SOIL PREPARATION

because penetrometer resistance measurements are strongly affected by soil water content and texture, measurements must be done under well-defined conditions to be useful for studying soil compaction. In a more recent study in vineyards, bulk density, soil strength (using a penetrometer), shear strength (using a pocket vane tester), infiltration (double-ring) and soil hydraulic conductivity (mini-disc infiltrometer) were compared (Figure 3.7) to measure compaction on three sites in the Western Cape (Hoffman et al ., 2016). Bulk density measurements gave consistently good results. The pocket-vane tester did not perform well on a gravelly soil and the mini-disc infiltrometer gave unrealistically high values in some cases due to preferential flow paths. Results obtained with saturated hydraulic conductivity and bulk density measurements did not agree consistently. Bulk density and penetrometer resistance were more sensitive to soil preparation effects than saturated hydraulic conductivity. The penetrometer results also corresponded well with the compaction pattern indicated by the bulk density values. Current results confirmed the findings of previous studies namely that penetrometer resistance is a sensitive, accurate and quick method to measure soil strength. Automated constant speed penetrometers are expensive, but manual instruments capable of giving equally usable results do exist.

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FIGURE 3.7: Measurement of soil physical properties namely a) bulk density, b) water infiltration rate, c) shear strength and d) unsaturated hydraulic conductivity (Photos: J.E. Hoffman, Stellenbosch University).

42 | DETECTION OF ROOT IMPEDIMENTS