How good is the evidence that soil-applied biochar improves water-holding capacity?

Biochar application to soil is suggested as a way of enhancing soil fertility by increasing the availability of nutrients and water. The former is perhaps better documented while the latter has less experimental support. This review critically investigates the recent literature which focuses on determining whether biochar induces increases in plant available water and that this provides part of the explanation for possible increases in crop yield. A number of studies suggest that biochar increases crop yields, and this is linked to the enhancement of soil water content and increased crop growth. However, many of these studies fail to fully consider if the measured biochar increases of 10-30% in soil water content were actually responsible for an increase in plant available water for crop growth. There is also limited evidence of increased crop yields when biochar is used in field experiments. While biochar soil application may increase soil water content, this appears to most likely occur with free draining coarsely textured sandy soils. As yet there is limited evidence that biochar improves soil water content in temperate soils and even less that it facilitates plant tolerance to drought stress. More recent literature shows the use of methods which quantify soil biochar changes with respect to plant water availability. However, despite some advances in our understanding of biochar's mode of action, there are still only a few studies which link increases in plant available water with increased crop yields, and particularly with respect to the longer term use and functionality of soil-applied biochar.