Completion of Soil Fertility Paradigms Evaluated through Collaboration On-Farm and On-Station
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. Data analysis continues under this, the second of two SARE grants.
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.
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 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 10 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.
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.
Impacts and Contributions/Outcomes
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.
Organic Cropping Specialist
Department of Horticulture
147 Horticulture Hall
Iowa State University
Ames, IA 50011
Office Phone: 5152947069
USDA National Soil Tilth Laboratory
Iowa State University
Ames, IA 50011
Office Phone: 5152943336