Spring 1993 (v5n3)
Scavenging Nitrogen
in Orchards
by Chuck Ingels, SAREP and Rick
Miller; Agronomy and Range Science Department, UC Davis
Nitrate levels in many wells
throughout California exceed established health standards, largely due
to percolation of highly contaminated water from agricultural soils. Soil
water beneath fertilized orchards, for example, can exceed 100 milligrams
per liter (mg/l) of nitrate-nitrogen, while health levels in drinking
water have been set at 10 mg/l. Nitrate leaching may be especially serious
in tree crops, since the proportion of nitrogen removed in the harvested
crop to the fertilizer applied is often far less with tree crops than
in other crops. Many practices aimed at improving nitrogen-use efficiency
have been developed, although many gaps in information still exist. Overfertilization
in tree crops is still common because most growers rely on synthetic fertilizers.
One strategy that will reduce
nitrate leaching is the use of winter annual cover crops. These covers
grow during the period when uptake of nitrogen by trees is at a minimum
and percolation from rainfall is greatest. While few growers have economic
or legal incentives to grow cover crops solely to prevent nitrate leaching,
it is one of many benefits of cover cropping and should be an integral
part of orchard floor management decisions.
Timely Growth
The amount of nitrogen taken
up by the cover crop is closely related to the biomass production of the
cover crop. Cover crops must grow vigorously in the fall and winter to
be most effective. In some orchards, this requirement maybe difficult
to meet. For example, drip irrigated orchards are usually dependent on
fall rain for cover crop germination, but rain often does not occur until
November. Also, late harvesting in walnut orchards can delay
cover crop planting until November.
Effects of Cover Crop Species
Resident Vegetation. Using
resident vegetation, or weeds, can be an inexpensive and simple way to
capture soil nitrate. However, the amount of nitrate taken up by weeds
varies greatly among species. Since weed species composition among orchards
varies greatly, resident vegetation will vary in its nitrate uptake capacity.
Chemical mowing in the aisles often results in a weed population with
little biomass and therefore a negligible effect on nitrogen cycling.
Nonlegumes Nonlegume
cover crops will often accumulate about 80 pounds of nitrogen per acre.
However, they have been shown to accumulate up to 100 to 150 pounds. Most
of the research on nitrate scavenging in nonlegumes has focused on a few
key species. Of the grasses, cereal rye has received the most attention,
although others have been successfully used, such as barley, annual ryegrass,
oats, and wheat. The crucifers showing the greatest potential include
oilseed radish, mustard, rape, and turnip. Another cover crop species
which has shown promise in lettuce rotations is phacelia, also called
bee phacelia.
Legumes. Legumes will
use symbiotic nitrogen fixation to obtain nitrogen, but only when soil
nitrate levels are low. While legumes usually recover less nitrate than
nonlegumes, one study showed that vetch acquired 125 pounds of nitrogen
per acre from a high-nitrogen soil. The remainder of the nitrogen in the
vetch-about 70 pounds-was derived from nitrogen fixation. Mixing grasses
and legumes is a practical strategy for both conserving and adding nitrogen.
Effects on Tree Nutrition
While cover crops can greatly
affect nitrate leaching, it is also important to consider their effects
on tree nutrition. The carbon-to-nitrogen ratio (C/N ratio) of residues
is an indicator of subsequent nutrient availability. With the incorporation
into the soil of a high C/N ratio material (e.g., grass residue), microorganisms
will scavenge available nitrogen from the soil, creating a temporary deficiency
for the trees. If a low C/N ratio material is incorporated into moist
soil, it will decompose rapidly and will release a large flush of available
nitrogen in the soil within a few weeks. The time of incorporation (spring)
corresponds to the time of high demand by tree crops. The C/N ratios of
some residues are as follows:
| Residue | C/N ratio |
| Legume | 15:1 to 20:1 |
| Mustard | 20:1 to 30:1 |
| Grass | 40:1 to 80:1 |
Under nontillage, residues
with a high C/N ratio will decompose slowly due to a lack of nitrogen
source for microorganisms which decompose surface mulch. The nitrogen
contained in the residue will become available to plants only after the
residue has become incorporated into the soil organic matter pool. If
low C/N residue is left on the surface, microbial decomposition will be
more rapid because of the favorable nutrient status. About 80 to 85 percent
of the nitrogen in cover crops is contained in the above-ground portion.
Limitations on Effectiveness
Several obstacles can limit
cover crop production and nitrate scavenging in orchards. These limitations
usually do not preclude the ability to grow cover crops. However, they
can reduce their ability to scavenge nitrate.
Herbicide strips. The
ability of cover crops to remove nitrate is reduced by the use of herbicide
strips. For example, an orchard with rows 24 feet apart and with a 6-foot
herbicide strip has only 75 percent of the soil planted to cover crops.
In addition, some growers fertilize in the treated strips only, since
this area contains a large portion of the tree roots. During the winter,
fertilizer nitrogen remaining in the strips may easily leach beyond the
tree root zone, especially if late summer or fall applications were made.
Shading. Deciduous
orchards with a complete canopy cover can cast considerable shade on the
orchard floor in the fall and spring. Shading from the branch structure
of deciduous trees probably has minimal effect on cover crop growth during
the winter. Cover crops grown in citrus orchards, however, may receive
little or no direct sunlight, especially in orchards with east-west row
orientation.
Winter orchard operations.
Cover crops in orchards can rarely be left undisturbed during the
winter. The traffic required for pruning and spraying can seriously diminish
the growth capacity of the cover. Most species can tolerate a moderate
amount of traffic, however, especially when they are older. One operation
that can nearly eliminate most cover crops is pushing brush out of the
orchard. This practice should be done before planting or seedling emergence.
Cover crops could also be planted in alternate rows, with brush stacked
on rows with resident vegetation. Alternatively, large wood can be removed
and smaller wood can be shredded with equipment now on the market.
Cover crops offer an important
way to conserve excess soil nitrate during winter and to supply nitrogen
when needed by the trees in the spring. Because of these and other benefits,
orchardists should consider using cover crops rather than relying solely
on synthetic nitrogen fertilizers for managing nitrogen. Cover cropping
can then become a tool for integrated floor and fertility management.
FOR MORE INFORMATION:
Jackson, L.E., L.J.
Wyland, J.A. Klein, R.F. Smith, W.E. Chaney, and S.T. Koike. Management
of cover crops with reduced tillage in lettuce production systems. Manuscript
submitted to California Agriculture.
Janzen, H.H. and
S.M. McGinn. 1991. volatile loss of nitrogen during decomposition of legume
green manure. Soil Biol.Biochem. 23(3):291-297.
Hargrove, W.L. (ed.).
1991. Cover Crops for Clean Water. Soil & Water Conservation
Society, 7515 N.E. Ankeny Rd., Ankeny, IA50021-9764.
Shennan, C. 1992.
Cover crops, nitrogen cycling, and soil properties in semi-irrigated vegetable
production systems. HortSci. 27(7):749-754.
Weinbaum, S.A.,
R.S. Johnson and T.M. Dejong. 1992. Causes and consequences of overfertilization
in orchards. HortTechnology 2(1):112-121.
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