Turning sand to sponge: Scientists find optimal biochar-compost mix to combat drought

The Challenge of Sandy Soils

With drylands covering over 40% of the Earth's land area, improving the agricultural potential of sandy soils is a critical global challenge. These soils, common in arid and semi-arid regions, are notoriously poor at retaining water, making it difficult for crops to survive and thrive, especially with increasing drought periods due to climate change. For decades, scientists have explored organic amendments like compost and biochar—a charcoal-like substance made from pyrolyzed biomass—to improve soil quality. While promising, the exact recipe for success and the best methods for testing their effects have remained unclear.

A "Goldilocks" Recipe for Water Retention

A new study published in Carbon Research has identified an optimal mixture for significantly enhancing the water-holding capacity of sandy soils. Researchers found that a specific blend—consisting of 0.6% biochar, 5.4% compost, and the remainder sand—yielded the best results. This composition increased the volume of water available to plants at field capacity by 38% compared to untreated sand. This substantial improvement means that amended soils can store more water from rainfall or irrigation for longer periods, giving plants a crucial lifeline during dry spells.

Counteracting Water Repellency

A potential drawback of using organic amendments is that they can sometimes make soil hydrophobic, or water-repellent, which would counteract the benefits of water retention. The research team investigated this risk by comparing various mixtures of sand, compost, and biochar. They discovered that while compost alone could slightly increase water infiltration times, adding biochar to the mix significantly improved the soil’s wettability. This finding suggests that biochar not only contributes its own porous structure for holding water but also helps reduce the potential water-repellent properties of compost.

Finding the Right Tool for the Job

A key part of the study was to determine the most reliable method for measuring soil wettability in this context. The scientists compared three different laboratory techniques: the Water Droplet Penetration Time (WDPT) test, the Wilhelmy Plate Method (WPM), and the Capillary Rise Method (CRM). Their results showed that the WDPT test was the most suitable for evaluating the real-world wetting behavior of biochar-compost amended soils. The WDPT test proved superior because it could be performed on minimally disturbed soil columns at various moisture levels, providing a more accurate reflection of field conditions.

Practical and Accessible for Field Use

The endorsement of the WDPT test is a significant outcome, as it is a relatively simple and low-cost method that can be conducted easily, even in the field without access to a high-tech laboratory. This makes it a practical tool for farmers, land managers, and researchers working to improve soil health in remote or resource-limited areas. The study highlights that while the test is effective, careful execution is needed to ensure accurate measurements, especially for very short infiltration times.

Implications for a Drier World

The findings offer a tangible strategy for climate change adaptation in agriculture. By providing a tested recipe for biochar and compost application, this research empowers farmers in arid regions to make their soils more resilient to drought. Improving soil water storage can lead to reduced irrigation needs, lower costs, and enhanced food security. The study concludes that applying biochar-compost mixtures is unlikely to cause ecologically harmful water repellency and provides a clear pathway for turning unproductive sandy soils into more fertile and sustainable agricultural land.

Corresponding Author:
 

Sara de Jesus Duarte

Original Source:
 

https://doi.org/10.1007/s44246-022-00032-2

Contributions:
 

Duarte, S. J. was responsible for writing the manuscript, Glaser, B. contributed to conducting the research, the discussion of results, and improvement of the manuscript, and Alena Maria Hubach contributed to the experimental design and conducting the experiment and writing. The author(s) read and approved the final manuscript.