Soil Health Indicators Related to Soil Properties, Climate, and Crop Yield

As stated in previous articles on this website and in myriad professional and trade journal articles, soil health (SH) is recognized as an integral and important part of the continued capacity of a site to produce high-yielding crops. Soil health indicators include biological components such as organic matter (OM), physical indicators such as available water holding capacity (AWHC), texture, and water-stable aggregates/aggregation (WSA), and chemical indicators such as P, K, and pH that are determined by soil testing. The assessment of SH can be used as an indicator of sustainable land management at agricultural sites.

It is now recognized that soil response to management is dependent on specific factors at the site of production of crops, and that these factors must be considered when using and interpreting SH measurements. These factors include tillage intensity, management practices, weather, and cropping system/history. The interactions among biological, physical, and chemical soil properties involve more than just soil nutrient quantities, and thus should be assessed to diagnose and quantify the soil properties and processes that make up the totality of SH.

In an article titled “Reanalysis validates soil health indicator sensitivity and correlation with long-term crop yields” by van Es and Karlen (Soil Sci. Soc. Am. J. 2019:83:721-732), the authors report findings that assess the use of SH indicators as related to crop yields.

•    The premise of the analyses was to demonstrate that relationships between SH and crop yield could justify adoption of certain management practices such as reduced tillage and altered crop rotations.

•    Data were obtained from three long-term experiments conducted in three distinct physiographic regions of North Carolina. The three trials focused on different agronomic management practices and used different crop sequences. Yield data used in the analysis were collected from corn and soybean crops.

•    Biological indicators associated with labile C (“new” OM that breaks down quickly and is major food source for soil microbes) and N showed a strong linear relationship with yield of both crops. Their analysis indicated that different management practices differentially affect different aspects of SH, especially those associated with labile OM.

•    The analyses suggested that OM quality vs. OM quantity may be more relevant to crop yield since long-term average yields showed a strong linear relationship with SH indicators related to OM quality.

•    The authors concluded that: 1) comprehensive SH assessment was able to determine effects of agronomic practices such as tillage; 2) biological indicators associated with labile C and N were most impacted by management practices; and 3) SH indicators can be related to corn and soybean yields, but scoring curves for SH likely should be regionalized.

Carbon dioxide (CO₂) flush after rewetting of dried soils is now recognized as a SH indicator. However, this measurement has not been related to most soil properties and crop yields. In an article titled “Carbon dioxide flush as a soil health indicator related to soil properties and crop yields” by Sainju et al. that appears in Soil Sci. Soc. Am. J. 2021 (https://doi.org/10.1002/saj2.20288), the authors report findings from research that address this issue.

•    Two long-term experiments were established at dryland farming sites in eastern Montana.

•    The effect of cropping system and N fertilization on CO₂ flushes after 1 and 4 days of incubation following rewetting of dried soil was related to soil physical, chemical, and biological properties and crop yield.

•    Reduced crop residue input resulting from the absence of crops decreased both 1- and 4-day flushes of CO₂. Thus, cropping system can affect both the quality and quantity of residue returned to the soil.

•    Their results were compared to those from the above studies, and it was surmised that both air temperature and precipitation likely will affect residue production which will in turn affect CO₂ flush potential from a site.

•    A positive correlation between CO₂ flush and soil aggregation and stability suggests that soil aggregation can stimulate CO₂ flush because of increased microbial growth. Likewise, a positive correlation between CO₂ flush and soil water content indicates that microbial activity increased as soil water content increased.

•    A negative correlation between CO₂ flush and soil bulk density indicates that soil respiration is reduced by compacted soil with concurrent decreased porosity.

•    The positive correlation between CO₂ flush and most soil nutrients suggests that increased biological activity increases the availability of soil nutrients.

•    Positive correlations existed between CO₂ flush and most of the measured soil biological and biochemical properties that included total C and N concentrations, SOM, total and potentially mineralizable N, and organic C. This suggests that CO₂ flush is an indicator of substrate availability, N mineralization, and enzyme activity.

•    Because of the rapid and inexpensive measurement and increased sensitivity to management practices, soil properties, and crop yields, the authors determined that measurement of CO₂ flush after 1 day of incubation can be used as a SH indicator in routine soil testing for dryland cropping systems in the region.

Soil Organic Carbon (SOC) is recognized as a major indicator of SH. An article titled “The soil health assessment protocol and evaluation applied to soil organic carbon” by Nunes et al. appears in Soil Sci. Soc. Am. J. 2021:85:1196-1213. In this article, the authors propose the Soil Health Assessment Protocol and Evaluation (SHAPE) tool that accounts for edaphic (i.e. soil properties/conditions) and climatic factors at the continental scale when assessing SOC values. The authors state that this tool “is a flexible, quantitative tool that provides a regionally relevant interpretation of this key soil health indicator”. Here are some details about this tool and its use to accurately/properly evaluate SOC content in myriad soil types.

•    The tool was developed using a soil health database that includes 14,680 SOC observations from across the U.S., and accounts for edaphic and climatic factors at the continental scale. Soil variables included order, suborder, texture, and drainage class. Climate variables included mean annual precipitation, mean annual temperature, potential evaporation, and wetness index.

•    The tool was developed with the recognition that dynamic soil property responses to management and land use are highly dependent on site-specific soil and climate factors.

•    This flexible and quantitative tool was developed to provide a better, more comprehensive soil health index that accounts for continental variation in climate and natural soil conditions.

•    The model was used to develop scoring curves for soil health indicators, in this case SOC. The SOC observations in the dataset were from a large range of land uses that included agricultural production, and from management practices that reflected variable tillage intensity, residue management, and addition of soil amendments.

•    Based on SHAPE scores developed from information in the database, the authors concluded that soil SOC concentration can be improved depending on land use and management.

In a FarmProgress article titled “3 challenges to carbon measurement accuracy”, author Shelley E. Huguley discusses points that should be considered to ensure accurate measurement of soil C. They are: 1) depth of sampling–e.g. deep vs. shallow sampling to accurately determine the potential of the soil to store long-term C that will not be as affected by disturbance and microbial activity in the shallow soil depths; 2) consistent sampling methodology to ensure that only organic C components are measured; and 3) consistency in time of sampling to ensure that samples are taken at the same time of year between growing crops.

The short summaries of the above-cited articles in no way imply that SH determination is a simplistic process. In fact just the opposite is true. However, all of the above lead to the following generalities. 1) Soil C content can be used as an indicator of SH. 2) Sampling methodology for soil C should be consistent and adapted to measure only organic soil C. 3) Soil C content should be assessed in relation to edaphic and climatic conditions at the site of collection.

Composed by Larry G. Heatherly, Sept. 2021, larryh91746@gmail.com