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Putting P, K fertilizer in its place

Every fall we get questions about fertilizer placement, and as prices escalate, the discussions get longer and more complex. Broadcast applications of P and K have been the norm and standard for decades, but like other agronomic practices that have evolved as yields rise, we are all looking for ways to save money, increase profit and protect our environment when it comes to fertilizer applications.

A consistent issue that comes up among growers and agronomists is the “broadcast P and K versus deep banding” question. Our P and K expert at Iowa State University is Antonio Mallarino, professor of agronomy with research and Extension responsibilities in soil fertility and nutrient management. He’s been working on this placement issue for several years.

He says deep-banding P and K has some potential advantages; concentrating these nutrients in bands could increase nutrient uptake in soils with unusually high capacity to fix P and K and when root growth is restricted by unfavorable soil conditions.

Particularly with no-till or ridge-till management, deep banding also could increase nutrient uptake and yield compared to broadcast or banding with the planter when the soil surface layer becomes drier. Do these potential advantages equate to increased yields, profits or environmental benefits? Mallarino has some answers.

In his work, the P placement method seldom influenced grain yield for any crop or tillage. While the banded P almost always increased corn early growth in no-till and ridge-till fields, no yield difference between the placement methods was observed with application rates ranging from 28 to 120 pounds P2O5 per acre. Although a few fields tested very low in P, no field tested less than 7 ppm (Bray-1 test).

Results of another study in a north-central Iowa field testing very low in P (2 ppm) showed that at low P application rates (25 pounds P2O5 per acre) banding with the planter produces higher yields than broadcasting. At that site, however, only currently recommended broadcast rates or a combination of both placements achieved maximum yield and maintained soil test values in the optimum range.

While he didn’t see differences in P placement, it is interesting that deep-banding K increased corn yield over the broadcast or planter-band placement. The differences were smaller for soybeans, and the two band placements often did not differ. The differences between the placements were similar for K rates ranging from 35 to 140 pounds K2O per acre.

Across all fields and years, the corn yield advantage of the deep banding over the other two placement methods was approximately 8 bushels per acre for ridge till, 4 to 5 bushels per acre for no-till, and about 2 bushels per acre for the chisel-disk tillage.

The corn response to deep-band K usually was larger and more consistent for ridge-till fields than for the other tillage systems, and in many trials, even high rates of broadcast K did not increase yield at all over the control. In no-till fields, the response to deep-band K was consistent across fields, but its magnitude varied markedly.

Responses frequently were observed in high-testing soils and could not be solely explained by K stratification. It is likely that the responses were related to weather conditions, particularly soil moisture.

Under normal conditions, corn grown with no-till or ridge till draws more nutrients from the soil nearer the surface, because of the higher nutrient levels present. However, when surface soil layers become drier, root development in deeper portions of the soil profile increases. When this happens, the portion of the root system actively taking up nutrients can be below the zone of highest nutrient concentration.

When does deep banding pay?

To boil all this down, Mallarino provides some recommendations. Deep banding of K may provide distinct yield advantages by making K more available in ridge-till and no-till fields, even on soils that test optimum to high in K. Growers can deep-band P at the same time to increase early corn growth. Deep banding increases fertilizer application costs, but the magnitudes of yield responses suggest deep banding will be cost-effective in most ridge-till and many no-till fields, but not in a chisel-plow system.

The economic benefit of deep banding can be increased in two ways. An ongoing on-farm project in no-till fields shows that a combination of fall strip tillage, anhydrous ammonia application, and deep banding of P and K produces higher yields and does not increase costs significantly compared with a separate broadcast P and K application. Also, deep-banding the needed P and K once for the two-year corn-soybean rotation will produce yield increases in both crops equivalent to application before each crop.

We continue to do a wide range of research at ISU on fertilizer rates, timings, placements, sources and many other variables dealing with soil fertility. Two great places to check out are the ICM News website at www.extension.
iastate.edu/CropNews
and our Integrated Crop Management Conference proceedings at www.aep.iastate.edu/icm/homepage.html.

McGrath is partnership program manager of ISU’s Corn and Soybean Initiative.

This article published in the November, 2011 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.