Some have linked nitrogen imbalance with quality-robbing pithiness in drip-irrigated celery, but Tim Hartz, Extension vegetable crops specialist at the University of California, Davis, says the true reason is water stress, either too little or too much.
Speaking to a group of celery growers at Salinas recently, Hartz described his trials with drip irrigation and nitrogen fertility in nine commercial celery fields in Santa Maria and Salinas.
Drip irrigation is being used on about one-third of the California acreage (6,800 acres, 2003 fall season) and is increasing as methods and practices are refined.
Hartz said he found instances where certain growers were using portable drip systems that were poorly designed and operated.
Using drip tapes of various flow rates on various row lengths, those growers were attempting to irrigate different soil textures on the basis of frequency of irrigations rather than the actual volume delivered to the crop. Pithiness of up to 20 percent occurred with these faulty systems.
Pithiness, or a breakdown of internal tissue density, is a major source of quality loss and is particularly important for export markets.
In addition to water stress, the condition can be caused by cold stress, root infection, or other pre-harvest conditions. Storage at 32 degrees after harvest is particularly vital in delaying pithiness.
Approximately 85 to 90 percent water distribution uniformity, Hartz explained, is realistic from a portable surface drip system, and when distribution uniformity is below that range, significant over-irrigation will be needed to supply the driest part of a field.
Same bad effect
When plant roots are left in standing water from excessive irrigation, the physiological effect is the same as insufficient moisture.
"Regardless of whether you over-water or under-water, you still have the threat of serious pith development," he said.
"Drip irrigation," said Hartz, "is most efficiently managed by using a combination of two systems: water budget calculation and soil moisture measurement."
Water budget calculation is estimating the amount of water the crop requires based on crop growth stage and weather conditions. Soil moisture measurement is monitoring the depletion of available water in the crop root zone.
In general, for determining irrigation frequency, Hartz said maximizing growth rate and quality of celery requires that no more than 20 to 30 percent of available water in the primary root zone, or the top foot of soil, be depleted between irrigations. Since drip irrigation does not wet the entire soil volume, it must be applied more frequently than sprinkler irrigation.
For soil moisture monitoring, he pointed to use of tensiometers as one of the best methods. Installed in the plant row 10 to 12 inches deep in several different parts of a field, tensiometers measure soil-water tension in centibars, which increase as available soil water is depleted.
Celery, he added, is most sensitive to water stress during the final four to five weeks before harvest and particularly during high evapotranspiration conditions. "Celery maturing from late fall through early spring seldom develops significant levels of pithiness, even if some degree of transient water stress is encountered."
Celery has traditionally been the most heavily fertilized vegetable crop grown in California. Under sprinkler irrigation, applications of 300 to 350 pounds per acre of nitrogen have been common and some growers have applied more than 400 pounds per acre.
Hartz said those rates account for not only the high nitrogen use of the crop, typically 200 to 280 pounds per acre, but also the leaching of a significant amount of the applied nitrogen below the reach of celery roots.
"If you limit the amount of in-season leaching with well-managed drip irrigation nitrogen, fertility rates can be reduced correspondingly."
For a crop maturing in summer, he said, drip irrigation is used for the final nine weeks before harvest. Up to the point where the drip system is started, the total crop nitrogen content averages only about 20 to 30 pounds per acre. Afterward, as the crop growth rate increases, the nitrogen uptake peaks at about 35 pounds per acre per week just before harvest.
The current grower practice, he continued, is to top-dress 80 to 120 pounds per acre of nitrogen under the drip tape just before the system is installed, the theory being that the drip irrigation will carry the nitrogen to the root zone.
"However, it will be several weeks before the crop can utilize this amount of nitrogen, and any leaching in those first few weeks of drip irrigation can flush much of this fertilizer below the root zone."
A more efficient approach, Hartz suggested, would be to either eliminate the top-dressing, or apply only 20 to 50 pounds per acre and concentrate on applying more by fertigation later in the season when the crop is better able to utilize it.
Detailed guidelines for the above procedures and other calculations for drip irrigation and fertigation management of celery are available from the UC, Davis Vegetable Research and Information Center at www.vric.ucdavis.edu.