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Sally Brown

While you might be exceptionally proud that your compost has been certified by the US Composting Council’s Seal of Testing Assurance (STA) program, it is highly likely that your potential customers (your mom excluded) care much more about what that same compost will do for their soils than looking in detail at your test scores.  I will talk about those test results and please do post them on your refrigerator. STA certified compost is something to be proud of.

But the critical thing is to look at the test results for your potential customers.  Here I mean the SOIL test results. These test results will not have the STA stamp and will likely have gone to very different labs than the ones that analyze composts.  However, the scores from these tests can help let you know how the soils will respond to compost.

For many cases, you don’t need to see test results to know compost will do the job. New construction is one example.  Construction projects almost always destroy topsoil and soil structure. Adding compost is a way to build back in an instant. Roadway right of way is another no questions asked; compost will do the trick.  However, use the soil test results if you are trying to market composts to gardeners and growers.  It is a way to respect the work that they do to make their soil healthy – evidenced by the fact that they are asking about compost in the first place.

Adding compost to soils is very different from adding fertilizer. Fertilizer recommendations are relatively easy, particularly for agronomic crops like corn and wheat. There are a wide range of standardized tests to see if your soil is deficient in the three major nutrients – N, P and K aka nitrogen, phosphorus and potassium.  If your soil is nitrogen deficient, you add N and the plants grow. Without the N, those plants may try to grow but will typically turn out yellow and stunted. In contrast, compost is typically added as a “soil conditioner” – a multipurpose vitamin if you will.  When the receiving soil is worn down, the response can be exceptional. For a healthy soil, however, that compost multivitamin is likely to go unnoticed. There is too much kale in the diet already for the compost to make much of a difference.

For this two-part  column, I’ll go through soil and compost testing results that Andrew Carpenter (Northern Tilth) has kindly provided.  I’ll start with a summary spreadsheet that he sent that has results from four soils.

Soil and Water

One of the biggest ways that compost can help a soil is in its relationship with water. Standard soil testing doesn’t directly measure that. However, there are things to look for to see how your compost might provide a benefit. The first is soil texture. If soil texture has been included in the soil testing, you will get a result that provides the name of the textural class, for example, loamy sand, loam, silt loam, and silty clay loam.

The way that compost can help soil with water can be split into two fractions.  For coarse textured soils, it is often difficult for water that enters the soil to stay there. Coarse texture in soils means a fair amount of sand. Water entering these soils will infiltrate quickly and likely disappear just as quickly. Compost will aid these soils in water retention. On the opposite side of the spectrum are the fine textured soils. These typically contain a fair amount of clay. For these soils, getting water to soak into the soil is typically the primary problem. Once water has infiltrated, there are plenty of nooks and crannies and charge sites for it to hang around in. What compost can offer these soils is faster infiltration as a result of improved soil structure. If you look at the texture classes of the soils above, you’ll see one that includes “sand” and another that includes “clay.”  For that soil with sand, you would expect that a benefit associated with compost would be improved water holding capacity.  With the soil whose texture includes clay, you would expect faster infiltration as a result of better soil structure.  Both benefits would be the result of added organic matter from the compost.  For the soils without either the loam or silty loam, those are the ideal textures for a fertile soil, so unless it has been disturbed or eroded, it is not clear how much bang they would get for the compost buck.

The soil tests include other measures that can back up your ability to deliver. How much water the soil holds, especially at field capacity (when all excess water has drained but there is still plenty for plants to drink) has been seen as the best measure of soil hydraulic properties (Bagnall et al., 2022b). Here the soil labs measured plant available water. In this case, it is as predicted by the texture (Table 1).  The soil with clay in the texture has the highest % plant available water (% PAW) and the one with sand has the least.  A national study on soil health found that increasing soil organic carbon by 1% increased soil water holding capacity by 0.03 m³ per m³ across all textures (Bagnall et al., 2022a). 

Table 1. Plant available water based on soil texture

Another measure that was used for these soils was the % WSA or water stable aggregates.  Aggregates are clumps of organic and mineral material. Soil aggregation is critical to the physical properties of the soil including soil structure and bulk density (weight per unit volume). A well-structured soil will have plenty of pore space to allow for rapid water infiltration and root growth. The WSA is a measure of how many of the soil aggregates will hold up to getting wet and being shaken. The more WSA the better. The same national group that measured changes in water holding also evaluated different tests for aggregate stability (Rieke et al., 2022). They found that more organic matter (i.e. more compost) meant more stable aggregates. 

