Irrigation water from many wells on the central coast contains a significant amount of nitrate-nitrogen (NO₃-N); recycled water from the Monterey Regional Water Pollution Control Agency, the sole water source for approximately 12,000 acres of prime Monterey County farmland, is high in both NO₃-N and NH₄-N.  Growers historically have been reluctant to modify their N fertilization practices on the basis of irrigation water N content because it is unclear how one can reliably calculate the ‘fertilizer value' of this N.  This issue has taken on added significance with under the current ‘Ag Order' that was adopted by the Central Coast Region Water Quality Control Board in 2012.  The Ag Order requires tier 2 and 3 growers who produce vegetable and berry crops to report the total amount of nitrogen applied to crop land, including N contained in irrigation water.  Baseline numbers from the first two years of reporting on the Central Coast clearly showed that a majority of vegetable producers applied significantly more nitrogen than their crops took up.

Water quality regulators would like growers to take proactive steps to reduce nitrogen inputs to their crops.   One way many growers could make significant reductions in their use of fertilizer N is by off-setting their nitrogen fertilizer rates by a portion of the N applied through their irrigation water. The term “pump and fertilize” has been used to describe this practice because conceptually a grower is pumping water and using the water as fertilizer for the crop.  The benefit for the grower is lower fertilizer costs, and the benefit for the environment is reducing nitrate loading to groundwater.

 In the Salinas Valley, the nitrate concentration of water pumped from agricultural wells averages more than 20 ppm N.   This N concentration would translate to 37 lbs of N per acre for a lettuce crop receiving 8 inches of irrigation water.   A significant number of wells have nitrate-N concentrations in the range of 30 to 50 ppm, and the amount of N that potentially could be applied to crops by irrigating with this water could be as high as 55 to 90 lbs per acre.   Since most lettuce crops take up 130 to 150 lbs of N per acre, high nitrate water could substitute for one third to half of the fertilizer N normally needed to produce a crop.

Despite the potential benefits of implementing “pump and fertilize,” adoption by growers has been slow.   One reason is due to doubts that the nitrate in irrigation water is completely available to crops.   Chemically speaking, nitrate in fertilizer and ground water are exactly the same.  Nevertheless, growers are concerned that N concentrations in high nitrate water may still be too low to be absorbed by crop roots.   Fertilizer N applications usually boosts the N concentration of the water in soil pores to levels much higher than are found in irrigation water.

Another reason that growers are reluctant to account for the N in irrigation water is because they are uncertain about how much fertilizer credit to take when water is applied to leach salts that build up in the soil.  Some growers have also expressed concern that the nitrates significantly increase the salinity of water, making it less beneficial to their crops.  Finally, growers who use multiple wells to irrigate their fields have difficulty estimating the average N concentration of the irrigation water applied to their crops.    

Research trials

Unfortunately, a limited body of research documents the efficiency of crop uptake of N from irrigation water upon which to base an estimate of ‘fertilizer value' under normal irrigation and N management practices.  During the past 3 years we conducted replicated field trials to evaluate how much of the nitrate in irrigation water could be taken up by head lettuce (cv. Telluride) and broccoli (cv. Patron).  Crops were seeded in two rows on 40-inch wide beds, and germinated with sprinklers.  The only N applied at planting was from an anti-crustant application ranging from 17 to 22 lbs N/acre. Crops were drip irrigated after establishment using water of nitrate concentrations ranging from 2 to 44 ppm N.   Water-powered injection pumps were used to enrich all drip applied water to target nitrate concentrations.  Injected NO₃-N was a blend of Ca(NO₃)₂ and NaNO₃ to maintain the cation balance in the water.  These water treatments were compared to an unfertilized control and standard fertilizer treatment (150 and 225 lbs N/acre AN20 for lettuce and broccoli, respectively). In addition, a water treatment dominated by NH₄-N was included in the trials to simulate the N composition of recycled water. To observe the interaction of irrigation efficiency and crop nitrogen recovery, each N treatment was evaluated at two rates of applied water:  1. Standard water rate of 110% of crop ET  2. High water rate of 160% to 180% of crop ET, which corresponded to a 40% to 50% leaching fraction.   

A second set of field trials directly compared crop N recovery from irrigation water and fertilizer.   Irrigation water with concentrations of 14, 25, and 44 ppm NO₃-N were compared with fertigation applications of AN20 of seasonal totals equal to 0, 20, 60, and 150 lbs N/acre in lettuce and 0, 40, 80, and 200 lbs N/acre for broccoli. 

