Are You Thinking of Applying Compost to Your Soils?

Mar 22, 2023


Recent price increases and supply chain shortages for fertilizers, chemicals, and energy have impacted farm profitability and viability. As a result, many Indiana farmers are interested in alternative and more sustainable approaches to maintaining crop productivity and building soil health, such as applying composts to their soils.

Applying compost to the soil is common in many farming systems. Composting involves the controlled biological decomposition of organic materials into nutrient-rich soil amendments or mulches. These materials (feedstocks) may be rich in nitrogen, such as manures and legume plant residue, or rich in carbon sources, such as leaves or straw. An ideal carbon-to-nitrogen ratio for composting is 30:1. Lower ratios can result in excess nitrogen loss as ammonia gas and carbon loss. Nitrogen is insufficient for microbial decomposition at higher C:N ratios, which slows the composting process.

Composting requires moisture, aeration, time, and the proper carbon and nitrogen feedstock mixture to optimize soil fertility. When properly managed, compost applications can build soil health by increasing soil organic matter, increasing microbial diversity, suppressing soil pathogens, and reducing nutrient leaching into groundwater. Generating compost on-site and applying it enhances on-farm sustainability by reducing the contribution of organic feedstocks to landfills that lead to elevated greenhouse gases in our environment.

Not all composts are created equal

It is essential to know what is in your compost by analyzing it or requesting that information from suppliers. Test results will help guide how much and when to apply compost to build soil health. Organic amendments like compost do not supply plants with readily available nutrients (apart from inorganic N, which is immediately available) because they are released slowly through microbial decomposition. It may take several years to break down and release nutrients for plant uptake. The decomposition rate is affected by environmental conditions such as soil moisture and temperature. It is important to consider differences in compost decomposition rates for soils in high tunnels compared to open fields. To maximize plant nutrient uptake, apply compost in the Spring. As soil temperatures increase, mineralization will release nutrients during the growing season. Summer compost applications can be beneficial for hay and pasture areas. However, fall application typically increases nutrient loss unless soil temperatures are low enough to immobilize soil nutrients until the following Spring.

Protecting your investment in soil fertility

Farmers must consider long-term seasonal conditions and short-term weather events before compost applications. Many farmers we spoke with are concerned that heavy precipitation can increase the runoff of composts and other fertilizer amendments, leading to nutrients flowing off-site. One way to mitigate this is by applying mulches to rows or beds to ensure those nutrients remain near plant roots for uptake. Incorporating compost with shallow cultivation might be necessary to ensure the compost is present in the root zone. Keep in mind that it is easier to incorporate compost in light-textured soil.

What happens if I overapply compost?

When significant, affordable quantities are available, it may be tempting to overapply composts in depleted, compacted, or nutrient-deficient soils. However, over-applying compost, especially manure-based compost, can result in nitrate leaching into groundwater, excessive phosphorus soil concentrations, and high soil salinity. Typically, soils have more phosphorus than crops need when composts have been over-applied for several years, but nitrogen may be lacking. The high phosphorus concentration can reduce the crop’s micronutrient uptake ability (particularly iron and zinc) and reduce yield. Manure-based composts typically have a pH greater than 7, so monitoring pH in heavily composted soils is essential because elevated pH can reduce crop yields. In addition to the cost of making, transporting, and applying compost, these concerns have led many farmers to search for additional sustainable farming practices to maintain crop productivity and build soil health.

Is cover cropping an alternative?

There is an increased interest among vegetable farmers to incorporate cover crops into crop rotations in addition, or as an alternative, to applying compost. Using fast-growing annual cover crops such as different types of grasses, mustard, sunn hemp, clovers, vetches, rye, and radishes, to mention a few, can also build soil health by increasing water holding capacity, increasing organic matter and trapping excess nitrogen. Cover crops tend to increase phosphorus availability without leading to excessive soil concentrations. However, farmers may perceive the timing of planting, determining optimal application rates, time spent planting and killing the crop, and costs as potential barriers to using cover crops. Farmers have commented that it can be challenging to determine optimal application rates for cover crops when considering both nutrients already present in the soil and the nutrient needs of crops that will be planted. Some farmers may need to learn how to amend soil fertility after terminating the cover crop. Small-scale farming operations may not be willing to incorporate cover crops into their management plan if they are concerned that it could reduce yield, and farmers on short-term leases may be concerned that they won’t reap the rewards associated with the long-term benefits of building soil health. It may not be profitable for small-scale farmers to use cover crops and take land out of production for an entire season because this can lead to a loss of income.

Is it worth testing your soil’s health?

We noticed an increased interest in building and maintaining soil health in conservation circles and farm press but limited research on farmer perspectives. Still, farmers want to better understand the potential benefits and outcomes before investing time and money in resources like composts and cover crops. Farmers who use soil fertility tests to manage their crops and soil health have asked us whether soil health tests are worth the additional expense. Soil health tests offer farmers a comprehensive evaluation, or “score,” based on soil health indicators such as microbial soil respiration, water-soluble organic carbon and nitrogen, and their ratio. Many soil health tests are available at commercial testing labs, but the costs can be expensive compared to traditional soil fertility tests. We asked ourselves: How effective are soil health scores in determining variety selection and soil health management strategies? Will soil health test results give them additional insights and opportunities for management strategies beyond soil fertility tests? We hypothesized that applying composts and using cover crops would yield better soil health scores and reduce inputs over time, leading to more conservation-based, sustainable approaches to soil health management.

The Soil to Market research

We initiated research in 2021 at three locations in Indiana, one in the north (Wanatah), central (Lafayette) (Figure 1), and the south (Vincennes). Climatic conditions and soil type differ at these locations. Treatments included organic or conventional fertilizer sources, manure or plant-based compost sources, summer cover crops (mustard, sorghum Sudan grass, and sunn hemp) in year 1, and a mix of cereal rye and hairy vetch cover crops in the fall/winter. Some treatments received no additional biomass during the experiment, except for pepper crop residue, some received a lot with the summer cover crop in year 1, and some continue to receive more with the fall/winter cover crop. We conduct soil tests every Spring and Fall and use crop removal estimates to determine application rates. We aimed to manage fertility with similar soil nutrient levels in each treatment. That requires using conventional and/or organic fertilizer in compost and cover crop treatments. We experienced several issues during 2021. We aimed to implement the research by prioritizing sustainable and organic farming practices. So, we decided not to use plastic mulch on the raised beds, and we did not use any herbicides. We soon ran into weed issues. Grass took over the research plots, and we could not keep up with weeding activities. The weeds scavenged nutrients intended for the crop, and we lost nutrients due to runoff and leaching (Figure 2 & 3). Due to a lack of rain, we could not establish a mustard cover crop at the Lafayette location.

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