Agriculture Reference
In-Depth Information
that is managed sustainably to improve defi cits in any of these traits in
support of plant growth without degradation, such as erosion or saliniza-
tion. One of the most important indicators of a healthy soil is the level of
organic matter. Like other carbon-based fuels, organic matter is consumed
more rapidly by soil microbes at high temperatures and high levels of oxy-
gen, so while good drainage and aeration are important for root growth,
the cultivation required to support annual cropping hastens the breakdown
of organic matter. Chemical fertilization stimulates the microbial mineral-
ization of soil organic carbon [33], but the greater agricultural productiv-
ity achieved with synthetic nitrogen fertilization can result in higher crop
residues and increase soil organic matter. However, high levels of nitrogen
and phosphorus fertilization can undermine soil stability and lead to in-
creased erosion [34].
Organic matter in the soil is a repository not only of carbon, but of the
nitrogen and phosphorus compounds contained in the fl ora and fauna of
which it is comprised. Soil organic matter as humus glues soil particles
together, creating structure that is resistant to collapse and therefore is es-
sential to drainage and aeration, soil organic matter has more surface area
and a far greater capacity to absorb water than the mineral components
of the soil [35], so it not only adsorbs nutrient cations (the soil cation ex-
change capacity) but can hold nearly its own weight in soil water. Since
the quantity of water used by a given plant species is directly proportional
to its leaf growth [36], and therefore to its vegetative productivity, the abil-
ity of a soil to absorb and release water is a critical trait.
4.2.3 THE IMPACT OF GRAZING MANAGEMENT ON
FORAGE PLANTS
To the extent that grazed or conserved forages comprise a greater amount
of the feed of organic ruminants than non-organically fed ruminant live-
stock, the health and performance of these animals may be significantly
influenced by the mineral composition of forages. The uptake of nutrients
by forages is a function of rooting depth, root surface area, and forage
plant species. In general, the root length and density of perennial forages is
greater than that of annual crops such as soybean, corn or the cereal grains