Treatments in all trials were replicated 4 times and individual plots measured 4 beds × 40 ft.   All trials were harvested when the highest fertilizer treatment reached commercial maturity.  Above ground fresh and dry biomass yield were evaluated in the center two beds of the plots.  Whole plant N content was determined so that crop N uptake could be estimated.  Crop N uptake was plotted against N applied in the water and fertilizer N treatments.  The amount of N applied in the water treatments was calculated by the equation:

Applied N (lbs/acre) = applied water (inches) × NO₃ concentration of water (ppm N) × 0.23

Note, that based on this equation applied N from the water treatments will increase as the amount of water applied and concentration of NO₃ in the water increases.

 Crop N uptake response to nitrate in water

Results of the lettuce trial demonstrated that the concentration of nitrogen in the irrigation water significantly affected plant size, N content of tissue, biomass yield (data not presented) and confirmed that a significant portion of the N in the irrigation water was taken up by the crop (Fig. 1).   The crop was able to utilize concentrations of NO₃-N as low as 12 ppm in the irrigation water.   Similar results were also observed in the broccoli trial (Fig. 2).   As shown by the regression curves fit to the data, crop N uptake from irrigation water increased as the concentration of N in water increased.  The fertilizer treatment (square symbols) indicated that crop N uptake would likely level off at high N concentrations. 

The N uptake from the water treatments in lettuce was similar for high and standard water rates, indicating that the volume of water applied did not affect the recovery of N (Fig. 1) and that all of the applied water could be credited as having N value for the crop.   For broccoli, N uptake was lower under the high water rate (180% ET) than the standard water rate (110% ET).  However, the recovery of N from the standard fertilizer treatment was also less under the high water rate (Fig. 2).  

Figure 1.  N uptake by lettuce from water and fertilizer sources of N at standard (110% ET) and high water rates (160% ET).  Symbols represent the mean N uptake of water and fertilizer treatments. N concentration of water treatments is displayed next to the symbols

Figure 2.  N uptake by broccoli from water and fertilizer sources of N at standard (110% ET) and high water rates (180% ET). Symbols represent the mean N uptake of water and fertilizer treatments. N concentration of water treatments is displayed next to the symbols.

Nitrate and ammonium sources of N in water

The source of N in the irrigation water (NH₄ vs NO₃) had no significant effect on N recovery by the crop (Fig. 3).  Presumably under normal summer temperatures NH₄ would quickly transform to NO₃ when added to the soil. Nitrate did boost the salinity of water, but the amount was small: approximately 0.07 dS/m for each 10 ppm increase in Nitrate-N concentration.   Hence, water with a 45 ppm NO₃-N concentration would have a small boost in electrical conductivity equal to 0.31 dS/m.   

Figure 3.  N uptake by lettuce from water with nitrate and ammonium sources of N at standard (110% ET) and high water rates (160% ET). 

Crop recovery of N from water vs fertilizer 

The second set of trials confirmed that crop recovery of N from irrigation water and fertilizer was similar in lettuce (Fig. 4) and in broccoli (Fig. 5).  Symbols in both figures represent the mean N uptake response from the water treatments, and the regression curves were fit to the N uptake response to the fertilizer treatments.   The observation that crop N uptake of most water treatments was equal to or greater than values represented by the regression line demonstrated that crop recovery of N from irrigation water was equal to or greater than from fertilizer.   Similar to the previous trials, crops were able to recover N from water with concentrations of nitrate as low as 14 ppm N.  N recovery was similar under high and standard water rates for lettuce but as found in the earlier trial with broccoli, crop N recovery declined for both the fertilizer and the water sources of N under the high water rate (Fig. 5).

Figure 4.  N uptake by lettuce from water and fertilizer sources of N at standard (110% ET) and high water rates (170% ET). Regression lines represent N uptake response to fertilizer treatments. Symbols represent average N uptake from water treatments. N concentration of water treatments is displayed next to the symbols.

Figure 5.  N uptake by broccoli from water and fertilizer sources of N at standard (110% ET) and high water (180% ET) rates.  Regression lines represent N uptake response to fertilizer treatments. Symbols represent average N uptake from water treatments. N concentration of water treatments is displayed next to the symbols.

