Fall is a good time to incorporate organic matter like compost into soil. Increasing soil organic matter improves soil structure and adds nutrients. This in turn improves plant growth, efficient use of water, and water infiltration into soil to reduce runoff.

Plant roots grow in soil pore spaces. Pore spaces are where oxygen is found. Just as plants cannot grow without water, they cannot grow without oxygen. Soil pore space is important to plant growth and efficient water use.

Soils with good structure have adequate pore space making them well drained while still having good water and nutrient holding capacity. Ideal soils are about 45% mineral particles, 5% organic matter, and 50% pore space for water and oxygen. An important aspect of soil structure is soil aggregation.

Soil aggregates are groups of mineral particles (sand, silt, and clay) bound together by beneficial fungi, earthworm secretions, soil glues and more. Aggregates help create larger pore spaces for better oxygen recharge and water infiltration into the soil.

How well soil aggregates form and are maintained is highly dependent on organic matter and biological activity in soil. Avoiding excessive tillage and reducing soil compaction is also important to soil structure.

On soil tests, a goal to aim for is five percent organic matter. On many soil tests, it is common for organic matter to only be one to two percent. It is a good idea to have soil tested at a soil lab every 3 to 4 years. 

The addition of organic matter is not a one-time practice since it breaks down over time. Sources include plant debris like grass clippings, tree leaves and wood chips, well-rotted manure, composted yard waste, peat moss, and green manures (plants planted in fall and tilled under in spring).

Almost all soils benefit from the addition of organic matter. It improves drainage in clay soils and increases water holding capacity of sandy soils. It provides energy needed by soil microorganisms to build soil structure. They, along with earthworms, create soil glues that help aggregates form.

For compost, which is fully decomposed organic material, spread a two to three-inch layer of compost on the soil surface and incorporate it six inches deep. Use a rototiller or spade to work it into soil. Remember excessive tilling damages soil structure.

Use less if incorporating undecomposed plant material, like grass clippings or tree leaves. This has to break down to improve soil. During decomposition, microorganisms use nitrogen and this can lead to a temporary deficiency. Shredding organic material, such as running a lawn mower over dried leaves, before spreading it on the garden in fall will promote faster decomposition.

The addition of manure into garden soil is recommended during fall to avoid food safety issues with bacteria. If manure is used as an organic matter source, use manure that has been composted for one or more years and incorporate it into soil during fall.