Why Organic Matter Matters
Fact number one:
Plants are the mainstay of ALL life on earth.
Organic matter begins with plants when they use carbon dioxide (CO2) and water to capture sunlight energy. The energy is stored in chains of carbon atoms we call sugars, starches, fats, oils and proteins. Humans, other animals, insects and microbes consume plants to extract the sunlight energy as nourishment.
Strictly speaking, organic matter is plants and all the living things they support. However, in everyday use, organic matter is dead and decaying plants and animals. It is found in mulch, compost, manure, bio-solids, straw, hay, stubble and many other sources.
What is organic matter in soil?
Organic matter in soil is the sum total of live, dead and decaying plants and animals, microbes, insects and other critters in your soil.
Organic matter is the food that drives all life in the soil. It is seen as leaf litter and decomposing material on the soil surface and contributes to the dark color as you dig further down. We can’t see it but it is there and is vital for soils to work. Without it, our soils are dead.
As organic matter is consumed it breaks down into finer and finer particles. This occurs faster when the soil is moist and warm. Some organic matter is stored in a very stable form called humus which holds water and nutrients.
Fact number two:
Warning! Do not confuse organic matter with carbon
These two are often confused in soil tests and in conversations. Soil organic matter is usually about 58% organic carbon. If your soil test result shows % organic carbon then you can calculate organic matter as follows.
Organic matter (%) = Total organic carbon (%) x 1.72
There are three forms of soil carbon; organic, inorganic and elemental. For practical purposes, just stick with organic matter.
Why is soil organic matter important?
Organic matter has the capacity to supply ALL the nutrients pastures and crops require.
Prior to synthetic fertilizers, organic matter was what farmers relied upon until the early 1900’s. Good farmers treated organic matter with respect and maintained levels year-to-year by incorporating stubble, animal manures and using rotational and cover cropping to ensure as much organic matter was retained so crops received a full compliment of nutrients.
Synthetic fertilizers were, and still are, a quick and easy application to provide the “big three” essential nutrients (NPK) in modern cropping practices. In totality, more than 10 other nutrients are required for optimum health of soil, plants, animals and humans.
Continuous applications of only three nutrients (NPK) results in soils devoid of life, especially when routine applications of herbicides and pesticides also are also used in crops effectively killing all forms of life.
However, more land managers are realizing that maximum soil productivity needs organic matter, not just nitrogen, phosphorous and potassium (NPK) and the odd dose of sulfur and calcium and maybe, some trace elements.
Organic matter influences many soil characteristics including nutrient holding capacity, nutrient turnover and soil aggregate stability (structure) which, in turn, influence water storage potential, efficiency, aeration and workability.
Soil structure improves when organic matter provides a food source for microbes. Sticky secretions from microbes, fungal hyphae and plant roots all contribute to better soil structure. Soil particles bind together into aggregates or ‘peds’ that allow movement of air and water down into the soil, instead of running off the soil surface.
Fact number three:
An increase of only 2% organic matter in the top 30cm of the soil profile will store about 150,000 extra litres of water per hectare. The most effective method to increase organic matter over time is adhering to the rule – 100% living groundcover 100% of the time. Pasture cropping is one system which has proven this in many countries around the world.
Bonus. The extra 2% organic matter builds better soil structure which means rainfall will run into the soil to be stored instead of running off down into the creek.
How can you keep increasing soil organic matter on your farm?
Think of soil organic matter like water in a leaky bucket; the only way to fill the bucket is to keep adding organic matter faster than it leaks out.
The best way to add organic matter is to keep pastures and/or crops growing in your soil all the time with no fallow periods. Even in areas with cool weather and unreliable rain, organic matter can still be lifted. For example, pasture cropping is being used in Norway and very cold areas of the US. As one advocate for building organic matter says, the only restriction is the willingness to try it and to increase your ecological literacy.
A diverse population of plants aids the accumulation of organic matter especially when there is a mix of perennials and annuals. Learning from traditional methods of farming illuminates the practices where as many plants as possible are grown in the same area – known as polyculture (exact opposite to monoculture.) Biodiversity also has other benefits as it plays a role in animal health and human health.
Organic matter can supply the full range of nutrients required by pastures and crops, not just N, P & K and levels can be increased with mindful management.
Once soil tests have been undertaken, the addition of extra nutrients, such as high quality compost can accelerate the accumulation of organic matter.
For best results in medium to heavy soils, aim for at least 5% OM (2.9% organic carbon) in your soil test results. This level is ideal in the top 10cm but far more effective when it reaches a depth of 30cm.
Things you need to know
Moist, hot and well-aerated soil conditions favor rapid decay of organic matter. If the rate of organic matter addition is greater than the rate of decay, the organic matter in a soil will increase. Conversely, if the rate at which organic matter is added to soil is lower than the decay rate, the organic matter will decline.
Continuous perennial vegetation builds organic matter quicker than other rotations, especially if you use timed grazing rather than set stocking, and avoid over-grazing.
Moving plant and animal produce off farm or anything that slows down plant growth will slow down the increase in soil organic matter.
Erosion removes topsoil which contains the bulk of a soil’s organic matter. This can take years of management to replace and costs money. Best practice is not to lose it in the first place.
The absence of organic matter in farming soils is the cause of poor crop nutrition and hard soils. Have you ever wondered why more powerful tractors are needed? Not for speed but to bust through the poorly structured soils – when soils are soft and friable, sowing implements are easily pulled through.
All up, in years to come we will start to value organic matter as being more valuable than gold. It is stored solar energy and nutrients, the natural currency of living systems and agriculture.