Implications for reducing N inputs

The results of these field trials demonstrated that N in irrigation water has fertilizer value for both shallow (lettuce) and deep (broccoli) rooted vegetables, even when the N concentration in the water was low (12 to 14 ppm N).   The trials also showed that the volume of water applied did not affect the crop recovery rate of N from water more than from fertilizer, suggesting that it is reasonable to credit all the N applied in water as having fertilizer value to the crop.   These results were attained under well-managed drip irrigation with a high application uniformity and frequent irrigations so that irrigation volumes were small, which likely minimized leaching losses of any single irrigation event.   It is possible that under poor water management or less efficient irrigation methods (eg. furrow), recovery of N would be less than was reported in these trials.  

Although the results of these experimental trials confirmed that growers can confidently take credit for background levels of nitrate in the irrigation water, one should still be cautious when implementing this practice.    Experimenting on fields where the water source is known to have a consistently high concentration of nitrate but is not excessively high in salts is recommended.  Drip provides better control of irrigation volumes than sprinklers and furrow systems, which may minimize excessive leaching, and also offer more opportunities for fertigating N to correct observed deficiencies.  Because the crop water use is low during the first weeks after planting, it is also reasonable to wait until after establishment to take credit for the nitrogen applied in irrigation water.  Soil nitrate levels should be monitored after crop establishment to determine if the soil has a sufficient supply of N.   If using multiple water sources for a crop, the nitrate concentration of the blended water needs to be determined in samples collected at the field.  Finally, applied water volumes need to be accurately monitored to estimate the amount of N that was applied through the irrigation water.

With water quality regulations continuing to become stricter for agriculture, it makes sense for growers to start implementing practices that can both lower farming costs and are beneficial for the environment.  By accounting for the nitrate in irrigation water and using the soil nitrate quick test to monitor soil N levels, growers may be able to make significant progress in reducing the amount of fertilizer nitrogen needed to produce their crops, and demonstrate that they are addressing water quality concerns.

Acknowledgements

We thank Sharon Benzen and David Lara of the USDA-ARS in Salinas, CA for assistance with the field trials. This project was funded by a grant from the California Department of Food and Agriculture's Fertilizer Research and Education Program (FREP) and the Fertilizer Inspection Advisory Board.

Mark your calendar and register online for the upcoming UCCE Vineyard Pest and Disease Management Seminar to be held on Nov. 4, 2016 in San Luis Obispo. This meeting has been approved for 4.5 hours of DPR Continuing Education credits and CCA credits have been applied for.

Registration webpage:

http://ucanr.edu/ucceviticulture

Meeting information:

Date: Nov. 4, 2016

Time: 9:00 - 15:00

Location: Veterans Memorial Building, 801 Grand Avenue, San Luis Obispo

Cost: $100 per person (includes refreshments and lunch)

Schedule:

8:30 - 9:00 Check in and refreshments

9:00 - 9:10 Introduction and overview of pest and disease issues

Mark Battany, UCCE San Luis Obispo

9:10 - 9:30 Development of disease resistant rootstocks

Andy Walker, UC Davis Viticulture and Enology

9:30 - 9:50 Impact of Red Blotch Disease on grape and wine composition

Anita Oberholster, UC Davis Viticulture and Enology

9:50 - 10:10 Update on Red Blotch Vectors

Cindy Preto, UC Davis Entomology and Nematology

10:10 - 10:30 Developing grapevine leafroll management strategies

Monica Cooper, UCCE Napa

10:30 - 10:40 Break

10:40 - 11:00 Pierce's Disease biology and management in California vineyards

Matt Daugherty, UC Riverside Entomology

11:00 - 11:20 Managing weeds to avoid resistance issues

Paul Verdegaal, UCCE San Joaquin

11:20 - 11:40 Bud necrosis and fertility

George Zhuang, UCCE Fresno

11:40 - 12:00 Sprayer calibration

Lynn Wunderlich, UCCE Central Sierra

12:00 - 13:10 Lunch

13:10 - 13:30 Powdery mildew overview

Lindsay Jordan, UCCE Madera

13:30 - 13:50Sulfur for powdery mildew control

Larry Bettiga, UCCE Monterey

13:50 - 14:10 Powdery mildew and botrytis research update

Allison Ferry-Abee, UCCE Tulare

14:10 - 14:30 The integration of phenology screening into pest resistant rootstock breeding programs

Jean Dodson, Cal Poly Wine and Viticulture

14:30 - 15:00 Improving insecticide controls for mealybugs - following the movement of insecticides in the vine

Kent Daane, UCCE Kearney Ag Research Center

Cabbage maggot (Delia radicum) (Fig. 1) is a serious and destructive pest of brassicas in the Salinas Valley of California. Brassica crops damaged by cabbage maggot are broccoli, cauliflower, cabbage, and Brussels sprouts. Cabbage maggot flies lay eggs in the soil around the base of a plant.  Legless, white maggots feed on the taproot and affect plant development. After feeding for about 3 weeks, the maggot pupates in the surrounding soil for 2-4 weeks before emerging into an adult fly. The symptoms of cabbage maggot feeding in the root are yellowing, stunting, and slow growth.

