Early Season Irrigation of Processing Tomatoes

Tim Hartz, Extension Specialist, UC Davis

In my experience, irrigation management is a crucial part of processing tomato production; more potential yield is lost due to improper water management than to any other factor that a grower can control. Here is some background information on early season irrigation management; elsewhere in this newsletter are some suggested practices for preharvest irrigation management to maximize fruit quality.

It is important to begin the season with a full soil moisture profile. Tomatoes are deeply rooted, capable of drawing water from as deep as four feet or more. However, once the tomatoes are established and actively growing it becomes increasingly difficult to irrigate sufficiently to replenish deep moisture without overirrigating and inducing Phytophthora root rot or fruit rots. Therefore, with the combination of pre-irrigation and sprinkler irrigation to establish the plants, enough water should be applied to thoroughly wet the profile. The crop should start out with a full bank account.
Although tomatoes will tolerate significant moisture stress with minimal visual symptoms on the vine, early season water stress can reduce yield potential and increase the incidence of blossom end rot on fruit.

It is difficult to condense all the issues involved with effective irrigation into a newsletter article; a good overall reference on irrigation scheduling is 'Scheduling Irrigation: When and How Much', which can be purchased from the UC Web site at http://anrcatalog.ucdavis.edu/. This manual has a full discussion of evapotranspiration, crop coefficients, soil moisture monitoring techniques, etc.

The one message that I would underscore here is that until fruit set is complete, no more than half of the available moisture in the top 2 feet of soil should be depleted between irrigations. The only good way to evaluate soil moisture depletion is through direct monitoring.
The old 'feel' method of determining soil moisture is quick and easy, but less quantitative than using one of the monitoring tools available. Several high-tech devices have come on the market in recent years, but for cost-effectiveness and ease of use I prefer electrical resistance blocks. The most widely used resistance block is probably the WatermarkTM block. These blocks are calibrated in centibars (cb) of soil moisture tension. Fifty percent available moisture depletion is equivalent to about 40-50 cb in sandy soils, and 60-90 cb in heavier soils. In practical terms, maintaining at least 50% of available soil moisture in the top two feet of soil usually means irrigating approximately every 7-14 days (depending on weather conditions and soil texture) once the plants begin to set fruit. The water intake rate of some soils slows dramatically over the course of the season, so on some irrigations you may get less recharge of the root zone than you expect. Soil moisture monitoring is the only way to be sure you are successfully maintaining deep soil moisture.

Once fruit set is complete (usually about 6-7 weeks before harvest) it is desirable to induce some moisture stress to increase fruit soluble solids. At this point soil moisture stress will not cause additional blossom end rot, but can marginally reduce the average fruit weight, which will reduce yield; it is a balancing act to achieve soluble solids high enough to satisfy the processor while keeping yield loss to a minimum.

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