Nitrate leaching in lettuce production – may be difficult to interpret - Salinas Valley Agriculture

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Nitrate leaching in lettuce production – may be difficult to interpret

Author: Richard Smith
Research Assistant: Aaron Heinrich

Published on: April 23, 2010

Nitrate leaching from vegetable production along the Central Coast is under greater scrutiny and is the subject of proposed regulations by the Regional Water Quality Control Board (RWQCB). The regulations as written have stipulated that leachate from agricultural lands should not exceed the public health limits for nitrate in drinking water of 10 ppm nitrate-nitrogen.

To date there has been little information developed on the quantities of nitrate contained in leachate from lettuce production. In 2009 we conducted a nitrogen fertilizer trial in which we applied 10, 75, 150, 225 and 300 lbs of N/A and water was applied at 116% of evapotranspiration. In order to measure leachate from the plots, suction lysimeters were installed to a depth of 2 feet deep in the soil (photo 1). During each irrigation, suction in the lysimeters was maintained at 20-25 centibars which was assumed to be the leachable fraction of soil water. After each irrigation, leachate was collected and analyzed for nitrate concentration.

The 10 lbs N/A was a low N treatment (and yielded substantially lower than other treatments), but even in this treatment had leachate nitrate-N concentrations substantially greater than the 10 ppm nitrate-N drinking water standard (see graph below) for the majority of the early season. The concentration of nitrate-N in this treatment declined to below the drinking water standard for the final third of the growing season. These data give us a glimpse into nitrate levels of leachate from vegetable production fields. Even treatments with little applied N can have substantial quantities of nitrate in the leachate. This indicates that monitoring of the concentration of nitrate in the leachate may not be a consistently useful tool for understanding the quantity of N leached.

Figure 1: Nitrate-N concentrations in leachate over the growing season of romaine lettuce (for simplicity, we pooled the leachate nitrate levels of the highest three nitrogen fertilizer treatments 150, 225 and 300 lbs N/A, as they were not significantly different from each other)

Tags: leaching (1), lettuce (38), lysimeters (1), nitrate (7), nitrate leaching (2), nitrogen (7), nitrogen management (9), regulations (1), vegetable production (1)

Comments: 4

Comments:

by William R. Horwath

on April 27, 2010 at 2:06 PM

Interesting data! The treatments do differentiate from each other with higher rates leaching more nitrate. My question is "What was the previous N management"? Is the low N addition rate being influenced by residual fertilizer N from the previous crop?

by Richard Smith

on April 28, 2010 at 2:21 PM

Response to William Horwath: The field was winter fallowed prior to the trial. The soil at the site has 2.1% organic matter and may have mineralized the sizable quantities of nitrate observed early in the crop cycle.

by Mark Bolda

on April 29, 2010 at 11:53 AM

Hi Richard,

I'm not so clear on what the last line of this excellent presentation means. By saying that "monitoring of the concentration of nitrate in the leachate may not be a consistently useful tool", do you mean that the numbers tend to bounce around too much? It does seem though at the lower application rates there is a trend downwards over time- I am assuming this is because the plant is larger and more able to pick up more N as the season progresses. Thanks for putting this out.

by Richard Smith

on April 30, 2010 at 1:32 PM

Mark: The comment refers to the fact that the RWQCB has proposed to monitor nitrate concentration in leachate as a measn of measuring nitrate leaching from agricultural fields and has set the drinking water standard of 10 ppm nitrate-nitrogen as the upper limit. The data in this trial would suggest that concentration may not be a reliable way to measure nitrate leaching because even the control treatment had nitrate leaching levels that exceeded the drinking water standard during a large part of the growing cycle. We are just beginning to increase our understanding of nitrate leaching through the use of lysimeters. It may be that it is premature to set limits for nitrate in leachate based on concentration and that focusing on load of applied nitrogen may be simpler and more accurate approach.