be used as the sole nutrient source. It should first be noted
that using organic fertilizer like manure can result in exces-
sive levels of these micronutrients and other trace elements.
Repeatedly applying animal manure to cropland can lead to
such high levels of copper and zinc that it becomes toxic to
plants. Applying too much of the wrong kind of manure can
make a field less fertile. This does not happen quickly, though.
Livestock manure consistently applied for ten or even twenty
years at reasonable levels does not lead to excessive levels of
Most fields receiving only chemical fertilizers continue
to increase in productivity, so these micronutrients do not
appear to threaten food production yet. One researcher found
that for one particular region, the soil's store of nitrogen could
feed a crop for twenty years, while its store of micronutrients
could feed a crop for thousands of years. Fields receive micro-
nutrients from atmospheric deposition as well. This helps
explain why farmers can harvest more and more crops over
decades or even centuries without depleting the soil of micro-
nutrients. Still, we know many of these micronutrients will
one day need replacing, and some sooner than others, like the
impending need for copper in the SanJaoquin Valley. These
micronutrients pose so few problems that it is very difficult
to find information about their decline over time, making it
problematic to predict when most soils will require some-
thing other than N, P, and K.
Sometimes a micronutrient problem exists even though it is
prevalent in the soil. This is because not all nutrients are avail-
able to plants. North Dakota soils have large amounts of iron,
but much of it is unavailable because the soil's pH is too high.
The solution is not to apply more iron but to lower the soil's
pH, or to apply a chelate, which helps transport iron from the
soil onto the surface of plant roots.
We do have an answer for what will happen when a soil
becomes deprived of micronutrients, because in a few areas
it has already happened. In the last twenty years some