The effects on a number of soil physical and aeration parameters of compaction during spring pre-sowing operations were measured on a clay soil (49% clay). A soil-tyre contact stress of 200 kPa was applied by tractor tyres.Yield of an oat crop was reduced by 30% as a result of compaction. Total porosity of the soil was reduced by 6% vjv owing to loss of pores > 60 pm, and water retention was increased. The resultant decrease in air-filled porosity greatly reduced gas diffusion and air permeability coefficients of the soil, and, for a time, 0, content of the soil atmosphere was significantly lowered in the compacted treatment. Penetrometer resistance after sowing was 3.5 MPa in the control and 4.5 MPa in the compacted treatment; in the latter, root growth was inhibited until the soil dried and cracked. By the end of June, canopy temperature measurements indicated water stress in the oat crop on compacted soil but not in that on the control. The results obtained indicated that air permeability, measured in the field, of 1 mm s s ' provides a satisfactory single value below which crop growth is likely to be reduced.

Two small cold water reservoirs with stable water levels were compared with two reservoirs scheduled for complete drawdown to determine the effects of drawdown on invertebrate abundance and fiih abundance and density. An extensive literature review supplemented the results with other applicable information. No conclusive evidence of effects of drawdown on invertebrates or fish was found. Characteristics of a reservoir and its drainage basin which obscure or lessen drawdown effects are discussed. Analysis and observation of drawdown procedures showed that duration and scheduling of drawdown periods are important in determining whether drawdown will be harmful to fauna in a given reservoir. (KEY TERMS: drawdown effects; drawdown patterns; small cold water reservoirs; invertebrates; trout; suckers; reservoir ecology.) LITERATURE CITEDAass, P., 1960. The effects of impoundment on inland fisheries.

No abstract

The Metro Denver Chapter of the

SUMMARY The effects on soil aeration of varying both the intensity and quantity of water applied per irrigation were investigated in a field experiment on a clay soil. Soil physical and soil aeration parameters were measured before, and for up to 7 days after irrigation. Irrigation increased the volumetric water content of the surface layer by approximately 8% v/v. However, periods of waterlogging were short on this soil. Significant decreases in soil oxygen content in the days after irrigation were caused by increased soil and root respiration. In terms of subsequent crop growth, the practice of applying 30 mm irrigation water at an intensity of 5 mm h−1 was shown to be superior to irrigations providing half this quantity (15 mm) or double this intensity (10 mm h −1). The lower intensity irrigations decreased soil permeability by homogeneous wetting and swelling, and thus more of the water applied was recovered in the soil. Approximately one third of the water applied at the higher intensity was lost through cracks, and wetting of the soil was less uniform.