Using Extracellular Polymeric Substances (EPS) from Bacteria to Make Soils More Drought-Adapted.

Crops—and plants in all ecosystems—depend on healthy soils that can supply water and nutrients even when it is not raining (or irrigating). One possible strategy for helping agriculture adapt to drought is to learn from soil bacteria. When soils dry out, bacteria are known to release extracellular polymeric substances (EPS) into the soil environment that potentially act as sponges to increase water retention, glues to increase aggregate stability, and pipes to increase diffusion of soluble resources. However, nothing is known about the link between soil EPS and plant health. What if we could augment EPS in soil during drought as a way of helping crops adapt? This question was studied using soils from Sea Smoke Vineyard near Lompoc, California. Pinot noir grapevines were grown in a greenhouse with soils of varying amounts of xanthan gum. Xanthan gum is a bacterial EPS polysaccharide that is organically certified and commercially produced. We captured close-up images showing how strands of xanthan gum connected dry soil particles that were otherwise hydrologically disconnected. Laboratory tests showed that a xanthan gum concentration of at least 0.5% (by mass) was needed to increase soil water-holding capacity and aggregate stability. And greenhouse trials with xanthan gum in vineyard soils showed reduced irrigation demand without sacrificing nitrogen supply (i.e., ammonium and nitrate) or plant health. Because xanthan gum can increase nutrient retention while reducing irrigation demand, bacterial EPS (whether amended or naturally secreted) may be a viable strategy for dealing with water scarcity and nitrate runoff in arid land agriculture.