16.6  Concluding remarks

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Documented concern with plant nutrition in Australasia has a history that roughly coincides with European settlement. Research in this area was initially driven by an urgent need for agricultural self-sufficiency and in subsequent decades by commercial motives. Extensive development of agriculture well beyond naturally rich soils was eventually underpinned by a productive mix of chemistry, soil science and plant physiology, all with a clear mission.

Having identified plant needs for macronutrients and applied those principles to agricultural practice with good effect, spectacular results from new knowledge followed successful correction of micronutrient deficiencies such as Cu, Mn, Mo and Zn. Those practical outcomes are a tribute to painstaking and exacting research in this technically difficult area of nutritional physiology. Such research soon gained prominence in plant science worldwide due to shrewd definition of issues in soil–plant relations, and boosted by a newfound availability of radioactive isotopes for tracer experiments.

Following pioneering research in Australia that ascribed new functional roles to key elements (e.g. Mo for N2 fixation), field research established their essentiality in agricultural and pastoral ecosystems, providing an excellent example of early application of new knowledge. Attention then turned to adaptive features of native plants that enabled them to maintain stable ecosystems on impoverished soils. Functional requirements of key elements were found to be qualitatively similar to those of crop plants, but native plants carried highly specialised adaptive devices for enhanced uptake and long-term retention of key nutrients. Some remarkable biotic associations were found that enhanced acquisition of sparse nutrients on depauperate sites. Symbiotic associations with mycorrhizal fungi are a case in point, and that knowledge now finds direct application in plantation forestry where seedlings are routinely inoculated with appropriate organisms prior to planting. Improved establishment has been confirmed, and knowing that fungal associates also enable plant access to organic forms of N as well as mineralised N, later-age forests may also benefit from these same symbioses.

In contrast to natural ecosystems that have come to equilibrium with meagre soil resources, high-input agricultural and pastoral communities now confront plant science with a new generation of problems that derive from human-induced changes in soil properties. Heavy cropping and pasture improvement on light soils can lead to rapid acidification and thus to Al and Mn toxicities. Impaired root growth on soils so affected results in crop losses, but remedies are in prospect from a wider knowledge of soil processes involved in acidification, plus genetic improvement of crop species. Soil ameliorants offer an enduring solution; tolerant genotypes would provide immediate alleviation of crop losses, and prospects for genetic improvement are encouraging.

In a good example of process-based plant selection surplanting empiricism, excretion of organic chelating agents by plant roots is now known to forestall inhibitory effects of toxic elements, and to be heritable. This adaptive feature might even be amenable to molecular methods so that future crop plants could be endowed with traits found in nature that confer tolerance to acid soils or to heavy-metal toxicity on serpentine soils or mine tailings. Moreover, in a reverse application of processes underlying biotic associations and acquisition of sparse nutrients, heavy-metal tolerance by tree seedlings is also improved via mycorrhizal associations. In principle at least, crop plants might be similarly protected once useful symbioses are developed.

As shown above, heavy metals, non-essential elements and even nutrient ions can prove toxic to plants if root-zone concentration is high enough. Examples cited generally referred to plant growth on soil variants within major groups, to localised sources of particular heavy metals, or to human-induced change in productive regions. Equally restrictive, but of lesser extent, salt-affected soils limit plant growth by osmotic stress and ion toxicity as well as by poor physical structure. Sodium and chloride ions are largely to blame for these compounding stresses and have led to an entire class of salt-tolerating plants in nature (halophytes), as well as many generations of novel solutions for managed communities. Salt is considered next (Chapter 17).