Fall Fertilization


By Harry Brook

With the 2021 harvest behind us there is actually some time available to do fall work. That just might include fall application of fertilizer for next year’s crop. Often, but not always, fertilizer is cheaper in the fall than the spring. In a lot of years, fall fertilization makes economic sense and it also saves time the following spring. Before you order your fertilizer (which is extraordinarily expensive at the moment) you should do a little homework.

The dry conditions this summer, coupled with reduced crop yields (in some cases) means there may be significant amounts of fertilizer still left in your soil. It would be a good time now to take a fall soil test, a 0-6”(15 cm) plus a 6-12”(15–30 cm) and even one from 12–24”(30–60 cm). Reduced yields this year should mean there will be carryover of 2021 fertilizer into the 2022 crop year.

Whichever soil testing system you use, make sure it adequately represents your field. Whether it be benchmarked or an average sample, it is valuable information on the nutritional state of your fields. With the information from the soil test you can also use a tool called AFFIRM, which can aid you in planning your fertilizer budget for 2022.

Fall fertilization has a few dos and don’ts. Nitrogen has the highest risk for losses of all nutrients. It can be lost to the atmosphere, called volatilization; it can be lost in saturated or warm soils (denitrification) and lost in soil water moving down the soil, out of the root zone (leaching). Applying the fertilizer in a tight band reduces a lot of these losses but does not eliminate the risk. It is recommended fall fertilizer application be done when soil temperatures reach 5˚ C or less, but before freeze-up. Cold soils reduce the activity of nitrifying bacteria, those that are responsible for taking fertilizer N and making it into nitrate where it can be lost.

Fall application of other nutrients are not nearly so delicate. You can band phosphate, potassium and sulfur with a minimum concern of losses. Sulfur in the sulfate form can leach with water but the phosphate and potassium will stay put.

There are also several products that can be applied to fertilizer to slow down the conversion of fertilizer nitrogen to nitrate. Nitrification inhibitors delay the conversion of nitrogen to nitrate and the losses that potentially affect the nitrate form. Coated fertilizer prills with a polymer can also effectively reduce nitrate conversion to nitrogen gas which is then lost to the atmosphere. There is a time and place to use each one of these products but they do increase the cost of the fertilizer product.

You can also use nitrification inhibitors with anhydrous. With the dry soils this fall you can still get good nitrogen retention in dry soil but you need to ensure the soil is sealed up behind the shank. Cloddy soils or those with big lumps will have much higher nitrogen losses to the atmosphere.

Due to this year’s increasing fall fertilizer prices the idea of fall fertilizer application may not work. Regardless of whether or not you plan to apply fertilizer this fall, it still is an excellent opportunity to get your soils tested and see what is left from this year’s crop. Using some of the tools available, such as Affirm, can help take that information and make prudent fertilizer plans for the future without costing the farm. For more information on Affirm, please contact the County.

Harry Brook is Flagstaff County’s Agricultural Fieldman. He can be reached via email at: or by phone at: 780-384-4138.