Tomatoes respond to simple drip irrigation schedule and moderate nitrogen inputs

Timothy K. Hartz, Michelle LeStrange, Donald M. May

Publication Information
California Agriculture 48(2):28-31. DOI: 10.3733/ca.v048n02p28. March-April 1994.

Author Affiliations
T. K. Hartz is Extension Specialist, Department of Vegetable Crops, UC Davis. M. Le Strange is Farm Advisor, Tulare County. D. M. May is Farm Advisor, Fresno County.


Scheduling drip irrigation of fresh market tomatoes according to CIMIS ETo data and plant canopy development has proved simple and efficient. Using an easy calculation, maximum yields were produced in both a mild coastal climate and the San Joaquin Valley. In similar trials, researchers confirmed that drip-irrigated tomato crops need only modest levels of nitrogen fustigation and successfully tested a new, portable device that will enable growers to measure petiole nitrogen without leaving the farm.

Full Text
The fresh market tomato industry in recent years has experienced a drip irrigation revolution. In the mid-1980s, less than 20% of California’s acreage was drip irrigated; today, the majority of growers of fresh market tomatoes have made the switch. This rapid change has been driven in part by a concern over water availability, but mainly by the advances in drip irrigation technology, which have reduced costs (relative to other production inputs), improved system performance and manageability, and dramatically improved yields.

Now the challenge for fresh market tomato growers is to manage drip irrigation for high crop productivity while maximizing the efficiency of irrigation and nitrogen fertigation. Finding the most appropriate drip irrigation and nitrogen schedules was the goal of the studies we conducted from 1990 to 1992 at the UC South Coast Field Station (SCFS) in Irvine and the West Side Field Station (WSFS) near Five Points.

Scheduling irrigation

A 2-year trial at SCFS examined three methods of scheduling irrigation: Two treatments were based on estimates of evapotranspiration (the loss of water through evaporation from the soil surface and plant transpiration, abbreviated here as ETo) generated by the California Irrigation Management and Information System (CIMIS) computerized weather network. In one of them, ETo values were modified for crop growth stage by multiplying by predetermined crop coefficients (fig. 1) adapted from those in the UC Cooperative Extension Leaflet 21427, which were developed by compiling information from many tomato irrigation studies. In the other treatment, CIMIS ETo was modified for crop growth stage by multiplying by the estimated percentage of the soil surface covered by foliage; foliage cover was estimated by measuring the average plant canopy spread (in inches) and dividing by the bed width (60 inches).

The advantage of using a crop canopy coverage percentage instead of crop coefficients is that it more closely tracks the plant vigor of a particular field and is unaffected by field configuration. The published crop coefficients reflect “standard” field conditions and can require considerable modification for efficient use. The plants in both CIMIS ETo studies were irrigated either daily or three times per week.

The third approach was designed as a deficit irrigation treatment, replacing less water than the plants use, to impose a moderate level of water stress. The plants were irrigated only when 20% of available moisture was depleted, measured by tensiometers at a 12-inch depth. During canopy development, each application was 0.15 inch; once the crop canopy was fully developed, applications were increased as required to maintain available soil moisture at 80% of ETo. Irrigation frequency varied from every 4 to 7 days early in the season to daily at the end of the season.

The trial plots were arranged in a randomized complete block design, with four 50-foot replications. The soil was San Emigdio sandy loam with 11% (1.3 inches/ft) available moisture capacity. A single drip irrigation tube was buried 6 inches deep in the center of each 60-inch bed. A preplant application of 50 lb N and 60 lb P2O5/ac was supplemented by eight weekly applications of 15 lb N/ac applied in the irrigation water, for a seasonal total of 170 lb N/ac. Transplants of ‘Bingo’ tomato were set in the field on April 26 in 1990 and April 19 in 1991, and grown unmulched under bush (nontrellised) culture. After three weeks for transplant establishment, the different irrigation treatments were initiated. Tensiometers, installed in all plots at 12- and 18-inch depths, were read daily. Routine pest control practices were followed.

Breaker-stage fruit were harvested twice, followed by a final harvest of breaker and mature green fruit. Fruit were separated by size and condition according to USDA standards.

There was no significant difference in either year between the crop coefficients treatment and the percent canopy coverage treatment with respect to total or marketable yields (table 1), or fruit size distribution; all plots showed high yields and a high percentage of extra large fruits (>2 3/4- inch diameter). Daily irrigation did not improve crop productivity compared with irrigation three times per week. The deficit irrigation treatment based on soil moisture depletion produced yields equivalent to the other treatments in 1991 and showed only a modest yield decline in 1990. In neither year was the incidence of blossom end rot or other water stress-related defects significant.

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