Efficient nitrogen management in drip-irrigated lettuce production

Drip irrigation is revolutionizing lettuce production on the coast. The transition to drip brings opportunities and challenges. In theory, water management should be more efficient with drip, and nitrogen can be applied with no cultural constraints; this means that good lettuce yield and quality should be attainable with lower rates of both water and nitrogen application than with sprinkler irrigation.

However, those benefits are not automatic. Our work several years ago with celery growers transitioning to drip suggested that growers use widely differing practices, and that significant improvements in both drip irrigation and N fertigation management were possible.

In 2007 we began a series of trials examining irrigation and fertilizer management in drip-irrigated lettuce. Two field trials were conducted in Monterey County, both planted in June for August harvest. In each field we installed valves in individual drip lines so that we could create a ‘reduced N fertigation’ treatment by turning them off during selected fertigation events.

Individual plots were 4 beds wide and 200 feet long, and were replicated 4 times in each field. Prior to the first N fertigation, soil samples were collected to determine the amount of residual soil NO3-N present in the root zone. To document the irrigation volume applied water meters were installed in individual drip lines in each field and monitored for the final 4-5 weeks of the season.

We also evaluated whether phosphorus fertilization was required for maximum crop productivity. Our prior research showed that, in sprinkler-irrigated fields, P fertilization was not required when soil test P level was greater than 50-60 PPM (based on the Olsen extraction method); we wanted to document that this held true in drip-irrigated fields as well.

In the first trial the grower did not apply P because of the high soil test level; we created four plots in which P was broadcast and incorporated preplant. In the second trial the grower applied an acid-based P fertilizer at planting as an anticrustant; we established four plots that did not receive the anticrustant spray.

In neither field did P fertilization improve lettuce yield. This confirmed our prior research, and suggests that growers can skip P fertilization in fields with Olsen soil test P > 50 PPM, at least in warm soils. Since soil P is less available in cool soil, using a threshold of 60 PPM during spring planting is a reasonable safeguard.

Soil test P level in fields not receiving P fertilization will decline slowly, but highly enriched fields may not require P fertilization for several years. Continuing to fertilize soils above this agronomic threshold will tend to increase the P concentration in field runoff. Water quality monitoring throughout the Salinas Valley has shown persistently high PO4-P concentration; eliminating P fertilization of high P soils should over time improve water quality.

In both fields the growers were judicious with water application, applying 80% of reference evapotranspiration (ETo) over the final month of the season. In both fields there was also substantial residual soil NO3-N at the time the growers began N fertigation.

As a rough approximation, each PPM NO3-N in a soil sample representing the top foot of soil represents approximately 4 lb N/acre, so the residual NO3-N in these fields represented approximately 80-100 lb of available N/acre. This level of residual N is quite common in coastal lettuce fields, and represents a ‘free’ resource that growers can utilize. N fertigation totals were 127 and 153 lb N/acre in fields 1 and 2, respectively. In both fields the reduced N fertigation treatment had statistically equivalent lettuce yield compared to the grower N treatment, while saving 77 and 107 lb N/acre.

This ability to reduce N application to quite low levels (below a seasonal total of 100 lb N/acre in both fields) without affecting lettuce yield can be attributed to several factors. 1) Due to careful irrigation management, leaching volume, and consequently nitrate loss, was low; 2) Significant residual soil NO3-N was present in both fields; and 3) lettuce does not require large amounts of N uptake for maximum growth.

When we sampled at harvest the total amount of N in the above-ground biomass was only around 100 lb N/acre. This fits with our previous experience; over more than a dozen lettuce fields that we have monitored over the years, N uptake has consistently been in the range of 90-120 lb N/acre. Of the average of 92 lb N/acre that the growers applied above the reduced N fertigation treatment, only a tiny fraction of it was even taken up by the crop; the rest remained in the soil, susceptible to leaching.

Fertilizer prices are going through the roof, and regulatory pressure to reduce nutrient concentration in surface water is building. Drip irrigation can help lettuce growers address both issues, but only if it is managed efficiently. Efficient drip management requires: 1- conservative irrigation to limit leaching volume; 2- use of an N fertigation plan that recognizes that lettuce has only a modest N requirement; and 3- willingness to adjust that general N fertigation plan to utilize residual soil NO3-N.

We will be conducting additional drip trials in 2008 to refine management practice recommendations.

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