Keep tight reins on crop nutrients

Written by Dr. Jason Weirich on .

Nutrient management is a leading topic for farmers and ag retailers. It’s going to stay that way for a while. If you look at what’s going for agricultural industry, you see increasing restrictions and requirements on fertilizer manufacturers and suppliers. If you look at the farm level, you see increasing scrutiny on how we use commercial fertilizers.

A 2012 a study from Purdue found that 40.5 percent of row-crop expenditures were spent on fertilizer. That’s a major part of a crop-year investment. Clearly, to be a good manager fiscally, environmentally and agronomically, you need to prevent as much fertilizer loss as possible.

MFA’s Nutri-Track bases recommendations on soil test and yield removal. The program allows you to apply variable-rate N, P and K based on your yield goals and measured fertility levels across the field. Nutri-Track focuses on putting nutrients where they are needed and avoiding over-application in areas that won’t perform.

While some growers assume this kind of management will reduce overall fertilizer application rates, that’s not the case most of the time.

Yield is never even across a field. But what is the variability in nutrient removal from your fields? We know that corn uses 0.45 pounds of P and 0.25 pounds of K per bushel grain produced. Soybeans use 0.9 pounds of P and 1.5 pounds of K per bushel.

How much difference does it make?

Try this example. You have a corn field that has a 150-bushel-per-acre average yield, but varies throughout the field from 80 to 190 bushels per acre.

If you made nutrient plans on a flat 150-bushel-per-acre removal rate, the areas that yielded 80 bushels per acre would receive an extra 31 pounds of P and 17 pounds of K. The areas that yielded 190 bushels would be short 18 pounds of P and 10 pounds of K.

Or, you have a soybean field with an overall 50-bushel-per-acre average but a range of 20 to 80 bushels per acre. If you use the flat 50-bushels-per-acre removal rate, the parts of the field that yielded 20 bushels would receive an extra 27 pounds of P and 45 pounds of K. Areas that yielded 80 bushels would be short 27 pounds of P and 45 pounds of K.

The trick is getting nutrients in the right place at the right time and in the right amounts.

Keep that nitrogen

The other aspect of making sure we are good stewards of our nutrients is stabilization nitrogen. Most recently, I’ve written about the importance of protecting nitrogen from volatilization losses. However, there other modes of loss. Let’s focus on denitrifiction and leaching.

Ammonium (NH4+) sources in the soil go through a process called nitrification. Ammonium is converted to nitrite (NO2-) by nitrosomonas bacteria, and nitrite is further oxidized to nitrate (NO3-) by nitrobacter bacteria. A majority of the nitrogen taken up by the plant is in the nitrate form, however most plants can also take up ammonium (NH4+). Once in the nitrate (NO3-) form, the nitrogen is subject to leaching and denitrification losses. Nitrate moves freely throughout the soil profile with moisture. In coarse-textured, well-drained soils nitrate can leach below the root zone where they become unavailable to the crop. Nitrate is also subject to denitrification losses. Denitrification is a biological process that converts nitrate to gaseous forms of nitrogen that are lost to the atmosphere. It occurs in soils that become waterlogged.

Currently there are two proven nitrification inhibitors on the market: nitrapyrin and dicyandiamide. Nitrapyrin has been used since the 1960s. It has long been marketed as N-Serve and most recently as Instinct, an encapsulated product for dry and liquid fertilizers. Instinct can also be used in liquid manure. The other proven nitrification inhibitor on the market is dicyandiamide (DCD). DCD is the nitrification inhibitor in Agrotain Plus and Super U.

Growers often ask me just how long N-Serve protects nitrogen in the soil. A general rule of thumb is 90 days for fall-applied nitrogen. Keep track of those days by counting application until soil temperatures drop below 40º F. Resume counting in spring when soil temperatures warm above 40º F.

For spring nitrogen application, expect 8 weeks of activity from an April 15 application; 7 weeks from a May 1 application and 6 weeks from a May 15 application.

Research indicates about a 7 percent yield advantage from fall-applied and a 5 percent advantage from spring-applied nitrogen.

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