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Source: Pioneer news release

Corn plants with yellowing leaves along the edges of fields can be a common sight this time of year, but growers may be unsure as to the cause, especially if they've checked their soil nutrients and found them adequate for corn growth. Potassium deficiencies are often blamed, however, that's usually not the correct culprit, especially in the drought conditions many farmers are currently experiencing.

"Our environment is dynamic," said Matt Montgomery, Pioneer Field Agronomist. "Yes, these are potash deficiency symptoms, but the issue isn't potash. The issue in this case is moisture availability, and that's because we've had this combination of drought and grass roots competing for moisture. If we looked at nothing but symptoms, we would have jumped to incorrect conclusions."

Yellowing leaves are often a sign of a problem with the nutrient uptake process, either due to water competition from nearby grass or drought conditions. Corn plants get 13 of the 16 required nutrients for growth through the soil, either through direct contact as roots grow or via mass flow or diffusion. Both mass flow and diffusion require water in the soil to function.

Mass flow is the process by which nutrients are dissolved in water and taken in by the plant roots. Mass flow is responsible most transport of nitrate, sulfate, calcium and magnesium.

Diffusion is the third nutrient uptake process and the one most likely responsible for potassium-deficient corn plants displaying yellow leaves. Diffusion occurs when nutrients move from a high-concentration area to a low-concentration area to reach equilibrium. Higher concentrations of nutrients in the soil move via a thin film of water molecules to the lower concentration root surfaces where they can be taken up. This process is particularly important for the transport of phosphorous and potassium.

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