3.2  Extracting water and nutrients from soil

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Soils exhibit sharp variations in water and nutrient supply which must be accommodated by root distribution and activity. Furthermore, the supply of water and nutrients is not constant over time, with diurnal opening and closing of stomata influencing water flow to roots and affecting, in turn, mass flow of nutrients from soil to plant. Remarkably, roots can modify this heterogeneous soil solution to generate consistently large amounts of sap to support shoot activity. Plants growing in soils with adversely low or high levels of water or ions are confronted with even greater regulatory challenges.

While nutrient flow through soil and the long-distance pathways of plants is dependent on sustained water flow, other factors also exert an influence on nutrient supply to shoots. For example, ion acquisition by roots is subject to soil chemical factors such as vast variations in solubilities and mobilities of the main nutrient ions (Section 3.3). Orthophosphate, for example, diffuses through soil at least 50 times slower than potassium and 500 times slower than nitrate! Ultimately, though, any nutrient ion dissolved in the soil solution is available to diffuse along gradients in concentration or be swept to the root surface in a mass flow driven by water uptake. Once an ion arrives at a root surface, uptake processes begin to exert their influence, generating a sap which will enter xylem vessels and be delivered to shoots. These processes will be discussed in following sections.

Bulk flow of water, with its cargo of nutrients, is central to long-distance transport and will be the theme of this section. Water flow will be analysed in terms of hydraulic gradients and resistances through the soil–plant–atmosphere continuum. While any quantitative description of flux is dependent on the species and environmental conditions, the principles governing water and ion movement into plants are universal.