Anything you read about healthy garden soil and how to improve your soil mentions organic matter. While only a small fraction of agricultural or garden soils—between 3 and 6 percent—consists of organic matter, it plays a crucial role in soil productivity. Being knowledgeable about organic matter is important even if you have “good” garden soil because all soil needs to be replenished with organic matter from time to time. Amending soil with organic matter is not a one-time project but an ongoing process.
This article explains what organic matter is, what differentiates it from organic material and fertilizer, and why organic matter is beneficial for soil productivity in many ways.
What is Organic Matter?
Generally speaking, organic matter comes from living materials that fix and store carbon and deliver it as a source of energy to the soil. More specifically, organic matter is divided into three types, depending on the time it takes for the organic matter to fully decompose. Active organic matter consists of fresh plant and animal residues that take from a few months to a few years to decompose. This type of organic matter is soil that is very much alive because it is filled with lots of active microorganisms. At the other end of the spectrum is passive organic matter, also called humus. It is the stable form of organic matter where the decomposition has already been completed and there is no longer any microbiological activity. Slow organic matter is somewhere between active and passive—it is organic matter that takes decades to decompose, such as bones.
Organic Matter vs. Organic Material
Organic material is the source of organic matter: leaves, compost, manure, plant residues etc. As organic material decomposes, it changes its form and mass; it is an unstable material. About 90 percent of the organic material disappears during the decomposition process into organic matter. That’s why soil formation, of which the integration of organic matter is an initial step, takes so long—up to 100 years to form one inch of topsoil.
The Benefits of Organic Matter
Adding organic matter to soil improves the soil texture, regardless of the type of garden soil you have. Sandy soil has the largest soil particles, which affects its ability to hold water and nutrients and hence healthy plant growth. Adding organic matter to sandy soil improves its ability to retain moisture so that it is there long enough for the plants absorb. Clay soil, on the other hand, has extremely fine soil particles. While clay soils are often rich in nutrients, they are also dense because the soil particles stick together, which leads to poor drainage and a lack of oxygen, which negatively affects the root growth. Adding organic matter improves aeration by loosening up heavy clay soil.
Even if your soil is loamy, which means it has a balanced mixture of different-sized particles, adding organic matter is still beneficial for soil texture, as soil compacts over time.
Soil is not a dead material but very much alive. It needs organic matter to feed the microorganisms and insects that make up its balanced ecosystem. Organic matter creates the environment for the many types of soil microbes and organisms that enhance plant growth and health.
Also, the more beneficial microorganisms are in your garden soil, fewer bad organisms will survive. That's because these good guys feed on harmful microbes, such as nematodes and certain soil-born diseases.
Beneficial microorganisms also release nutrients into the soil when they die and decompose. The more beneficial microorganisms that are in the soil, the more nutrients will be in the soil.
Adding organic matter improves the soil’s Cation Exchange Capacity (CEC) so it can better hold the positively charged ions of key nutrients such as calcium, magnesium, potassium. Organic matter contains acids that can make plant roots more permeable, improving their uptake of water and nutrients, and it can dissolve soil minerals, making them available to plant roots.2
Organic material also fends of changes in the soil pH, an effect called buffering. The pH also plays an important role in nutrient availability and uptake.
How to Add Organic Matter to Soil
As long as it has been decomposed into humus, you can add any type of organic matter directly to the soil, such as mature compost, well-aged manure, and peat moss.
You can also add other types of organic matter to the soil but keep in mind that high-carbon organic matter can take a long time to break down.
Work the organic matter into the soil when it’s dry. Thoroughly mix it into the soil by rototilling or digging, which prevents layering.
There are no fast and hard rules for the maximum amount of organic matter that you can add to your soil, as it depends on the quality of the existing soil. For a garden or any areas where crops are grown, you cannot go wrong by adding up to 2 to 3 inches of organic matter every year, and working it in.