Cover crop effects on spring soil water content and the implications for cover crop management in south coastal British Columbia
Soil water content in the spring is an important factor in making decisions on cultural operations. A 2-year study was conducted to determine the effects of fall planted cover crops on spring soil water content dynamics and to relate the water content to cover crop management in terms of field trafficability. Each year, cover crops were planted at two planting dates in the fall, late August and late September on a loamy, mixed, mesic Humaquept on the Fraser River delta, south coastal British Columbia. Treatments were bare (control), wheat, rye and spring barley. Soil samples were collected at approximately bi-weekly intervals beginning in early spring to determine soil water content. Plant samples were also obtained late in the spring to determine C:N ratio. Treatments affected soil water content differently at each sampling date. No differences in soil water content at depths 20�40 and 40�60 cm were observed. Critical soil water content value for trafficability was determined to be 0.32 kg kg-1. Cover crops attained critical C:N ratio of 31.7:1 when the soil water was still above the critical value. The results suggest that terminating the cover crop growth in the spring could best be done by either mowing or spraying the cover crops as opposed to incorporation in order to minimize compaction since the critical C:N ratio for net mineralization is achieved when the soil is still too wet.
Winter Cover Crops for Sod‐Planted Corn: Their Selection and Management1
Winter cover crops for sod-planted corn (Zea mays L.)were studied in 11 location-year field experiments in Virginia during the 1962-1966 period. Conventionally tilled corn with cover crop turnplowed and sod-planted corn were also compared. Rye (Secale cereale L.) was the most satisfactory because of superior winter hardiness, susceptibility to herbicidal killing, and the production of relatively large amounts of persistent mulch. Wheat (Triticum aestivum L.) and oats (Avena sativa L.) were comparable as mulches except at higher elevations where oats winterkilled. Barley (Hordeum vulgare L.) was the least satisfactory winter grain tested because of resistance to herbicides. Inclusion of crimson clover (Trifolium incarnatum L.) or hairy vetch (Vicia villosa Roth.) in winter grain seedings slightly increased mulch forage but not corn yields. In general, highest corn yields occurred where largestamounts of cover crop mulch occurred, except for Italian ryegrass (Lolium multiflorum Lam.). The additional mulch produced by late versus early killing of particular cover crops tended to increase corn yields, though not always significantly. Planting corn in rye stubble after removal of forage for hay or silage reduced corn yields an average of 47% in three of seven comparisons with rye left as mulch, while yields in the remaining four were comparable. Yields of sod-planted corn in rye sod averaged 44% higher than conventionally tilled corn in 4 of 13 comparisons and were comparable in the remaining nine. More soil moisture was found under sod-planted corn than under conventionally tilled corn, especially during the first half of the growing season. Corn yields were increased or at least maintained, in comparison to conventional tillage, by sod-planting in rye over a wide range of soil and moisture conditions, with the added advantages of improved soil and water conservation.
Energy skin effect of propellant particles in Electrothermal-Chemical Launcher
A numerical model of the radiation has been employed by a Monte Carlo method and statistical physics to simulate the process of a capillary plasma source for Electrothermal-Chemical (ETC) Launcher. The effect on propellants with different physical parameters is discussed. The plasma-propellant interaction is also discussed when combined with a thermal model. Results show that radiant energy only causes a small field around the plasma injector in the propellant bed. The responses of energy flux and propellant particles on radiation are both in the order of picosecond. The strong instantaneous radiation is responsible for the transmission of energy to the propellant particles leading to ignition. Compared with conventional ignition, the energy absorbed by propellant particles is used to increase quickly the temperature in the surface layer of propellant particles. This energy skin effect in the propellant particle surface appears to be the main cause of plasma ignition.