Composting for Soil Health

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Numerous farmers in our area use composted manure to improve their pastures or crops. In fact, applying compost to fields or garden beds:

  • Improves water infiltration and water holding capacity for soils.

  • Improves soil tilth.

  • Decreases nutrient runoff and leaching into waterways.

  • Jumpstarts soil microbial communities.

  • Suppresses weeds and soil diseases.

  • Increases yields.

  • Increases nutrients and organic matter.

While compost can provide some nutrients initially, it has lower fertilizer value and is often called a soil conditioner or amendment. This is because most of the nutrients in compost become available for plants over a longer time scale—over months or years—unlike mineral fertilizers which are quickly available to plants.

In some ways, applying compost is similar to putting money into a savings account or emergency fund— the benefits are longer-term and create resilient soils that respond better to extreme events like drought.

However, there are unique challenges to purchasing and applying commercially produced compost to agricultural fields. Specifically, farmers have identified compost price, spreading (equipment and time), compost delivery, plastic contamination of compost, and lack of information as barriers to using compost.

From 2011-2016, the Washington State University (WSU) Compost Outreach Project worked to increase knowledge and awareness of compost in Snohomish county and also partnered with composters and farmers to run experimental compost trials on farms. Overall, most farmers experienced improved soil quality and crop production in compost treatments.

In 2015, 62% of farmers said they were motivated to keep trying compost. In 2017-2019, researchers at WSU further investigated the compost cost barrier by estimating the net return for five different crops grown in compost and non-compost treatments in Snohomish and King counties. Researchers found that raspberries and mixed vegetables had positive net returns in compost-amended soils, although compost costs outweighed the profits for yield increases in blueberries and soft white winter wheat. 

To reduce costs and still receive the benefits, you may want to consider producing your own compost instead. However, there can be a learning curve to managing decomposition.

Composting relies on a balance of air, water, and nutrients to heat up your materials to hot temperatures and keep microbes decomposing quickly. For manure, reaching upper temperatures of 130-165 F for at least three days is especially important when trying to eliminate pathogens and weed seeds.

Here are some general tips:

Air

You can add air to compost by regularly turning it over with a shovel or a tractor, adding pipes to get air into the center of the pile, or by rolling it if you have a tumbler system. This will also help make sure that everything in your pile/container gets composted (hotter temperatures tend to be in the center of the compost pile). You could also set up a blower system to actively run air through your pile/container.

Water

The compost’s moisture should feel similar to that of a wrung-out sponge. A dry pile (low moisture) will decrease the speed that materials break down, but a wet pile (high moisture, low oxygen) will break down slowly and can be messy and smell bad. In Western Washington, this means that composting buckets, bins, or piles need to be covered from the winter rain but sprayed with a hose in our dry summers.

Nutrients

The manure, yard waste, food scraps, paper, etc. you are trying to compost have varying levels of nutrients. For successful composting, it’s best to pay attention to carbon and nitrogen amounts. This is often referred to as parts carbon to parts nitrogen or a carbon to nitrogen ratio. In general, the sweet spot for composting is a mixture of materials with a carbon to nitrogen ratio between 20:1 and 40:1 although some have suggested an even narrower range (25:1 and 30:1). Nitrogen heavy materials (carbon to nitrogen ratio 20:1 or less) include food waste, grass clippings, and most manures and the more carbon heavy inputs (~50:1 or higher) include wood-based yard waste, straw, shredded paper, and autumn leaves. If you want to try calculating your own materials C:N ratio (and moisture content), try out Washington State University’s composter calculator tool.

Temperature

Use a thermometer to see how temperature changes and to make sure your compost is hitting that pathogen and weed seed-killing level (130-165 F for 3-5 days). Temperature can also be a great indicator for issues with air, water, and nutrient content.

Final Thoughts

In the past, crop residues, manure, and compost were commonly used to add nutrients back to the soil. However, many of the products and systems we developed to make agriculture more efficient have cut out these older practices. Now, people are rediscovering the benefits of adding compost to their lands and seeing their soil’s health improve. With modern equipment and technologies and creative problem-solving, they are working through the new challenges that continue to persist. 

Interested in applying your own? After working with Snohomish farmers, WSU outlines how. Not sure how it could work on your property? A helpful collection of compost images by WSU illustrate different techniques to apply compost.

Additional Resources

If you’re interested in reading more, check out these resources: