the soil profile, leaching, and getting into groundwater. One variable that can minimize the risk of leaching is organic matter.

Pesticides typically are designed to adsorb (attach) to organic matter. The more organic matter your soil has the greater chance that pesticides are being held in the soil and available for its intended use. Lower soil organic matter leads to higher risk for pesticide leaching into groundwater. As more water enters the soil profile through precipitation or irrigation pesticides may become “desorbed”. Desorbed is when pesticides become soluble and detaches from the soil organic matter.

Partition Coefficient, PC, value is the measurement of pesticides adsorption to the soil. The value is defined as the ratio pesticides adsorbed to soil particles to the pesticides dissolved in the soil water. The higher the PC value the tighter the pesticide attaches to the soil and organic matter. The smaller the value the better chances that the pesticide will dissolve and the higher the risk for leaching. Table 1 is a brief listing of PC values for certain pesticides used in Michigan.

Table 1.

Pesticide generic name PC
Dicamba 11
Aldicarb 12
Carobfuran 29
2,4-D 32
Captan 33
Fonofos 68
Dinoseb 120
Simazine 138
Atrazine 163
Cyanazine 168
Alachlor 190
Carbaryl 229
Diazinon 580
Linuron 863
Lindane 1161
Malathion 1797
Phorate 3200
Terbufos 4796
Parathion 7161
Chloropyrifos 13490
Trifluralin 14000
Heptachlor 24000
Chlordane 38000
DDT 243000

Low organic matter can also pose a risk to over application of pesticide that will have a negative effect on the crop. Pesticides labels may state that a low pesticide rate should be used for low soil organic matter. Some pesticide labels may even state that soils with a low organic level should not be treated with the pesticide.

Fields in Michigan can have many different soils. If a field has a dominant soil type the organic matter percentage can be determined by that soil. If a field has many different soils with no one dominant soil the fields leaching potential should be determined by the most vulnerable soil i.e. the one with the lowest soil organic matter.

To determine the soil organic matter the easiest way is to look it up in the county soil survey. This number may not be accurate if management practices have been done on the field that added or subtracted the percentage of soil organic matter. If that is the case a soil organic matter test can be done. MSU Soil and Nutrient Testing Lab will test soil organic matter for $6 or you can have it done along with the regular soil test for and additional $5. Once again be aware of variability’s of soils in your field and test accordingly. You may need to test in more than one area in your field.

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