Why Add Organic Matter to Lawns?
Lawns should be able to generate their own organic content if the soil is not compacted and the grass roots grow full and deep, and if there is a healthy population of earthworms, other invertebrates, and microorganisms. Then the level of organic material in the soil will be a balance of the addition of dead roots, stems, blades, and soil organisms. Roots, earthworms, and microorganisms are constantly multiplying and dying. However, for most lawns our mowing and removal of grass clippings tips the balance towards lowering the level of organic matter.
Because organic matter, whatever its type has a spongy, loose texture, when it is introduced into soil, it creates pore spaces between its particles, lightening its texture. This is especially helpful in clay soils but is good for all soil types. Plant roots can penetrate this soil more easily and grass is healthier.
The soft, loose texture of the organic matter in a lawn mulch improves the ability of the soil to both hold moisture and drain away excess water. Because of this virtue, organic material is particularly useful in enhancing water retention of sandy soils.
Healthy lawn soil contains 3-5% organic material. That level can be maintained only if you add organic matter to the soil at the surface to replace that removed. The life in the soil such as grass plants, earthworms, invertebrates, and microorganisms need that additional matter to sustain a healthy soil.
Few home garden lawns have soil this healthily rich in organic material. Many New Zealand lawns contain less than 1% organic material. This is most often because no new organic material has been introduced, as the most abundant natural sources, leaves and grass clippings, were systematically collected and disposed of or composted for use in flowerbeds.
A healthy soil needs a steady source of new organic material. Organic material in the soil is continually decomposing and releasing nutrients for the grass, and needs to be replaced for healthy soil and lawn. The black fibrous material (called "humus") that results from the decomposition of dead grass, roots, stems, clippings, and other organic material eventually has little value remaining.
Mulching grass clippings into the lawn provides some organic material, however, it is rarely sufficient to consistently provide the 3-5 % needed for a healthy lawn. So, you should provide more.
How Organic Matter Mixes with the Soil
When you add organic material, or mulch the lawn by leaving grass clippings and chopped leaves on the lawn when you mow, the organic material eventually decomposes, and earthworms pull it down into the soil and eat it, helping break it down. Earthworms excrete their own weight in nutrient rich castings every day.
Worms not only produce this valuable natural fertiliser, but they also spread it evenly through the topsoil. In many cases they travel much deeper, sometimes as far down as 2 metres. The upper layer of rich soil is where most nutrient absorbing grass roots and micro-organisms are found.
Adding Organic Matter Improves the Lawn Grass
If you do not routinely add organic matter to your lawn it is time to begin. It contributes to the development of healthy soil by stimulating soil life to provide nutrients and by loosening soil structure so that it holds air and moisture.
Healthy soil means healthy grass. Growing lawn grasses in soil with 3-5% organic content helps grass plants develop their root systems, penetrating deeply into the soil to tap more dependable supplies of water and nutrients. The grass becomes more self-reliant, and less dependent on added nutrition and moisture. The grass will be less susceptible to stress. This will also result in fewer weeds, diseases, and pests.