Sally Brown and Andrew Carpenter

Top Photo: Courtesy of Northern Tilth

In Part I of this column, we talked about the fact that the testing results from a US Composting Council (USCC) Seal of Testing Assurance (STA) analysis may not be the key factor in determining if a potential customer can benefit from compost use. But scores can be intriguing.  You may want to pore over your test scores.  What if, in an attempt to show that potential customer how wonderful your compost is, you ask for their soil testing results’?  A fine example of “I’ll show you mine if you show me yours.”  

Directly comparing the two tests may lead to confusion rather than a clear demonstration of what makes your compost superior for their soil.  A test of compost quality and a traditional soil test are designed to measure different things. Let’s focus on how the tests are different and how to do a productive comparison.

Total Versus Available Metals

A first and easy way to compare the tests is to look at their measures of metals. Compost testing for metals is done to make sure that those composts do not have excess of any of the metals regulated to assure the safety of the product. The soil test is looking to see if there are sufficient concentrations of plant available metals (some of which are critical plant micronutrients) to meet the nutrient needs of the crop. The compost test will pour concentrated acid on the compost to bring ALL of the metals into solution. The soil testing lab will pour a specially designed concoction (typically a weak acid) over the soils to mimic what plant roots will see when they go looking for food.  Table 1 has results from Control Laboratories of metals in a compost and the soil fertility tests for some of those same metals (plus a few others).

Table 1. Metals testing results

Click table to enlarge.

Testing for total metals will always give you a higher number than testing for plant available metals.  In many cases, the total metals in compost will be higher than both the plant available and total metals in soils.  That is a really good thing for nutrients like iron, manganese copper and zinc.  It means that adding compost will increase the soil’s ability to provide those vitamins.  

Salts

Salts are something present in all composts that one does not want in high concentrations in soil. Salt is measured by measuring the conductivity of the compost (or soil). The ability of the soil to conduct electricity is a way to see if there are a lot of ions floating around in soil solution.  No chance of being electrocuted by a soil with high conductivity. Just a chance of killing seedlings or making it harder for plants to grow.  

Salinity is typically measured as conductivity. Of the different metals that are responsible for saltiness in soil, sodium (Na) is the one to really be concerned about. It is not necessary for plants and can hurt soil structure. Compost tests report on sodium in percentage concentration, not parts per million. The 0.36% Na in the compost  shown in Table 2 is equivalent to 3,600 ppm.  Guidance from the soil testing lab says that sodium in excess of 100 ppm is cause for concern. That is much lower than what is in the compost. However, there are multiple factors that let you rest easy.  A salty compost – and all composts will have higher electric conductivity (EC) than soil – is typically applied at low rates relative to the weight of the soil. That will effectively dilute the high EC in the compost.  

 Table 2. Salts testing results

Click table to enlarge.

The compost is also going to contain other salts that will dilute the impact of the Na. This particular compost contains 18,000 ppm potassium, 22,000 ppm (2.2%) calcium (Ca), and 9,500 ppm magnesium (Mg).  Finally, for many of the places where compost is used, there is sufficient rain and/or irrigation water to wash any added salts from the compost out of the soil. Note that the soil test from Maine (data used in Parts I and II) didn’t even measure conductivity. Why? In Maine, excess salts are not anywhere close to something to think about. Conversely, parts of Arizona, California, Nevada and New Mexico are a different story. The high Ca and other salt concentrations in the Maine compost, however, can typically reduce issues related to excess salts in soils even with elevated EC in the composts. On another note, when using compost as a high percentage of a potting soil blend, using a compost with lower EC is critical.

Soil Biology

In soils, measures of soil biology (see Part I) are surrogate measures of soil health.  You want high numbers. High respiration rates mean a large microbial community, implying plenty of food (carbon) and good soil structure.  In compost, plenty of carbon is pretty much guaranteed.  You want measures of respiration to be on the low side. Composting itself is a microbial eating orgy.  A critical measure for an STA compost is its stability. You want to be sure that the feasting has settled down enough to provide a relatively sedate amendment for the soil. The soils from Maine had respiration reported as CO2-C in mg per kg of soil ranging from 50 to 140. The compost had measures of CO2-C of 0.96 mg per GRAM of total compost. Converting to the same units gives that compost a respiration rate of 960 mg CO2-C per kg of soil. And that is considered to be stable. Always check your units!

