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

Project Overview

Project Type: Research and Education
Funds awarded in 1998: $59,027.00
Projected End Date: 12/31/2002
Matching Federal Funds: $40,300.00
Matching Non-Federal Funds: $25,956.00
Region: North Central
State: Iowa
Project Coordinator:
Dr. Rick Exner
Practical Farmers of iowa

Annual Reports


  • Agronomic: corn, oats, soybeans


  • Crop Production: organic fertilizers
  • Education and Training: demonstration, farmer to farmer, on-farm/ranch research, participatory research
  • Farm Business Management: budgets/cost and returns
  • Production Systems: general crop production
  • Soil Management: organic matter, soil analysis, nutrient mineralization, soil quality/health


    Stakeholders compared the economic and agronomic consequences of two contradictory approaches to soil fertility – the cation ratio paradigm (CR) and “sufficient level of available nutrients” (SLAN). A total of nine private farms and two ISU farms participated. Soil mineral parameters were most sensitive to soil amendments. Crop leaf tissue nutrients were less so. Grain quality and weed biomass were least affected by treatments. Yields averaged somewhat greater in the CR treatment; however, input costs averaged $10.42 per acre greater in the CR treatment than in the SLAN treatment. Analysis of the data is continuing under a second SARE grant.


    State land grant universities regard fertility in terms of “sufficient levels of available nutrients” (SLAN), while many crop consultants and a number of private laboratories give primacy to the proportion of cation nutrients on the soil cation exchange (“cation ratio,” or CR). Because of the dearth of communication between these two camps, farmers are often left with no objective way to choose which approach to use in fertilizing for crop production. Our objectives in this study were to implement both approaches accurately and credibly in side-by-side comparisons and, involving stakeholders on both sides of the question, to evaluate the economic and agronomic consequences of these two philosophies.

    A paired-comparison study was initiated at eight Iowa sites evaluating the economic and agronomic outcomes of fertilization according to these two competing paradigms of soil fertility. The project involved in total nine private farms and two outlying farms of Iowa State University. At each site approximately six replications of two treatments were implemented. The treatments, representing the CR and SLAN approaches, varied with the soil analysis and the farming preferences of the individual producer. Four of the nine of the private sites were in organic production, one is transitional to organic, and four were sustainable variations of conventional production. After year one of the project, three producers transitioned out of the study and three new ones were included; each year eight sites were involved. The project monitored the following crop and soil quality parameters in the two comparison systems: leaf tissue (12 nutrients), grain (crude protein, crude fat, crude fiber, ADF, TDN, net energy, and five minerals), biomass of broadleaf and grassy weeds, soil aggregate stability, bulk density, soil particulate organic matter and microbial biomass, soil P1, K, Mg, Ca, S, Zn, Mn, Fe, Cu, B, OM, pH, and buffer pH.

    SARE grant LWF 62-016-03806 was funded for two years of study. SARE subsequently funded a third year of study through LNC 01-198.1. The proposal for the third year stipulated that funds remaining from the first grant would be applied toward year three costs. Data analysis are continuing under the second grant and may detect patterns not discernable through the univariate analysis so far employed.

    Statistical analysis of soil mineral status, grain yield and quality, crop leaf tissue nutrients, and weed biomass show that the parameters most sensitive to treatment effects are those directly related to the nutrients contained in the soil amendments – zinc, calcium, and potassium. Soil mineral status showed the greatest treatment effect, crop leaf nutrients were somewhat less sensitive to treatment, and grain quality and weed biomass were least affected. Soil microbial biomass and organic matter fraction data will complete the picture, and multivariate analysis will help us identify patterns of treatment response.

    Yield results were equivocal. Although few site-years gave significant treatment effects, yields did average higher in the CR (cation ratio) treatments by approximately 1.8 bushels in corn and soybeans and by 0.9 bushels in small grains. It is not clear whether this can be ascribed either to the quantity of fertilizer nutrients present or their relative proportions. The CR philosophy also led to higher input costs; expenditures on amendments averaged $10.42 per acre greater due to applications of calcium, zinc, potassium, and lime that were deemed unnecessary by the SLAN approach. The breakeven grain prices for this additional input cost would be $5.66 per bushel for corn, $11.58 per bushel for soybeans, and $5.85 per bushel for small grains. Some organic producers could approach these prices, but most farmers can not.

    The lack of a strong or consistent treatment effect in grain quality or weed biomass suggests that if the CR treatment did increase yields, it could have been due to “sufficiency” responses rather than effects on the cationic nutrient balance. Analysis is continuing.

    This study took place in the public eye, with field days, meetings, print and electronic publications employed to stimulate discussion of the farm management issues involved. Producer discussions examined the merits of these two paradigms of soil fertility and fertilization. The project was designed to bring out the agronomic and economic consequences that farmers are likely to experience as a result of pursuing these two approaches.

    Project objectives:

    1. Initiate a process, one involving stakeholders on both sides of the question, to compare the economic and agronomic consequences of two contrasting philosophies of soil fertility, termed here the sufficiency (SLAN) approach and the cation ratio (CR) approach.
    2. Implement a series of side-by-side comparisons of the two management styles, with both approaches 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.