The lack of rain in the Salinas Valley brings many concerns. The lack of runoff into lakes San Antonio and Nacimiento of course is a concern for the availability of water to run down the river to recharge the ground water for irrigation purposes. In addition, the lack of rain will affect the levels of salts that remain up in the root zone of the crops. Soil nitrate (NO3-) is one of the anions that will remain in the soil if leaching by winter rains does not occur. Nitrate is highly mobile and can be easily leached with just one or more significant rain storms; figure 1 illustrates nitrate leached from the top foot of soil by a series of storms that delivered 2.0 inches of water over the course of one week in the winter of 2010. High residual soil nitrates may come from several sources: 1) unused fertilizer from the previous crops or fall preplant nitrogen applications; 2) mineralization of crop residues from the previous crop; and 3) mineralization of soil organic matter over the winter (mineralization of soil organic matter is much slower during the winter but will still occur to a minimal degree).
We recently surveyed several soils looking for a site to conduct a fertilizer trial and observed that residual soil nitrate levels were routinely over 20 ppm nitrate-nitrogen. These levels were in contrast with levels that we observed last year following a wet spring where, in general, residual soil nitrate levels were in the 5 – 10 ppm nitrate-nitrogen range. The difference in conditions between a dry winter like this and a wet winter like last year is that it has implications for planning nitrogen fertilizer programs; with soil residual nitrate levels this high, the nitrogen fertilizer needs of the first crop fields will behave like second crop fields in that the robust amounts of residual soil nitrate in the soil that can provide for the crop needs and allow you to reduce nitrogen fertilizer programs.
To illustrate this point, we observed a great difference in the fertilizer needs of first vs second crop spinach during the 2011 growing season. In a first crop spinach planting, residual soil nitrate levels were at 5 ppm at the beginning of the trial. Spinach responded to at-planting applications of nitrogen up to 40 lbs nitrogen/A (Figure 2). The second crop spinach planting had initial levels of residual soil nitrate of 28 ppm nitrate-nitrogen which allowed the grower to skip the at-planting nitrogen application; he made one top-dress nitrogen application two weeks after planting to bring the crop to harvest. The results of a top-dress nitrogen evaluation indicated that there was no improvement in yield beyond 25 lbs nitrogen per acre (Figure 3).
These results indicate the importance of deep percolation of water on residual levels of soil nitrate. Winter rains have the beneficial effect of leaching salts from the soil. It is very unfortunate that nitrate is one of the salts that is leached with the water, but that is the case. In many of the discussions that we have had over the last several years regarding managing nitrogen fertilization more efficiently, we have emphasized that testing for residual soil nitrate is generally most effective for the second crop of the season. However, given the extreme lack of leaching rain events this winter, residual soil nitrate levels are also high at the beginning of the first crop in many areas in the valley and can be taken into consideration when planning nitrogen fertilization.
Figure 2. Yield response of first crop spinach under five
application rates of at-planting nitrogen (0 – 80 lbs N/A)
Figure 3. Yield response of second crop spinach under five application rates of top-dressed nitrogen (0 – 105 lbs N/A); no at-planting nitrogen was applied to this planting.
- Author: Richard Smith, Vegetable Crop and Weed Science Farm Advisor
In recent weeks a number of samples have come into our office of lettuce plants that have the following symptoms: stunting, yellowing outer leaves and occasionally with wilting during the afternoon (Photo 1). The symptoms superficially resemble Lettuce Dieback caused by Tomato Bushy Stunt Virus, but Steve Koike has not detected this virus in these plants. Affected plants also typically have roots that are no longer than 1.5 to 2.0 inches long (Photo 2). Upon careful examination of the root tissue, it can be seen that the roots once extended further, but were burned off at this point in the soil. The death of the tip of the root was not caused by a disease. In nearly all cases that I have seen so far, this problem occurs on heavier clay loam to clay type soils.
Based on the uniformity of the depth of the point of death of the tap root, it appeared that this problem was associated with an application of fertilizer. Given that fertilizer is a salt, it is capable of damaging young root tissue. These symptoms are distinct from ammonium toxicity which damages lettuce root tissue by the toxic action of the ammonium on root tissue (see Blog entry April 26 by Steve Koike). Ammonium toxicity causes distinct symptoms on affected roots; however, these symptoms are distinct and appear to be caused simply by salt burn of fertilizer. (Photo 3).
To confirm this hypothesis, last summer I worked with a cooperating grower to recreate these symptoms on lettuce. I used a pipette to inject fertilizer 0.5, 1.0 and 2.0 inches from the base of lettuce plants, and 1.5 inches deep in the soil; all applications were applied at the thinning stage. We observed that there were higher levels of plants with the tap root burned off in the plots where the fertilizer was applied 0.5 inch from the plant than farther from the plant. These results are not surprising, but the question is why do these symptoms occur at all? Tractor applied fertilizer is spaced 2-3 inches from the plant to avoid fertilizer burn. One possible explanation on how the fertilizer may reach the lettuce roots has to do with soil type. As I mentioned the problem seems to occur on heavier soils; these soils are more prone cracking which can permit liquid fertilizers to flow a short distance towards the seedline during the application. If the material flows close enough to the taproot of the young plant, then it can burn the tap root at the level of injection in the soil. This explanation may explain why the problem occurs at more or less a uniform depth in the soil and why affected plants are scattered in the field (e.g. scattered plants or 2-3 affected plants next to healthy plants) (Photo 4).
