By Kelly Feehan, UNL Extension Horticulture Educator
Good soil is the foundation to efficient water use and healthy plants. Quality soil 1) allows water to infiltrate rather than run off, 2) has good water-holding capacity so less water is lost to gravitational drainage, and 3) is well drained to allow excess water to drain away so oxygen is available to roots along with moisture.
People often ask what they can add to soil to improve it, especially if they have a more clay or more sandy soil. Along with good management practices, the addition of organic matter is one of the best ways to improve soils. Generally, soils with 3-5% organic matter have better structure.
While a soil test will determine the percentage of organic matter a soil has, and can help determine the best source of organic matter to use, organic matter is one amendment that can be incorporated without first taking a soil test.
Benefits of Organic Matter
Increasing soil organic matter increases infiltration rates in clay soils to reduce runoff of water and nutrients. It improves the water- and nutrient-holding capacity of sandy soils to decrease water and nutrients lost to drainage. Organic matter has the added benefit of promoting soil organisms like earthworms that aerate soil and beneficial Mycorrhizal fungi that enhance water uptake by roots.
Organic matter can be used fresh or composted. Examples are compost, manure, grass clippings, tree leaves, wood chips, sawdust, and green manures.
Amending Soil with Compost
Fresh materials are best incorporated in the fall to allow time for decomposition. Soil microorganisms require nitrogen for the decomposition process; hence the use of fresh materials during the growing season can cause a temporary shortage of nitrogen for plants.
Composted materials can be incorporated during fall or just prior to planting. Decomposed organic matter is ideal because it can be more easily mixed into soil. Soil amendments need to be thoroughly worked into soil, not just buried beneath the surface
The easiest and most effective way to incorporate organic matter is prior to lawn and landscape installation. It is important for organic matter to be incorporated throughout the planting site or bed. Organic matter added to individual planting holes only creates conditions conducive to girdling roots and plant death.
To incorporate organic matter deeply and thoroughly into soils, use double digging or tillage. This will break up hardpan, encourage beneficial microorganisms and improve water infiltration and drainage.
Double digging is the practice of manually removing the topsoil, loosening the subsoil, then replacing the topsoil. Typically, about 6 inches of topsoil is removed and set aside in a wheelbarrow or on a tarp. The soil below the topsoil, subsoil, is loosened with a spade or shovel. Organic matter can be added to the subsoil at this time if needed.
The topsoil is then placed back over the loosened subsoil with additional organic matter incorporated. It is important the subsoil and topsoil are both loosened, but not mixed with each other.
To determine the amount of decomposed organic matter to add, use the 2:1 ratio rule of thumb. For every 2 inches of soil depth tilled, add 1 inch of decomposed organic matter. For example, when working soil to a depth of 8 inches, plan to add 4 inches of compost evenly over the soil surface prior to tilling.
On established landscape beds, increasing soil organic matter is more challenging. Organic matter cannot be readily incorporated because tillage around established plants will damage roots.
Instead, remove any existing mulch and then topdress by spreading a 1-2 inch layer of well-rotted organic matter over the area. Adding organic matter in this way is a much slower process but does not damage established roots and can be done any time during the growing season.
After topdressing, add 2-3 inches of mulch to the soil surface to reduce evaporative water loss from the soil. Organic mulches, like wood chips or shredded bark, will slowly add organic matter to soils as they decompose. Replenish mulch as needed to maintain a 2-3 inch layer.