The effect of bulk density on moisture content at 5 0 mb tension in four soils of different textures was studied. The volumetric water content increased linearly with bulk density over a wide range of densities. Depending on texture, a maximum bulk density was reached above which continued compaction decreased the water content. This is shown to be the point at which the air capacity of the soil at this tension approaches zero. Accepting that the gravimetric wilting point depends mainly on texture, the available water capacity varies in a manner similar to the 50 mb water content.If the relationships described are valid in the field, the available water capacity and air capacity may be optimized using cultivation techniques to adjust the bulk density. The available water capacity of coarse-textured droughty soils may be increased by increasing the bulk density providedthat the air capacity remains above acceptable lower l i m i t s (10-15 per cent). Conversely, the air capacity of compacted soils with large available-water capacities could be increased by reducing the bulk density to a value corresponding to an acceptable available-water capacity. In very compacted soils a decrease in bulk density will benefit both available-water capacity and air capacity.
The bulk density, available water (Av), air capacity (C,), and retained water capacity (8,) were determined for 158 A, B, and C horizons of field soils. Clay (< z pm) and silt (2=60 pm) were also determined. Statistical analysis of the results demonstrated that bulk density exerts a profound influence on A,, C,, and O,, but the effect varies between texture groups and horizons. Significant negative correlations were obtained between bulk density and C, for most texture and horizon groups. In B and C horizons A, and 6, also decrease with increasing density, whereas in A horizons A,and 8, tend to increase with bulk density except in silty soils. Within a limited range it is feasible to control these parameters by using field techniques to achieve optimum bulk density for particular soils.
Picloram (4‐amino‐3,5,6‐trichoropicolinic acid) was applied to a ryegrass (Lolium perenne L. cv S23) sward at 0.05, 0.28, and 1.68 kg ai (active ingredient)/ha in June 1967 and again in August of 1969 and 1970. Small amounts of residue were found in soil samples 1 year after application of the low and medium doses. On the highest dose plots 5 to 6% of the amount applied was usually recovered 1 year after spraying. Following the final application this residue degraded slowly over the following 3 years. Analyses of the final samples (taken 222 weeks after spraying) indicated that the residue had declined to about 0.5% of the total amount applied. In stratified soil samples, residues were found to the maximum sample depth of 90 cm 1 year after the initial application, but 69 weeks after the final application no residue was found below 30 cm.Crops grown across all plots in 1974 did not differ in yield. However, on the plots that had received the highest dose, leaf abnormalities were noted in beans and potatoes and color differences were observed in kale.
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