At the 2005 Annual Meeting
of the American Society for Horticultural Sciences in Las Vegas, Nevada,
a workshop titled “Reconciling Productivity and Environmental
Stewardship: The Challenges of Production Horticulture in the BMP
(Best Management Practices)” was conducted. The following is
an exert from the session, “Vegetable Production BMP to minimize
nutrient loss” by Dr. T.K. Hartz, the University of California
at Davis. This is the first of a series of articles to be presented
from Dr. Hartz’s presentation.
Commercial vegetable production presents a unique environmental challenge.
Vegetable crops have high product value, and exacting market standards
for size, color and quality; high fertilizer rates and frequent irrigation
are typically employed to ensure optimal growth. Many vegetable crops
are shallowly rooted, which limits fertilization and irrigation efficiency.
Extensive tillage is practiced, and fields often have little or no
foliage cover for extended periods. Consequently, vegetable production
carries substantial environmental risk. Nitrate pollution of groundwater
is a widespread problem in vegetable producing regions of the U.S.,
as is the runoff of nitrogen (N) and phosphorous (P) into surface
waters. Across the country, sediment and nutrient loss from vegetable
fields has become a focus of regulatory interest, and more stringent
regulation of production practices is likely.
Nationwide, regulatory agencies are urging adoption of BMPs to protect
water quality. Presented here are five BMP concepts that are widely
applicable and, if applied appropriately, can dramatically reduce
nutrient and sediment loss from vegetable fields.
BMP1. Use preplant soil testing to determine
P fertilization
Soil testing for P availability has been an established practice for
decades. Growers of agronomic crops commonly consider soil test P
(STP) when developing field-specific fertilizer programs. However,
many vegetable growers ignore STP when determining P application rates.
A recent survey of lettuce (Lactuca sativa) and cauliflower (Brassica
oleracea var. botrytis) fields in the coastal valleys of California
(T.K. Hartz, unpublished data) found no correlation between STP and
P fertilization rate. While some growers assumed soil P sufficiency
and eliminated P application in fields with STP as low as 40 mg-kg-1
bicarbonate-extractable P, others continued to apply P in fields with
STP more than three times that level. This results in needless expense
for the grower, and progressive enrichment of soil P status. Given
the strong correlation between STP and P loss in runoff or leaching,
greater reliance on soil testing to determine P fertilization will
be essential to reduce P loss fro vegetable fields.
Among the reasons growers are reluctant to use soil tests to guide
P fertilization are lack of confidence that laboratory extraction
tests accurately estimate soil P bioavailability, and uncertainty
as to what soil test level represents the crop response threshold.
While it is true that no common laboratory test (e.g., bicarbonate,
Mehlich, or Bray extraction) precisely predicts P bioavailability
across a wide range of soils and environmental conditions, these extraction
tests are closely correlated with more direct measurements of soil
P bioavailability such as P absorbed on anion exchange resin or on
ion-impregnated paper strips. There is contradictory information regarding
crop response thresholds; for example, reported STP thresholds for
lettuce response have ranged from 25 mg-kg-1 to approximately 50 mg-kg-1
bicarbonate-extractable P. Growers may be warranted in using a small
“insurance” application on P in fields with marginal STP.
However, many vegetable growers persist in fertilizing fields with
STP far exceeding the crop response threshold. This observation was
corroborated in a series of P fertilization trials in California lettuce
fields, found that the cooperating growers applied P in 9 of 11 fields
in which there was no response to P fertilization, including fields
with >90 mg-kg-1 bicarbonate extractable P.
To improve P management within the vegetable industry, both additional
research to confirm crop-specific STP response thresholds and an expanded
grower education campaign are needed. To be maximally effective in
changing grower behavior such research, and any associated field demonstration
projects, should be conducted on commercial farms because many growers
harbor suspicions that research conducted in small plots on university
facilities does not represent the “real-world” conditions
on their farms. Also, growers may be more amenable to change when
confronted with evidence that their neighbors are utilizing soil test
results to guide P application. Some growers have indeed eliminated
P application in high-P soils. Convincing growers who continue to
fertilize high-P fields to simply emulate their more efficient neighbors
could significantly reduce P pollution potential.
