farmgate: When Will Your Fertilizer Be Applied This Fall?
With your crop still several weeks from being ready to harvest, and the onset of frost and cold weather coming in less time than that, your priority is on getting your barely mature crop out of the field and into the bin. The last thing you are thinking about is fall fertilizer application.
Your farming plate is full enough as it is without adding fertility issues right now. However, this is the time of year when crop nutrient issues are addressed, and there is always the possibility that a very late fall will allow a timely harvest as well as fall fertilizer application and tillage. With that potential, it would not be inappropriate to make some plans about building fertility for your 2010 crop. Iowa State fertility specialists John Sawyer and Antonio Mallarino use their recent newsletter to provide some issues for consideration.
Soil sampling. Your need for P and K is generally guided by the result of a soil sample, as well as determining how much, if any, lime is needed based on soil pH. However, with crops growing in a field it is difficult to collect soil samples. It also takes time to get them analyzed, get the fertilizer delivered and spread while the soil is still tillable. Certainly, those issues can be schedule for the spring, but based on the past several years you may not have the chance to get any fertilizer spread. And the agronomists say it is a good practice to collect soil samples in the same season that potash needs are addressed.
Soil samples should represent no more than 10 acres each unless there is very little soil type variation in the field. Many Cornbelt fields are rife with different soil types. The agronomists say composite grid samples should represent areas smaller than 3 to 4 acres and having multiple test results per field helps determine uniform application rates and aids the effort to have site-specific application rates for fertilizer, lime and manure. Fertility recommendations are based on samples taken in the top six inches of the soil and cores should be taken at a consistent depth. They recommend 10-12 cores per sample with uniform blending before the sample is sent to the lab for testing.
P & K application. Among the reasons to apply P & K in the fall is typically the availability of time, unlike the potential issues which farmers will face this year. The soil would probably be dry and the fertilizer could be applied before tillage. The nutrients would not be locked into the soil before their use in the spring, and even a two year application could be made. With rainfall usually less in the fall, there is little worry about loss of nutrients to leaching or erosion. And rain will help P incorporation into the soil. The only downside is the amount of nitrogen applied when P is applied in the form of DAP or MAP, and the ammonium form of the nitrogen will allow higher amounts of nitrates to be formed and lost to the environment.
N application. While nitrogen can be applied successfully in the fall, it has several requirements that are key to that success. Soil temperatures have to be under 50ยบ to prevent the conversion of the ammonium to nitrate. If soils are warmer, then a nitrification inhibitor is required to slow that conversion rate. Many farmers will apply anhydrous ammonia in the fall under those conditions and have nearly all of it available to boost corn growth the following spring. Other N fertilizers, such as urea and UAN will nitrify too quickly and should not be fall applied. Coated urea can be used if managed properly.
Manure application. Manure is a good nutrient source but because of variable N, P, and K content, handling and rate of application can be challenges that reduce its success rate. While the maximum rate of application for one nutrient will limit the application of the other two, those others will have to be bolstered by a commercial fertilizer. The agronomists say that giving priority to the amount of N in the manure means that manure will best be applied in the fall after soils cool, and that will not affect P and K availability. With the potential for N loss, the manure N should be injected or immediately incorporated to reduce loss with surface application. Additionally, the manure form of P should also be managed to prevent runoff, and also prevent excessive application that results in a high P index rating.
Summary:
With the lateness of the 2009 crop, fall fertilizer application may not get made by many farmers, although fall may be the best time to do so because of drier soils, chance for good incorporation, and usually the spring fieldwork is already rushed. Assessing fertility your fertility requirement begins with a soil sample, and sufficient numbers need to be taken to adequately analyze the need for P and K and lime. P & K can be applied every two years because of their stability in the soil, unlike nitrogen. With less rainfall in the fall, fall-applied P has less of a chance to leach out or run off and will be incorporated. However, with the use of DAP or MAP, there may be nitrogen that is also applied, and if the soil is too warm, it converts to nitrates and is lost to the following crop. N application can be successful in the fall if soils are cool or a nitrification inhibitor is used. Manure is also a good nutrient source, but phosphorous may reach its maximum before other nutrients reach adequate levels.
Posted by Stu Ellis on September 15, 2009 1:06 AM to farmgate