Utilize your soybean “cover crop” in damaged
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
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
The general feeling among farmers has been that damaged
soybean fields were a complete loss; however this dark cloud may have a
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
220 pounds of N (fixed from
the atmosphere by the nodules).
40 pounds of phosphate
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
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.