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

Final Report for LNC98-138

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
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Project Information


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.


Click linked name(s) to expand/collapse or show everyone's info
  • Kathleen Delate
  • Douglas Karlen
  • Richard Thompson


Materials and methods:

M&M for Objective 1. Our process included a range of stakeholders, including three from the Iowa Organic Crop Improvement Association (OCIA), five from Practical Farmers of Iowa (PFI), and several unaffiliated. (Memberships also overlap.) Representatives of OCIA and an organic marketing co-operative have given presentations at field days organized through the project. Project team members include Mr. Keith Cuvelier, a crop consultant who makes recommendations based on the CR approach to fertility. Both the ISU team members and Cuvelier have participated in field days.

M&M for Objective 2. A total of nine private farms and two Iowa State University outlying farms were involved in the study. The only part of Iowa not represented by sites was the southeastern part of the state. The two ISU farms were in southwest Iowa and central Iowa. Based on soil tests done in the fall of 1998, in the spring of 1999 fertility treatments were implemented according to the two paradigms. Field-scale plots were established in order to enable farmer participation in harvesting. Nine of the 11 sites had six replications, one site had seven replications, and one had five. For various reasons, three of the 1999 cooperators did not participate in 2000, so an additional three cooperators were recruited to the project.

Research results and discussion:

Amendments and Input Costs
The following tables (Tables 1-3, Appendix) show the types of inputs used in each treatment and farm in the three years of field work. Amendments and rates varied from site to site and from year, since recommendations were based on annual soil tests and the philosophy that was operant in each treatment. The tables also show the costs minus delivery for each treatment. Total fertilizer costs are shown as are costs specifically for lime, calcium, potassium, and zinc. Total fertilizer here consists of all amendments applied through the study and does not include manure and some nitrogen applied to the whole field. In the interest of crop production, the project did facilitate application of a certain amount of phosphorus fertilizer of either organic or synthetic origin; however, an attempt was made to keep rates the same across the two treatments.

Input cost was the single strongest distinguishing feature between the two treatments of this study. As Figure 1 (Appendix) indicates, the average difference in input cost between the treatments over the study equaled $10.42 per acre; with the CR approach averaging $9.61 more in fertilizer and $0.81 more in lime cost. In fact, theses figures are based on local prices for limestone. In some parts of Iowa, calcitic limestone is not locally available, and producers can expect to pay approximately $0.15 per ton per mile additional to transport this material from another community.

Soil Mineral Analysis
Tables 5 and 6 in the appendix show soil mineral analyses for 2001 and over the whole period of the study, respectively. Because treatment effects may not evidence themselves immediately, data from the last year of the project are shown separately. Eighteen parameters were analyzed, and the tables show an almost random pattern of treatment responses. These will be subjected to multivariate analysis to test for relationships with other data.

Soil Analysis Over Years
In general, the soil tests most readily show the effect of zinc fertilization in the CR treatments; all farms but #6 showed a zinc treatment effect that approached statistical significance (Table 2, Appendix).

Four of 11 farms showed significantly greater soil sulfur levels in the CR treatment; each of these farms received calcium sulfate (gypsum) as part of the CR treatment. Elevated calcium on the soil cation exchange of four farms is likewise attributable to applications of calcium sulfate and/or calcitic limestone. Three of these four farms also yielded elevated soil tests for manganese. In each of these three cases, elevated manganese in the SLAN treatment was accompanied by a lower soil pH.

Two farms showed higher soil tests for potassium in the CR treatments than in the SLAN plots. On Farm #6, fertilizer 0-0-60 was applied to the SLAN plots at almost double the rate of the CR plots; however, lime rates were much greater in the CR, and this may have affected soil test values for potassium. On Farm #11, limestone was applied at equivalent rates in the two treatments, and soil test potassium reflects the application of 0-0-60 in the CR treatment.

Both Farms #6 and #11 showed higher levels of available soil phosphorus in the CR treatment. However, the Farm #6 experiment received no phosphorus fertilizer, and the P fertilizer rate on Farm #11 was uniform across the experiment.

Soil Analysis 2001
Soil analysis trends in 2001 were similar to those from other years (Table 5, Appendix). Ten of 11 farms displayed elevated soil zinc in the CR treatments; the one that did not was the only farm not to receive zinc as an amendment. There was an overall significant treatment response in soil sulfur, and the two farms with significant differences had received calcium sulfate. On two of the three farms showing significant differences in potassium, application rates were the likely cause. The third farm (#2) lies in the western part of the state, where soil potassium levels are adequate, and no potassium fertilizer was ever applied to either treatment.

Weed Biomass
Weed biomass was collected because it has sometimes been asserted that soil calcium/magnesium ratios affect certain weeds, perhaps through soil structure effects. The Appendix provides broadleaf and grassy weed biomass in the trial, with Table 7 showing 2001 results for each farm and Table 8 showing results combined over years. Data generally appear only if the difference between treatments was significant at close to the ten percent level of significance.

In 2001, three of eight farms exhibited differences in grassy weed biomass. In two of these, weed biomass was greater in the SLAN treatment, while in the third, grassy weed biomass was greater in the CR plots. This farm (# 11) also showed a significant difference in grassy weed biomass when the data were analyzed across the years of the study. No other site showed such a difference in grassy weeds. However, Farm #1, which only participated in year one, yielded significantly grater broadleaf weed biomass in the SLAN treatment.

