Slow-release fertilizers may be good insurance against nitrogen loss to leaching, but don't expect them to show a return every year, according to Tim Hartz, vegetable crops specialist at the University of California, Davis.
Hartz, who worked on Salinas Valley trials with slow-release fertilizers on broccoli last winter, said he saw no significant yield advantage over standard practices.
However, he said, “There are differences in release rate and temperature-sensitivity among these products.”
The trials, which will continue in 2002, showed that some products release their nitrogen content in the first four weeks, and others release as little as 20 percent during the same length of time.
Broccoli was selected for the trials because it is extensively grown in the Salinas Valley during winter, when growers struggle with wet fields to make timely application of side-dressed fertilizers.
Definition of slow-release and controlled-release varies, even in the fertilizer industry, he said. Slow-release is generally applied to materials having nitrogen forms that are not readily available and must be mediated by microbial action. Composted manure qualifies, as do some chain urea compounds.
Controlled release products, on the other hand, have a coating encasing a soluble, synthetic fertilizer like urea. The coating controls the rate the fertilizer dissolves.
‘Pretty good match’
Slow-release products commonly available for use on coastal vegetable crops are “a pretty good match for what we need,” and soil type appears to have no influence, Hartz said.
Hartz collaborated with Richard Smith, Monterey County farm advisor, in the project supported by the California Department of Food and Agriculture's Fertilizer Research and Education Program (FREP). Hartz summarized their work, which was also funded by the California Lettuce Research Board, during a recent FREP Conference in Tulare.
Smith did his field trials (including earlier work with lettuce) with the products Polyon and Duration at preplant rates from zero to 200 pounds of nitrogen per acre. He charted nitrate-nitrogen movement from the root zone from December to February. Meanwhile, Hartz carried out laboratory leaching studies with the two products used in the field trials and seven others.
They found that slow-release fertilizer applied at the beginning of the season maintained higher levels of nitrate-nitrogen during the season than either a combination of slow-release and standard fertilizer or the standard treatment.
But, just as with lettuce trials earlier, there was little to conclude about increasing yields. “In the broccoli trials, there was virtually no difference, yield-wise, between the treatments. There was a marginal trend for an increase in average weight of heads with some of the slow-release treatments,” said Hartz.
Throughout the history of slow-release fertilizers, he explained, consistent results haven't been observed in every field, every year.
One reason is leaching loss due to rainfall, which has been relatively light in recent years. Another is the difficulty in detecting an overall nitrogen response in highly enriched cropping systems.