Intensive tillage effects on spatial variability of soil physical properties

Studies evaluating intensive tillage effects on the spatial variability and sample size requirements for assessing soil physical properties have been limited. Research to address this need was conducted on a Bertie silt loam (fine-loamy, mixed, mesic Aquic Hayludult) at Wye Research and Education Center near Wye Mills, Maryland. The field was subsoiled twice at a 38-cm depth, chisel plowed to a 20-cm depth, and disked at a 15-cm depth using controlled wheel traffic patterns. Rye grass (Secale cereale L.) was planted in October 1987. Undisturbed soil samples were taken at 1-m intervals in April 1988 along two parallel 45-m transects in the East-West direction and along two parallel 37-m transects in the North-South direction. Soil samples (3.0-cm length X 7.6-cm diameter) were taken from soil depths of 6 to 9 and 27 to 30 cm for determination of particle size, bulk density (P-b), soil strength (SS), mean pore size (MPS), and saturated hydraulic conductivity (K-sat). Coefficients of variation were 4.17 and 7.14 for P-b, 48.00 and 36.42 for SS, 21.54 and 23.65 for MPS, and 172.9 and 231.4 for K-sat, for 6- to 9-cm and 27- to 36-cm depths, respectively. All of the soil physical properties exhibited spatial structure at both depths. All soil physical properties except K-sat were weakly spatially dependent for the 6- to 9-cm depth. For the 27- to 30-cm depth, the properties exhibited moderate spatial dependence. The number of samples needed to estimate the true mean for K-sat was large; however, properties that exhibited low to moderate variability required fewer samples. More intense tillage at the 6- to 9-cm depth reduced the number of samples required to estimate true mean when compared with the 27- to 30-cm depth. Knowing the intensity of tillage in a given field is critical when sampling for soil physical properties.