A Final Report submitted to the Organic Farming Research Foundation, December 2000

Mark Schonbeck, Virginia Association for Biological Farming
439 Valley Drive NW, Floyd, VA 24091

Project Summary

The purpose of this project is to determine whether an unfavorable balance among soil potassium (K), sodium (Na), calcium (Ca) and magnesium (Mg) might be limiting vegetable production on some organic farms in the southeastern United States. These cation (positively charged) nutrients are held in exchangeable (plant-available) form on the soil's negatively-charged clay and humus particles. The Albrecht formula, utilized by "eco-farming" consultants and by many private soil test labs, recommends that the soil's cation exchange capacity (CEC) be occupies by cations in approximately the following ratios: Ca 65-75%, Mg 10-15%, K 2-5%, Na 0.5-3%, and hydrogen (H, or acid) 10-15%. This is called "base saturation ratio."

In Virginia and other southeastern states, many cultivated soils show Ca base saturation below 65%, with Mg and/or K well above their recommended ranges. This imbalance is believed to "tighten" the soil and degrade crumb structure, hamper aeration and drainage, cause surface crusting and hardpans, inhibit beneficial soil organisms and humus formation, aggravate weed, pest and disease problems, and hurt crop and livestock health. Applications of high calcium lime or gypsum to restore the balance are claimed to correct these problems; to enhance soil biological activity and organic matter levels; to increase availability of phosphorus (P), nitrogen (N) and other nutrients; and to improve produce flavor, nutritional value and shelf life.

Between 1998 and 2000, on-farm studies and a literature review were conducted to evaluate some of these claims. Field trials were conducted on five organic vegetable farms in Virginia and eastern Tennessee to determine whether applying Ca amendments to soils showing high Mg (average 24%) and slightly low Ca (average 63%) would benefit soil tilth, soil life or marketable vegetable yields. Treatments (low-Ca = control; high-Ca = Ca amendments applied four times between summer 1998 and summer 2000) were replicated three times at each site.

The high-Ca treatment shifted the average base saturation ratio to 70% Ca and 18%Mg, and slightly reduced K saturation. Vegetables grown on the amended soil had slightly higher foliar Ca and lower Mg levels. However, the high-Ca treatment had no detectable effect on soil organic matter, biological activity, crop uptake of N, P and micronutrients, abundance of weeds, incidence of disease or insect pest damage, or Brix (percent soluble solids, an index of produce quality) in broccoli or tomato. Broccoli yielded about 11% more in the high Ca treatment in the 2000 season, whereas treatment effects on tomato and squash yield have been inconsistent.

Soil bulk density (degree of compaction), moisture content, water infiltration rate (indicates drainage and porosity) and soil strength (resistance to root growth) were measured in fall 1999, and spring and fall 2000. When results from the five sites were averaged, the high-Ca treatment did not seem to affect any of these soil attributes. However, when results were considered site by site, some possible trends emerged at two sites. On a clay-loam soil in the Blue Ridge foothill region, the high-Ca treatment seems to have improved water infiltration and slightly loosened a pre-existing subsurface hardpan. Conversely, the high-Ca treatment apparently tightened hardpan and slowed water infiltration on a Tidewater sandy loam.

Two creek-bottom loam soils in the Appalachian region showed the "worst" base saturation ratios (Mg 27-28%, Ca 58-59%), but the best physical properties: low bulk density, high porosity and moisture-holding capacity, and no hardpan in the top 24 inches. Tilth has remained excellent regardless of Ca treatment. The clay soil at the fifth study site showed only a moderate Ca-Mg imbalance, but had a severe hardpan at 4 to 12 inches, and drained slowly after heavy rain. Ca applications have brought base saturation ratio close to the Albrecht formula, but have not improved soil tilth or loosened the hardpan.

A review of over 100 published studies and conversations with several soils consultants revealed evidence that proper cation balancing is inherently site specific. Most soils apparently do not need to conform to the Albrecht formula to be healthy and productive. Sandy Tidewater soils actually require somewhat higher Mg and K saturation for optimum crop nutrition. Some soil scientists warn that growers may be spending money and natural resources for lime or gypsum applications that their soils do not need. Both vegetable and agronomic crops thrive at a wide range of Ca and Mg levels, and soil tilth deteriorates only at extremely high Mg levels. However, too much soil K relative to Ca and Mg can tighten some soils, upset plant nutrition, and increase susceptibility of vegetable crops to some diseases and physiological disorders. Excessive soil K is a common problem in intensive vegetable production (both organic and conventional), and is often related to heavy use of off-farm inputs.

The current project was initially undertaken to validate the Albrecht formula in organic production in the southeastern US. Findings to date have led to a shift in focus toward developing a holistic, site-specific and resource-conserving approach to soil nutrient balancing. Often, growers can remedy cation balance by reducing inputs (e.g. excess K), and may not need Ca amendments to "correct" high soil Mg if soil and crops are already healthy. Information sheets have been developed to help to growers understand the cation balancing controversy, and to implement site-specific, cost-effective cation nutrient management on their own farms. Project findings were presented at three farm field days and one regional sustainable agricultural conference in 2000, and will be presented again at two more conferences in early 2001.

Additional funding will be sought to evaluate possible longer-term effects of the calcium treatments on soil health and marketable vegetable yields, and to refine cation nutrient management strategies.

Acknowledgements

This project has been conducted between 1998 and 2000 with support from the Organic Farming Research Foundation of Santa Cruz, Ca, and from the USDA Southern Region Sustainable Agriculture Research and Education (SARE) Producer Grant Program. The project also received an in-kind match from Soil Foodweb Inc. of Corvallis, OR, which conducted soil biology analyses for half the normal fees.

I would like to thank the following growers who participated in the field trials: Charlie and Miriam Maloney, Bo Holland, Ron Juftes and Polly Hieser, Joy Legendre, Ann Shrader, Cathy Guthrie, Chris Rasman, Ellen Polishuk and Hiu Newcomb. I am also grateful to Dr. Ray Weil, Professor of Soil Science at University of Maryland; Dr. Elaine Ingham soil micro-ecologist at Soil Foodweb, Inc; Steve Diver, Technical Specialist at ATTRA; and Margaret Merrill, reference librarian at Virginia Tech for technical assistance and support.

Mark Schonbeck, Project Coordinator