As growers in California central coastal counties adapt to new water quality control regulations, experiments with polyacrylamide (PAM) compounds suggest a better way to manage nutrient and sediment content in irrigation runoff water.
Trials last year by Michael Cahn, Monterey County farm advisor, indicated that significant reductions in nutrients and sediment leaving vegetable and row-crop fields can be achieved with PAM-treated water through sprinkler irrigation.
Products containing PAM are used widely, from waste water treatment and processing of fruits and vegetables to manufacture of cosmetics and clarification of juices.
Under regulations of the Central Coast Regional Water Quality Control Board, Cahn said, growers in the region, which extends from Santa Barbara to San Mateo counties, must have a conditional waiver for irrigation water runoff.
To obtain a waiver, a grower must develop a farm water quality plan, implement some practices for water quality protection, and complete 15 hours of training in farm water quality to help develop a management plan for his operation.
The regulations propose maximum total maximum daily amounts of nutrients and sediments and require implementation of best management practices to minimize impairments to surface and ground water quality.
“One of the broader issues for our growers in Monterey County is the erosion of sandy-loam, decomposed granite soils that under solid set sprinklers crust easily and have runoff with a lot of sediment,” Cahn said.
“They also have high levels of nitrogen and phosphorus that drain into various streams, into the Salinas River, and on into Monterey Bay. We have one of the most important agricultural areas in the world next to very important marine and wildlife sanctuaries.”
The regional board is monitoring water quality at many locations along the river to develop baseline data on release of sediment and compounds into the bay.
“In July of 2009,” he said, “the board will reevaluate conditions to see if progress is being made to improve water quality. One of the ways growers can show evidence is data from their farm water quality plan. They can use best management practices and keep records to show they are improving water quality.”
Cahn said PAM, briefly described, is a polymer molecule that links with suspended sediments in water and causes them to drop out of suspension. “It binds soils together to improve water infiltration and reduce runoff, and it also binds nutrients to soil and keeps them in place.”
He said it has been researched extensively on erosive soils under furrow irrigation at the USDA-ARS facility at Kimberly, Idaho. “They found that its granular form took care of erosion problems, reduced runoff, and clarified the discharged water.”
Cahn wanted to see if PAM injected through sprinklers common for germination of vegetable crops in the Salinas Valley could have the same effect. “The challenge for us was whether PAM could hold the soil structure as the droplets repeatedly impact the soil.”
The test plot trials on cooperating growers' fields from south of Salinas to King City showed that the product Soilfloc 300E, a liquid, water-soluble form, could be injected into the water source for sprinklers at concentrations of from 2.5 to 5 ppm when runoff began. Runoff was analyzed for volume and composition of sediments and nutrients.
In a trial near Chualar, results of three sprinkler irrigations showed that “PAM reduced sediment and turbidity levels in the runoff for all irrigations, which corresponded to a 95 percent reduction in sediment loss,” he said. He also recorded a significant reduction in runoff during the third irrigation.
“We observed that it dropped sediment, the soil remained in place, and the water leaving the field was essentially clear,” Cahn said. Variations were seen according to the soil type and salinity of the water, the product working best at an EC of 0.5 or above and an SAR of less than 4.
However, the trials also showed that PAM, when applied in excessive amounts, causes greater runoff because it increases water viscosity and slows infiltration. For that reason, Cahn is advocating the lower rate.
Since the trials last summer, Cahn has enlarged demonstrations for growers on fields ranging from 2 to 10 acres. “We know this works and now we are concentrating on demonstrating it for interested growers.” He is continuing to look for growers interested in conducting demonstrations on their fields.
The most significant runoff in the Salinas Valley occurs with applications made for germination of crops. “Ideally, at this point, we'd like to see growers use PAM with sprinklers up to the thinning stage and then go with drip irrigation. We are not suggesting they use it up to harvest. We are cautioning growers to get approval from their buyers before using the sprinkler method after thinning.”
$20 per acre cost estimate
Rough, preliminary estimates of costs of PAM, he said, are about $20 per acre for four applications, including labor and equipment costs. Cost sharing of up to 50 percent is available through the National Resource Conservation Service.
“These numbers need to be refined with ag economists and we need to work more with growers to find out what their real costs are. But there is also a cost for growers to use alternative methods like settling basins for tailwater management.”
At the least, he added, use of PAM could reduce the need for larger retention ponds commonly in use now. While the basins do improve water quality by containing the sediment and nutrients, at times, runoff exceeds pond capacity and overflows into streams.
“Although some of the sediment, such as sand and large particles, does settle, clays remain in suspension and bind to phosphorus and pesticides molecules. This water can be used for wetting roads or for preirrigation, but few growers would want to use it on a fresh-market vegetable crop and risk food safety issues,” he said.
Cahn said by 1999 nearly one million acres of farmland in the Pacific Northwest were treated with PAM annually. The materials have also been used in the San Joaquin Valley to reduce soil erosion during irrigation.
Funding for Cahn's research was provided by the California Department of Food and Agriculture's Fertilizer Research and Education Program.