Agriculture Reference
In-Depth Information
subsequent deposition of synthesized C compounds on and in the soil. A major
factor contributing to this imbalance is the accelerated rate of SOM decom-
position under cultivation as soil aggregates are disrupted. When aggregates
break down, organic matter that was previously protected because of its inac-
cessibility, and even chemically recalcitrant materials, become more vulnerable
to microbial attack. Nutrients such as N, phosphorus (P), and sulfur (S) that
are released during SOM decomposition are taken up by plants and removed
in plant products. Hence, a decrease in SOM under cultivated crops gradu-
ally leads to a decline in soil fertility. There are also undesirable changes in soil
structure.
Vineyards in which the mid-rows are clean-cultivated will experience a
decrease in SOM over time, especially if the soil has a light texture (sandy to
sandy loam). Examples of this condition occur in the Bordeaux region, France,
as illustrated in figure 5.5. Another example is shown in figure 2.16, chapter 2.
On sloping land that is not terraced, continuous cultivation of the mid-rows and
under vines predisposes soil to erosion, as seen in an example in the Côte d'Or
region, France (figure 5.6). Although these soils are of medium texture (clay loam
Brown Earths) and high in SOM in their natural state, they have been markedly
depleted in C through cultivation and erosion. The next sections discuss ways of
slowing or preventing SOM depletion in vineyards.
Figure 5.5
Sandy, gravely soil of low organic matter content in an overcultivated vineyard on
the right bank of the Gironde River, Bordeaux region, France.