Research showed that infestation by cabbage maggots in direct-seeded broccoli could be severe throughout the growing period, except the first 30 days after seed was planted.  Typically, insecticide targeting cabbage maggot is applied immediately after planting seeds and before sprinkler is turned on.  Efficacy studies with at-planting application of insecticide did not provide adequate cabbage maggot control.  This suggested that insecticide applied at planting might be early relative to cabbage maggot incidence and thus, delaying application might be more effective.  

In 2014 and 2015, replicated experiments were done in a commercial planting of baby turnip. The treatments were one chlorpyrifos application at planting and 2 weeks after planting seeds.  A tractor-mounted sprayer was used to apply insecticide. Samples were collected and were transported to UCCE entomology laboratory where roots were evaluated for damage by cabbage maggot. 

Results suggested that delayed application of effective insecticide suppresses cabbage maggot (Fig. 2).  In a previous study, Joseph and Martinez (2014) showed cabbage maggot flies did not lay many eggs at the base of brassica plants until 3 weeks after plant emergence (Fig. 3), despite adult cabbage maggots in the field during early stages of plant development.  Also, cabbage maggot infestation tend to be continuous after 3 week stage depending on local pest pressure and crop disturbances (e.g., harvest) in the surrounding fields (Joseph and Martinez 2014). 

Delaying insecticide application would increase the likelihood of intercepting cabbage maggot larvae seeking roots.  In the Salinas Valley of California, use of organophosphate insecticides including chlorpyrifos is regulated.  This stringent regulation is forcing growers to seek alternate insecticides for cabbage maggot control.  Previous study showed that clothianidin, thiamethoxam, and spinetoram as well as pyrethroid insecticides such as zeta-cypermethrin, fenpropathrin, bifenthrin, lambda-cyhalothrin, and pyrethrins were effective against cabbage maggot larvae, and efficacy was comparable to chlorpyrifos (Joseph and Zarate 2015).  However, alternate insecticides are likely to be less persistent because they break down quickly (e.g., spinetoram) or become immobile in soil under field conditions because they bind to organic matter in contact (e.g., pyrethroid insecticides).  Thus, as fewer effective older chemistries (e.g., organophosphate insecticides) are used against cabbage maggot because of use restrictions, delayed application of insecticide might be more critical.

For more details on this study, please read the published paper. http://cemonterey.ucanr.edu/files/248875.pdf

References

Joseph, S. V.  2014.  Efficacy of at-planting and basal applications of insecticides on cabbage maggot in seeded-broccoli. Monterey County Crop Report. January/February 2010-2013. http://cemonterey.ucanr.edu/newsletters/i__b_  Monterey_County_Crop_Notes__b___i_50471.pdf

Joseph, S. V.,and J. Martinez.  2014.  Incidence of cabbage maggot (Diptera: Anthomyiidae) infestation and plant damage in seeded brassica fields in California's Central Coast. Crop Prot. 62: 72-78.

Joseph, S. V., and J. Zarate.  2015.  Comparing efficacy of insecticides against cabbage maggot (Diptera: Anthomyiidae) in the laboratory. Crop Prot. 77: 148-156.

Fig. 1. Cabbage maggot larvae feeding on broccoli roots

Fig. 2. (A) Number of cabbage infested turnip roots, and (B) severity of feeding injury

Fig. 3. Number of cabbage maggot eggs collected from the crown area of the plant following weeks after planting

A series of laboratory and field studies were conducted to determine if the insecticides coated lettuce seeds are an option to control key lettuce pests in the Salinas Valley: springtail (Protaphorura fimata; Fig. 1A), leafminers (Liriomyza spp.; Fig. 1B) and western flower thrips (Frankliniella occidentalis; Fig. 1C). In addition, a laboratory test was conducted to determine if “primed” lettuce seeds reduced springtail feeding damage.

