[Edited_by_A._Ciancio,_C.N.R.,_Bari,_Italy_and_K.(Bookos.org)

IPM IN PECAN PRODUCTION
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3.3. Soil Building and Sustainability
Clover can improve orchard soils in a number of ways. Protection against erosion is
the most obvious benefit, but providing organic matter is an equally important, and
more long term goal. Clover can provide habitat and/or food source for important
soil organisms, break up compacted soil layers, and help dry out wet soils.
Erosion deprives orchards of topsoil, the most fertile portion of soil with the
highest amount of organic matter. When soil particles are dislodged by rainfall, they
are more vulnerable to runoff. Cool season legumes can reduce the impact of rainfall
on bare ground, slow the action of moving water, increase the soil’s ability to absorb
and hold water, and help stabilize soil particles. Crimson clover produces more dry
matter (6,237–6,683 kg/ha) than many other legumes and is recommended for soil
erosion control because of its high early autumn dry matter production.
Grass/crimson clover mixtures combine fibrous surface roots with long tap roots and
have been observed to reduce herbicide runoff by 94–100%.
As the soil–plant–atmosphere continuum cycles plant nutrients, some are
recovered via plant uptake, some are incorporated into organic matter, some are
adsorbed to mineral and organic surfaces, and some are precipitated as solid
minerals. Excess nutrients are “lost” or removed from the field by runoff or
subsurface flow, potentially ending up in water supplies. Nutrients such as N, that
are the most water soluble and mobile in the soil, have the greatest potential for the
pollution of ground and surface water (Havlin, Beaton, Tisdale, & Nelson, 2005).
Over-fertilization is common in orchard crops because most growers rely on
synthetic fertilizers. The amount of nitrogen removed by the crop in proportion to
that applied is often far less with tree crops than with more traditional crops.
Therefore, the leaching of nitrates into groundwater may be especially serious in
orchard crops (Weinbaum, Johnson, & Dejong, 1992). Wiedenfeld, Fenn,
Miyamoto, Swietlik, and Marlene (1999) suggested that sod alone on the orchard
floor does not sufficiently reduce nitrate leaching. Green manures like cool-season
legumes reduce the need for N application and aid in nutrient conservation by
utilizing excess fertilizer not assimilated by the pecan trees, preventing undesirable
nutrient levels in streams or lakes.
Winter cover crops such as crimson clover grow primarily during a period of
tree dormancy, when N uptake by the crop is at a minimum and percolation from
rainfall is often the greatest. Although pecan growers have historically had little
economic incentive to grow cool season legumes solely to prevent nitrate leaching,
it is one of the many benefits they provide.
Orchard floor covers can affect nitrogen uptake by the main tree crop if C:N
ratios for the floor cover are high. In such cases, the micro-organisms responsible
for decomposition of the highly carbonaceous plant matter require nitrogen to do so,
and can lead to the unavailability of N for the main crop (Brady, 1974). Residue
from a grass/legume mix has a higher C:N ratio than the legume alone. However,
under humid conditions, the C:N ratio of such a mix is less than sod alone. In fact,
ratios for the grass/legume mix are such that the release of N is slow, a scenario in
which many perennial tree crops perform well. Slow release of nutrients also causes
nutrients in the orchard soil to be less vulnerable to loss.