Long-term study gauges water, energy, carbon inputs

University of California, Davis, researchers are modifying the goals of a unique long-term study of different farming systems to emphasize differences in water and energy use, and carbon footprint.

While most university trials run a few years, researchers at the UC Russell Ranch facility outside Winters began 19 years ago with their 100-year study, a look at the different long-term impacts of a conventional, organic and an intermediate system of farming.

"Twenty years ago, things were quite different in terms of what people were concerned about, so we had to change how we do the long-term project. We decided looking at energy inputs, water inputs and carbon inputs are good parameters for looking at the farming systems," said Kate Scow, UC Davis Russell Ranch Sustainable Agriculture Facility director.

Scow discussed the new science plan for the long-term study with the farmers and researchers who came to look over the trials during the annual Russell Ranch Sustainable Agriculture Field Day.

The first two decades of the 100-year study have already yielded a bounty of unique information about how farming systems gradually change the soil.

Tomatoes and corn have been grown for 19 years using a conventional system, an organic system including a winter cover crop, and an intermediate system using synthetic fertilizer and a winter cover crop.

As the tomato-grain rotation approached 20 years, the soil in the organic plots reached 3.1 percent organic content, compared to 2 percent in the conventionally farmed plots.

"After about five years, the organic system had higher soil organic material, and that's holding. It's about 3 percent organic material in the organic system, compared to about 2 percent in the conventional system," Scow said.

The long-term study has also shown that the application of mineral fertilizers gradually acidifies the soil, as the conventional ground had a pH of 7.0 compared to a pH of 7.4 in the organic ground.

"It actually has long-term impacts on microbial communities," Scow said.

The potassium levels in the soil were also higher on the organic side, at 238 parts per million compared to 216 parts per million in the conventional plots.

But among the greatest differences was in phosphorous, as ground farmed conventionally for nearly 20 years had less than 20 parts per million phosphorous, while nearby ground that had been farmed organically with the same rotation had nearly 80 parts per million phosphorous.

"You'll see we had higher phosphorous content in the soil in the organic system," Scow said.

Scow joined with Stanford University professor of biological sciences Peter Vitousek and Stanford graduate student Gabriel Maltais-Landry to examine how composted poultry manure and legume cover crops affected long-term soil phosphorous dynamics under different management practices.

"Alternative fertilization strategies are needed to reduce costly mineral phosphorous inputs while maintaining yields and soil fertility. Increasing the use of compost and cover crops could help reduce mineral phosphorous inputs. Compost recycles phosphorous found in wastes, whereas cover crops reduce soil phosphorous losses and could mobilize soil phosphorous bound to soil minerals and organic matter," the researchers reported.

While this phosphorous represents a bounty in nutrients available for future crops, it also represents a potential water quality problem that has to be managed.

"Accumulation in organic plots increases risks of phosphorous loss via leaching and runoff while phosphorous depletion in rain-fed systems reduces soil fertility," the researchers cautioned.

The study will now aim to learn more about how inputs can be balanced with productivity, how water and energy can be economized, and carbon sequestered, without sacrificing yield. This new science plan at Russell Ranch was developed over years of conversations among farmers, researchers and extension advisors.

Researchers' preliminary estimates are that a two-year tomato and corn rotation grown conventionally will require, when everything is taken into account, nearly twice as much energy as the same rotation grown organically with a legume cover crop grown in the winter and incorporated for fertility.

Researchers also anticipate that the organic system will put nearly 60 percent more carbon into the ground during the two years.

But even with a rain-fed winter cover crop, the organic tomato and corn rotation is expected to use nearly 30 percent more water for this two-year rotation. The long-term study at Russell Ranch has already documented that soils that have been cover-cropped use more water because they have higher infiltration rates.

Processing tomato yields have been similar in the conventional and organic plots in the first 19 years of the trial, but corn yields have been significantly higher in the conventional system.

Yields have usually been highest when a cover crop was added to the conventional system.

This yield bonus looks to come from the improved water infiltration in ground that is cover-cropped every winter.

Two systems with virtually no inputs will be included in the study as reference points to compare with the conventional and organic systems.

"We also have a native grassland, so you have something to compare to a managed system," Scow said.

The grass is "native" but it will take a few years of intense management to rid the native grassland plots of invasive plants before it can stand as an unmanaged control, she said.

There will also be plots of wheat grown with no irrigation, fertilization or cover crops for comparison purposes.

"Somewhere out here you will see plots of wheat grown with no inputs. It's a bookend that no farmer would do, but it's important for scientific reasons. You'll see what the wheat that receives no input is producing," Scow said.

The long-term study will also include water and energy use, and carbon inputs, in a minimum tillage system and in a six-year rotation including four years of alfalfa followed by tomatoes and corn.

Researchers have already begun using data loggers to carefully record fuel usage for standard processing tomato operations at three commercial farms, as well as at Russell Ranch.

"We have measured the fuel use to grow processing tomatoes in three growing systems: conventional, organic and intermediate," said Emma Torbert, UC Davis Agricultural Sustainability Institute postdoctoral associate.

Fuel use for all the operations in three systems was around 50 gallons an acre except for the cover crops, which used significant amounts of fuel to plant and incorporate.

"The fuel used was similar except the fuel used to incorporate the cover crop added 12 gallons of diesel per acre, which made fuel use higher in the organic and intermediate systems," Torbert said.

But that figure is incomplete because it does not include the substantial energy used to produce synthetic fertilizer for conventional systems, according to Torbert.

(Bob Johnson is a reporter in Monterey. He may be contacted at bjohn11135@aol.com.)