Sustainability is a term that is being used more and more in relation to soybean production in the U.S. In fact, it is now an integral component of U.S. soybean production as outlined in the Soy Sustainability Assurance Protocol (SSAP) that was compiled by the American Soybean Association and the United Soybean Board working with the U.S. Soybean Export Council. This document contains guidelines that will support SSAP certification of soybeans that are produced in the U.S.
An oft-mentioned aspect of production sustainability is soil health (also referred to as soil quality), which is defined by the USDA-NRCS (click here) as “the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans”. They further state that “this definition speaks to the importance of managing soils so they are sustainable for future generations”.
Understanding soil health involves assessing and managing a soil’s inherent properties of fertility, structure, microbial activity, etc. so that it functions to support optimal plant growth, both now and in the future. This means that changes in soil health must be constantly monitored so that soil is not degraded, but rather is managed using a set of practices that are both sustainable and promote soil sustainability for the long term.
Soil is an ecosystem that not only holds and provides nutrients and water for plant growth, but also provides habitat for soil microbes that are an integral part of the soil’s interaction with plants that are growing in it. These microbes are key to decomposition of chemical and plant residues and resultant nutrient cycling, as well as being integral to soil components that affect soil structure, aeration, porosity, and water holding capacity.
Microbial activity is an integral part of soil health. As stated in the overview of the USDA-NRCS soil biology primer, “The creatures living in the soil are critical to soil health. They affect soil structure and therefore soil erosion and water availability. They can protect crops from pests and diseases. They are central to decomposition and nutrient cycling and therefore affect plant growth and amounts of pollutants in the environment.” Other chapters of this primer give additional detail about the living component of soil and how it contributes to agricultural productivity and sustainability, and air and water quality. The USDA-NRCS has a playlist of videos that cover all aspects of soil health. They also have numerous articles on the various aspects of soil health that can be accessed here.
A recently published article titled “Do soil health tests match farmer experience? Assessing biological, physical, and chemical indicators in the Upper Midwest United States” by O’Neill, Sprunger, and Robertson appears in Soil Sci. Soc. Am. J. Vol. 85:903-91 (2021) (https://doi.org/10.1002/saj2.20233). The contents of this article are the summation of an assessment of whether soil health test scores resulting from analyses conducted by the authors align with farmers’ experiences with their self-defined “best” and “worst” fields in Michigan. A summary of the research conduct and its results follow.
• Soil health has emerged as a framework for linking soil management practices to agronomic performance. The soil health paradigm has resulted in a new soil testing regime that is more closely linked to principles of ecological management with the ultimate goal of improved crop growth and productivity, soil carbon (C) sequestration, and reduced nutrient loss.
• Understanding how soil health tests align with producers’ assessments of a particular field’s agronomic performance can lead to more informed recommendations that emanate from testing results.
• Traditional soil testing for soybean production is primarily focused on soil inorganic chemistry–i.e., pools of needed nutrients and pH. Soil health tests include these components while also including key measures of soil’s biological and physical properties such as available water capacity (AWC), aggregate stability (AS), dynamic soil C fractions such as mineralizable soil C (MINC), and cation exchange capacity (CEC).
• The alignment of producers’ knowledge of soil characteristics and their function as related to crop productivity with their interpretation of soil health test results is paramount for the adoption of appropriate soil health management practices.
• The specific objectives of this research were to 1) quantify variability in on-farm soil health scores, and 2) evaluate the degree to which soil health parameters align with producers’ assessments of field performance. The authors hypothesized that physical and biological soil health indicators will better align with those assessments than will chemical soil traits alone since fields differing in productivity may not have measurable nutrient differences.
• Participating farmers were asked to identify three fields that included a “Best” field, a “Worst” field, and a reference non-row crop field (NRC). NRC fields typically have higher soil health scores and thus can be used to compare soil health metrics across cropped fields.
• Management sensitive parameters measured in each field were: 1) Penetrometer readings to measure surface and subsurface compaction; 2) Soil pH; 3) Total soil organic matter (SOM); 4) Soil P and K; 5) Cation exchange capacity (CEC); 6) Soil texture; 7) Wet aggregate stability (AS); 8) Available water capacity (AWC); 9) Permanganate oxidizable C (POXC); 10) Mineralizable C (MINC); and 11) Potentially mineralizable nitrogen (PMN). Items 1-5 above were grouped as chemical properties, items 6-8 were grouped as physical properties, and items 9-11 were grouped as biological properties.
• Participant growers were interviewed to discuss: 1) management history of each field, including crop rotation, tillage, and farmer-specific management decisions that allowed categorization of fields as Best or Worst; and 2) specific test results for each field with the intention of integrating soil test results with farmers’ knowledge and experience for each field type. Nearly all Best and Worst fields on each farm had the same crop rotation, but tillage practices tended to differ more between field types.