Grain Quality Analysis
Grain samples were analyzed for 16 parameters related to feed quality (Tables 9 and 10, Appendix). In general, grain quality was uniform across the treatments. On Farms #2, #6, and #11, however, crude fat differed somewhat between the CR and the SLAN treatments. On two farms crude fat was greater in the CR treatment, and on the third farm it was greater in the SLAN treatment. In both the 2001 data and the analysis overall, comparison of nutrient levels among the three categories of grains (corn, soybean, and small grain) shows no consistent trend for zinc, calcium, magnesium, or potassium, the nutrients most closely related to the treatments.

Crop Leaf Tissue Analysis
Tables 11 and 12 in the Appendix provide overall and by-farm values for 12 leaf nutrients for 2001 and over the whole study, respectively. It should be noted that the overall data are often averages of two or three different crops. Neither the overall results nor those from 2001 show a strong association with the amendments associated with the two treatments. In 2001, three of eight farms did show elevated leaf potassium in the CR treatments; in two of the three this is consistent with fertilizer rates applied, and in the third (#6) a high rate of lime in the CR may have increased potassium availability.

In two of 11 farms overall and in three of eight in 2001, leaf magnesium was lower in CR treatment crops; however leaf Mg was higher on one farm in 2001. Two of eight farms in 2001 and two of 11 in the study overall showed elevated leaf iron in the Cr treatment; however, in 2001 two other farms yielded higher leaf iron levels in the SLAN treatment. Zinc levels were significantly higher in the CR treatment on only one farm in the overall analysis and on no farms in 2001.

Grain Yield
Table 12 provides grain yields by form for each year of the study. There were four site-years in which the CR treatment was associated with yields that may have been statistically greater then SLAN, and there was one site-year in which the reverse was true. Farm #6 saw greater yields in the CR two years of three. This was the farm in which soil potassium and phosphorus levels were higher in the CR treatment, but levels were in the adequate range throughout the field. Additional analysis is needed to examine the factors associated with these four site-year yield differences.

Research conclusions:

The sustainable agriculture movement is discovering new relationships and mechanisms relating to pest management, systems functioning, weed dynamics, and so on. Soil is an area of great interest to practitioners pursuing sustainability. A number of models, or paradigms, have been proposed to describe the nutrient-supplying functions of soil. Each paradigm has its proponents, and industries have grown up to supply materials and testing services consistent with each model. Farmers are frequently "caught in the middle," not knowing which approach will best serve them in their pursuit of profitability and sustainability.

Given the cogency of this question for sustainable agriculture, it is perhaps surprising that this study is only the second SARE project to address the issue at all, and the first in field crops. This project was designed to generate reliable comparative data regarding the "sufficiency" approach to soil fertility developed and recommended by university research, and the "ratio" approach used by a number of crop consultants and testing laboratories. The project was also intended to develop a dialog on the topic involving farmers, scientists, and information providers on both sides of the question. In accomplishing these two objectives, the project is helping sustainable producers to supply their crops with needed nutrients in a more cost-effective manner and is increasing their capacity to make effective management decisions for their farms.

Several studies in Iowa have shown that a decade of cropping is necessary to deplete soils testing high in potassium and phosphorus to the point that additional fertilizer provides a yield increase. Nine of 11 farms tested high in soil potassium when they entered the study, and none tested below optimum. In the course of this three-year study, it was not possible to determine whether both treatment approaches would provide crops adequate potassium over the long term. There was a non-significant trend for crop yields to be greater in the CR treatments, but this trend is not easily ascribed either to the quantity of nutrients present or to their proportions.

As stated earlier, analysis is continuing on the agronomic data. Soil microbial biomass and organic matter fractionation are still in process and not yet available. The data presented here have been subject to univariate analysis. Because of the number of variables and site-years involved, some results would come up significant by chance alone. On the other hand, strict adherence to a five percent or a ten percent confidence level could lead one to overlook broad trends in the data. For this reason we look forward to subjecting the dataset to multivariate analyses. We hope thereby to identify patterns in the results that would not be found by a series of single-variable analyses. This work will proceed under the second of the two grants received in this SARE research project.

Economic Analysis

As discussed above, input costs differed by $10.42 per year between the two treatments of the study. While there were few significant yield differences on participating farms, taken as a whole, yields averaged somewhat higher in the CR treatments (1.8 bushels per acre in corn and small grains, 0.9 bushels per acre in soybean: Table 4, Appendix). Notwithstanding the lack of statistical significance, if these yields are taken to be treatment effects, then at current prices, these yields make up for somewhat less than half of the additional input cost incurred in the CR treatment.

Farmer Adoption

We did not anticipate an immediate change in farmer practices as a result of the research or the 13 field days carried out as part of this study. We do look forward to increased discussion of these soil fertility issues in the producer community, and as data analysis reaches completion under the second SARE grant, we will pursue additional opportunities to foment this discussion at workshops, field days, and conferences.

Participation Summary

Educational & Outreach Activities

Participation Summary:

Education/outreach description:

Five of the eight participating farms hosted farm field days in 1999, four did so in 2000, and four held field days in 2001, with total attendance of more than 450. At these events project collaborators and farmer cooperators describe the project to those in attendance. Follow-up information has been provided at the annual winter conference of Practical Farmers of Iowa, in the organization's quarterly newsletter, The Practical Farmer, and ultimately will appear on the organization's Website, http// When data analysis is complete, collaborators will also disseminate project findings through professional meetings and publications.

Project Outcomes


Areas needing additional study

Considerable research has taken place in controlled settings on these soil fertility paradigms. While there may be remaining questions that demand such research, the greater need is in the realm of farm systems management. Producers are more interested in outcomes that they can see and count than on theories. Producers need to be involved in the process of deriving this management information, both so that it is credible to them and to build their capabilities. Additional projects, in different cropping systems and in different regions, will shed further light on the question and cultivate farmer's ability to manage soil fertility intelligently and sustainably.

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.