Is Compost Application an Effective Soil Health Practice?

Soil organic matter has been one of the fundamental benchmarks and metrics to building healthy, functioning, and resilient soils. Compost is often recommended as an effective amendment to increase soil organic matter (SOM) while providing organic nutrients and added fertility. It has often been referred to as the gardener’s best friend. Compost application is now being recommended and used as a practice to sequester carbon at scale, and farmers are even getting paid to apply compost through the California Healthy Soils Program.

Many no-till market gardeners have adopted the practice of compost mulching as a key soil health practice to no-till gardening. However, it is also known that certain types of compost and too much compost can cause big issues with crop production due to undesirable nutrient levels, high pH and salts, and water quality nutrient loading.

I am frequently asked my opinion about compost and if farms should be using it. In full disclosure, I have applied thousands of tons of compost to farmland and gardens under my management. However, I do not consider it a primary soil health practice but rather a tool (one of many) and I welcome its use within the context of each farm. I now feel compelled to provide some perspective as to why farmers and decision makers should not emphasize the use of compost as a regenerative or soil health practice, but as an amendment that may be useful under specific contexts. Farmers and policy makers should consider the regenerative principles and how we can use specific tools to help us successfully implement those principles. The tools that we can utilize to help us include compost, no-till, cover crops, etc. All of these tools must be used within their context to be effective at building lasting soil health and sequestering carbon.

To better understand the Principles of Regenerative Agriculture go to: https://understandingag.com/the-6-3-4tm-explained/

Many reading this article likely have extensive knowledge about compost. I believe we can agree that quality, nutrient value and microbial benefits of compost are variable and are dependent on the feed stock, and how the compost was made. So, not all compost is made equal, and some composts will provide more soil health benefits than others. For example, a more fugally populated compost is considered to be of higher quality and more beneficial to nutrient cycling.

Most farms purchase their compost from a large composting facility. Once delivered, a farmer must use fuel, labor and equipment to apply the compost. This can be quite expensive on a per-acre basis. With a five-ton per-acre application, which is less than a 1/8-inch-deep coverage, costs can balloon to over $600 per acre. Most purchased composts average $60 per yard cost, including purchase, delivery and application.

Applying compost generally comes with disturbance to the soil from equipment traffic, and tillage to incorporate. So, compost is relatively expensive, and the positive impact it has to a farm’s productivity and economical sustainability must be closely evaluated. To properly evaluate compost as a smart investment we must evaluate how critical it is to building soil health within each farm’s context.

Let’s quickly review the principles of soil health and consider how compost might help us achieve any one of these principles: 1) context, 2) minimize disturbance, 3) maintain a continuous living root, 4) armor the soil surface, 5) maximize diversity, and 6) integrate livestock. The most important principle is often context, and we must consider the uniqueness and complexity of each farm and ranch within its own context, so this should be at the top of our mind when considering the cost and use of compost.

How does the application of compost help achieve these goals? Is it providing a strong armor or ground cover? If using as a mulch, is it the best bang for your buck to provide ground cover? Are you minimizing disturbance with the application of compost? Is it adding diversity? (Maybe diversity of organic matter?) Could it cause chemical disturbances? Do you have an analysis of the nutrient concentrations, soluble salts and pH of this compost? Will the application improve plant growth, ground cover and facilitate living roots?

Compost certainly provides benefits, and with highly degraded soils, the addition of compost is a great tool to add organic matter. It also can jumpstart or “reboot” soil function, providing energy and food for microbes to start doing their work. This may create a temporary improvement to soil aeration and water holding capacity. The key is to recognize that these benefits are temporary if we do not also implement the principles of soil health along with the compost application.

Since compost is used as a source to add organic matter, let’s take a quick dive into the different forms of organic matter. Many farmers and gardeners believe that compost adds lots of organic matter that will quickly improve their soil. If you apply compost to the soil surface as a compost mulch, or even with incorporation, and then pull a six-inch depth soil sample and send it to a lab, you indeed will see an immediate increase in the soil organic matter levels. So, does this mean your soil has improved? No. Are the microbes that are measured in that compost going to populate the soil in the field and immediately lead to improve soil microbial function? No. In most cases the compost soil microbes will die off within a week, so soil health principles must be implemented for soil health improvements to take place.

