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Mississippi

Utilize your soybean “cover crop” in damaged fields.

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A drive through any of our actively farmed areas can be a shock these days since thousands of acres of soybeans remain unharvested, with field after field of gray beans standing as a reminder of last year’s disaster.

The disease we refer to as pod and stem blight was responsible for most of the damage. Weeks of rain and warm weather provided the perfect incubator for development of this and other diseases. We see varying levels of damage almost every year; however in 2009 the extent of damage was greater than ever before.

The general feeling among farmers has been that damaged soybean fields were a complete loss; however this dark cloud may have a silver lining.

Soybeans grew very well in most cases, nodulated heavily, and fixed huge amounts of nitrogen from the atmosphere. They drew the necessary phosphorus, potassium, and a full complement of secondary and micro-nutrients from the soil. Having accomplished this, they have since been deteriorating, with plant contents being released into the environment.

In a “perfect world” we might claim that there was no nutrient loss from these soybean fields, and that all of the nutrients contained in the seed and vegetative tissue will be recycled for use by other plants. However, this is certainly not a perfect scenario, and some of the nutrients like nitrogen may be carried away as vapor. More of the nitrogen and elements less prone to vaporization will be carried away in runoff water; or they may be leached downward through the soil and out of the root zone.

In fact, there is no way to accurately estimate the amounts of nutrients that may be recycled to be used by this year’s crop. Neither will soil tests produce a good answer since the process is not yet complete. There are simply too many variable factors, including soil type, soil pH, soil organic matter level, slope, vegetative cover, cropping and tillage history, weather, and others.

Assumptions are tricky; but I believe we can accept two as we attempt to make an estimate of carryover nutrients. They are that the carryover amounts are greater than zero and less than 100%. Just for the sake of discussion, let’s say that half of the nutrients contained in the plants may be recycled.

According to the PPI table for nutrient uptake, a 40 bushel per acre soybean crop (fairly common or even higher in 2009) should contain about:

  • 220 pounds of N (fixed from the atmosphere by the nodules).

  • 40 pounds of phosphate (P2O5).

  • 140 pounds of potash (K2O).

When we apply our 50% recycle estimate to these numbers we come up with:

  • 110 pounds of N.

  • 20 pounds of P2O5.

  • 70 pounds of K2O.

It is likely that higher percentages of phosphate and potash will be retained than N; but this is a starting point.

Our estimate suggests that we may have enough recycled nutrients following many of our abandoned soybeans to supply most of the nutrients for a cotton crop; although I expect that some supplemental phosphate and potash may be needed. For corn following these beans, there may be about half enough N, and around a third or more of the phosphate and potash.

Soil tests should be done, and the results combined with estimates of carryover to arrive at a better estimate of crop needs for 2010.  We essentially grew a soybean cover crop last year where fields were not harvested.  Take advantage of it to reduce costs this year.  Thanks for your time.