Soil Health in Relation to Grazing
J. Volk, W. Barker, and J. Richardson, NDSU Animal and Range Sciences and Soil Science Departments
Soil, the most important natural resource in the Coteau region, determines the economic returns and the success of the rancher. Soil health and grassland vegetation should be managed in a manner to promote soil quality and overall rangeland quality. Rich, deep soils result in productive bio-diverse grasslands. Conversely, the grasslands produce the rich deep soils, an example of mutual synergism. Healthy soils need root turnover to promote soil microbial activity and provide soil organic matter, and vegetation needs healthy soils with their stable structure, and water penetration and retention properties that result in deep robust root growth. Specific land management practices promote the synergistic soil health and grassland biodiversity.
Russians call grassland soils Chenozems or black soils, Canadians call them Orthic black soils, in the U.S. we call them Mollisols or soft soils to point out their mellow nature. Regardless of the name, their fertility and tilth mean they are the world’s most productive soils. Prairie vegetation contributes a large amount of organic matter into the soil due to a large root biomass and rapid turnover. The conversion of organic matter results in the black color, soft stable soil structure, and an increase in water infiltration properties.
In this study we used six soil cores taken from each of eight land uses: light continuous grazing, historic short duration grazing, heavy early grazing, intensive continuous grazing, moderate continuous grazing, heavy late season grazing, idle (CRP) and non-use native range, and twice over rotation grazing. The soil cores were evaluated in terms of soil macro- porosity, soil color, root distribution and depth, and additional observations of root fragility and soil texture. Only two range sites which represented nearly identical landscape positions were used: silty and overflow.
Results and Discussion
Our criteria in regards to soil health indicate that twice over rotation grazing promotes the “best” soil health. The abundant root growth over 15”deep was observably greater than other systems. The large connected pores allowed for high water infiltration and root penetration to at least that depth and probably deeper. Deep active roots allow for better plant nutrient acquisition because they have larger soil volumes from which to gather nutrients. Roots from grasses receive large amounts of translocated carbon from photosynthesis adding to the soil organic matter. The highest herbage production, highest biodiversity, and most native plants resulted from twice over rotational grazing. Soil health and ecosystem health therefore improved.
We rated the light continuous grazed pasture next best. There was very little root matting at the surface of the soil resulting in good root distribution and robust roots. Soil structure was well aggregated with large pores deeper than 15”. We noted that pastures with twice-over rotational grazing and light continuous grazing had high biodiversity. We concluded that soil health was positively influenced by biodiversity. Diverse plant communities do not produce root bound conditions nor root matting. The presence of legumes promotes the addition of organic matter over time along with the natural fixation of nitrogen.
The third group included historic short-duration / miscellaneous pastures. The conditions involved here were interesting because these pastures seemed to be drier than other pastures, even though all pastures have approximately the same topography. The water retention was lower than the above land uses. Root matting was observed but these soils did not appear to be root bound. Overall, these pastures had good soil structure, fair root disposition, and fair water permeability.
The fourth group included the heavily grazed late season pasture. Plant diversity was high on this pasture which positively influenced root distribution. The root mats were present but not to the point where the roots are root bound at the surface. Approximately a 1.5” root mat was observed on these sites. The soil structure still consisted of large aggregated pores allowing for deep water infiltration and deep robust root disposition.
The fifth group included the moderate continuously grazed pasture. This land use typically produced a root mat of 1.5” to 2”. Root disposition was not very deep and roots were thin and fragile. Below the root mat, the observable vertical porosity was poor, which impeded water percolation. These sites are shallow soils for the respective range site and soil development appeared to be poor. We concluded that the lack of good aggregation below the root mat and large massive, weak soil structure represents soil deterioration. A change in color of the soil from black to gray at approximately 10” occurred. This layer impedes water percolation and root growth past that point. Overall we rank this soil as fair at best.
The sixth group included the intensive continuously grazed pasture. The root mat of this pasture was approximately 2” to 3”, but still promoted fair root growth within that zone but not deeper. The soil itself below the mat was very hard with poor observable vertical porosity. This poor porosity leads to low water percolation and fragile root development. The saving grace of this soil is the plant diversity. A good mixture of grasses, forbs, and legumes on this site makes the soil health better than one should expect.
The seventh group included heavy early grazing. As observed in the moderate grazed pasture there is a 1.5” to 2” dense root mat. The vertical soil structure was weak and nearly clod-like with very few vertical pores. The lack of vertical pores did not allow roots to penetrate deep and we assume water percolation would be impeded. Roots were extremely fragile and did not penetrate deep into the soil due to the degraded soil structure. This site was dominated by Kentucky bluegrass, which is a sod former. The difference between heavy early season grazing and intensive grazed system is the plant diversity impact on the soil health. A more diverse plant community occurred with the intensively grazed pasture.
The poorest soil that we observed was on the non-use native range and the idle (CRP) lands. These sites had a 2”-3” very dense root mat and high surface litter content. The roots are kinked and fragile and only penetrate approximately 6”. There is poor vertical structure impeding soil porosity and retarding water penetration or root growth past 6”. There is a drastic color change at approximately 6” from a black to a light colored gray. The grey color soil acts almost like a water barrier impeding any further root penetration and deep water infiltration. We assume that the water flowed off laterally. The vegetation on these sites is virtually a monoculture of smooth brome. We concluded that CRP and idle native range need grazing disturbance and increased plant diversity.
In summary, soil health depends on root growth and grazing and plant species diversity. The development of excellent porosity and soil organic matter relates directly to roots growing to depths greater than 12” and biodiversity observed with rotational grazing systems or light/moderate continuous grazing. The difference between the soils of the twice over rotation and idle (CRP) land can be seen by the naked eye. In particular, abundant vertical pores and root distribution were easily viewed in soils of the rotational grazed system.
The low soil quality of the non-use native range and idle (CRP) lands stem from a dense litter mat lying on the soil surface that is not being incorporated into the soil profile. Therefore, recommendations to positively influence the soil and vegetative health of idle and CRP lands would be grazing and species diversity. Moderate grazing reduces litter build up, increases water and root penetration and so incorporates the soil organic matter in the soil and not on the soil surface as in idle and CRP lands.