Fall is a fine time to apply phosphorus fertilizer but a risky time to apply nitrogen. Despite recommendations to wait until spring for nitrogen fertilizer application many producers still apply anhydrous ammonia in the fall because soil conditions are better for injection of the fertilizer and fall application can decrease the workload in the spring.
Most producers have learned, some the hard way, to use precautions when this is done. The two main precautions are to wait until soil temperatures have cooled to below 50 degrees and to use a stabilizer that slows the biological conversion of ammonium nitrogen, which is bound to soil particles, to nitrate nitrogen which can leach.
The phosphorus fertilizers we commonly use in Iowa all contain nitrogen, and most phosphorus fertilizer is applied in the fall. When the phosphorus fertilizers are applied at crop removal rates, substantial quantities of nitrogen can also be applied. The precautions that are widely used when anhydrous ammonia is applied are seldom considered when phosphorus is applied.
Applications begin in September as soon as crops are harvested and often onto warm soil where the ammonium in the fertilizer can fairly quickly become nitrate and be at risk of leaching and deteriorating water quality. Producers tend to disregard the nitrogen in fall applications of phosphorus fertilizer either because they are unaware of it or because they can't count on it being in the root zone the next year when the crop can use it.
But, because our choices for phosphorus fertilizer in our local marketplace all contain nitrogen, we unavoidably are adding fertilizer N in the fall to the residual and mineralizing nitrogen pools in the soil, and then many of us use expensive measures to attempt recovery of it in the spring. It would be far better to avoid applying it when we don't want it there. We should be able to apply phosphorus without nitrogen. We don't currently have any options in our marketplace to do that.
Fortunately, options exist that could be brought into our marketplace if demand develops for them. Triple Superphosphate (TSP) is a high analysis, competitively priced fertilizer that contains no N, but it is not available locally because there is no local demand for it. Simple Superphosphate is a lower analysis fertilizer that contains no nitrogen but does contain sulfur.
These are both good options for phosphorus fertilizers that can be fall applied without being counterproductive to our efforts to reduce the nitrogen load escaping our farms. Increased awareness of these options could increase demand for them. When we go to our fertilizer vendors to order fertilizer we tend to order off the menu just as we do in a restaurant.
But, the next time you talk to your vendor ask about these options. After all, simply not applying nitrogen when or where you do not want it has to be one of the least cost means of meeting the goals of the Nutrient Reduction Strategy.