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.
1 The relationship between demographic factors â population size, distribution, and composition â and environmental change.  The mediating factors that influence this relationship: technological, institutional, policy, and cultural forces.