Composted manures offer promise as beneficial soil amendments for vegetable growers. New research results show that composted manures can increase vegetable yield, influence crop diseases, and bring about changes in soil microbial life.

These research findings are from an on-farm collaboration between vegetable growers and research scientists at UW-Madison. The project began with Richard DeWilde and Linda Halley of Harmony Valley Farm, Vernon County growers who raise 50 acres of fresh-market vegetables. Because they raise certified organic crops, they use composted manures, cover crops, and mineral supplements allowed by organic certification rules.

In 1991, they noticed an increase in foliar diseases after they stopped using composted manures following the sale of their livestock. Their observations agreed with greenhouse studies which suggest a link between use of composts and disease suppression. But no research had clearly demonstrated how to generate and use disease-suppressing composts under field conditions.

As a result, DeWilde and Halley were hesitant to invest heavily in the equipment necessary to generate the large quantities of compost they would need for their expanding operation without being certain that the compost would improve their crops’ health and yield.

During a tour of their farm for a Center for Integrated Agricultural Systems (CIAS) Citizen’s Advisory Council meeting, they found a common interest in soil health with Robert Goodman, professor of plant pathology at the UW-Madison and CIAS Faculty Associate. Part of Goodman’s research focuses on investigating soil microbial communities which play a critical role in soil health.

With encouragement and financial backing from CIAS and from the USDA Sustainable Agriculture Research and Education producer grant program, DeWilde, Halley, and Goodman developed a plan to conduct joint research at Harmony Valley Farm with two goals. The first goal was to conduct on-farm trials to assess whether compost amendments influence crop health and productivity at Harmony Valley. The second goal was to collect soil microbial data on the farm where links between microbial indicators and yield could be investigated. Goodman hoped to use soil from the trials as starting material for developing DNA-based methods for assessing soil microbial communities.

Crop yield findings

During the summers of 1997 and 1998, DeWilde and Halley established replicated trials with the assistance of Beth Kazmar, a research specialist with Goodman in UW-Madison’s Department of Plant Pathology. Kazmar advised DeWilde and Halley about trial design and execution and monitored crop disease responses to field treatments. DeWilde and Halley assumed responsibility for managing the trials. In early 1997, they purchased a used compost turner and a hydrostatic tractor to pull it.

Bedded manure was purchased from neighboring farms, shipped to Harmony Valley, and mixed with cornstalks to bring the carbon to nitrogen ratio to about 20 to 1. The material was composted for eight weeks and rested for two weeks before being applied to the experimental plots by a four-spinner manure spreader.

Four treatments were studied in the field trials: an untreated control, composted goat manure, composted dairy cow manure, and a commercially-available feathermeal product mixed with soybean meal which was included as a NPK fertility control. To help isolate compost’s non-nutrient effects, researchers varied compost application rates between 10 and 15 tons per acre to match compost nutrient levels and those of the NPK control.

The same field was used in both seasons, allowing observations on cumulative effects of compost applications in the second year. Each year, two crops were planted in adjacent strips.

The results were promising. One or both composts increased yield of three of the four crops studied (see table below). Several factors that influence crop health appeared to be involved. The composts increased soil organic matter and were a source of minerals such as nitrogen, phosphorous and potassium. DeWilde and Halley believe nitrogen is the limiting nutrient in their cropping system. The NPK control supplied the highest levels of soil nitrogen, yet increased yield less than the goat manure compost, suggesting that mineral supply is not the only benefit of the compost applications.

Crop health findings

Some diseases were less severe in plots treated with the composts, but other diseases, particularly those involving the fungus Rhizoctnia solani, were more severe (see table above). In one case (root lesions on beets), disease incidence was lower in the compost amended plots relative to the control. The significant disease suppression observed in several cases is likely to have contributed greatly to greater plant productivity. For example, Cercospora leaf spot of beet is a major pathogen, and can limit yield if the disease attacks early. Both dairy cow and goat manure composts appeared to limit this disease in 1998.

In contrast, compost use increased several diseases caused by the fungus Rhizoctonia solani. During the two years of the experiment, Kazmar monitored a variety of crops at Harmony Valley and recognized that diseases caused by this organism were problematic on many crops. But the increase in disease on crops with compost amendments was often accompanied by an increase in yield.

The composts appear to be a useful part of soil management for Harmony Valley, but should be used in conjunction with other techniques to manage levels of Rhizoctonia, such as extended rotations of susceptible crops and rotation to grasses where feasible.

A striking finding of the study was how crops responded to compost in dramatically different ways. Few field experiments have compared the effects of compost on crops planted into the same plots. Interestingly, in 1998 the beet crop responded strongly to both composts and the NPK control. In contrast, the carrot crop planted six feet away responded most strongly to the goat manure compost. The mechanism remains a mystery, one of the many as yet unanswered questions about compost management.

Is compost an economical option for Harmony Valley? “Yes,” says DeWilde, “after seeing the results of our experiments, we definitely plan to continue making and using compost. We expect to eventually be able to reduce our application rates as the compost accumulates in the soil, improving soil structure and tilth. These are important long-term benefits with our sandy loam soils.”

DeWilde figures that if he can average a ten percent yield increase for all crops using compost, he will increase average gross sales by $800 per acre. This would make it well worth the costs of making the compost including equipment depreciation, even at $25 per ton or $300 to $400 per acre. “Economic benefits are even greater when long-term effects are accounted for,” says DeWilde.

Soil microbiology

What was learned about changes in soil microbiology? Researchers at the commercial firm Soil Foodweb, Inc. examined soil samples from both years for an extensive array of microbial qualities. One test (the ratio of total fungal to total bacterial biomass) was a strong predictor of yield. This measure was capable of distinguishing the soils collected from the amended and unamended plots, revealing that the dairy compost had indeed changed the soil microbial community.

The Goodman lab continues their investigations with soils collected from the trials. To date, research conducted by Jeff Dillen of the UW-Madison Institute for Environmental Studies on samples from another farm showed that a DNA-based method could detect microbial community shifts in response to long-term, 30-year differences in crop management. An extensive collection of microbial DNA samples from the trials at Harmony Valley has been archived for future analysis.

The farmer-scientist collaboration

DeWilde and Halley found this collaboration especially rewarding because they were included as active partners. The UW scientists also valued the collaboration. Kazmar said, “We try to mimic the decisions a grower makes during university research station trials, but it’s inevitable that there will be differences. We found it valuable to evaluate our results in the context of Harmony Valley.”

Contact CIAS for more information about this research.

Published as Research Brief #45
December, 1999