Spring, 1992 (v4n3)
By Chuck Ingles, SAREP
(Editor's Note: This is the second part of a two-part series
on citrus production and groundwater contamination. Part 1 appeared
in Sustainable Agriculture News, Vol.4, No.2, Winter 1992. Information
from this series will be included in a SAREP citrus publication
due out at the end of 1992.)
For citrus growers, prevention of nitrate leaching can be a far
more difficult task than that of herbicide leaching: farmers cannot
simply stop applying nitrogen (N). The problem is a concern to
both conventional and organic growers, since nitrate can leach
below the root zone from organic and synthetic fertilizers. Several
management practices exist to reduce the potential for nitrate
leaching.
Determine N supply and demand. Citrus trees typically require
about 100 to 150 pounds of N/acre/year, however lemons
can require up to double this amount. Annual leaf analyses can
indicate the need for less N; fertilizing on the basis of leaf
analysis can result in less nitrate leaching while maintaining
fruit quality and yield. Equally important, and often overlooked,
is the need to sample irrigation water to determine nitrate content,
and to reduce N fertilizer use accordingly.
Time of application. The time of maximum N uptake is during
the summer. Because flowering and fruit set require high levels
of N in the tree, however, N must be made available to the trees
shortly before flowering. Fertilizer is usually applied in late
winter and early spring, with split soil applications frequently
made in February, March, and April. Some growers, however, apply
N fertilizer in the fall. This practice stems from the tradition
of applying raw manure in the fall so that winter rains can move
nitrate into the root zone. A substantial portion of fall-applied
N will leach beyond the root zone as a result of winter rainfall,
water applied during frosts to warm the orchard, and reduced root
N uptake during the winter.
Choice of materials. The choice of N fertilizer can affect
the accumulation and leaching of nitrate. Synthetic nitrogen
fertilizers supply N in the form of nitrate, ammonium,
and/or urea (which is rapidly converted to ammonium in the soil).
Some ammonium is taken up by the plants directly and some is
adsorbed onto soil particles, thus delaying N loss through leaching.
However, most of the ammonium is converted to nitrate within a
relatively short period of time. How much nitrate leaches below
the root zone depends on the rate and frequency of application
as well as the depth to which irrigation water percolates.
Bulky organic materials such as manure and compost release N more
slowly as the organic matter decomposes. However, excessive applications
of manure can also result in leaching losses. Finished compost
supplies a more stable and slow-release form of N than conventional
fertilizers or manure. Compost applications can therefore be
made at any time of the year without significant loss to leaching.
Compost also provides the soil with humus, a stable form of organic
matter, which is invaluable in nutrient cycling. Because of low
N content and expense, most growers who use compost also use supplements
of chemical fertilizers and/or cover crops. Slow-release synthetic
fertilizers are also available; these can reduce the nitrate pollution
potential. A significant problem with slow-release fertilizers,
including organic materials, is that a continuous N supply can
cause fruit quality problems such as regreening. A moderate application
of compost, together with spring applications of soluble materials,
may strike a balance between maintaining fruit quality and yields,
reducing nitrate leaching, and improving soil quality.
Cover crops can be useful in converting mobile soil nitrate into
immobile plant organic N. Winter annual cover crops can utilize
rainfall for growth and remove excess soil nitrate during winter,
when nitrate leaching potential is the greatest. (For more information
on cover cropping in citrus, see UC SAREP's Components, Vol.2,
No.3, Summer 199].)
Foliar applications of urea result in vastly reduced nitrate
pollution potential compared to soil-applied N, and yield and
fruit quality can be maintained or enhanced. A practical fertilization
strategy might be to supply half the N to the foliage (two to
four sprays) and half through soil applications. Foliar sprays
may be applied in the winter and early spring, but researchers
at UC Riverside have recently shown that January and February
applications may lead to the highest yields. Organic materials
such as fish emulsion may be useful as foliar sprays, although
they contain less N and are far more expensive than urea. The
practice of applying N to the foliage is not universally accepted.
Some growers believe that the nutrient pathway is and should
be through the roots.
"Fertigation." The application of water-soluble
fertilizers through the irrigation system is called "fertigation."
If the depth of wetting can be limited as well, fertigation can
provide a "steady state" of N availability only when
N is needed by the tree, with greatly reduced potential for nitrate
leaching. Fertigation can be used in small amounts up to 15 times
through the spring. (Often only three to five applications are
made.)
Growers of all crops are being challenged to produce safe, inexpensive
food without damaging the soil, the air, or the water. They must
also maintain their economic stability. In the long term, however,
economic stability is dependent on environmental and social well-being.
Citrus growers are confronted with numerous obstacles to sustainability.
Yet many practices have been developed to reduce the environmental
impacts. Should we wait until regulations force us to be nonpolluting?
Thanks go to John Freeman,
Jim Gorden, Ralph Jurgens, Rob McGill, Neil O'Connell, John Pehrson
and others for information used
in these articles.
References
Embleton, T.W., W.W. Jones, and R.G. Platt. 1983. Leaf analysis
as a guide to citrus fertilization. Soil and Plant Tissue Testing
in California. Univ. of Calif., Division of Agric. Sci., Bull.
1879, pp.8-13.
Embleton, T.W, M. Matsumura, L.H. Stolzy, D.A. Devitt, W.W Jones,
R. ElMotaium, and L.L. Summers. 1986. Citrus nitrogen fertilizer
management, groundwater pollution, soil salinity and nitrogen
balance. Applied Agric. Res., 1(1):57-64.
Lovatt, C. 1991. Maximizing the "growth regulator" properties
of foliar-applied urea to increase flowering, fruit set and yield
(without a reduction in fruit size) in the "Washington"
navel orange. Progress Report to the Citrus Research Board and
UC Citrus Research Advisory Commirtee.
Pehrson, J. 1985. Irrigation scheduling in San Joaquin citrus.
Citrog. 70:183-84,188.
Pehrson, J. 1988. Vegetation management in citrus groves. Citrog.
74:15-18,20.
Pickett, C.R., L.S. Hawkins, J.E. Pehrson, and N.V. O'Connell.
1990. Herbicide use in citrus production and ground water contamination
in Tulare County. Calif. Dept. of Food & Agric. pub. no. PM
90-1.
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