EDITOR'S NOTE: The Following information was developed by Tim Hartz, and Paul Johnstone, Vegetable Crops Department, University of California, Davis and Mike Cahn, UC Cooperative Extension, Monterey County.

Efficient fertilizer management has been a concern for California coastal vegetable growers for years. Most attention focused on nitrogen management, a response to elevated nitrate concentration in groundwater in several of the coastal production areas.

Phosphorus fertilization practices are now being scrutinized more carefully as the Regional Water Quality Control Boards move to implement non-point source pollution control measures mandated by the Clean Water Act. Currently there are several dozen water bodies in California listed as “impaired” by excessive nutrient concentration (N and / or P), including portions of the Salinas and Pajaro River systems.

Coastal vegetable growers have traditionally applied phosphorus to each crop to ensure peak production. Over decades, soil P concentration has been dramatically enriched; fields with a long history of vegetable production now commonly have extractable soil P levels of more than 60, with some fields exceeding 100. While this generally does not cause agronomic problems, runoff or drainage from such fields can be a contributor to high P concentrations in streams and rivers.

Lettuce phosphorus

In the past two years we have taken a comprehensive look at the issue of phosphorus management for lettuce production, from both an agronomic and environmental standpoint. Our interests were in documenting the validity of the standard bicarbonate (Olsen) soil test as a predictor of plant-available P, and in determining the relationship between soil test P levels and P pollution potential. This research has shown that the Olsen test is a good indicator of soil P status, and that phosphorus use in the coastal vegetable production areas can be significantly reduced without loss of crop productivity. Furthermore, reducing P fertilizer use will reverse the trend of increasing soil P levels in coastal soils, and will minimize P losses to the environment.

To determine whether P fertilization is necessary in fields with elevated soil test P levels, a total of 12 trials were conducted in commercial lettuce fields from Salinas to King City in 2002 and 2003. Fields were chosen which had moderate to high bicarbonate extractable soil phosphorus (53-171); this soil P range encompasses the vast majority of vegetable fields in the coastal production areas. In two of the fields the growers did not apply pre-plant P fertilizer; within both of these fields four plots receiving a pre-plant application of 130 pounds P205 per acre were established. In all other fields the growers applied pre-plant P, and we established 4 plots per field in which this pre-plant P application was skipped. Crop response to P was determined by comparing the performance of plots with and without pre-plant P.

Planting dates varied from late January through late July, with harvests varying from early May through late September. A range of iceberg and romaine cultivars were encountered in these trials. Both 40- and 80-inch bed configurations were used.

A statistically significant yield response to P fertilizer was observed in only one of the 12 fields. This field had among the lowest soil test P levels (54), and was planted relatively early in the season (cool soil limits P availability). Averaged across all fields, lettuce yield in plots receiving no pre-plant P was virtually identical to that of plots receiving P. Plant vigor was poor in both treatments in several fields, so P fertility was apparently not the growth limiting factor.

Marginal uptake

Tissue analysis revealed that pre-plant P fertilization resulted in only marginally increased plant P uptake, and cupping stage leaf P concentration of both treatments in all fields was above the generally recognized sufficiency standard (0.30-0.40 percent); phosphorus concentration of harvested heads was even higher.

Although we did not conduct storage tests to determine if P fertility affected post harvest quality, the high P concentration in harvested heads, and the lack of clear differences in P content between P treatments, suggests that differences in post harvest quality would be unlikely. P removal from the field in the harvested lettuce averaged approximately 10 pounds phosphorus per acre (23 pounds P2O5 equivalent), about one-third of what the growers applied.

These results demonstrate that lettuce is not likely to respond to P fertilization in fields with elevated Olsen P levels. A small pre-plant P application is justified in fields with low to moderate soil P (up to 50-60) planted in cool weather. P fertilization of fields with higher soil test P, particularly if planted in warm soil conditions, is unnecessary and serves only to increase the potential for off-field movement of P.

The majority of soil P is not in a soluble form, ready for plant uptake. Most is bound in soil organic matter, or is in chemical forms that are only slightly soluble. The Olsen extraction technique is the standard laboratory testing procedure to estimate plant available soil P in neutral to alkaline California soils. In this test soil is extracted in a sodium bicarbonate solution at pH 8.5; the high pH guards against solubilizing P compounds that would remain insoluble at the pH of the soil.

In theory the Olsen test estimates the pool of plant available P, but does not directly measure how readily additional P will become available as the crop removes P from soil solution.

We evaluated an alternative soil test procedure purported to more directly measure P bioavailability. In this test a thin membrane of anion resin is placed on the surface of moist soil for 16 hours; as phosphate ions from the soil solution are adsorbed onto the membrane (mimicking the action of plant roots), additional P is solubilized. The amount of P adsorbed on the membrane over time ranks the soil's P supplying power. In testing a set of 30 representative vegetable crop soils from the Salinas Valley we found a strong correlation between Olsen P and P bioavailability as measured by the anion resin test. This confirms that the standard Olsen soil test is a reasonable predictor of plant-available P.

Phosphorus saturation

It has been widely assumed that in alkaline soil fertilizer P to a large degree becomes chemically bound, and therefore at low risk of leaving the field in runoff. To determine the degree to which Salinas Valley soils can tie up applied P we evaluated the percent phosphorus saturation of the same 30 soils. Soil was shaken in a solution containing a 1.5 ppm soluble P. The amount of soluble P adsorbed by the soil was determined, and the percent P saturation calculated. This value ranks the degree to which the soils' P absorption capacity has been diminished; the higher the percent P saturation, the lower the soils' remaining ability to tie up fertilizer P.

The percentage of P saturation was closely correlated with Olsen P. Soils at or above 100 Olsen P had virtually no remaining capacity to tie up P, and several of the soils actually contributed P back into the solution.

Using six of these Salinas Valley soils we then directly evaluated the potential for P loss to the environment. Each soil was used to fill six plastic containers; half of these were seeded with a variety of oats commonly used as a cover crop. The oats were allowed to grow until they reached about six inches tall, then all the containers (both planted and fallow) were subjected to simulated rainfall. Runoff from the containers was collected and analyzed for soluble P concentration, and for the weight of sediment contained in the runoff

Although the correlation isn't perfect, it is clear that soils with higher Olsen P have significantly higher soluble P concentration in runoff. To put these numbers in perspective, the surface water quality target for P concentration will likely be less than 0.2, meaning that the four high-P soils had runoff P at least 4 to 10 times higher than the likely water quality standard. It is also clear that the presence of the oat cover crop significantly reduced P concentration in runoff, and was even more effective in reducing sediment loss. Sediment loss from high-P fields is problematic because after it reaches a waterway it will continue to release soluble P.

This project points out that coastal vegetable growers have both an opportunity and a challenge with respect to P management. The opportunity is to save money by reducing P fertilizer use on fields where a crop response is unlikely. The challenge is to minimize P loss from the lands they farm. Over the coming decade, the judicious use of P fertilizer can reduce the very high soil test P levels now common.

But until that happens, management practices that reduce runoff volume and sediment loss will help meet water quality goals. Those practices are best targeted toward fields with the highest Olsen P, particularly if those fields have characteristics (slope or poor water infiltration rate) conducive to generating runoff.