 SOIL SURFACE MANAGEMENT OF ANNUAL RANGELANDSSoil surface conditions have direct and indirect effects on water infiltration, soil organic matter, soil aeration, soil compaction, runoff, and evaporation. The micro-environment at the soil surface influences seed germination and seedling establishment, thus influencing plant species composition and plant succession. Plant Succession On annual rangelands, plant species composition is heavily influenced by soil surface conditions. Most seeds germinate on the soil surface or at depths to 0.4 inches below the surface. Dormant seeds and small seeded legumes require heat treatment to break dormancy. Moderation of summertime diurnal temperature variation and reduction of maximum temperature at the soil surface by litter cover significantly reduces the softening rate of filaree seed in the soil, and thus reducing germination. There is a tendency for dominance by forbs, including many range weeds and wildflowers, when little or no residue is present on the soil surface in the form of standing and surface litter. When mulch is allowed to accumulate, species composition tends toward the taller annual grasses, such as soft chess brome and wild oats. Apparently, mulch alters botanical composition through influence on physical conditions at the soil surface and on the bulk density of the surface soil horizon. Mulch has little influence on pH and phosphorus and nitrogen content in the grassland system. Managing the Soil Surface Soil porosity influences soil aeration and water-holding capacity. Under heavy grazing conditions bulk density decreases because of trampling, reduced residue leading to lower soil organic matter, reduced root and shoot production leading to lower organic matter, and shallower rooting depths. Raindrops on bare soil tend to seal the soil surface and reduce water infiltration and increase runoff. Cracked soil surfaces tend to seal over following the first big autumn storm. Managing residual dry matter is the rancher’s main means of influencing soil surface conditions and ultimately soil permeability, water holding capacity, and plant productivity. However, this approach to range improvement can only be a long-term process with little apparent change from year to year. If pasture plants are vigorous above and below ground, productivity will increase, thus providing more forage for animal production. More surface residue and increased root productivity and rooting depth will result. Gradually, deepening rooting systems should increase soil organic matter and porosity at greater depths. Over time the volume of soil contributing nutrients to forage will increase. Increasing the vigor of grazed range plants can be accomplished by maintaining light to moderate grazing. At heavy grazing intensities, intensive grazing management systems that use pasture subdivision and rotation to provide periods of rest between grazing may be appropriate. Pasture rotation allows grazed plants a period of rest following grazing. The length of this rest period depends on the intensity of grazing. Heavily grazed plants will regrow slowly, requiring longer rests than lightly grazed plants. Adapted from UCCE & NRCS Rangeland Watershed Program Fact Sheet No. 30.
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