3.1.3  Root architecture and uptake of water

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Figure 3.5  Sample of dense subsoil containing a biopore occupied by a root. Such biopores are passages for roots through otherwise almost impenetrably dense soil

The dense root systems common in topsoils (Table 3.1) extract water effectively from surface soil layers. Extracting water from subsoil layers is more difficult. Australian subsoils are typically inhospitable to roots. They are dense, have a large resistance to penetration and are often sodic, that is, sodium dominates the exchange complexes on soil particles, affecting structure and water availability. Moreover, subsoils can be acutely deficient in some nutrients that are required locally by roots (e.g. zinc) or poisonously high in others (e.g. boron). Native vegetation overcomes these difficulties by forming biopores in the subsoil — highways that enable roots to create and maintain a path to water deep in the subsoil, possibly even as far as a water table 20 m below the surface, as in the jarrah forests in Western Australia. Clearing native vegetation for agriculture has disconnected subsoil biopores from the topsoil, and the roots of crops may have trouble in finding them. Nevertheless it is common for roots of crops to colonise biopores; a particularly clear example is given in Figure 3.5 which shows dozens of roots clustered together in a pore about 5 mm in diameter that traverses a soil core collected from subsoil under a wheat crop.

Clearing native perennial vegetation for agriculture has generally replaced deep-rooted perennials with shallow-rooted annuals. These annuals, whether crop or pasture, do not have time to grow their deep roots into the subsoil, and consequently considerable amounts of rain may percolate beyond the reach of roots. Such water moves slowly through the deep subsoil, moving laterally as well as vertically, and in so doing often transports salt to lower parts of a landscape, discharging at the surface and leading to dryland salinity. A major challenge facing Australian agriculture is to prevent long-distance movement of this escaped water. Any long-term solution will depend on manipulating root-system architecture. Replanting agricultural landscapes with trees sometimes helps but is not a universal solution because water will not move to roots through the unsaturated zone of the soil over distances greater than about 1 m. Effects from trees are therefore quite local unless tree roots can reach a water table where lateral movement of water is rapid.