Management Practices as they relate to Soil Productivity

By Thembi Borras

The proverb, an ounce of prevention is worth a pound of cure, is worth repeating in reference to soil conservation. Preventing soil loss, organic matter loss and soil compaction is far more cost effective than building soil. That being said, minimizing rather than preventing these impacts is more realistic in the industries that supply the goods, which occupy a soil medium, which society demands, including oil, water, food and forest products. In the context of forestry, minimizing soil impacts is a function of the type of silviculture, harvesting, site preparation, slash management and road management implemented.

Paul Zinke, Professor of Forestry at UC Berkeley ranked silvicultural systems and harvesting methods by their degree of soil disturbance. Of the six silvicultural systems, which include single tree selection, group selection, shelterwood, seed tree, clearcutting and biomass harvesting, single tree selection, with 10% of the area affected and 5% of the trees removed, was ranked lowest. Clearcutting, with 90+% of the area affected, was ranked highest and clearcutting where the crowns as well as the logs were removed had a greater impact on soil fertility because the foliage, twigs and branches contain the highest above ground nutrients of the tree. Of the harvesting methods evaluated, feller-bunching and tractor or skidder logging produced a higher degree of soil disturbance than cable and horse logging which produced a higher degree of soil disturbance than helicopter logging.

The type of silviculture, harvesting, site preparation, slash management and road management interact with each other, the topography, e.g. steepness of slope and aspect, and the characteristics of the soil, e.g. shallow, bare, infertile and/ or saturated, to further minimize or maximize soil impacts. For instance, implementing single tree selection will not generate as much slash as a clearcut and therefore may not require the practices typically used to prepare a site after a clearcut. To facilitate planting the next generation of trees after a clearcut piling and burning, broadcast burning, terracing and ripping may be employed, which can lead to topsoil loss, erosion and volatilization of nutrients.

No matter the silvicultural system or the harvesting method, the creation of slash is inevitable. Slash management includes leaving slash in place, burning, lopping and scattering, chipping, packing slash on skid roads or removing slash from the site altogether. Generally, leaving slash in place is best because the nutrient cycle and soil fertility is impacted less.

Roads can significantly contribute to soil loss, organic matter loss and compaction. Minimizing roads, using roads in the season for which they were designed, draining roads well and frequently onto stable surfaces, diversion proofing crossings, and designing crossings for the large episodic event are road management practices to minimize impacts on soil. Moreover, in tractor yarding, restricting equipment to well-located portions of the existing skid road network and prescribing equipment operators to move no more soil than is necessary minimizes soil impacts.

A portion of this production was gleaned from Sustaining Site Productivity on Forestlands; A User's Guide to Good Soil Management published by the University of California.