Dr. Dan Talks Agronomy

Keep It For the Crop

Daniel Davidson , DTN Contributing Agronomist
Keeping nutrients in the soil for the crop and out of the water benefits everyone. (DTN photo by Chris Clayton)

There are a lot of titles being put on property nutrient management today. Keep it for the Crop (KIC), nutrient loss reduction strategies (NLRS), 4R, conservation stewardship -- it doesn't matter what you call it, it is all about getting more nutrient use from what you apply.

The efforts are all being driven by the goal to reduce the amount of nitrogen (N) and phosphorus (P) getting into rivers and surface water bodies. Since nutrients are soluble in water, they follow the water flow to their natural discharge points. Preventing hypoxia in the Gulf of Mexico, Lake Erie, Chesapeake Bay and many other small water bodies and reducing nitrate loads in rivers such as Iowa's Raccoon River is our responsibility. The challenge is to do it without short-changing the crop.

Nitrogen (N) is the biggest culprit we have to deal with since it is most vulnerable to loss and corn is so nitrogen needy. Nitrogen undergoes two important transformations in the soil -- nitrification and denitrification.

Any ammonium forms of nitrogen, supplemental or organic, will convert to nitrate. Nitrate will leach down into groundwater and drain out tile lines. When soils are saturated with water, nitrate will denitrify and release nitrous oxides -- a greenhouse gas. While denitrification doesn't contribute to hypoxia, it does contribute to greenhouse gas load in the atmosphere and it is a net loss of nitrogen to the grower.

The upside of this natural phenomenon is that this same loss mechanism can be a tool when bioreactors are installed to remove nitrates from tile line discharges.

To manage nitrogen and keep more of it for the crop, the first step is to move away from fall-applied ammonia and apply nitrogen in the spring and early summer. We all know the arguments against that -- ammonia may be cheaper in the fall, growers have more time and the soil is in better condition for application. However, protecting the environment means relearning how we think and what we do.

NITROGEN KNOW-HOW

Applying ammonia after the soils consistently cool down below 50 degrees Fahrenheit in the fall and using a product such as N-Serve or NutriSphere-N keeps the N in the ammonia form longer before it converts to nitrate the following spring. Eventually, that ammonia will convert and losses will occur if there are heavy spring rain events before the crop begins to take it up.

Better options are to apply nitrogen in the spring preplant, at plant or sidedress using either ammonia, urea or UAN (urea ammonium nitrate) that has been stabilized with N-Serve, Instinct, NutriSphere-N and Agrotain PLUS to protect surface-applied and non-incorporated urea from ammonia loss. The key is to apply N in several doses and stabilize it to prevent losses. Corn takes up the bulk of its N beginning at V8 so applying N closer to its rapid uptake period means less N loss and more for the crop.

PHOSPHORUS PLANNING

Phosphorus is not mobile in the soil and doesn't leach like nitrate. However, it is as important a culprit of hypoxia as N. Phosphorus moves with surface runoff. When it rains, phosphorus moves with soil particles and flows with sediments into streams and surface water. There are commercial P amendments (Avail, Titan) that slow fixation and keep it available to the crop. However, none of these products sequester P away in the soil and keep it for the crop or prevent discharge losses.

Gypsum, or calcium sulfate (either a natural mined mineral or FGD byproduct) can be used for this purpose. Gypsum is soluble and a salt and breaks down into calcium and sulfate. Calcium and phosphate (commercial form of fertilizer P) have a natural affinity for each other and can trap phosphate on the landscape so it won't be lost or discharged by surface runoff. Gypsum also provides sulfate for the next crop.

CONSERVATION CONSIDERATIONS

Soil conservation is one the best strategies for protecting against nutrient loss. Contour farming and contour strips reduce runoff. Conservation tillage and no-till keep more residue on the surface and protect against soil loss and nutrient discharge. Any good soil conservation practice that keeps soil on the landscape also keeps nutrients on the landscape. This is one of the primary mechanisms for keeping P in the field.

Grassed waterways and buffer strips have been a part of production agriculture for decades. While they take up acres and sometimes make planting and spraying difficult with 60- and 90-foot planters and 90- and 120-foot sprayers, grassed waterways and buffer strips slow water movement and capture nutrients before they reach streams. Even seeding natural waterways every fall with a cover crop is a viable solution for slowing runoff and discharge.

Bioreactors are another tool. They can be installed on tile outlets, and microorganisms and woodchips denitrify nitrates before they contaminate surface waters. USDA posted a blog on the topic at http://bit.ly/….

Cover crops offer a lot of potential as a solution. Besides protecting the soil and suppressing weeds, they scavenge free nitrates, ammonia, phosphate and other nutrients from the soil. When the covers are terminated, these nutrients are mineralized and available to the crop. In addition, they add more carbon back to the soil, promote microbial activity and improve soil health.

Nutrient loss reductions should become part of your everyday nutrient management practices. There are many tools at your disposal, but what is important is adopting a system of practices that keep nutrients on the landscape without short-changing the crop so it goes hungry.

Dan Davidson can be reached at AskDr.Dan@dtn.com

(PS/AG)

Dan Davidson