Photo 1: Typical symptoms of plants with fertilizer burn on the roots
Photo 2. Plant with the tap root burned of 1.5-2.0 inches down in the soil
Photo 3. Close up of the burned of tap root (note that the remainder of the root tissue is healthy)
Photo 4. Pattern of the problem in the field
The draft Agricultural Order issued by the Central Coast Regional Water Quality Control Board (CCRWQCB) on November 19 increased the regulation of discharges of nitrate-nitrogen to surface and ground water from agriculture. As written, all vegetable operations that produce over 1000 acres of lettuce, cole and several other ‘high risk’ crops and that use chlorpyrifos or diazinon are placed into Tier 3 compliance category which has specified regulations concerning the movement of nitrates to surface and ground waters. The new regulations may require growers to implement a certified Irrigation and Nutrient Management Plan (INMP) to document information on nitrogen applied to crops vs nitrogen removed by crops. This information would be used to calculate a nitrogen balance ratio and growers are given three years to demonstrate nitrogen balance ratios of 1.0 for annual rotations that are double cropped. In other words, if double cropped lettuce annually removes 240 lbs of N/A (120 lbs N/A/crop), the annual amount allowed to grow two crops of lettuce in order to comply with the nitrogen balance ratio would be 240 lbs N/A/year. Given current production practices and traditional fertilization programs, complying with these new restrictions will require many growers and their fertility consultants to make a shift in their approach to fertilization of leafy green vegetables.
The ultimate goal of the regulations issued by the CCRWQCB is to reduce the load of nitrate that is added to agricultural operations in the hope to improve the quality of ground and surface waters in the valley. It is therefore important for us to explore ways that we can be more efficient with applied nitrogen fertilizer and reduce unnecessary loading of nitrate when possible. The good news is that there are tools that can help growers manage nitrogen and safeguard yields in this new regulatory era. If anyone is interested in more information on this subject, there are a number of Monterey County Crop Notes articles that deal with this issue. One point that I will make today regarding the lowest hanging fruit in terms of nitrogen savings in lettuce production is the use of fall applied preplant nitrogen. We observed rapid and nearly complete loss of this nitrogen in a series of rain storms in January of this year (Figure 1). I realize that fall nitrogen applications are often mixed with decision regarding phosphorus and potassium applications; for more information on phosphorus applications to cool season vegetables check out the following publication.
The draft rules issues by the CCRWQCB are yet to be finalized by the full board in March 2011. Whatever shape the final rules take, it appears that we have to begin the process of rethinking our approach to fertilizing lettuce and other leafy greens. The good news is that there are solid tools that can help the industry cope with this new regulatory era. Click here to view the draft Agricultural Order.
Figure 1. Fate of fall applied nitrogen in one storm even in January, 2010
Nitrogen use to grow vegetables along the Central Coast is the subject of proposed regulations by the Regional Water Quality Control Board (RWQCB). As a result there is increased interest in improving the efficiency of applied nitrogen (N) fertilizer. Understanding the uptake pattern of lettuce is critical to careful N management. Lettuce typically takes up 120-140 lbs of nitrogen in the above ground biomass with the higher uptake levels occurring on 80 inch beds with 5-6 seedlines. The uptake patterns of romaine and head lettuces are similar. Figure 1 shows the uptake of N in a lettuce crop over the growing season. At thinning (app. 31 days after planting) the crop had taken up less than 10 lbs N/A. The biggest increase in nitrogen uptake by the crop occurred between 39 days and 52 days after planting. The most difficult task in managing nitrogen fertilization of lettuce is to supply adequate levels of nitrogen during this period of high demand by the crop, but to not leach it during irrigations.
In second crop lettuce it is possible to take advantage of residual soil nitrate and use it in place of applied fertilizer. Figure 2 shows a typical pattern of nitrate-N levels in Salinas Valley soils. The data indicates that typically by the time you get to the second crop, levels of residual nitrate-N can increase above 20 ppm (which is equivalent to 80 lbs N/A) due to left over N applied to the prior crop and mineralization of nitrate-N from crop residue. Whatever the source of the residual nitrate-N, it is a substantial quantity that can be utilized by a growing lettuce crop.
Residual soil nitrate can be monitored by use of the soil nitrate quick test (see photo 1). The test provides information on levels of soil nitrate-N and can help you to decide whether you need to apply fertilizer as planned or if you can reduce This test is most effective if done at thinning to determine residual soil nitrate levels for the first post thinning fertilization and can be done again prior to subsequent fertilizer applications if desired.
Figure 1. Nitrogen uptake by lettuce over the growing season.
Figure 2. Nitrate-N in soil over the course of the growing season January to December – mean of six fields
Photo 1. Soil nitrate quick test
- Author: Richard Smith
- Research Assistant: Aaron Heinrich
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)