ISSUE 2 May 12, 2005
SOYBEAN FERTILITY NOTES
Although some soybeans are in the ground, there are many additional acres yet
to be seeded.
Briefly, here are soybean fertility guidelines-
- Nitrogen is only needed on first-year soybeans, or soybeans where
nodulation was poor the last time the field was in soybeans, or in fields
where iron chlorosis has been a serious problem in the past. Rates including
residual soil N should be about 50 lb N/acre. Soybeans in fields well
nodulated in the past and in fields where chlorosis is only a spotty problem
do not need supplemental N regardless of soil test nitrate level.
- Phosphate is needed at soil tests below medium levels. Broadcast is
desirable, even in no-till fields, but banded P can be applied if rates of N
in 7-inch rows do not exceed 10 lb/acre and do not include urea-N. MAP is
probably most safe, but DAP can be used if rates are low.
- Micronutrients have not been found to be helpful in North Dakota
generally. Sometimes iron applied with the seed or foliar has helped greenup
plants, but EDDHA, the soil applied form most helpful, is too high priced to
be practical at rates required for yield increases. Zinc has not been found to
increase yields in local studies.
- Inoculation is most useful in first and second-year soybeans. Thereafter,
if nodulation was good in the first years, only occasional inoculation is
FARMING MYTHS AND LEGENDS
Myth and legend 1- Soybeans build soil organic matter
Soybeans only build soil organic matter if they are tilled under before
harvest as a green manure. Rotation studies have shown that when soybeans are
introduced, organic matter levels decrease at a greater rate than previous
non-annual legume rotations. Research has suggested that under soybeans, rate of
organic matter breakdown is increased, evidently helping soybeans by providing
additional soil N so that the soybean carbohydrate contribution to nodules is
reduced. The increased rate of organic matter breakdown and N mineralization is
part of the source of our soybean previous crop N credit we give crops following
NDSU Extension Soil Specialist