Four certified organic producers in the eastern Klamath Basin of Oregon want to refine their understanding of managing soil fertility so they can optimize their potato crop yield and quality and, at the same time, support organic soil quality and biological diversity.
Yields for organic potato growers sometimes fall short of expectations because there’s not enough mineralized nitrogen available during tuber bulking, a critical phase of crop development. In organic farming, as in conventional farming, over- or under-fertilizing can harm tuber quality, farm profitability, soil health and water quality.
To address those fertility concerns, project coordinator Woody Deryckx of the Klamath Basin Organic Farm Improvement Association developed a SARE-funded project that would test composted chicken litter and industrial rapeseed meal. The project is part of a long-term series of field trials to determine optimal organic methods and systems for employing fertilizer, green manure, crop rotations and disease management.
Participating farmers, Richard Rajnus and Adolph Drazil, each chose the rate of application and the layout for test plots. The amendments were applied in the spring on standing green manure crops, mainly cereal rye. The nutrition of the potato crop was monitored with petiole sampling and analysis. One of the studies included a bag test to measure nitrogen mineralization.
The participants analyzed the test plot soil to gauge the effects of the chicken litter and rapeseed meal on soil ecology. They also analyzed soil from a neighboring farm that used conventional methods, including application of Telone II, to compare it with the test plots for the rapeseed’s effects on nematodes.
All treatments provided enough nitrogen nutrition to maintain healthy vines and provide good yields of Legend Russet and Russet Burbank potatoes. Petiole nitrogen levels were within acceptable levels all season and peaked in mid August in all fields. On Richard Rajnus’ field, compost at 5 tons per acre provided vigor and higher yield than 2,000 pounds of rapeseed meal per acre, but the differences were modest and the trial lacked replication.
In the mineralization study, 4,000 pounds per acre of rapeseed released 27.7 parts per million of nitrate nitrogen in 70 days. The compost at 7 tons an acre mineralized an average of 18.4 ppm. However, the distinction had little impact on tissue analysis results or gross yield.
The research found large differences in biological activity between the organic treatments and the neighboring conventionally managed field, a difference the participants will continue to assess, along with differences in nitrogen nutrition between the two management styles.
Experiments like this can help producers, both organic and conventional, find fertility treatments that will provide not only improved yield and quality in their potatoes but enhance soil biology and reduce nitrogen leaching.
FARMER ADOPTION AND DIRECT IMPACT
As this is an ongoing study that will further test the impact of varying soil amendments on different cultivars, farmer adoption of the practices will likely occur only after enough data have been collected to offer recommendations.
FUTURE RECOMMENDATIONS OR NEW HYPOTHESES
In the future, says Deryckx, studies will pay attention to the quantitative relationships between the green manure cover crops and the applied organic nitrogen sources to find the ideal nitrogen mineralization curve for potato crops. The researchers also intend to compare fertilized plots with unfertilized control plots as well as intensify the bag tests for mineralization comparisons. The research will add different potato cultivars to assess responses to the amendments and look at non-chemical control for corky ringspot, which is emerging as the most severe threat to quality potato production.
DISSEMINATION OF FINDINGS
The results of these studies were presented at the Ecofarm Conference in Monterrey, Calif., Jan. 20, 2000, and in a poster at the Western SARE conference in Portland March 7-8, 2000. A field day is planned to present the results compiled after two years of study.