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J.D. DUTCHER ET AL.
Clover and other legumes are capable of supplying significant amount of N to
orchard soils. Studies in Oklahoma suggest that a mixture of legumes including
crimson clover, hairy vetch, red clover, and white clover planted to the orchard floor
provided over 100 kg of N/ha. In the same study, pecan leaf N concentrations were
maintained above a threshold of 2.25% by the above mixture, as well as by crimson
clover alone Blue lupine can fix as much as 168 kg of N/ha, while common vetch is
capable of fixing 92 kg of N (Smith, Shiferaw, & Rice, 1996).
Pecans are an “irregular bearing” crop, meaning that they tend to bear heavy
crop loads for 1–2 years, followed by very light crop loads, depending upon various
physiological and environmental factors. A general rule of thumb for the N
requirement of pecan trees is 4.5 kg of N for every 45.4 kg of expected crop (Wells,
2007). Depending upon the degree of irregular bearing, mature pecan trees in the
Southeastern USs may require from 78.5 to 168.1 kg of N annually for optimum
production.
Nitrogen uptake in the pecan tree is driven by demand. There are two critical
periods of nitrogen demand during the season the first at early foliage growth and
the second at kernel filling. The early spring foliage flush is nourished primarily
from reserves held within the tree, while the nitrogen demand during the kernel fill
stage is usually satisfied from soil uptake. If N is limited at kernel filling, then the
tree will mobilize N from the foliage to the kernels.
Studies have suggested that crimson clover over-winters dependably in the
southeastern US and much of the US pecan belt. A vigorous stand of crimson clover
will contribute between 78.5 and 168.1 kg of N/ha. When aided by moisture and
warm weather to speed up decomposition, up to half the N available from legumes
can be released within 7–8 weeks. In the light crop or “off” years, the N supplied by
the clover alone, would be adequate for optimum production. Heavy crop or “on”
years may require low supplemental N rates in order to bring marketable nuts to
maturity and provide a return crop the following year.
3.2. Enhancement of Pest Management
Conserving and encouraging beneficial organisms is key to achieving sustainable
pest management. The deep red blossoms of crimson clover attract various species
of bees, which feed readily on the abundant nectar. In addition, blooms may harbor
beneficial insects such as the minute pirate bug. Pea aphids and blue alfalfa aphids
are commonly associated with crimson clover. Although these species are not pests
of pecan, they serve as alternative food sources for beneficial predators such as
ladybeetles, green lacewings, soldier beetles, predaceous stink bugs, damsel bugs,
and hover flies. As the clover declines with the onset of warm weather in June, these
beneficial insects move into the trees to feed on pecan aphids and other insect pests,
reducing the need for insecticide application.
In addition to reducing insecticide inputs through enhancing beneficial insect
populations, the use of legumes in the orchard can also reduce herbicide use. As
crimson clover grows, it forms a thick, living mulch. This helps to smother and
shade out more troublesome, competitive weed species.