Minerals are absorbed to form the soil solution (see Chapter 21). The plants take up only water-soluble material so all supplies of nutrients including fertilizers and manures must be in the form of ions (charged particles). The movement of the elements in the form of ions occurs in the direction of root cells containing a higher mineral concentration
than the soil, i.e. against a concentration gradient. The passage in the water medium across the root cortex is by simple diffusion, but transport across the endodermis requires a supply of energy from the root cortex. The process is therefore related to temperature and oxygen supply (see respiration p118).
Nutrients are taken up predominantly by the extensive network of fine roots that grow in the top layers of the soil (see Figure 9.1). Damage to the roots near the soil surface by cultivations should be avoided because it can significantly reduce the plant's ability to extract nutrients. It is recommended that care should be taken to ensure that trees and shrubs are planted so their roots are not buried too deeply and many advocate that the horizontally growing roots should be set virtually at the surface to give the best conditions for establishment.
The surface thickening that occurs in the ageing root does not significantly reduce the absorption ability of most minerals, e.g. potassium and phosphate, but calcium is found to be principally taken up by the young roots.
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