Measures of compost biology also include tests for salmonella and fecal coliform. These are never included in a soil test.  You want to make sure pathogens in the compost are long dead and buried (or eaten) before that compost is sold.  This is never something that is measured in soils. 

Nutrients

Most people send in their soils for testing to see if they need to add fertilizers to grow a crop.  The soil tests from Maine that we’ve been using specify corn for one and High Power Pasture for another. Fertilizer recommendations are based on the available N and P in the soil and the expected nutrient demands of the crop. In contrast, compost is most often applied as a soil conditioner, not as a primary source of nutrients. The fertilizers added to soil based on a soil test will be there and ready when the plants need them. For nitrogen at least, that can often lead to nitrate leaching to groundwater.  

In contrast, the fertilizers included in compost only come to the table over time.  The amount of N in compost that will become available to plants will vary based on the C:N ratio of the compost, the nature of the carbon, the climate, soil characteristics where the compost is applied and likely four or five other factors. A few columns on this topic were written in BioCycle a while back. Adding N to the soil with compost is like taking out a long- term CD where you can use the interest. In contrast, adding fertilizer N to the soil is like Apple Pay. 

Table 3. Nutrients in sampled compost and soil

Click table to enlarge.

For the soil in Table 3, the soil test lab recommends adding 100 lbs/acre of N and 50 lbs/acre of P.  Say you add 5 dry tons of compost per acre to this soil.  That is adding 60 kg or 132 lbs of total N, well over what the soil test lab recommended. However, you still might get some yellow (N deficient) plants. Of that 132 lbs, typically about 25% or less will become plant available the first year. (Note the C:N ratio of the compost is 12:1, suggesting it will have available N to spare.)  That 25% comes to 33 lbs or well below the recommended fertilizer needed.  

An appendix at the end of the compost test results gives you calculations to use to figure out how much of the total N is likely to be plant available (Figure 1). Not that these take into account the respiration rate and the C:N ratio.  The higher the respiration rate, the more active the microbes and the greater the transformation from organic N to mineral N (aka plant available N). 

Figure 1. Calculating amount of plant available N 

Click figure to enlarge.

Phosphorus is more complicated. Soils tend to bind P so that the quantity that is plant available is typically much less than the quantity you add. How much is bound and how tightly it is bound depend on characteristics of the soil. As a result, there are more ways to test for phosphorus than fingers and toes. A bit of an exaggeration but not too far from reality. You can test for total P, plant available P in cases where plants are very hungry, plant available P after the plants have already had a snack, and then environmentally relevant P which is equivalent to excess after a five-course meal. Plant available P extractions will vary based on expected characteristics of the soil. 

Early on, the two primary tests were Bray (neutral to acidic soils) and Olsen (neutral to basic soils). Now there are additional tests including Mehlich I-III and Modified Morgan. Tests for P that focus on potential excess are also in use including, among others, iron strip P and water extractable P.  If you look at the fine print or call the testing lab, they’ll be able to tell you what test they use. In Maine, it is the Modified Morgan. In composts, most of the P is tied up in the organic fraction. Only as that gets eaten by soil bacteria and mineralizes does it become plant available. It doesn’t take into account the specifics of the receiving soil. The P reported from a compost analysis is most likely total P.  The P reported from a soil test is likely the P dissolved in a particular extract that is appropriate for the region where the compost will be used. 

Hopefully this guide has helped you to understand the difference between compost test results and soil test results. This isn’t quite apples to oranges, but more like comparing an algebra exam to a trigonometry test.  Related, building on each other, but not the same. And congratulations on being STA certified.  Even more power to you if you can read and understand your test results. 

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.