Soil health and fertility

soil food web jpeg large

Proper grazing management enhances the soil food web and the natural systems that create fertility and functional soil structure. 

Once a piece of land has become healthy and fertile, it can be used for cropland in rotation with pasture for livestock; crop yields will be more abundant than could be expected on land that has not been managed by rotational grazing.  Studies have shown that rotational grazing produces more herbage biomass than one all-season paddock or non-grazing, and that the additional herbage translates into increased weight gain of livestock that are grazed in a rotation rather than conventional  grazing.

Fertility

Though providing soil nutrients to plants involves complex chemical processes, the basic pasture management goals are simple: to establish deep-rooted plants, and to foster abundant soil life, of which mycorrhizal fungi and associative bacteria are key. The same grazing practices that increase  fertility also increase carbon sequestration, which is the precursor to fertile soil.  Microbes will convert the carbon to humus.

When energy from the sun begins photosynthesis, carbon is channeled into the plant’s roots. Deep roots, in addition to providing mechanical support, transfer between 5% and 21% of all the photosynthetically fixed carbon to the soil, well below the surface. In a two-way transfer that utilizes a microbial “bridge,” the roots also allow water/nutrient uptake for healthy plant growth.

This bidirectional flow happens through a symbiotic relationship between plants and mycorrhizal fungi.  The fungi link with the cell walls of the plant roots and grow into them, creating structures that allow for the transfer of nutrients between the plants and the soil. Carbon flows out from the plant host to the fungi on the roots. The fungi have long hyphae that can extend several yards into the soil to access and exchange carbon for minerals.  The soil minerals, which are essential to plant health, are then transferred from the fungi to the plant. And the cycle continues as these nutrients  in turn facilitate an elevated rate of photosynthesis and increased production of carbon.

Micorrhyzal fungi and associated bacteria are all important to nutrient transfer and soil fertility.
Micorrhyzal fungi and associated bacteria are critically  important to nutrient transfer and soil fertility.

The fungi can become the primary organ acquiring mineral nutrients for the plant. These nutrients are phosphorous, organic nitrogen, and calcium, as well as trace elements such as zinc, boron, and copper, and plant growth stimulating substances.

But this critical bidirectional flow between plants and the soil is often hindered by conventional agricultural practices, including (1) soil disturbance, such as plowing, (2) application of nitrogen or water-soluble phosphorous, and (3) application of herbicides and pesticides that kill essential soil microbes. When the carbon supply becomes limited by the loss of this primary pathway for sequestration, soil loses its physical, chemical, and biological functions, and this causes mineral depletion in food. The extent of this nutrient decline in food over decades has been documented by a study in the UK.

Other important sources of fertility are (1) manure, which the cattle spread evenly over the land, and which supplies microorganisms as well as soil nutrients, (2) weed residues and green manures that decompose, and (3) the root material that the plants shed when they are grazed in order to re-establish equilibrium between their root and leaf areas.

Soil structure and water retention

A recent study conducted by the USDA NRCS (National Resource Conservation Service) illustrates the dramatic difference in water infiltration capacity on land managed by rotational grazing as opposed to cropland or land managed by conventional grazing. A video of an experiment on three South Dakota fields representing these three scenarios showed that the same amount of water took 31 minutes to infiltrate the cropland soil, 7 minutes to infiltrate conventionally grazed pasture, and 10 seconds to infiltrate soil managed by rotational grazing. See the 6-minute video from the NRCS here.

A key to the soil health and water retention capacity of the land managed by rotational grazing is the abundance of soil microbes, notably the Glomales fungi. Glomalin is made by Glomales fungi as part of their stress response. High levels of carbon dioxide levels in the air stimulate the fungi to produce glomalin.  These fungi colonize plant roots and make a protective waxy coat out of glomalin that improves water infiltration and water retention in the soil and keeps soil carbon from escaping. Glomalin lasts 7 to 42 years, depending on conditions. The production of glomalin has been linked to grazing.

Glomalin holds 27 percent of the soil’s stored carbon.  Discovered in 1996, it is a glycoprotein, bound together with iron and other ions.  It permeates organic matter, storing it in both its protein and carbohydrate sub-units, and is 30-40% carbon.

Healthy soil results in lush pasture that can withstand drought.
Healthy soil results in lush pasture that can withstand drought.

Glomalin also glues together silt, sand, or clay soil particles, forming large granules or aggregates that improve soil condition, providing  what is commonly known as “tilth.” Aggregates protect soils from the eroding forces of winds and water. Previously  plowing was thought to increase tilth, but now it is understood that soil disruption actually contributes to soil compaction and that no-till practices increase soil glomalin.

A 2013 study of three grazing scenarios and three cropland scenarios found the highest concentration of glomalin in native grassland pastures managed by rotational grazing.