Organic matter is carbon-based matter that results from decomposition of plants, microbes and other organisms. Organic matter in soil plays an important role in plant nutrition and soil health. Healthy soil has a number of soil organisms, including beneficial soil nematodes, earth worms and a number of microbes that are responsible for nitrogen transfer from the soil to plant roots. Organic matter is more than fertilizer. Organic matter often has beneficial microbes and trace elements that are critical to soil and plant health.
Unless replenished, organic matter can become exhausted after several planting cycles. In nature, leaves and other plant wastes are the source of organic matter replenishment. In gardens, however, the natural cycle is interrupted. In addition, in gardening and agriculture, plant densities are often higher than they would be in nature. In general, 5 percent fresh organic material in soil will ensure good crop production.
Organic materials in the soil offer additional benefits. In sandy soils, water drains too quickly. The water drops below plant roots before it can be absorbed. Adding organic matter to sandy soils can help to both increase soil fertility and increase the soil's ability to hold water. Although the exact percentage will depend on the crop and a professional soil analysis, adding 20 to 30 percent organic matter to soils that start out at around 50 percent sand can help the soil retain water.
Clay soils offer very few nutrients and can retain too much water for many plants. This additional water retention can cause root problems. In addition, clay soils can be very hard, making it more difficult for roots to grow in the soil. When working with a primarily clay soil, adding organic matter can help to add nutrients. However, because of their poor drainage, you will likely also need to add some sand to improve drainage. Add equal parts organic matter, like compost, and sand to heavy clay soils to get the percentage of clay below 40 percent.
Annual Soil Replenishment
If you are gardening heavily, your soil may need annual replenishment of organic matter. In most cases, compost is the most convenient form of organic replenishment. By adding between 2 and 6 inches of compost to your garden soil, you can add fresh organic nutrients to your garden soils.
Other Sources of Organic Matter
Organic matter can come from a number of sources. Although compost, whether from a home compost pile or from a commercial compost source, including plant-based compost and compost manures, is popular, a number of other gardening practices can help add organic material to your garden. Organic mulches help keep weeds down and help retain soil moisture. In addition, they break down over a season. As they break down, they add organic matter to your garden soils. Common organic mulches include grass clippings, dry leaves, wood chips and straw. An inch or two of mulch over a year can serve multiple purposes.