Our technical support staff has been getting a few calls lately about how much nitrogen can be safely applied in the fall on sandy soils. Because sandy soils are prone to leaching, fall N application is not a good management practice (BMP). Applying nitrogen fertilizer in the fall when soil temperatures are low reduces the risk of N leaching losses on these sandy soils, but the overall risk is still too high.
There is some confusion about the CEC (Cation Exchange Capacity) of sandy soils and how that affects the rate of fall N that can be applied safely without risk of N leaching. All soil particles have a negative charge which gives soil the ability to hold onto cations like calcium, magnesium, potassium and sodium which all have positive charges (like a battery). While ammonium forms of nitrogen fertilizer also have a positive charge and can be held by the negative charge of the soil, this is a temporary situation. All ammonium fertilizer is eventually converted to nitrate nitrogen. Nitrate nitrogen can easily be leached downward in a sandy soil and lost from the root zone in the fall or spring. It does not matter what the CEC of a sandy soil is once the ammonium based nitrogen fertilizer converts to nitrate. So the next time somebody suggests that there is a safe rate of nitrogen fertilizer that can be applied in the fall on a sandy soil based on the CEC, ask him or her how much of their money they would like to give you if the N is lost due to leaching!