DOES THE UNIFORM PACKING OF SAND IN A CYLINDER PROVIDE A UNIFORM PENETRATION RESISTANCE - A METHOD FOR SCREENING PLANTS FOR RESPONSES TO SOIL MECHANICAL IMPEDANCE

Sand packed to a constant dry bulk density, is frequently used as an a rtificial growth medium in which to simulate the effects of a constant mechanical impedance on root growth. This research aimed to determine whether conventional packing resulted in constant mechanical impedanc e and to test alternative packing regimes. Perspex cylinders 300 mm ta ll with a 49 mm internal diameter were packed with moist sand to unifo rm and varying bulk densities to examine which type of packing gave th e greatest uniformity of penetration resistance (PR) with depth. The c ylinders packed to a constant bulk density (1.48, 1.55, and 1.6 Mg m(- 3)) all had measured PR profiles which increased markedly with depth b y approximately 1, 1.5 and 3 MPa, respectively, within the top 100 mm. Between 100-300 mm depth, these same cylinders showed reductions in P R of up to 1, 2 and 2 MPa respectively. These results show that sand p acked to a constant bulk density with depth would not provide a unifor m mechanical impedance to plant roots. By packing sand to different bu lk densities at different depths, we obtained packed cylinders that ha d much more uniform PR profiles (with average values of 0.25, 1.40 and 2.30 MPa). Below a depth of 50 mm, the coefficients of variation for replicate cylinders packed in this way were 12%, 5% and 18% for the 0. 25, 1.40, and 2.30 MPa treatments respectively. For experiments with s ingle plants, the lower PR values that were unavoidable near to the su rface (< 50 mm) can be avoided by sowing seeds at the base of a funnel inserted into the cylinder. Treatments such as these can provide repr oducible growth media, with adequate water/nutrient and aeration statu s for the study of plant response to uniform mechanical impedance.