AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Fall 1997, No. 1
Use fall conditions to your advantage. Wet spring conditions can arise suddenly and cause mayhem when soil sampling, fertilization, tillage, and planting are all scheduled in a short period of time. Completing some of these tasks in the fall will better prepare growers and dealers for any future surprises. Wet spring conditions can also lead to soil compaction as growers and applicators are forced to run equipment over ground that is just dry enough to keep equipment from getting stuck. Compacted soils can result in severely reduced yields. Root growth becomes restricted, and plants are unable to take up the water and nutrients needed to optimize yields. Such effects can last several years if left uncorrected. Running application equipment over typically drier soils in the fall will reduce the risk of compaction.
Fertilizer applications, regardless of when they are performed, should always be based on soil test results. The results can be misleading, however, if proper samples are not taken. Sampling under the drier fall conditions provides better reproducibility of sampling depth. Wet soil cores tend to compact within the tube, and deeper portions of the sample may simply be pushed farther down, rather than entering the tube. Reproducible, representative soil samples must be collected if fertilizer recommendations based on them are to be accurate.
Aglime should be applied in the fall prior to spring cropping. Aglime will not alter soil acidity immediately. Soils have the ability to resist such changes. Longer reaction times overwhelm the soil's resistance, and acidity eventually is reduced. Fall applications allow the time necessary for these events to occur.
Get the most from fall nitrogen applications. This will depend on soil type, form of nitrogen fertilizer, and soil temperature. Nitrogen should not be applied in the fall on sandy soils with a high leaching potential. Only spring applications are appropriate for such soils. In semi-arid climates, nitrogen should not be applied in the fall if preplant irrigation is necessary. Most universities recommend that ammonium forms of nitrogen be used for fall applications. In many areas, the use of a nitrification inhibitor is also beneficial. Ammonia is recommended because it reacts with soil water to form ammonium, a form of nitrogen that is held in place by the soil. Nitrification inhibitors, such as nitrapyrin, help keep ammonium from being converted to nitrate, which can easily be lost during the winter because of leaching. Research has shown that soil temperatures below 50øF extend the time that ammonium and nitrapyrin remain in soil. Therefore, fall nitrogen applications of ammonia should be made when soil temperatures have dropped below 50 degrees F and are expected to remain at these lower temperatures until spring.
Like ammonium, potassium is also held in place by soils. However, in sandier soils, the capacity to hold potassium is reduced. If the quantity of potassium applied is greater than the soil's ability to retain it, leaching losses can occur. For this reason, fall applications of potassium are not recommended on sandy soils. In addition, research has shown that fall applications of potassium on organic soils is inappropriate, because leaching losses can occur during the winter.
Applications of phosphorus are well suited to the fall because phosphorus is immobile in soils and will not leach, except on certain sandy soils in high rainfall areas. For the most part, phosphorus can be lost only through erosion. Consequently, phosphorus should only be applied in the fall on soils where appropriate erosion control measures have been implemented.
Perhaps the most common method of applying phosphorus and potassium in the fall is a broadcast application on stubble followed by primary tillage. Excellent results, however, have been demonstrated for fall band applications in ridge-till and no-till systems. Banding fertilizer 3 to 4 inches below the surface in either system and then planting over the bands in the spring maximizes the availability of both nutrients during the initial stages of crop growth.