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Often overlooked: Monitor soil fertility in soybean fields

A lot of uncertainty exists for the 2006 Mississippi soybean crop. Fertilizer prices, fuel costs, and commodity prices will all play roles in determining the crop mix for the upcoming growing season.

The bulk of the soybean crop in Mississippi is grown on heavy clay soils. A portion is grown on other soils, but the bulk by far is on clay. We have increased the portion of the crop that is rotated with other crops, predominately rice, but the most is soybeans following soybeans.

Cropping changes over the last few years have allowed us to produce above-average yields. Higher-yielding varieties, earlier-maturing varieties, earlier planting dates, Roundup resistance and, most importantly, increased management have all played major roles. However, the above-average yields have not come without a cost.

Although we have capitalized on technology, some basic production inputs are often overlooked. One area is soil fertility. This is not the concern in other crops that it can be in soybeans. Often fertilizer is not applied to soybeans when, in fact, a 40-bushel soybean crop will remove more P and K than a 2-bale cotton crop. One positive aspect of the heavy clay soils is higher P and K levels. That is not true in every scenario, but it is more indicative of clay soils than mixed to sandy soils.

In recent years, long-term cropping and higher yields have seen soil fertility levels in the Delta reduced. Think about history. How many crops have been grown in a field? How often and how much fertilizer was added?

In the case of soybeans, has fertilizer ever been applied? The answer many times is “never.”

Soybeans do need and will respond to fertilizer. However, one advantage they have is that they prefer residual fertility — fertility that is carried over from previous years or levels released as crop residue breaks down. Although soybeans prefer residual fertility, that does not mean zero fertility.

A bushel of soybeans will remove 0.8 pound of P and 1.4 pounds of K. These numbers represent crop removal in the grain, not crop uptake. Although uptake is higher, levels above this will be released back into the soil following harvest.

Using the above figures, a 40-bushel crop will remove 32 pounds of P and 56 pounds of K. Given these levels, I think you can see why most soybean fertilizers are applied in a 1:2 ratio. In comparison, a 2-bale cotton crop will remove 28 pounds of P and 40 pounds of K.

As I stated, the removal figures will allow you to maintain soil fertility levels in the absence of a soil test. I hate to guess but when forced to do so, I look at the history of crop yields and the amount of fertilizer applied to the previous crops.

This is no substitution for a good soil sample. In the last few years, intensive sampling has allowed us to fine-tune fertility needs, essentially putting fertilizer where needed. Areas that exhibit lower levels are often related to soil productivity. In other words, lower levels may represent the most productive areas of a field. Higher-yielding areas remove greater amounts of fertility.

High yields come at a cost. So if you are considering planting soybeans to save on fertilizer costs, you might be able to with nitrogen but maybe not with P and K.

You may can skip a year or two but at some point you will need to replenish the soil. The one exception may be some heavy clay soils where fertility levels are quite high and cropping can be accomplished without adding fertilizer. For this to be successful, an intimate knowledge of each field will be essential.

Alan Blaine is the Mississippi Extension soybean specialist. e-mail:

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