Nutrient Management Research Database

http://horttech.ashspublications.org/content/16/3/398.abstract

Nutrient loss from commercial vegetable fields has become a significant environmental issue in all the major vegetable-producing regions of the United States. Growers are facing potentially disruptive regulations aimed at improving the quality of both surface and ground water. Significant improvement in nutrient management will be required to meet this regulatory challenge. This paper discusses five practical, low-cost nutrient best management practices (BMPs). These BMPs are widely applicable, relatively inexpensive to implement, and can dramatically reduce nitrogen and phosphorus loss from vegetable fields. However, even with careful application of these BMPs, runoff and leachate from vegetable fields may periodically exceed environmental water quality standards, which are very stringent.

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• Best management practice (BMP) to use preplant soil testing to determine P fertilization can reduce costs from P fertilization while concurrently reducing P loss from vegetable fields.
• Data shows that vegetable growers in Coastal California routinely over fertilize with P, beyond the threshold for crop response.
• Reasons growers are reluctant to use soil P tests to guide P fertilization are lack of confidence in accuracy, and uncertainty about what soil test level represents in terms of the crop response threshold.
• BMP to use an appropriate crop N fertilization template can improve the accuracy of crop fertilizer programs and reduce excessive N fertilization.
• University fertilizer recommendations can serve as a general information “template” to plan fertilizer programs. Industry averages may also be used as an upper limit to N fertilization.
• The fertilizer template of a particular crop should be modified according to previous crop, preplant soil nitrate, soil texture and soil organic matter content.
• In-season N monitoring is essential to confirm N fertilizer requirement.
• Pre-sidedress soil nitrate testing (PSNT) can help eliminate the uncertainty associated with pre-plant nitrogen availability to crops (in both drip and furrow irrigated systems).
• The use of PSNT can be tailored for different crops; originally developed for corn, the test has been adapted and used for cabbage, celery, lettuce, sweet corn and tomatoes.
• The potential to reduce N fertilization by using PSNT can be substantial (over 40% in some cases).
• Cost/ benefit ratio of PSNT in these California studies estimated that a reduction of 10 kg/ ha (9lbs/acre) would more than offset monitoring costs.
• PSNT is most beneficial in situations where large sidedress N applications are made early in the cropping season, and in which the risk of in-season leaching is not significant.
• In areas where the risk of leaching is high, other monitoring techniques (small N applications throughout the season and/or use of slow-release fertilizer) are more useful.
• The benefits of plant tissue analysis are limited by the fact that both leaf N and petiole N are insensitive indicators of current soil N availability.
• Efficient irrigation management is essential to minimize in-season nutrient leaching.
• High soil N levels (greater than 20mg/kg nitrate) that are maintained to ensure crop growth in sandy loam soils are likely to exceed surface and groundwater standards in leachate and runoff.
• Efficient irrigation requires high distribution uniformity as well as high irrigation efficiency (defined as the percentage of water applied that is beneficially used either by the crop or for leaching salts).
• Higher distribution uniformity can be achieved through management practices including land leveling, shortening furrow lengths, use of surge valves and adjusting sprinkler spacing.
• To achieve higher irrigation efficiency, water application rate should match infiltration rate and take into account the rooting zone of the crop.
• Efficient furrow and sprinkler irrigation practices are very difficult to achieve while drip irrigation systems can achieve distribution uniformity and irrigation efficiency of around 90 percent.
• Compared with other crops, vegetable crops have a higher nutrient pollution risk.
• Cover crop rotation and the use of cover crops in furrows can help minimize nutrient loss.
• Cover crop production in vegetables may involve costs, complicate tillage practices and disrupt planting spring planting schedule, but maybe be worth it in some systems that must reduce nutrient loss.
• A potential problem of organic production systems that apply manure and composted manure regularly is the buildup of phosphorus in the soil.
• Organic systems with high levels of P in the soil, should use organic materials low in P such as feather meal.
• While application of discussed BMPs will minimize nutrient loss, it may not be enough to meet water quality standards.