Agriculture Reference
In-Depth Information
time. Løes and Øgaard [21] studied the change in plant available phospho-
rus over time in organic systems, and determined that maintenance of soil
phosphorus is essential for the long-term productivity of organic systems.
Cornforth and Sinclair [22] demonstrated that in grazing systems, the high-
er the stocking rate, the greater the need for phosphorus inputs. Gosling
and Shepherd [23] also demonstrated the depletion of another important
nutrient, potassium, under long-term organic farming. Where long-term
manuring has increased plant-available potassium to undesirably high lev-
els, nutrient export from farms in the form of beef and dairy products will
have a long-term benefi t. In the northeastern US, where heavy manure ap-
plication has occurred for decades, nutrients such as potassium (and even
phosphorus) have built up to very high levels. It is not unusual for pastures
in the Northeast to have potassium levels in excess of 3%, high enough to
cause milk fevers [24]. The time required for plants to access phosphorus
mineralized from organic sources such as manure [25] and from allowed
mineral sources [17] compared with synthetic phosphate fertilizers varies
greatly, which means that long-term planning is required for the proper
maintenance of soil fertility in organic production systems.
Since the use of synthetic fertilizers is prohibited in organic systems,
producers must rely on animal manures, compost and cover crops to in-
crease soil nitrogen content. Nitrogen is needed in large quantities and is
often a limiting nutrient in organic pastures and crop fi elds. In non-organic
systems this issue is addressed by the use of nitrate or ammonia fertiliza-
tion. However, nitrogen in the nitrate (NO
3
−
) form is not held by the soil
and can leach below the root zone into groundwater, even when it is ap-
plied as split applications, two or three times within the growing season,
to minimize application in excess of plant need, and nitrate is a serious
contaminant of drinking water. Microbes can convert nitrogen into forms
that volatilize from the soil; nitrogen volatilized as nitrous oxide (N
2
O) has
300 times the greenhouse gas effect of carbon dioxide (CO
2
). A fundamen-
tal understanding among organic practitioners is that nutrients should be
recycled within each farm and the transfer of nutrients, leading to deple-
tion or excess across farm boundaries, should be minimized. Manure is
one of the best sources of nitrogen available to organic producers; manure
also contains approximately 90% water by weight, so it is challenging to
transport or distribute. Therefore, manure is commonly disposed of
