Soil - Agroecology: The Ecology of Sustainable Food Systems - page 106

up returning to the sea 3000 mi from where it originated.
Since the time necessary to build up sufficient sediments
of phosphate-rich rock and to go through the geological
process of uplifting is very much beyond the realm of the
human time frame, and since the known easily-available
phosphate reserves are quite limited, current practices of
phosphate fertilizer management in many modern agroeco-
systems can be said to be unsustainable.
For sustainable management of phosphorus to occur,
phosphate needs to pass quickly through the soil component
of the cycle and back to plants for it not to be fixed in
sediments or washed to sea. Ways must be found to better
keep phosphorus in an organic form, either in standing
biomass or in soil organic matter, and to ensure that as soon
as phosphorus is liberated from this organic form, it is
quickly reabsorbed by soil microorganisms or plant roots.
An additional component of sustainable management
of soil phosphorus has to do with the formation of insoluble
phosphorus compounds in the soil. Phosphates in the soil
solution often react chemically (especially with iron and
aluminum) to form insoluble compounds, or become
trapped in clay micelles out of reach of most biological
recovery. Low pH in the soil exacerbates the problem of
phosphate fixation in an insoluble form. At the same time,
however, these processes provide a strong mechanism for
retaining phosphorus in the soils of the agroecosystem;
phosphate fertilizers added to the soil are retained almost
completely. Some agricultural soils in California show very
high levels of total (through not easily available) phospho-
rus after several decades of farming. So leakage of phos-
phorus from agroecosystems can be quite small, but the
unavailability of phosphorus from the soil component of
the system once it is fixed requires further addition of avail-
able phosphorus in the form of fertilizer. Of course, biological
means of liberating this “stored” phosphorus might contrib-
ute better to sustainability. These means have a lot to do
with the management of soil organic matter.

Phosphorus
in rocks

Fertilizer
manufacture

Weathering

Soil phosphorus

Excreta and
dead
organisms

Uptake

Crop plants

Pathway 2

Pathway 1 Pathway 3

Pest
herbivores

Grazing
animals

Humans

Excreta

Excreta

Lost to ocean
sediments

FIGURE 8.4 Pathways of phosphorus cycling in agroecosystems.

parent material; therefore, the input of phosphorus into
the soil and the phosphorus cycle in agroecosystems is
limited by the relatively slow rate of this geologic process.
Inorganic soluble phosphate ions are absorbed by
plant roots and incorporated into plant biomass. The phos-
phorus in this biomass can be sent along one of three
different pathways, depending on how the biomass is con-
sumed. As shown in Figure 8.4, consumption of plant
biomass by pest herbivores, by grazing animals, or by
humans who harvest the biomass comprises the three path-
ways. Phosphorus in the first pathway is returned to the
soil as excreta, where it decomposes and enters the soil
solution. Phosphorus in the second pathway can be recy-
cled in the same way, but if the grazing animal goes to
market, some phosphorus goes with it. In the third path-
way, there is little chance of the phosphorus returning to
the soil from which it was extracted (except in much of
China, where human excreta is used as fertilizer).
Much of the phosphorus consumed by humans in the
form of plant biomass or the flesh of grazing animals is
essentially lost from the system. An example of what may
happen to phosphorus in the third (human consumption)
pathway may serve to illustrate the problem: phosphate is
mined from phosphate-rich marine deposits that have been
geologically uplifted and exposed in Florida, processed
into soluble fertilizer or crushed into rock powder, and
shipped to farms in Iowa where it is applied to the soil
for the production of soybeans. A part of the phosphorus,
in the form of phosphates, is taken up by the plant and
sequestered in the beans that are harvested and sent to
California, where they are turned into tofu. Following
consumption of the tofu, most of the liberated phosphate
finds its way into local sewer systems, and eventually ends

SOIL ORGANIC MATTER

In natural ecosystems, the organic matter content of the A
horizon can range up to 15 or 20% or more, but in most
soils it averages 1 to 5%. In the absence of human inter-
vention, organic matter content of the soil depends mostly
on climate and vegetative cover; generally, more organic
matter is found under the conditions of cool and moist
climates. We also know that there is a very close correlation
between the amount of organic matter in the soil and both
carbon and nitrogen content. A close estimate of soil
organic matter content can be obtained by either multiplying
total carbon content by 2 or total nitrogen content by 20.
Soil organic matter is comprised of diverse, heteroge-
neous components. Its living material includes roots,
microorganisms, and soil fauna; its nonliving material
includes surface litter, dead roots, microbial metabolites,