by Sally Colby
The results of a nationwide survey of organic farmers to identify research priorities didn’t hold any surprises. Mark Schonbeck, research associate at the Organic Farming Research Foundation, said organic farmers listed soil health, fertility and nutrient management as top priorities.
Organic farmers in the western U.S. identified more specifics, including how soil life affects fertility, soil microbes and nutrient cycling in desert climates; nitrogen fixing cover crops for the arid west and drip irrigated fields; liquid fertilizer management to reduce nitrogen leaching (especially in areas with high winter rainfall); and crop varieties that require less nitrogen and that will outcompete weeds.
With inadequate organic matter or prolonged bare surfaces, crops don’t receive adequate nourishment. “Excessive tillage can create subsurface hardpan and further burn up organic matter,” said Schonbeck. “If you’re using a lot of soluble fertilizer, it not only bypasses soil life, it has negative impacts on soil organisms.” Schonbeck said over the past 20 years, mainstream ag has an increased appreciation for soil health in crop nutrition, nutrient cycling and nutrient retention. However, there is also “leftover” information from the 20th century, including emphasis on feeding the plant directly with soluble fertilizers. “Although this could compensate for poor soil health in the short to medium term,” he said, “it does contribute to long-term problems of inadequate soil life and poor soil health.”
Soil testing is essential and provides a snapshot showing pH, CEC, plant-available phosphorus, calcium, potassium, magnesium and some micronutrients. Most soil tests also show percent of organic matter. Schonbeck said many farmers are relying on additional tests to fine-tune their nutrient program. “Standard soil tests don’t usually report soluble nitrogen,” he said. “Every time it rains, that drops. When plants take it up, it drops sharply. During a period of moist but not saturated or leaching conditions, or after fertilizer application, soluble nitrogen will go way up. However, there are stages in crop development when it’s very useful to take a soil nitrate test, such as a pre-sidedress or preplant nitrate test for corn or vegetables.”
The diverse soils and climate of the western region of the U.S. present challenges for organic growers who want to manage nutrients to meet organic standards. Schonbeck described the Maritime Pacific Northwest, which includes Oregon and Washington west of the Cascades, as having fertile soil that’s easy to work. “The biggest challenge is a lot of rain in winter,” he said. “If there’s leftover nitrogen from production, it’s subject to leaching and impacts water quality, and you lose the benefit of nitrogen for the next crop.”
The Mediterranean climate of much of California and the western half of Washington and Oregon is characterized by winter rains and extended dry periods in summer. Schonbeck said in regions with a fairly cold climate, most of the winter precipitation is snow, with potential nitrogen loss during snow melt. He added that volcanic soils (andisols) in parts of the Cascades and parts of Idaho are high in organic matter and have good fertility and excellent structure.
In semiarid zones of Montana, Wyoming and Colorado, more of the limited rainfall occurs in summer. In northern parts of the interior west where precipitation is 10 to 15 inches a year, prairie soils (mollisols) are fertile and high in organic matter. “They tend to be alkaline and similar to the andisols in that high calcium levels tend to tie up phosphorus,” said Schonbeck. “Cover crop and crop rotation options are limited by moisture availability.” Schonbeck said NRCS studies in eastern Washington and SARE research in Montana are aimed at cover crop management for long-term soil benefits.
“Things get really challenging in regions where there’s less than ten inches of rain a year,” said Schonbeck. “These desert soils (aridisols) tend to be saline or alkaline, and they can be shallow or poorly structured.” Production on such soil is dependent on irrigation, which means potential water quality challenges with salinity.
Schonbeck said nitrogen is the most challenging nutrient in organic systems. “Organic crops are often nitrogen-limited because the grower is dependent on soil biology to cycle nitrogen and release it to crops in a timely manner,” he said. “If soil life is depleted because you’re transitioning from conventional to organic or you’re entering a new field where soil health isn’t good, there will not be as much nitrogen from soil life and you’ll need to add more.”
Both reduced tillage and high-biomass cover crops benefit soil health. Studies have shown the greatest soil health benefits are in organic systems where cover cropping and reduced tillage are maximized to the greatest extent practical. “But very often, this reduces plant available nitrogen (PAN),” said Schonbeck. “It tends to slow mineralization unless you’re growing a pure legume cover crop. Even snap beans, which fix some of their own nitrogen, can find themselves short of nitrogen if they’re grown in residue that’s high in carbon.”
Broccoli is a very heavy feeder and presents unique challenges. Washington and California studies in both maritime and semi-arid climates focused on broccoli yield increases after organic nitrogen application. While these nitrogen sources are expensive, broccoli yield pays for it. “Each pound of feather meal nitrogen at a cost of more than $6/pound increased broccoli yield from 11 to 88 pounds at a market value of $2.50/pound pays for itself several fold,” he said. However, high application rates resulted in high soil nitrate and potential for leaching.
A northern California study examined optimizing nitrogen in an organic strawberry rotation. Broccoli was harvested and the remaining crop residue (leaves and stems) added substantial nitrogen. “In addition to the broccoli leaving quite a bit of residual inorganic nitrogen in the soil, there was a further jump as the broccoli residue broke down,” said Schonbeck. “Meanwhile, strawberries are planted in late fall but they don’t really get going in terms of growth and nutrient consumption until mid-spring. By the time strawberries needed nitrogen, the soil nitrate level was quite low and strawberry yields are nutrient-limited.”
Schonbeck discussed the role of cover crops in organic nutrient management. “Deep rooted cover crops will retrieve subsoil nitrogen before it leaches and is lost,” he said. “Legumes and buckwheat enhance the availability of phosphorus while grasses enhance the availability of potassium.” When levels are below optimum, cover crops make nutrients more available to following crops. However, with optimal or excessive phosphorus or potassium, the cover crop won’t aggravate that excess because it recycles.
After reviewing the priorities for western organic production, Schonbeck said there are still questions about which measures will build soil health and fertility in arid and semiarid regions. Optimizing cover crops and crop rotations along with fine-tuning organic fertilizer recommendations for diverse crops and climates will contribute to better soil life and crop yields. Plant breeding for nutrient efficiency and effective association with mycorrhizae and other soil organisms is also worth studying.
“Particularly interesting is tightly coupled nutrient cycling,” said Schonbeck. “Some gene-regulated enzymes in the tomato root were found to be playing a role in the root to soil microbe communication that promoted this high cycling. Those genes seem to be present in many plants and plant families, so there’s good potential to expand this concept and develop practical applications for dryland soils and climates.”