A series of laboratory and field calibration tests was carried out with a commercial double‐probe gamma gauge in an attempt to resolve difficulties encountered in routine field use of the equipment. The laboratory tests showed that transmitted count rate is affected by the extent of the medium surrounding the beam path, indicating that linearity between log of count rate and sample density is not sufficient proof of monochromatic radiation. The equipment was also found to be sufficiently sensitive to require the use of separate attenuation coefficients for soil solids and for water. A universal calibration equation for wet density can therefore not be used with this equipment. Due to the above two phenomena, the calibration stand supplied with the instrument does not yield a calibration curve adequate for accurate work in the field range of soil moisture. A valid calibration therefore requires measurements of moisture and density in a large soil volume over a range of conditions. In view of the technical difficulties in meeting these conditions in the laboratory, empirical field determination of a multipoint calibration curve appears to be the best choice. Once such a curve has been developed, simple two‐point calibration checks of unknown soils could be performed in the laboratory, provided the assumptions of linearity between log of count rate and medium density, and of the effective universality of the attenuation coefficient of soil may be taken as valid. When properly calibrated, the equipment tested is sensitive and reliable for the determination of soil bulk density in the field.
Soil crusts affect both the profile water regime and the fate of crop seedlings. The former is affected via modification of infiltration, evaporation, and redistribution, while mechanical crust impedance prevents or retards seedling emergence. Growers alleviate this problem by frequent, small irrigations which are costly in water, equipment, and labor. This study was conducted to determine whether surface soil properties could be modified by layers of stabilized, hydrophobic aggregates so as to render these irrigations unnecessary.Unstable aggregates of a clay‐loam soil were treated with 7% potassium siliconate and 1% polyvinyl acetate based on dry‐weight of soil and sprayed in water to wet the soil to 15% gravimetric water content. Layers of soil mixes with different aggregate‐size distribution were placed in transparent columns and given a simulated rainfall; then the progress of infiltration, redistribution, and drying was followed. The most promising mix was used in large outdoor tanks as a mulch for carrots (Daucus carotta L.), using 1‐ and 4‐cm mulch layers with 0, 25, 50, and 100% cover of the soil surface. The soil was given one post‐sowing sprinkler irrigation, and the soil moisture regime was monitored by microtensiometers and a two‐probe gamma gauge.A 100% cover of 4‐cm deep mulch of treated aggregates was the most effective for water conservation. It decreased water loss by 20% over a 14‐day period. Seedling emergence was close to 90% under a 1‐cm mulch and 50% under a 4‐cm layer, both of which were superior to the unmulched control treatment.
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