Optimizing Vegetable Fertilizer Programs

Recent farm visits, questions from growers, and observations of research plots have me thinking about nitrogen and other fertilizer programs for vegetable crops grown in open fields and high tunnels for fresh and processing markets. What are optimal ranges for each production situation, which factors influence optimal rates most significantly, and what steps can growers and others take to identify optimal rates for each farm and planting?

Ranges currently recognized as optimal are published in numerous guides, handbooks, and other resources. The Midwest Vegetable Production Guide for Commercial Growers, Southeast Vegetable Production Handbook, Mid-Atlantic Commercial Vegetable Production Recommendations, New England Vegetable Management Guide, and references available from Cornell (e.g., https://cropandpestguides.cce.cornell.edu/) and other universities are helpful in Ohio and the region. The publications provide operating fertilizer application targets and tips on how to reach them. Targets in the publications are the best available benchmarks. However, it is best to think of them as not fixed in stone and as needing to be validated for individual cropping situations. On-farm validation (adjustment by trial and error) using published, research-based and other reliable benchmarks as starting points saves time, money, and headache.

Indeed, since production conditions change continuously and research-based recommendations require years to develop, evaluating fertilizer programs (material, rate, timing, placement) often is good practice. Like effective crop protection programs, fertilizer ones are not static, they need to be updated as weather patterns, varieties, rotations, fertilizer materials and their costs, and other factors change. Observe crops now and through the remainder of the season and ask if you are convinced their fertilizer programs are optimal. If you aren’t convinced, consider experimenting carefully.

Experiments are most effective when they account for factors that tend to influence their outcomes most significantly and consistently. To refresh my memory on these factors, especially nitrogen application rate effects on watermelon and other Cucurbit crops, I looked at extension resources referenced above and reports from research completed in the U.S. and other countries. I was also very pleased to hear from Ohio growers on the same topic.

That input pointed to the following seven factors as most likely to shape optimal fertilizer, especially nitrogen, application rates for individual farms, soils, crops, and plantings.
1. Soil type and condition. Sandy, loam, or clayey? Organic matter level? Have a prominent plow layer or other condition affecting drainage, etc? Fertilizer programs must be calibrated to soil type and condition since they influence many facets of nutrient availability at any one time.
2. Fertilizer application approach. For example, will fertigation be used? Fertilizer application approaches influence which materials are used, when and where they are applied, and their likely efficiency.
3. Precipitation and irrigation. Soil moisture management is a very large percentage of nutrient management. Are the irrigation and fertilizer programs in sync? Is rainfall cooperating? Can the program be adjusted for weather?
4. Variety(ies). Shifting market expectations (e.g., large to personal-size melon) may have implications for the fertilizer program. Similarly, the program may also need to be adjusted to maximize gains from using grafted planting stock because rootstocks may differ in, for example, their abilities to obtain nutrients and water.
5. Cultural practices. Production on plastic-covered raised beds versus the flat. Standard versus strip- or reduced tillage approach. Row and plant spacings (plant populations). These and other factors are consistently mentioned as factors shaping the four R’s (material, rate, timing, placement) of all fertilizer programs. The fertilizer program may need to be tweaked if any of these factors are changed.
6. Crop growth stage. Especially important for fruiting vegetables, including Cucurbit and Solanaceous crops. Nitrogen and other macro- and micronutrient levels influence many aspects of crop biology directly impacting (fruit) yield and quality from seeding/transplanting to harvest. Metering nutrient availability by crop stage is a proven, essential tactic in soilless greenhouse production. The same level of control is impossible in soil-based field or high tunnel production; however, a realistic application of the principle can be beneficial in both systems.
7. Nutrient credits. There is often little need to apply what is already there. Basing planned applications on current, reliable soil test data is a cornerstone of successful, efficient, cost-effective fertilizer programs.

Finally, setting optimistic but realistic yields goals, especially for non-vegetable rotation crops, if any, is also beneficial. Realistic yield goals help avoid significantly under- or over-applying fertilizer, regardless of crop. Avoiding such deficiencies and excesses enhances the overall return on investments in the current and subsequent crops.

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