Completion of Soil Fertility Paradigms Evaluated through Collaboration On-Farm and On-Station

Project Overview

Project Type: Research and Education
Funds awarded in 2001: $20,288.00
Projected End Date: 12/31/2003
Matching Federal Funds: $22,067.00
Matching Non-Federal Funds: $12,357.00
Region: North Central
State: Iowa
Project Coordinator:
Dr. Rick Exner
Practical Farmers of iowa

Annual Reports


  • Agronomic: corn, oats, soybeans


  • Crop Production: nutrient cycling, organic fertilizers, application rate management
  • Education and Training: demonstration, farmer to farmer, on-farm/ranch research, participatory research
  • Farm Business Management: budgets/cost and returns
  • Pest Management: weed ecology
  • Soil Management: organic matter, soil analysis, soil quality/health


    On-farm and on-station, we compared economic and agronomic effects of two approaches to soil fertility – the cation ratio paradigm (CR) and $B!H (Bsufficient level of available nutrients $B!I (B (SLAN). Treatments consisted of soil amendments. Crop leaf tissue nutrients were less responsive to treatments than were the soil test values; grain quality, soil organic matter, and weed biomass were least responsive. Yields averaged slightly greater in CR, which may reflect higher rates of nutrients rather than the types of amendments used. Input costs, however, averaged $10.42 per acre greater in the CR treatment than in the SLAN treatment.


    Within sustainable agriculture, two contradictory approaches to soil fertility uneasily coexist – the cation ratio paradigm (CR) and the one referred to as $B!H (Bsufficient level of available nutrients $B!I (B (SLAN). SLAN proponents, and this now includes all of the U.S. land grant universities, concern themselves with whether the soil contains enough of each nutrient in forms that are available to the crop. In contrast, the CR approach looks not at the gross amounts of available nutrients but the proportions in which they are represented on the soil cation exchange. The SLAN and CR approaches can result in very different recommendations for a producer. By way of illustration, consider a Marshall series soil in western Iowa on which an organic farmer planned to raise corn. The table below shows soil test levels for several nutrients, a CR-based recommendation for this field, a SLAN-based recommendation, and costs associated with each. [Table here in hard copy] There is little communication between the two approaches because they use different terms and conceptualize fertility differently. It is the farmer who is forced to integrate these two information streams and make the financial judgements required in farm management. Sustainable agriculture must deal constructively with this schism if it is to grow in credibility and relevance. Literature Review To many people, there may seem only a fine distinction between the CR and the SLAN approaches to soil fertility. In fact, these acronyms represent very different models of soil-crop relations. Many crop nutrients in their plant-available forms are positively charged (cations). This includes potassium, calcium, magnesium, and some of the micronutrients. These cations are attracted to negatively charged clay and organic matter, which constitute a major available reserve of those nutrients. The ability of a soil to retain positively charged species in forms exchangeable with the soil solution is quantified as soil $B!H (Bcation exchange capacity $B!I (B (CEC) (Tisdale et al., 1985). In general, high CEC is a desirable quality in a soil, since it means a greater total reserve of cation nutrients is available to plants. The SLAN and CR schools regard this information in very different ways, however. The SLAN approach draws from work begun in the 19th and early 20th Centuries, with the concepts of limiting factors (Von Liebig), a crop fertilizer response asymptotic to a theoretical maximum (Mitscherlich), and a nutrient sufficiency level (e.g. Macy) (Macy, 1936; Tisdale, et al., 1985). SLAN proponents, and this now includes all of the U.S. land grant universities, therefore concern themselves with whether the soil contains enough of each nutrient in forms that are available to the crop (Rehm, 1994; Voss et al., 1996). In contrast, the CR approach looks not at the gross amounts of available nutrients but the proportions in which they are represented on the soil cation exchange. Beginning in the 1940s with research by Bear and associates in New Jersey (Bear, et al., 1945) and continuing through writings by Albrecht (1975), Skow and Walters (1995) and others, the concept has developed of an apparent golden mean of nutrients in a $B!H (Bbalanced soil. $B!I (B The ideal proportion of nutrients on the cation exchange is believed to be 65-85% calcium, 6-12% magnesium, and 2-5% potassium (Graham, 1959). The cation ratio adherents are not impressed by the argument that a particular soil cation ratio can result in wildly different amounts of available nutrients in soils of different cation exchange capacity, nor that the amendments based on CR may have their greatest nutrient effects through altering soil pH. CR proponents, including several commercial testing labs, respond that the ratio approach is efficacious, and that, conversely, the real benefit of liming is often from the addition of calcium (Albrecht, 1975). Moreover, this conceptual model resonates with many practitioners in the sustainable agriculture community who are drawn to the ideal of farming in concert with the natural order. Literature Cited in This Report Albrecht, W. A. 1975. (C. Walters, Jr. ed.) The Albrecht papers. Acres USA, Kansas City, Mo. Bear, F.E., A.L. Prince, and J.L. Malcolm. 1945. The potassium needs of New Jersey soils. New Jersey Agric. Exp. Stn. Bull. 721. Cambardella, C. A., and Elliott, E. T. 1993. Methods for physical separation and characterization of soil organic matter fractions. Geoderma. 56:449-457. Macy, P., 1936. The quantitative mineral nutrient requirements of plants. Plant Physiol. 11:749-764. Rehm, G. 1994. Soil cation ratios for crop production. North Central Regional Ext. Pub. 533. Minnesota Ext. Serv. Rice, C. W., Moorman, T.B., and Beare, M. 1996. Role of microbial biomass carbon and nitrogen in soil quality. In: J.W. Doran and A.J. Jones (ed.) Methods for Assessing Soil Quality. Soil Sci. Soc. Am. Spec. Publ. No. 49, SSSA, Inc., Madison, WI. Rzewnicki, P.E., R. Thompson, G.W. Lesoing, R.W. Elmore, C.A. Francis, A.M. Parkhurst, and R.S. Moomaw. 1988. On-farm experiment designs and implications for locating research sites. Am. J. of Alternative Agric. 3(4):168-173. Shapiro, C.A., W.L. Kranz, and A.M. Parkhurst. 1989. Comparison of harvest techniques for corn field demonstrations. Am. J. of Alternative Agric. 4(2):59-64. Skow, D. and C. Walters, Jr. 1995. Mainline farming for century 21. Acres USA, Kansas City, Mo. Tisdale, S.L., W.L. Nelson, and J.D. Beaton. 1985. Soil fertility and fertilizers. 4th ed. Macmillan Publishing, New York. Voss, R.D., A.P. Mallarino, and R. Killorn. 1996. General guide for crop nutrient recommendations in Iowa. Iowa State Univ. Coop. Ext. Serv. Bull. Pm-1688. Yoder, R. E. 1936. A direct method of aggregate analysis of soils and a study of the physical nature of erosion losses. J. Am. Soc. Agron. 28:337-351.

    Project objectives:

    1. Initiate a process with producers to compare the economic and agronomic consequences of two philosophies of soil fertility, the sufficiency level of available nutrients (SLAN) approach and the cation ratio (CR) approach.

    2. Implement a series of side-by-side, on-farm and on-station comparisons of the two soil fertility management strategies, with both regimens accurately and credibly represented.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.