4 Ways Compost is Used in Ecological Restoration

Compost can aid in soil rehabilitation in a variety of ways and in a variety of soils. Everything from a clay-heavy soil, a sandy soil, and even soils altered by urban landscapes and dwellings can be rehabbed with compost.  

In clay-heavy soils, bulk density can be high, water infiltration low, and nutrient availability can be a struggle depending on the type of clay found in your soils. While clay particles are small, they tend to clump together into large aggregates. When compost is added to clay soils, the organic matter in compost can essentially coat the large clay aggregates, helping to break them up. This increases water infiltration and soil porosity, giving plant roots access to plenty of water and air flow. Increased water infiltration and air flow also benefit those all-important soil microbes. Keep them happy and your soil and plants will thank you!  

Clay particles inherently have plenty of charged sites, but some clays are much better than others at holding onto key nutrients due to their chemical structure. In soils, the charged sites that retain nutrients are called the CEC, or cation exchange capacity, and the AEC, for anion exchange capacity. Cation exchange sites are negatively charged to retain positively charged nutrients, while the opposite is true for the anion exchange sites.  Plant-available nutrients also need a charge to hold onto soil particles and be taken up by plant roots. Soils, clays, and organic matter all have a net negative charge, meaning they have more cation exchange sites than anion exchange sites. The majority of plant nutrients are positively charged, although some major ones are negatively charged, like nitrate and phosphates. The addition of compost to clay soils breaks up the larger clay particles, giving them a greater amount of surface area, which contributes to more plentiful CEC and AEC sites to hold onto nutrients. This keeps the nutrients available to plants and microbes in the soil rather than these nutrients being lost to runoff or leaching out of the soil profile.   

Conversely to clay, sand is a much larger particle. While clay particles tend to aggregate easily, sand is much more of a loner in the particle world. Think of squeezing a handful of wet clay and handful of wet sand. The clay will generally mush together, but the sand remains, well, sandy. Because sand doesn’t tend to clump together as easily, sandy soils have opposing issues to clay soils. Sandy soils often have trouble holding onto adequate water for plants to thrive, whereas clay soils tend to hold onto too much water and can drain poorly. Because sandy soils have larger particles, they tend to have decreased nutrient availability due to a lack of surface area, meaning fewer charged sites are available. Compost can help remedy both of these situations! Just like organic matter can help to coat the clay particles in clay-heavy soils, organic matter, or compost, can coat sand particles. In regards to water-holding capacity, compost amendments can provide the necessary bulk, nooks, and crannies to retain water molecules in soil for longer. In fact, compost can retain up to four times it’s weight in water, allowing much more to remain in the soil longer, keeping microbes and plants hydrated.  

Soils depleted by urban activities, hardscaping, construction, tillage, or even plain ol’ landscaping and gardening can suffer from a variety of issues, like poor water infiltration, limited presence or complete removal of the A and O soil horizons (aka, the soil horizons that plants prefer to live in!), erosion, or compaction. In highly disturbed soils, poor water infiltration could be related to a hardpan clay layer, possibly caused by soil compaction from heavy machinery use, repeated tillage, or a previously existing poured slab for a structure. Clay is supposed to be good in soil, though, right? Yes, it can be! However, because clay has a net negative charge, it will bind very strongly to large, positively charged particles, like aluminum or iron. These particles in particular can be quite difficult to displace once occupying an exchange site due to their strong charge. A heavy clay layer can bind with aluminum and iron particles in the soil and create a very dense, nearly impermeable layer of clay, usually beneath the topsoil. Another practice that lends itself to hardpan production is heavy machinery and tillage, both of which also impact soil compaction. Repeated heavy machinery driving over grass will eventually flatten it; the same thing happens to soil particles! The air space (also called pore space) will be pushed out of the soil, creating a dense, heavy soil layer that is difficult to dig into and will support very little plant life. Compost applied before planting crops with large taproots (these can be vegetables or flowers), can help loosen compacted soil safely. Tillage is another big player in the soil compaction and hardpan clay production game. While tillage initially seems to fluff up soil, over time it actually fractures the soil, contributes heavily to soil erosion and topsoil runoff, and increases the mechanical impact of raindrops on soil beds. These might seem like inconsequential outcomes, but tilled soil particles can clog pore space deeper in the soil, resulting in impermeable layers of squished soil, causing soil and nutrient runoff, a halt in soil microbial activity, and poor downstream water quality. The good news is proper application of compost can help with ALL of these problems! Depending on the issue, small amounts of compost added regularly or large amounts added fewer times can help remediate these issues over time. Nothing is instant, of course, and whatever your soil’s history will affect the timeline.