• Sand content averaged 72% to 75% for the three field types, and the sand contents were not different across all three field types at all locations. 34 of 39 fields used in the study were classed as loamy sand or sandy loam, and three and two fields had soil texture classed as loam and sand, respectively. Thus, all fields had coarse-textured soils.
• Overall soil health scores for Best fields were significantly higher than scores for Worst fields, with physical and biological soil health variables having higher scores on Best fields compared to Worst fields.
• There were no significant differences in chemical soil health scores between Best and Worst fields. Ratings for P and K were not different between Best and Worst fields. Soil health P scores for Best fields were lower vs. NRC fields because of excess P inputs to cropped fields.
• Best fields had significantly higher ratings for AS and AWC compared to ratings for Worst fields.
• The significantly higher biological soil health ratings for Best vs. Worst fields were the result of greater values for both SOM and MINC on Best fields.
• Overall soil health scores for NRC fields were higher than those for both Best and Worst cropped fields, and this resulted primarily from higher soil biological scores for the NRC fields. The physical soil parameter AWC scored higher for NRC fields than for both Best and Worst fields.
• No clear trend in chemical soil health parameters existed between NRC and cropped fields.
• Both SOM and MINC were significantly greater in Best vs. Worst fields, and this coincided with producers identification of their Best and Worst fields. POXC and PMN did not distinguish between Best and Worst fields.
• The apparent most sensitive test to distinguish between Best and Worst fields was MINC, and the poorest match with producers’ perception of their Best and Worst fields was POXC.
• These results suggest that: 1) chemical soil health metrics do not align with producers’ designations of their Best vs. Worst fields–i.e., P, K, pH, and CEC did not significantly differ between the two field types; 2) physical and biological metrics were significantly different between Best and Worst fields; 3) AWC best delineated between the two field types for physical soil health, and SOM and MINC best delineated between the two field types for biological soil health; and 4) on-farm soil health testing can effectively distinguish between fields that are at the top and bottom of the performance ladder in relation to crop productivity/performance. All of this indicates that combining proper soil health tests with farmer knowledge of cropped fields should enhance the implementation of appropriate soil health management practices.
• Although both SOM and MINC values were consistently greater for the farmer-identified Best fields, the greatest contrast between Best and Worst cropped fields was for MINC. In fact, MINC on Best fields did not differ from that on NRC fields, and MINC best captured field variability and was best aligned with producers identification of their Best and Worst cropped fields. This suggests that MINC is a more meaningful measure for assessing field management decisions that enhance crop production.
Since MINC reflects microbial stimulation of CO2 production following the rewetting of soil, it is considered a strong indicator of nutrient release in soil and is thus considered a key indicator of potential agronomic performance by crops. However, according to results from research reported by Wade et al. in an article titled “Sources of Variability that Compromise Mineralizable Carbon as a Soil Health Indicator” that appears in Soil Sci. Soc. Amer. Journal [Vol. 82, p. 243-252 (2018)], the following are cautions when using MINC as a measure of soil health.
• Inter-laboratory variability for MINC is greater than for other commercial soil tests.
• Both water content and direction of rewetting (top vs. bottom or capillary wetting, which inhibits MINC) affect values of MINC. For reproducible and comparable results, the soil moisture status and the direction of wetting need to be consistent across samples.
• Analytical variability of MINC is highly affected by soil type or texture. This means that water amount to be added for the test should be done on a soil-by-soil basis.
• For MINC to be widely used as an accurate indicator of soil health, it is essential that a standard protocol be used for its measurement.
The following descriptions of terms used in this article should add clarity to the above presentation.
Mineralization. The decomposition of the chemical compounds in soil organic matter by which the nutrients in that material are released in forms that may be available to plants. Mineralization increases the bioavailability of the nutrients that were in the decomposing organic matter. It is the opposite of immobilization.
POXC. A technique for the determination of oxidizable carbon in the soil, and is an indicator of more stabilized soil C fractions. POXC correlates well with total soil C, and it is proposed that it relates to the longer-term buildup or storage of soil C. These passive pools of soil carbon have a slow turnover rate, are more resistant to decomposition and nutrient release, and act as binding agents to aid in soil aggregate formation.
MINC. Active carbon in the soil, or carbon forms in the soil that are more readily decomposed or metabolized by soil microbes, with the subsequent release of nutrients (from the decomposed material) that are available to plants. MINC forms are more readily decomposed than C forms that are determined through POXC. MINC is often associated with labile organic matter, or organic matter that is readily used or transformed by soil microbes to affect soil quality.
Composed by Larry G. Heatherly, Aug. 2021, email@example.com