Soil acidity (and acidity of anything else, for that matter) is measured on a scale of 1 to 14. Everything below 7 is considered to be acidic. Everything above is considered to be alkaline. Most garden plants thrive at a pH between 6 and 7.5. The reason a pH between 6 and 7.5 is optimal for garden plants is that between 6 and 7.5, phosphorus in the soil is soluble - meaning that it dissolves in water and is taken up by plant roots.
Some of the factors that affect the rate at which particles diffuse include: the particle's molecular weight, temperature, concentration difference, diffusion distance, permeability and surface area. Pressure is also known to affect the rate of diffusion since it increases the speed of molecules.
Facilitated diffusion is different from free diffusion in several ways. First, the transport relies on molecular binding between the cargo and the membrane-embedded channel or carrier protein. Second, the rate of facilitated diffusion is saturable with respect to the concentration difference between the two phases; unlike free diffusion which is linear in the concentration difference.