Oct 18, 2023 · A mix of two parts soil, one part perlite, and one part finely ground bark is recommended for blackberry plants in a raised bed. Soil pH should be between 5.5 and 6.5; Make a soil mix from two parts soil, one part finely ground bark, and one part perlite. Mix in 1 pound of this compost per square foot of soil (4.8 kg/square meter).