An earthworm navigates the root mass of annual ryegrass and arrowleaf cover plants under the remnants of a manure paddy.
We can all appreciate, on cold winter mornings, how an old truck’s temperature gauge rises and helps its engine smooth out and run at peak efficiency. It works much the same when there’s enough organic matter in a pasture’s soil. Soil organic matter, functioning as a kind of gauge, can even out a pasture’s soil performance and ensure that it functions at peak efficiency.
Scientifically, soil organic matter refers to the amount of carbon-based material in the soil. In a practical sense, however, the term quantifies the soil’s living component – roots, fungi, bacteria, earthworms, etc.
Lately, there has been a renewed interest in managing soil organic matter levels in pastures. Much of this interest has to do with ongoing drought stress, how climate change affects pasture productivity, and a resurgence in exploring alternative grazing methods.
However, good graziers have known the benefits of increased soil organic matter for some time. High-organic-matter soils act as sponges to hold more water; are less dense, or compacted; have a higher cation-exchange capacity; and are more resistant to acidification by nitrogen (N) fertilizers. They also have more stable temperatures, host more beneficial microorganisms, and provide a reservoir for the rhizobia that infect legume nodules and fix N.
Scientists have documented that soil organic matter is higher in pastureland than in cropland. Southeastern pastures have double – or more – of the soil organic matter, on average, of neighboring croplands. When cropland is converted to pasture, soil organic matter usually increases by 0.1-0.2 percentage point per year for 10-20 years following conversion. Within 10 years, the soil holds half of its maximum organic matter and, by year 25, 80% of it.
My University of Georgia colleagues and I have documented some of the highest rates of soil organic matter increase on record – about 0.35 percentage point in each of the first three years after cotton and peanut cropland was converted to a dairy practicing management-intensive grazing. The increase from around 1% organic matter to over 2% in three years was nearly unbelievable.
We have confirmed this rate of change and have been examining which part of the forage system contributes the most to this change. We’ve also been evaluating how much of the organic matter buildup is due to the roots, plant litter and animal manure by monitoring the radioisotope signatures of these biomass sources as they decompose and become incorporated into the soil.
Preliminary results indicate that root systems are the major source of soil organic matter improvements. These results support the findings of a consortium of American and European scientists in a recent review in the journal Nature. Their controversial report challenged the long-held belief that crop residues and other biomass on the soil surface are the sources of soil organic matter buildup.
Whether large or small, the roots and the fungal mycelia that grow in association with them thoroughly explore every nook and cranny of the soil, even the microscopic pores and crevices in soil particles. There also are root exudates that lubricate and protect the roots as they slip through the soil, and sugars and protein that the roots exude to feed their fungal dance partners.
Though these forms of carbon were once assumed to decompose easily, current research is showing them to be incredibly resilient and stable, often residing in the soil for 30-50 years after their creation.
So, what does all this mean and matter? It shows that improvements in grazing management that focus on encouraging root development, rather than accumulating plant litter, will provide the greatest improvements in soil organic matter. This means a focus on rotational grazing techniques is critical.
The root systems of plants in continuously stocked pastures areshort and shallow. These plants are commonly grazed every two to seven days, on average. Rotationally stocked pastures allow plants a recovery period long enough to permit a more robust root system to develop.
Soils in a pasture system have a distinct organic matter and health advantage over continuously cropped soils, but tremendous improvements over common, continuous grazing methods can still be had. Implementing advanced grazing management can make the pasture more resilient to drought and climatic stresses, as well as provide all the many other benefits of a rotationally grazed system.