Soil Rebuilding
Updated: Feb 5, 2019
Soils are by far the most important part of any landscape. And although it may be difficult to “see” healthy soil, it is easy to see the results of healthy soil. Healthy soils (and proper soil care) provide plants with all the nutrients that they need, help them adapt to drier conditions and fight off disease. Healthy soils also suppress weed growth. (Weeds love poor soils that ornamental and native perennials struggle in.)
Tilling
In the perfect world the soil would never be tilled. While standard practice is to till the soil to effectively mix in organic matter and break up clay, there are some negative consequences. In Colorado’s clay dominated soils, repeated tilling can create an impenetrable clay pan below the depth of the tiller blades. Clay particles are plate shaped, and repeated compacting of them by the tillers blades slipping over them aligns them so that they make a dense layer with little to no pore space. Pore space is what allows oxygen, water and organic matter to move through the soil.
Tilling can also destroy the soil structure because it breaks large aggregates of soil into smaller pieces or even powder. When smaller pieces settle or are compacted, they have far less pore space to allow for oxygen, water or organic matter. The larger aggregates of soil are held together by delicate fungal hyphae and tunnels from invertebrates which are destroyed when chopped up. These fungi and pore spaces are important to healthy soils.
Everything gets mixed up. Soil microorganisms are tiny. Displacing them even a centimeter is like you or I moving to another planet. Plant roots ‘farm’ these microorganisms and have a very specific group of microorganisms that they work with. The organisms provide different nutrients that the plants need and species vary from centimeter to centimeter as they fill important niches in the soil ecosystem.
Invertebrates get chopped up. All those great earthworms, ground beetles and other beneficial insects that live in the soil are done for when the soil is chopped. Tilling disturbs and destroys the delicate soil food web which provides everything from disease resistance and fertilizer to greater moisture retention.
Weeds love tilling. Turning the soil exposes buried seeds to the sun and water that they need to germinate. (Bindweed, clover and buckthorn just to name a few.) It also chops up the roots of weeds that easily propagate via roots, and turns one into many. (Bindweed and cheatgrass are a couple great examples.)
Established Gardens
Soil improvement is just as important to established gardens as it is to new ones. Simply changing pruning and mulching habits can make a big impact on soil and plant health. Mulching your gardens with healthy garden debris cycles nutrients back into the soil. Think about it this way, plant roots “mine” nutrients and bring them up to the surface via branch and leaf growth. When the above ground growth is cut and left to decompose on the ground, you are essentially depositing nutrients from far below the soil surface at the top of the soil. Then invertebrates carry these nutrients back down into the soil. Viola! Nutrient cycling.
New Build Construction Sites
Construction sites create several of their own soil health challenges. The topsoil is generally scraped and stored in large piles for later use, or worse, sold. This topsoil is vital to the health of the future landscape because it contains a wealth of soil microorganisms that are native to that exact location and soil type. Once the topsoil is stored in piles over 12-18” deep an anaerobic environment is created and many of the organisms die. Often times the topsoil that is returned to the site has a great reduction in living organisms. Fortunately with proper storage and careful re-application, losses can be minimized.
After construction is complete bare subsoils may remain as the planting medium. These soils lack the organisms, nutrients and structure to support healthy plant growth. This is why turf in new neighborhoods needs to be watered twice a day for five minutes at a time; because excessive water quickly runs off due to the impermeable compacted clay that remains post construction. It is hard for new roots to penetrate this compacted clay layer. This stressed turf also requires a great amount of fertilizer and herbicides.
Compaction by heavy equipment and foot traffic is also a big problem. This compaction exacerbates other problems and turns the subsoil into a deep, impenetrable layer. By using a thick layer of mulch (or other preventative measures) prior to traffic, compaction can be minimized. Unfortunately it is not always possible to alleviate compaction. This makes post-construction soil building vital to the health of the future landscape.
Soil contaminants are another issue in construction sites. Oil from equipment, concrete and paint washouts and garbage are commonly found in these soils. With just a bit of care and designation, soils contaminants can practically be avoided.
All this said, soil building/rebuilding really doesn’t require a ton of work. The biggest impact is just in altering the actions that we are already taking. The bad news is that construction is really rough on soils, but the good news is that we can bring it back.