Measuring soil health improvements with cover crops in vegetable production

Chahal, I. and Van Eerd, L.L. 2019. Quantifying soil quality in a horticultural-cover cropping system. Geoderma 352: 38-48. doi:10.1016/j.geoderma.2019.05.039.
Research summary written by Cameron Ogilvie

Key messages

  • This study compared 3 soil health tests in a 6-year mixed cropping system with cover crops
  • The Haney Soil Health Test did not find differences with and without cover crops; the Cornell Soil Health Assessment and the Weighted Soil Quality Index did
  • After 6 years, cover crops improved soil health and tomato marketable fruit yield

The future of agriculture depends on us managing soil so it can be used for generations to come. To accomplish this, farmers grow cover crops – crops that improve soil quality, or soil’s capacity to grow crops. Cover crops protect soil from erosion and add more organic matter and nutrients to soil as they decompose. If farmers want to track their progress, they need simple and effective tests. But currently, scientists don’t agree on any one test that can show an overall improvement in soil.

Table 1. Measurements used in soil quality tests for this study.

MeasurementCASHHSHTWSQI  
Wet aggregate stabilityXP
Available water capacityXP
Organic matterXXC
Active carbonXC
Ace protein indexXC
RespirationXB
pHXXC
PhosphorusXC
PotassiumXC
MagnesiumXC
ManganeseXC
IronXC
ZincXC
Surface penetration resistanceXP
Sub-surface penetration resistanceXP
Solvita CO2-burstXXB
Water-extractable organic carbonXXB
Water-extractable organic nitrogenXB
Solvita labile amino-nitrogenXB
Abbreviations: CASH – Comprehensive Assessment of Soil Health; HSHT – Haney Soil Health Test; WSQI – Weighted Soil Quality Index; P – physical; C – chemical; B – biological

What they did

In this study, researchers from the University of Guelph Ridgetown Campus used three soil quality tests – the Haney Soil Health Test (HSHT), the Comprehensive Assessment of Soil Health (CASH) developed by Cornell University, and a Weighted Soil Quality Index (WSQI) (Table 1) – to track changes in soil with and without cover crops. The study was carried out in a mixed grain-vegetable crop rotation experiment where, for six years, four fields had cover crops and one didn’t. Oats, winter cereal rye, oilseed radish, and a mixture of oilseed radish and rye were the four cover crops used and compared to no cover crop.

What they found

The HSHT could not detect soil health improvements after growing cover crops, but both WSQI and CASH did. These two tests found that a mix of oilseed radish and winter cereal rye had the greatest impact on soil quality in 2015, and cereal rye alone had the greatest impact in 2016 (Figure 1). Unfortunately, neither of the test scores matched with tomato yields. In 2015, tomato yield was greatest following oat, and in 2016 tomato yield was greatest after oilseed radish. The tests did show that cover crops improved the amount of carbon stored in the soil, which can help reduce greenhouse gases in the atmosphere.

Figure 1. Impact of six years of cover crops on tomato marketable yield, CASH, and WSQI scores in 2015 and 2016.

Why it matters

Not all pre-made soil quality tests work from one farm field to the next, and sometimes the measurements included in pre-made soil quality tests aren’t important on a given field. In this study, there was too much variability in the HSHT score to pick up differences. Instead, CASH and WSQI both showed differences in soil quality with and without a cover crop, but WSQI was able to do this without needing any soil physical measurements (ex. penetration resistance, aggregate stability).

Because WSQI uses a weighted scoring system by first figuring out which measurements are important on that field, WSQI is recommended as a soil health testing method. And because cover crops improved crop yields and soil health, planting cover crops is recommended as a sustainable soil management practice.

More details on the soil quality tests

The HSHT uses three biological measurements to create a score, and the CASH uses 15 physical, chemical, and biological measurements. Both tests are “un-weighted” – meaning, they simply average the scores of the individual measurements to create an overall score. The WSQI uses a statistical test called a Principle Components Analysis to first decide which measurements are important (“weighty”) and then uses an average of these scores to calculate an overall score. In this study, five measurements showed strong differences between the cover cropped fields: Solvita CO2-burst, organic matter, pH, water-extractable organic carbon, and Solvita labile amino-nitrogen. Because these were important measurements according to the Principle Components Analysis, they were used in the WSQI and compared with other soil quality tests.

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