Soil structure and nutrient use efficiency

This talk focuses on soil physical constraints to nutrient use, and how these can be mitigated through better soil management and crop traits.  Soil structure degradation is widespread, leading to poor root growth and losses of nutrients.  In a broad survey of commercial farms in Scotland, we found about 20% had severely degraded soil physical structure that was easily detected visually. More in-depth analysis in another survey found a third of farms had mechanically impeded topsoils.  In such compacted soils, plant roots rely on macropores as growth pathways, having a large impact on root architecture that we found varied between crop genotypes.  Compaction decreases the effectiveness of fertilisers as poorer aeration accentuates denitrification, and physically constrained roots can explore less soil for available nutrients. Crops bred to root deep to capture leached fertilisers may remain restricted above the plough pan.

Plants have a great capacity to re-structure degraded soils, thereby altering the physical environment at the root-soil interface to improve nutrient capture.  Root exudates have well-known chemical and biological impacts to nutrient capture and cycling, but we have also found they can mechanically disperse soils, potentially aiding the release of immobile nutrients bound to soil.  By dropping the surface tension of pore water, exudates may ease capture of water and entrained nutrients, and they can act as hydrogels that mediate water stresses.  Over time, microbial decomposition of exudates and the hydromechanical action of the plant root creates a thin zone at the root-soil interface termed the rhizosphere.   Root hairs permeate into the rhizosphere, capturing nutrients and creating a highly aggregated pore structure that improves oxygen transport and biological habitats.

Great scope exists in crop breeding to exploit plant root traits like hairs, exudates and architecture to improve nutrient use efficiency.  Coupled with improved soil management, exploiting these traits could help mitigate the widespread soil structure degradation that has been found globally.