Researchers: Retta Bruegger, Megan Machmuller and Seth Urbanowitz
This project was funded by a Western Sustainable Agriculture Research and Education (WSARE) Professional-Producer Grant, received in 2020.
There is a lot of interest in improving soil health, sequestering carbon in agricultural soils, and using alternatives to commercial fertilizer. Research in California indicated a 1-time application of compost increased soil organic matter content, enhanced plant growth, and contributed to climate change mitigation in California’s rangelands (Silver et al 2018). Though promising, these results are untested in western Colorado, and were from rangelands, not irrigate pastures, which is more intensively managed.
With these interests and past research in mind, we were funded in 2020 to implement a 3 year treatment comparing compost with commercial fertilizer in irrigated pasture in western Colorado, specifically Mesa and Ouray Counties.
What rate are we using to apply compost?
We based the rate (tons/ acre) on the nitrogen demand of the crop relative to the available nitrogen in the compost. We also used assumptions about nitrogen mineralization rates and availability in the compost.
In coming to this conclusion, we found there isn’t great guidance on this topic. The amount of compost you need for this rate is quite expensive, ranging from 10 -16 tons per acre (compost was $45/ton + delivery), with 6 tons per acre used in the fertilizer plus compost treatment. This would be cost-prohibitive for most farmers and ranchers. However, influence on carbon and other characteristics would likely observable at such a high rate. As such, we decided to apply at these rates, because the goal of the study is to first answer the question ‘does it work?’ Further, one of the primary goals of the study is to test the practice in the context of emerging carbon markets. We would not assume farmers and ranchers would invest this much in compost without a payment for doing so in a carbon market framework.
How does carbon get into soils in the first place?
There are two main factors that contribute to a soil’s ability to increase soil organic matter, much of which is comprised of soil organic carbon. Vegetative growth of plants is the engine that drives below ground soil organic matter, and clay in the soil binds to the organic matter so that it stays in the soil. On a purely sandy soil, even if you have lots of production, you will not see accumulation of organic matter.
In our region, we certainly have the clay. Compost application hypothetically affects the first factor needed to increase soil organic matter below ground by increasing production. In short, the dominant pathway to increased soil organic matter belowground is production aboveground, and this is the basic assumption behind using compost. To a lesser degree (though still significant), compost may also allow microbes to more efficiently integrate carbon if the chemistry of the compost more closely matches that of their bodies (stoichiometry).
What are we measuring?
We are measuring grass hay yield, species composition and nutritional value. For soil we are measuring soil organic carbon, C:N ration, bulk density and more.
At left, Megan Machmuller demonstrates the Giddings probe, which we use to sample soils. We sample to a depth of 1 yard, and collect 3 cores/plot. We then create an aggregate sample per plot by 4 depths, which we sample in the lab for carbon, bulk density, etc.
Where is the experiment taking place?
The experiment is taking place at the Grand Valley Research Station in Fruita, CO, and at a private field near Ridgway. In each location, each treatment (i.e., compost, fertilizer, compost +fertilizer, or control) is replicated 3 times, for a total of 6 times between the two locations, and 24 plots total.
At right, Retta Bruegger and Megan Machmuller apply fertilizer to fertilizer-only plots. Fertilizer-only plots help contrast the impact of fertilizer versus compost.
What are the results so far?
We are collecting the second year of soil data in 2023. So far, results on yield show that compost lagged behind fertilizer in terms of yield (tons/ acre). We have not yet finished processing the soil health results.