How to Manage Pests

UC Pest Management Guidelines

Most agricultural soils in California contain sufficient potassium (K) and micronutrients to produce a lettuce crop. Most soils will require phosphorus fertilization, although in fields with a history of heavy phosphorus (P) applications, phosphorous fertilization may not be required. With efficient irrigation moderate nitrogen (N) fertilization rates are sufficient to produce maximum yields. Some fields may have toxic levels of certain salts.

Preplant soil testing is the primary tool for assessing nutritional needs and soil salinity status. Plant tissue testing can be useful to confirm the adequacy of fertilization. Nitrogen and phosphorous fertilization rates higher than that required for maximum growth can be detrimental to water quality. To avoid runoff of fertilizer, see MEASURES TO MINIMIZE WATER QUALITY IMPAIRMENTS TO SURFACE AND GROUND WATER.

Preplant Activities

Before planting, use soil test nutrient levels to determine what fertilizers will be needed. For the most accurate estimation of soil nutrient availability, collect and analyze soil from the most active rooting depth, which for lettuce is the top foot of soil. Collect a composite sample of a minimum of 12 soil cores from each field; if zones of different soil texture exist within the same field, take separate samples to represent the major soil types. Table 1 suggests appropriate soil analysis procedures and interpretation of laboratory results.

For phosphorous, potassium, and zinc, a low soil test value suggests the need to fertilize; with medium soil levels yield response to fertilizer application is possible, but not necessarily likely. At high soil levels yield response is unlikely. For phosphorous and zinc, fertilization is best done preplant or as a starter solution at seeding or transplanting. Potassium can be applied preplant, at sidedressing, or fertigated (injected into drip irrigation water) during the growing season.

The primary factors to consider in determining phosphorous fertilization rate are soil test phosphorous level and soil temperature. Soils with a very low level of bicarbonate extractable phosphorous (less than 10 ppm) may require as much as 200 lb P₂O₅/acre to maximize lettuce growth, particularly if planting is done in cold soil (less than 60°F mean soil temperature); soil phosphorous availability declines with decreasing soil temperature. Fields with soil test P greater than 30 ppm should not require more than 100 lb P₂O₅/acre. In lettuce production a relatively small amount of phosphorous is removed from the field in harvested product (typically less than 20 P₂O₅ equivalent/acre), so in fields with soil test phosphorous between 40-60 ppm a low rate of phosphorous fertilization will be sufficient to maintain soil phosphorous fertility and ensure peak plant productivity. Phosphorous application is not recommended in soil above 60 ppm bicarbonate phosphorous because lettuce growth is unlikely to be improved, and the soil will be further enriched; the risk that field runoff will contain environmentally damaging amounts of soluble phosphorous increases with increasing soil phosphorous level.

In fields with low soil test potassium, seasonal application of 50 to 100 lb K₂O/acre should be adequate to maximize yield. Even in fields with adequate soil potassium availability to support the current crop, application of 50 to 80 lb K₂O/acre would match crop potassium removal and maintain soil fertility.

Preplant soil testing for available nitrogen is not very useful, because rainfall or irrigation to germinate the lettuce crop can cause significant leaching of nitrogen from the root zone. Early season crop nitrogen requirements are modest, and the amount of nitrogen typically contained in common phosphorous fertilizers is sufficient to maintain the crop until sidedressing.

The issue with soluble salts (salinity) and boron is whether soil levels are high enough to be detrimental. Soils in the low range are desirable for both. As soil levels increase, the likelihood of crop damage increases; when high levels of either soluble salts or boron are present, take remedial actions such as leaching the soil profile and switching to a higher quality irrigation source to prevent further buildup (high soil salinity and boron is often the result of using marginal quality irrigation water).

Planting to Rosette
Fertilizer application at planting (whether a banded application of a standard fertilizer, a 'starter' solution applied over direct seeding, or a transplant drench) can be an effective practice, particularly for phosphorous. Standard, high analysis phosphorous fertilizers (10-34-0, 11-52-0, etc.) can be toxic to germinating seedlings because of the concentrated salts they contain. Banding such fertilizers several inches below and to the side of the developing plants is the only safe method of applying a high phosphorous rate. The use of phosphoric acid-based fertilizers for their anti-crustant properties is a common practice. At normal use rates these products pose little risk of phytotoxicity to developing seedlings and can be an efficient way to handle phosphorous fertilization where the phosphorous requirement is modest.

For sprinkler or furrow-irrigated production, the majority of seasonal nitrogen is typically applied in one to three sidedressings, with the first one applied post-thinning. Compared to most other vegetable crops, lettuce has a moderate nitrogen requirement, taking up on average only 100 to120 lb N/acre. Many replicated trials have demonstrated that, with efficient water management, seasonal nitrogen application of about 150 lb/acre should be adequate to achieve high yield and quality; in fields with significant residual concentration of nitrates in the soil even lower nitrogen rates can be adequate. Testing a composite sample of the top foot of soil just before sidedressing can guide nitrogen fertilization. If soil NO₃-N is greater than 20 ppm, sidedressing can be delayed until soil NO₃-N concentration drops below that level.

Nitrogen uptake by lettuce plants is slow until heading begins, so a modest sidedress amount (less than 50 lb N/acre) should be sufficient to carry the crop through the rosette stage. Because the root system is not yet well developed and crop water use is minimal, it is critical to efficiently control irrigation to keep applied N accessible to the young plants.

Some growers routinely sidedress with fertilizer containing phosphorous, which wastes money and can lead to excessive soil phosphorous levels and subsequently to pollution of surface water when field runoff occurs. The most critical time for soil phosphorous availability is during seedling growth; once a significant root system is established the crop can more efficiently extract P from the soil. If sufficient P has been supplied preplant or at-planting to support early growth, there should be no need for sidedress phosphorous application.

Rosette to Heading
In sprinkler or furrow-irrigated fields, a second and occasionally a third sidedress would typically be applied. Because the crop uptake rate of nitrogen is accelerating during this growth stage a larger sidedressing may be appropriate, depending on soil NO₃-N level. As long as soil NO₃-N level remains above 20 ppm, no sidedress is required. However, sidedressing is generally not done after heading because of the possibility of damaging the root system with the application equipment, so a modest sidedressing may be justified to ensure sufficient N supply to finish out the crop, particularly in light-textured fields where significant leaching is possible.

Where drip irrigation is used, it is typically installed during this growth period. Drip irrigation allows for application of nitrogen in synchrony with crop uptake, without the cultural restrictions of sidedressing. Drip irrigation also can limit in-season leaching, making nitrogen management much more efficient. Assuming that nitrogen availability have been adequate up to the rosette stage, in drip-irrigated fields two to three modest nitrogen fertigations (25 to 35 lb N/acre each) evenly spaced from rosette stage to about 10 days preharvest should be adequate to finish the crop. Even with drip, soil NO₃-N sampling can help fine tune the fertigation program.

Plant tissue testing can help determine crop nutrient status. The traditional analytical methods are midrib sampling for analysis of NO₃-N, PO₄-P, and K, and whole leaf sampling for total N/P/K analysis. It is important to understand the differences between these techniques. Midrib analysis is considered to be a measure of recent crop nutrient uptake, because the measurement is made on the tissue that transports nutrients to the leaves, and the measurement is of 'unassimilated' nitrogen and phosphorous forms (NO₃-N and PO₄-P are the common chemical forms that are taken up from the soil, and have not yet been assimilated by the plant into organic compounds). Unfortunately, factors other than soil nutrient availability can significantly affect midrib nutrient concentration, and therefore this measurement is of limited practical value in managing in-season fertilization. Values above the guidelines given in Table 2 can be reliably interpreted to mean that crop nutrient status is currently adequate, but because midrib nutrient concentration can change greatly within just a few days that measurement is not a sound basis on which to make future fertigation decisions. While midrib values far below these 'sufficiency' levels indicate limited nutrient availability, marginally low midrib nutrient concentrations are not sufficient information on which to modify your fertilization plan.

Whole leaf analysis provides a more reliable estimate of crop nutrient status than midrib analysis because it measures all forms of N and P. Leaf nutrient concentrations also change more slowly than midribs, and therefore provide a more stable basis upon which to base fertilization decisions. Leaf nutrient concentration substantially below the 'sufficiency' levels given in Table 2 would indicate potentially growth-limiting nutrient deficiency.

Heading to Harvest
In the final few weeks before harvest, the nitrogen application can be made in irrigation water, but in sprinkler or furrow-irrigated fields the uniformity of such applications is typically poor and the danger of loss to the environment in runoff is high; for these traditional irrigation methods, avoid late-season, water-run applications, if possible. In drip-irrigated fields late-season nitrogen fertigation is a normal practice. Crop nitrogen uptake in the final weeks of growth averages 3 to 4 lb N/acre/day, so schedule late-season fertigation to allow for uptake of applied nitrogen before harvest.

PUBLICATION

UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450
General Information
T. K. Hartz, Plant Sciences, UC Davis

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