There are four major components of SOM: 1) fresh residue, 2) living organisms, 3) decomposing OM (active fraction), and 4) stable OM (sometimes referred to has humic substances, and often called humus). A healthy, highly functioning soil contains lots of stable OM (30-50% of all OM), which is resistant to degradation and is well protected within soil aggregates. What we want to build in our soil is mature, stable, and resilient forms of SOM, which are formed under the soil surface in the rhizosphere. Specifically, within rhizo-sheaths, and within stable aggregates that are teeming with a diversity of bacteria, protozoa, actinomycetes and mycorrhizal fungi. The soil biology is further consuming and breaking down decomposed OM, including plant residues, root exudates or compost. These forms of decomposing OM, which are being consumed by microbes, are often referred to as “active organic matter.” This is also referred to as the easy to eat, “chocolate cake” of the soil organic matter pool. It is rich in nutrients and easily digested by microbes. The majority of compost sold consists primarily of decomposed-active organic matter, and does not provide much stable OM. Many types of compost, especially compost manufactured in large-scale windrow operations that require high heat and rapid turnover, do not contain stable and mature forms of organic matter that are critical for building soil aggregates. Slower, cooler, and more static compost piles can produce stable forms of SOM.

Stable OM does not provide a lot of food value to the soil microbial community.  However, stable OM composes the biological cementing agents that play a critical role in bonding soil particles and OM together, creating aggregates, and providing resilience to the soil at the micro-aggregate level.  With all this being said, the major work and manufacturing of stable OM is happening under the soil surface by the soil biology feeding on their favorite food source – plant root exudates.  It is the plant and their root exudations driving the process of organic matter formation. You could say the number one tool in the farmers toolbox for building soil organic matter is a living plant.  And it doesn’t cost a farmer too much money!

I went down this organic matter rabbit hole for a moment because our main soil health goal is to build stable macroaggregates. Good soil aggregation, a shovel full of soil that has a “crumb-like” and “cake-like” structure is one of the greatest indicators of soil health, and soil aggregation is best developed in a system with a diversity of plants, a living root, minimal disturbance. Compost is not required to build soil aggregates. We can build a plentiful reserve of active soil organic matter from crop and plant residues when the soil is continuously covered with living roots, and from animal manures (from actively grazing livestock), root exudates, and the microbial biomass. Once a farming system is a few years into properly implementing the principles of soil health and the rules of adaptive stewardship, we should have soil that is continually regenerating. This means a product like compost may not be necessary or even a smart farming expense. Plant diversity and a living root, pumping out liquid carbon (exudates) is our greatest tool.

Compost produced at large-scale facilities can pose some serious issues to plant and crop health, if overused. It may also contribute to soil chemical disturbance. Here are some issues to consider and look out for: 1) Most large scale compost has a relatively high pH, often over 8.0 pH, and relatively high-soluble salts or electric conductivity, which can exceed 2.0 ds/m; 2) The continual use of compost from municipalities or large-scale composting operations can lead to a buildup of metals such Al, Zn, Cu, Mn, and Fe; 3) High concentrations of phosphorus and potassium can build if copious amounts are applied; and 4) Broadleaf herbicide residuals can persist in compost using feed stock from landscape waste, and have been found to effect vegetable growers. These herbicides are generally at trace levels (ppb) in compost and can impact plants at levels that are non-detectable by lab analysis.

In summary, I am not trying to deter farmers or gardeners from using compost, nor am I suggesting that compost cannot provide benefits or aid in building soils. As mentioned previously, I have applied compost extensively in the past to gardens, orchards and vineyards. However, knowing what I know now, I would have applied that compost much more judiciously, and I would have taken a far more regenerative approach.

My goal is to provide perspective and education about building soil organic matter, prioritizing your soil health management practices, and making the right investment in your farm.  Compost may really move the needle in your context, or it may not.  If you are considering application to large-scale acreage, I encourage you to first run some check strips or field demos before making a huge financial commitment.  If you are a farmer and have the funds to apply compost to large acreage and assume you would be getting a benefit without question, you might be giving too much merit to compost in general.

Understand the quality of the compost you are sourcing and consider the potential compounding disturbances.  When feasible, produce your own high-quality compost.  Use static piles with a longer-term approach to create high quality, highly fungal compost.  Consider making compost extracts and teas to prime seed, feed crops and microbes, improve nutrient cycling, improve fertility and crop disease suppression.  And lastly, understand that no single product, not even compost, is a silver bullet.  The sun’s energy and a plant with its living roots, are the greatest tools in our toolbox to build soil health.  We can confidently improve soil health and ecological function when we fully commit to implementing the 6-3-4™.