Springtails. Springtail (P.fimata) is soil dwelling primitive arthropod primarily attacks germinating lettuce seeds, reducing the plant vigor or death, which cause patchy or area-wide stand loss. Most springtails possess a forked organ (furcula) in the rear-end, which is extended forward and backward to jump; hence, the common name, springtail. However, the springtail species, sampled from lettuce fields causing the stand loss, does not have furcula. This means it cannot jump.

The head lettuce seed ‘Regency' was coated with clothianidin, thiamethoxam, and spinosad (Table 1). The seeds were coated by Dr. Alan Taylor at Cornell University and coating technique mirrored commercial seed coating procedure. Laboratory studies were conducted in containers with springtail (P. fimata) infested soil. The data show that all three insecticides spinosad, clothianidin and thiamethoxam treated seeds significantly reduced the incidence of springtail feeding injury when compared with untreated seeds. Among insecticides, superior performance in efficacy was noted in the following order: clothianidin > thiamethoxam > spinosad (Fig. 2). Two field trials were conducted against springtails using the same seed treatments, however, the springtail pressure was so low that conclusive data were not obtained. Clothianidin (NipSit) in particular, is now registered on head lettuce and could be used for springtail control. This is an important information in that springtails attack the germinating seeds of lettuce especially in the spring time. During spring, we get some rain showers and the wet conditions in the field after planting makes insecticide application along seed line almost impossible. If the insecticide coated seeds are planted, the grower or PCA could avoid at-plant insecticide application which is typically targeted toward springtails. Application of insecticides such as neonicotinoids and pyrethroids along the seed line will protect the germinating seeds from springtail feeding. More field studies will be conducted in the following years to validate these results in the field.

Studies were also conducted to determine if there are any varietal effects exists (Table 2). The much needed attribute for springtail control is faster seed germination so that the springtail would not get sufficient time to feed and cause seed mortality. “Primed” lettuce seeds are used for uniform and a quick germination (cut short 2 to 3 days than “unprimed” seeds). “Primed” and “unprimed” seeds were evaluated to determine if the quick seed development would reduce springtail damage. Data show that germinating “primed” seeds were impacted with springtail feeding affecting their germination and were not different from the “unprimed” seeds when the springtail pressure was moderate to high (Fig. 3). The seeds used for this experiment were from same seed lot (“primed” and “unprimed”) for a lettuce variety. Also, there was no clear variety difference in springtail feeding damage.

Leafminers and western flower thrips. The leafminer eggs are laid within the surface layer of the leaf. The eggs hatch within couple of days and the maggots mine through the surface layer of the leaves. The egg laying and maggot mining creates stippling and mining injuries which make the leaves unmarketable. Although UC recommends few insecticides such as Agri-mek (Abamectin), Trigard (Cyromazine), Aza-direct (Azadirachtin) and Entrust (Spinosad), the management of leafminers are primarily relied upon on Agri-mek applications.

Thrips is another of the major pest of lettuce, and combination of direct feeding injury and viral disease [thrips-transmitted tospoviruses [Impatiens Necrotic Spot Virus (INSV)] can cause significant losses in lettuce production. In addition, because most of the export markets have set higher standards on prevalence of live and dead thrips in the produce, the lettuce industry is constantly battling ways to significantly reduce thrips in the produce targeted for export.

A replicated-field trial was conducted to determine the efficacy of seed coated insecticides (Table 1) on leafminers and western flower thrips incidence and their infestation. The results show that insecticide seed coating may not be an effective option for leafminers and thrips control in head lettuce (Table 3 and 4) under the conditions this experiment was conducted. There was no reduction of leafminer or thrips feeding with insecticide coated seeds compared with untreated control. Further evaluations under varying conditions might be necessary to validate the consistency of these results. 

Fig. 1. (A) springtail, (B) leafmine and (C) western flower thripsr

Fig. 2. (A) Set-up for springtail trial in the laboratory. Three insecticide coated seeds were compared with untreated seeds after 7 days of exposure. (B) The springtail containers retained in a controlled conditions in the laboratory. The treatment number in the container corresponds to the insecticide coating.

Fig. 3. Lettuce variety or "primed" and "unprimed" seed experiment: All the germinating lettuce seeds fed upon by springtails after 7 days of exposure. Please refer to Table 2 for details.

Table 1. Insecticide seed coating treatments used for springtails, leafminers, and western flower thrips experiments.

Table 2. The lettuce seed varieties used in the experiment.

Table 3. Effect of insecticide coating on head lettuce seeds for western flower thrips control in the field.

Table 4. Effect of insecticide coating on head lettuce seeds for leafminer control in the field.