Preplant Fertilizer

This portion of the study included two separate preplant fertilizer studies.

●          The first study was designed to determine whether or not levels of P, K, S, B, and Mn limit soybean yields in soils that test high or very high in nutrient levels.

The finding was that the extra application cost of about $80/acre for this approach vastly outweighed the yield benefits, which were very small (generally less than 2 bu/acre). Thus, soils that test high in required nutrients have sufficient levels of those nutrients to maximize soybean yields. Economic loss will occur if additional fertilization above this level is applied.

It should be noted that 90% of the sites that had sub-optimal levels of P and K prior to the study initiation did realize a significant yield benefit from application of P and K to achieve recommended levels. There was no economic analyses of this finding.

●          The second study was designed to determine if there is an advantage to applying preplant P and K ahead of soybeans instead of relying on carryover levels of these nutrients that were applied to a previous corn crop (this may not apply to a large acreage of Midsouth soybeans, but could if a soybean/corn rotation becomes prominent). In other words, would soybean farmers get a yield boost from applying P and K every year instead of biennially?

The finding was that increasing the frequency of P and K fertilizer application to an annual schedule did not increase soybean yield above that obtained from the biennial application. Again, these findings are from soybean/corn rotation sites that tested high or very high in P and K nutrient levels.

In the Midsouth, where monocrop soybeans are more the normal, preplant applications of K are recommended to bring soils to recommended levels on an annual basis.

Take-Home Message

Again, these studies were conducted on sites that either had or were fertilized to bring soil nutrient status to recommended levels before the project was initiated. So, they do not provide information about the economic gain or loss from producing a soybean crop on under-fertilized soils vs. those that are fertilized to reach recommended levels according to soil test results.

Thus, the tried and true adage that soybean yields will be maximized by having the recommended levels of nutrients held true in this 3-year study. Conversely, over-fertilizing beyond levels that are recommended in an attempt to obtain higher yields loses money.

In essence, the results from this portion of the study confirm the importance of following soil test recommendations to provide the proper levels of required nutrients for a productive soybean crop.

Composed by Larry G. Heatherly, Oct. 2012, larryheatherly@bellsouth.net