Seedbed

Abstract: There is considerable discussion about the influence of soil management techniques on soil erosion, water use and conservation, and more recently carbon dioxide sequestration and waste disposal. The soil–atmosphere interface, particularly the seed bed layer is of particular concern to agronomists and soil scientists because it is the focus of the physical processes affecting crop establishment and biological activity. This paper evaluates the current knowledge (1) in modeling seedling emergence and residue decomposition, (2) seedbed structure and its resulting physical conditions, and (3) tillage operations affect on seedbed structure and residue distribution.

Abstract: We examine the influence of (i) the spatial distribution and abundance of parent trees (as seed sources) and (ii) the abundance and favourability of seedbed substrates, on seedling recruitment for the major tree species in north - western interior cedar-hemlock forests of British Columbia, under four levels of canopy openness (full canopy, partial canopy, large gap, and clearcut). Substrate distribution varied with canopy openness, and substrate favourability was a function of both canopy openness and seedling species. Lack of suitable substrates was the predominant factor limiting seedling density under full canopies. Partial canopy and gap sites provided a broad range of favourable substrates in close proximity to parent trees, resulting in the highest observed seedling densities. There was much higher effective dispersion of seedlings away from parent trees in gaps than in the partially cut stands. Seedling dispersion to clearcut sites was poor with seedlings being tightly restricted to a narrow band along the forest edge. Thus, seedling recruit - ment in these forests was a reflection of the interaction between the abundance of seed and substrate favourability, and the relative importance of these factors varied significantly with canopy structure.

Abstract: The presence of detectable concentrations of pesticides in surface water has caused some concerns about possible effects on human health and aquatic biology. Herbicides are the class of pesticide most frequently detected. Several popular soil-applied herbicides can be commonly detected in surface water during the growing season and can sometimes exceed drinking water standards (standards based on daily lifetime exposure) following runoff events (1, 31, 37). Adoption of various Best Management Practices (BMPs) is being urged in many areas to reduce pesticide runoff. Conservation tillage is one BMP commonly suggested for this purpose. As defined by the Conservation Technology Information Center (CTIC) conservation tillage leaves a minimum of 30 percent of the soil surface covered with crop residue after planting (11). Conventional tillage, as defined by CTIC and as used in this article, totally disturbs the soil surface and buries crop residue. The moldboard plow is usually used in these systems. No-till systems leave the soil undisturbed before planting and complete planting in a narrow (3 to 8 cm or 1 to 3 inches) seedbed. In ridge-till …

Abstract: Nitrate N in the soil is susceptible to leaching with fall and winter precipitation and can pollute groundwater. Winter cover crops to scavenge residual NO 3 - through root extension are a possible solution. A 2-yr field study was conducted on a Greenville fsl (fine, kaolinitic, thermic Rhodic Kandiudults) in central Georgia to determine root distribution of legume and nonlegume winter cover crops and their ability to absorb soil NO 3 - and accumulate it in aboveground biomass. Cereal rye (Secale cereale L.), hairy vetch (Vicia villosa Roth), and crimson clover (Trifolium incarnatum L.) were planted in a prepared seedbed in the fall and accumulated biomass was incorporated into the soil in the spring. Seasonal variations in root distribution were measured by minirhizotron and soil separation methods. Soil mineral N concentration and aboveground biomass yield and N uptake were determined at regular intervals during the growing season. Total minirhizotron root count (MRC; no. roots cm -2 soil profile) at the 1- to 50-cm soil depth increased at the rate of 0.01 roots cm -2 d -1 in hairy vetch in the fall to 038 roots cm -2 d -1 in crimson clover in the spring, as temperature increased. Roots were well distributed to the 50-cm soil depth. Compared with the other cover crops, rye had significantly greater total MRC from Dec. 1996 to Feb. 1997 and total root length density (RLD; cm root length cm -3 soil) at the 0- to 30-cm depth from Nov. 1995 to Apr. 1996 and in Jan. 1997, and the subsequent NO 3 - or inorganic N concentration in the soil was lower and aboveground biomass yield was greater. MRC was positively correlated with RLD in Nov. 1995, Apr. 1996, and Jan. 1997. A significant positive correlation was observed between MRC and aboveground biomass yield or N uptake (r = 0.52 to 0.68, P ≤ 0.05) and a negative correlation between MRC and soil NO 3 - concentration (r = -0.51 to -0.55, P ≤ 0.05) early in the growing season. Rye had the greatest root density and aboveground biomass, and scavenged more soil NO 3 - early in the growing season. Nonlegume cover crops, such as rye, may be more effective in reducing residual NO 3 - and potential leaching of NO 3 - from the soil early in the growing season than are legume cover crops, such as hairy vetch or crimson clover.