3.2.1  Where are water and nutrients found in soil?

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Figure 3.6 Moisture content (m3 m-3) of a podsolised sandy soil in Tasmania measured by neutron moisture monitoring to a depth of 1.65 m throughout a season. A young Pinus radiata stand was growing on this site. Spatial and temporal variability in water status can be observed by this technique. For example, soil became progressively wetter until late winter then began to dry in spring. In spite of the high hydraulic conductivity of this sandy soil, surface layers of the profile wetted first (March to May) then deeper soil layers became wetter towards winter (May to July). (Sampling was on: 15 January; 14 March; 18 May; 17 July; 16 August; 18 September and 13 November) (Courtesy D. Sheriff; acknowledgements to CSIRO Forestry and Forest Products and ANM Forest Management)


Figure 3.7 Concentrations of mineralisable nitrogen and exchangeable potassium in the top 0.6 m of the soil described in Figure 3.6 showing strong gradients in concentration of these two major nutrient resources (Courtesy C. Carlyle; acknowledgements to CSIRO Forestry and Forest Products and ANM Forest Management)

Most soils are chemically and physically heterogeneous. Evaporation of water from the soil surface and extraction of water by roots can leave deep soil layers wetter than more superficial layers (Figure 3.6). Replenishment by rain showers first wets the surface soil, with progressively deeper layers becoming moist as water infiltrates the soil profile (Figure 3.6 — March to August). Nutrients too are often concentrated in surface layers of the soil where biological activity is high (Figure 3.7). Deeper soil layers can have toxic levels of ions such as sodium, chloride and boron.

Models developed to describe water and nutrient extraction from soil quantitatively must take into account the uneven distribution of resources in soil, transport properties along pathways from soil to shoots and feedback signals exerted by plants to coordinate supply with internal demand. Nutrient uptake depends on water flow through the soil–root–shoot pathways (Section 3.6); some nutrients remain in the transpiration stream throughout the pathway while others interact chemically with surfaces along the route. That is, resistance to long-distance transport is highly dependent on the inorganic nutrient species being transported and transpirational flow rate. Indeed, the transpiration stream as a whole passes through a series of variable resistances enroute to leaf surfaces (Section 5.2). Models for water and nutrient uptake attempt to quantify these resistances in order to predict resource delivery from soil to plants.

The basic laws describing water flow through soils described below show a relationship between water status and root density. Water used by crop species has been successfully modelled using this theory of water flow but natural eco-systems have much more variable soil profiles and root distributions. Building models for nutrient acquisition depends on developing comprehensive knowledge of water flow.