Soil health under study in Missouri

Published: November 18, 2016

Most growers realize the value of a typical soil test, especially to determine how much fertilizer to apply. And there is some indication available with these results on other physical characteristics (e.g. % sand, silt and clay). More understanding of soil can be obtained thru ‘characterization’ which deals with the physical and chemical properties of soil. However, it does not typically address the biological component. Since around 2010, interest in the biological component of soil has grown rapidly and this more holistic approach to the study of soil has been encompassed under the umbrella of Soil Health.

Last summer a program was initiated between the county based Soil and Water Conservation Districts and a new lab at University of Missouri (Soil Health Assessment Center), whereby comprehensive testing occurred across the state. Farmers took field samples and submitted them for testing, with the costs offset by conservation funds. (Funds provided by the Missouri Parks and Soils Tax and overseen by the Missouri Department of Natural Resources) This occurred throughout the fall of 2015 and just this November the results are being shared. This article is to discuss some of those results, highlighting four tests that are likely of most interest, and to share the results from specific counties in which some of the auction growers farm.

Each sample submitted received a report with over 15 separate test results, too many to discuss here. The four that seemed most relevant or of interest to a produce grower are:

• total organic carbon (TOC) 
• active carbon
• bulk density
• water stable aggregates.

The test for total organic carbon (TOC) is more carefully and accurately obtained than the ‘organic matter’ one gets with a typical soil test. But it can be roughly compared to soil organic matter with TOC being about 60% of typical organic matter. Most growers know more organic matter is better, that with higher amounts nutrients cycle better, more water is retained, and the environment is improved for microbes. Both organic matter and microbes help the soil to filter and buffer so that chemicals like herbicides are broken down.

Active carbon is that part of TOC which is likely to be broken down and used in the upcoming growing season. So again, more is better, as with this carbon breaking down, nitrogen is released to the crop. Higher active carbon is often associated with higher TOC. However, individual results vary greatly, and for some soils high active carbon levels are seen with lower TOC, and vice versa. There is a bit to be learned here.

Bulk density is an indicator of soil compaction and soil functions. A lower number is better, as soils with lower values allow roots to grow easier, have more pore space thus ‘breathes’ better, and holds more water. Bulk density will be influenced by the sand/silt/clay makeup. Sandier soils will tend towards higher values, less for silty soils, and then yet lower for clay. This runs counter to the notion that ‘heavy soils’ are often equated to those with more clay.

Water stable aggregates are soil particles bound together that resist breaking even when wet. A higher percentage is desired here, as it means the soil will be less likely to erode and crust over. They will allow water to infiltrate easier and retain more moisture. They will be easier to get across when wet and provide better seed to soil contact, which improves seed performance.

Please note the table below, which summarizes the soil samples which were submitted in 6 of the areas where there are many produce growers receiving this newsletter. There were no samples submitted from the counties of Dallas or Webster. The results for Daviess are quite good for TOC which can be typical of prairie soils.

Table One. Four soil health characteristics for selected counties and the entire state of Missouri.

Do any of these tests ‘biological component’? Yes, active carbon gives some indication of potential biological activity. There are other tests as will, such as mineralizable nitrogen and phospholipid fatty acid analysis. For these a consensus is still being sought on the best way to interpret or present the results.

How might this work on soil health be relevant to produce growers in Missouri? Well it isn’t likely you’ll want to submit a sample on your own ‘just to see the results’. The cost for a basic test is $30 and expanded is $80. For additional tests, some of which were performed for this study, the cost would be well over $100 per sample. One area of interest would be with a research project focused on sustainable agriculture, where samples from produce growers with some of the counties listed above are sampled. These could be compared to the samples from their own counties and the state average. With vegetable production being a somewhat differently managed cropping system, it would be interesting to see how they compare.

Interested in improving soil health? Suggestions were provided to improve the four characteristics presented with the soil health assessment report. Several of suggestions were mentioned three or four times, such as:

• Decrease tillage/disturbance
• Add manure, compost, or mulch
• Keep vegetation growing year-round
• Use double cropping
• Plant cover crops
• Add perennial crops (e.g. pasture or hay) to the rotation
• Add wheat or other small grain to rotation