Increasingly . . . emphasis is being laid on the direction of natural forces, on the conservation of inherent richness, on the acquirement of plant food supplies from the air and subsoil.
—J.l. hills, C.h. Jones, and C. Cutler, 1908
We used the term cycle earlier when discussing the
nutrients. Other flows are unplanned—for example,
flow of nutrients from soil to plant to animal to soil, as
when nitrate is lost from the soil by leaching to ground-
well as global carbon and nitrogen cycles (chapter 2).
water or when runoff waters take nutrients along with
Some farmers minimize their use of nutrient supple-
eroded topsoil to a nearby stream.
ments and try to rely more on natural soil nutrient
When crops are harvested and brought to the barn
cycles—as contrasted with purchased commercial
to feed animals, that is a nutrient flow, as is the return of
fertilizers—to provide fertility to plants. But is it really
animal manure to the land. Together these two flows are
possible to depend forever on the natural cycling of all
a true cycle, because nutrients return to the fields from
the nutrients to meet a crop’s needs? Let’s first consider
which they came. In forests and natural grassland, the
what a nutrient cycle is and how it differs from the other
cycling of nutrients is very efficient. In the early stages of
ways that nutrients move from one place to another.
agriculture, when almost all people lived near their fields,
When nutrients move from one place to another,
nutrient cycling was also efficient (figure 7.1a). However,
that is a flow. There are many different types of nutrient
in many types of agriculture, especially modern, “indus-
flows that can occur. When you buy fertilizers or animal
trial-style” farming, there is little real cycling of nutrients,
feeds, nutrients are “flowing” onto the farm. When
because there is no easy way to return nutrients shipped
you sell sweet corn, apples, alfalfa hay, meat, or milk,
off the farm. In addition, nutrients in crop residues don’t
nutrients are “flowing” off the farm. Flows that involve
cycle very efficiently when the soil is without living plants
products entering or leaving the farm gate are managed
for long periods, and nutrient runoff and leaching losses
intentionally, whether or not you are thinking about
are much larger than from natural systems.
Photo by iStock photo
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Building SoilS for Better CropS: SuStainaBle Soil ManageMent