Soil Organic Carbon and Soil Organic Matter

With any soil, a key indicator to the health of the soil and potential response to compost is the total soil organic carbon (SOC)  or total soil organic matter (SOM)  (Lipzin et al., 2022).  For soils that have a pH low enough (<7.0) so that calcium carbonates won’t be present, total carbon is typically measured by burning the soil – aka combustion.  Test labs use conversion factors to give % organic matter (OM) values once they’ve measured total organic C.  These conversion factors take into account that OM contains elements besides carbon.  Table 2 shows the complete data from these soil samples. 

Table 2. Percent organic matter plus total organic C
You can see here that the sand has the lowest organic matter, one of the loams comes next and then comes the clay.  The one soil here that is least likely to benefit from the compost is the loam because it has high plant available water, high total C and % OM.  The owners of the sand and clay soils will be very happy to get your compost, STA approved or not. The owners of the silt loam might be pleased over time.  The family that owns the loam will be ordering from you only to keep your mom happy. 

Bulk Density 

There are two tests that are typically not included in the offerings of soil testing labs that are very easy to do on your own.  They – not including time – are essentially free.  The first is bulk density.  That translates to how much a soil weighs.  While there are professional soil bulk density measuring tools, you can also do this with a can, piece of wood, hammer and a shovel.  Here are two handy links to a fact sheet and a You Tube video.

Your typical rock weighs about 2.65 grams per cubic centimeter.  A good soil should be no more than half rock (smashed of course into a mixture of sand, silt and clay particles. Another way to say that is that the bulk density of the soil should be less than 1.32 g cm³.  Ideally it should be even less than that so there is plenty of room for air, water and roots. If you are able, measuring the bulk density of the soil at a potential customer’s site will tell you how that soil will respond to compost.  If it is 1.4 g cm³ or more, it will really appreciate the compost.  If it is less than 1.1, unless the landowner is a firm believer in the need for annual compost applications, you might want to take your business elsewhere.

You can also measure how fast water soaks into a soil.  Here, while you can buy specially designed equipment, you can also just take the hammer and can from the bulk density measure and put them to work.  Hammer the can (top and bottom removed) into the soil surface.  Pour some water in and time how long it takes to disappear.  If it is gone in a flash, no need for compost to aid in infiltration.  If you can read a chapter on your Kindle while waiting, that soil is calling for your product.  On the other hand, if the water is gone in a flash, the soil might not be holding onto water and compost can provide more water-holding capacity.  The ideal is a soil that has good infiltration, but also is able to retain water for plants to use during drier periods of the growing season. 

In many cases, soil test results will also provide their evaluation of the status of your soil. What is considered good for a soil in Maine is very different from what a soil in Arizona can deliver. The soils that I’ve been talking about are all from Maine.  The soil tests were done at the University of Maine soil testing lab.  Here is their take on that Loamy Sand:

And the Silty Clay Loam:

That soil testing lab would clearly recommend compost for the sand, despite the relatively high plant available water.  It might not see much of a need for compost in the clay.  An infiltration rate measure could give you an answer on whether compost is called for. 

Notice too, that these tests give you results above and beyond organic matter for measures of biological activity.  POX C or permanganate oxidizable carbon is a way to test for microbially available carbon, respiration is a measure of how much the microbes are breathing (same measure as is used to test for compost stability) and potentially mineralizable N is a measure of N availability for plants and microbes.  These measures, while they may have some subtle differences in comparison to plain old organic matter, are generally well correlated to how much carbon is in the soil (Liptzin et al., 2022).

Hopefully after reading this you are still proud of your compost test results.  And you have a better idea of how to market that compost to prospective users.  And do show that STA test to your mom.  I promise she will be proud. 

Sally Brown, BioCycle Senior Advisor, is a Research Professor at the University of Washington in the College of the Environment. Andrew Carpenter, founder and principal at Northern Tilth in Belfast, Maine, provided the soil testing data.