In the foregoing discussion of the physical and chemical properties of soil attention has been drawn to the important part played by clay minerals and organic matter, with its end product humus, in regulating soil processes. In the course of this exposition nutritive elements have been referred to in a general way. Their availability to growing crops is of paramount importance. This chapter is therefore devoted to the consideration of some of the soil processes that regulate the availability of the main nutrients concerned in plant growth.
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