Because lettuce yield is reduced by mild deficits in soil moisture, the crop is often excessively irrigated, which may cause off-site transport of pesticides and nutrients into surface and groundwater.
| Pesticides and nutrients | Way of surface and groundwater contamination |
|---|---|
| The nitrate form of nitrogen | Very mobile in soil. Moves with water percolating below the root zone into tile drains or into ground water supplies. |
| Water soluble forms of nitrogen and phosphorus (nitrates and orthophosphates) |
Transported in surface run-off. |
| Non-soluble forms of nitrogen and phosphorus | Transported with sediments suspended in surface run-off. |
| Pyrethroid pesticides | Unlikely to leach, but adhere to sediments carried in surface run-off. |
| Organophosphate pesticides (e.g., diazinon) |
Soluble and transported in surface run-off. |
The following practices have been shown to minimize impacts of irrigation on the quality of surface and groundwater.
- Conduct regular audits of the distribution uniformity of the irrigation system. Less water needs to be applied using an irrigation system with a high distribution uniformity compared to a system with low distribution uniformity. An audit of the irrigation system during operation by a mobile irrigation lab will determine the distribution uniformity and suggest methods by which the distribution uniformity may be improved (increased).
- Use CIMIS evapotranspiration data and soil moisture monitoring to schedule irrigations. Irrigating too much can result in excessive tail water or leaching of nutrients beyond the root zone of the crop or both.
- Improve maintenance of irrigation equipment. Leaks from pipes and drip lines contribute to run-off and limit the distribution uniformity of the irrigation system.
- Train irrigators and foremen so that scheduling errors are minimized. Over and under application of water by personnel operating the irrigation system can interfere with attempts to improve irrigation scheduling. Irrigators and their immediate supervisors should be trained to operate the sprinkler and drip systems at optimal pressures. They should be trained on how to use pressure gauges and flow meters to assure that the irrigation system is operating correctly.
Practices for Control of Soluble Nutrients and Pesticides
The main water-soluble nutrients of concern for water quality in lettuce are nitrate-nitrogen and orthophosphate. Water-soluble pesticides of concern are organophosphate pesticides such as diazinon, which are usually applied to soil to kill insect pests of seedlings.
- Avoid applying nitrogen fertilizer through overhead sprinkler water. Tail water leaving the field when fertigating with sprinklers can carry high concentrations of nitrogen into surface water supplies.
- Use soil tests to determine if preplant fertilizer (N and P) can be reduced, and use the quick nitrate test to determine if side-dress applications of nitrogen can be reduced or skipped.
- If drip irrigation is used, fertigate nitrogen at rates that match the nutrient uptake pattern of the crop.
- For side-dress applications of nitrogen, fertilizer rates should be appropriate for the nitrogen uptake rate of the crop during different stages of development. The lowest nitrogen uptake rate is during the first 4 weeks of the crop (less than 1 lb of N/acre per day) and the highest nitrogen uptake rate is during the last 3 weeks of the crop (3 to 5 lb of N /acre per day).
- Capture surface run-off in retention basins. Reuse captured water for dust abatement, preplant irrigations or non-leafy green vegetable crops. (Note: presently the food safety metrics of the California leafy green marketing order do not allow reuse of tail water for in-season irrigation of lettuce)
- Establishing lettuce using drip irrigation or using drip early in the crop cycle can eliminate runoff that carries soluble soil applied pesticides.
- Operate overhead sprinklers in a manner to minimize run-off. Practices such as shutting off the sprinklers when the soil is obviously saturated, and not operating the sprinklers at excessively high pressures (greater than 60 psi) can reduce tail water runoff.
- When shanking pesticides into the soil, avoid leaving pesticide on the top of the beds or on the edge of the field where it is susceptible to being carried in run-off from overhead sprinklers.
- Apply enzymes to tail water to breakdown water-soluble organophosphate pesticides. Manufactured enzymes such as Landguard hasten breakdown of organophosphate pesticides including diazinon, and can greatly reduce the toxicity of these pesticides to water organisms. The enzymes can be metered directly into tail water using a dosing unit, or applied periodically to water captured in a retention pond before the water is released.
Practices for Control of Sediment-Bound Nutrients and Pesticides
The main nutrients of concern in lettuce production that move with suspended sediments in irrigation run-off are total nitrogen and total phosphorus, which includes the organic and inorganic fractions of nitrogen and phosphorus that are bound to sediments.
Pyrethroid pesticides also adsorb to sediments and therefore can move with suspended sediments in run-off. Strategies that retain sediments in the field can minimize water quality impairments caused by these nutrients and pesticides in surface water.
- Use drip irrigation as early as possible in the crop cycle to avoid surface run-off that often occurs from overhead sprinklers and furrow systems.
- Inject anionic polyacrylamide (PAM) polymer at low concentrations (about 5 ppm) in irrigation water to minimize sediment transport in surface run-off. Research studies conducted by UC have shown as much as a 95% reduction in suspended sediments and a 60 to 70% reduction in total P and N in irrigation run-off. Liquid emulsified formulations of PAM can be directly injected into pressurized irrigation systems using specialized pumps. Dry formulations can be added to head ditches of flood irrigation systems or to the water catchment ponds to settle out sediments before water is released off the farm.
- Planting vegetation in permanent and seasonal ditches to slow flows of irrigation run-off and drops out suspended sediments and adsorbed nutrients and pesticides. The effectiveness of this practice depends on the amount of run-off, length of vegetated ditch, vegetation coverage, and load of suspended sediments.
- Retention basins and sediment traps can drop out silt and sand-sized particles carried in tail water. These practices can be effective in removing large-sized suspended particles but are less effective for small-sized particles, such as clays, which may need a residence time of several days to a week to settle out. Water chemistry, dimensions of the retention basin, volume of tailwater, and soil texture, all factor into the effectiveness of retention basins to remove sediment from tail water.