A New Method for Measuring Plant Available Water Capacity Helps Document Benefits of Biochar-Soil Mixtures

By Karen Hills

This is part of a series highlighting work by Washington State University (WSU) researchers through the Waste to Fuels Technology Partnership between the Department of Ecology and WSU during the 2017-2019 biennium.

Five panels showing equipment, from small tubes to psychrometers.

Figure 1. Apparatus used to measure plant-available water holding capacities (PAWC) by conventional and centrifuge methods. A) pressure-plate apparatus for conventional field capacity measurements, B) soils in cups on top of pressure membrane, C) dew-point psychrometer for conventional wilting-point measurements, D) assembled centrifuge filter tube showing removable filter top containing soil to right, and E) rack containing large number of assembled and loaded centrifuge tubes. Source: Amonette et al., 2019.

Biochar has potential to draw down atmospheric carbon when applied to agricultural soils (as discussed in my previous article on this topic). There is currently not a robust way for farmers to be directly compensated for the benefits to society such drawdown provides. However, researchers have been exploring other co-benefits of using biochar as a soil amendment. One such co-benefit is biochar’s ability to increase the water-holding capacity of agricultural soils, and thus increase plant productivity in situations where water is limiting. However, documenting this effect has been limited by how time consuming and expensive it is to measure plant-available water-holding capacity (PAWC) by standard methods (See Figure 1 A, B, C). In an effort to alleviate this barrier, Jim Amonette at the Pacific Northwest National Laboratory and Washington State University’s Center for Sustaining Agriculture and Natural Resources led the development of a new, inexpensive, rapid method for measuring PAWC of soil-biochar mixtures (See Figure 1 D, E), based on applying a specific level of water potential to a sample using a centrifuge. The sample is supported by a filter membrane fixed midway in a centrifuge tube, thus allowing drainage into the bottom of the tube to occur.

Soil texture triangle, with scale of % sand, silt, clay on each side.

Figure 2. Soil textural triangle showing textural distribution of Washington A horizons in the USDA National Cooperative Soil Survey database (gray dots), and the nine natural Washington soils and one synthetic soil (borosilicate glass beads) used in this work (blue and yellow squares). Source: Amonette et al., 2019.

The new method was calibrated against standard methods and then applied to 72 combinations of soil and biochar: nine Washington soils of varying textures (Figure 2), each combined with four biochars, and at two different biochar application rates. Use of this new, rapid method for measuring PAWC allowed Amonette’s team to collect data in just five days, when use of standard methods would have taken several months. This new method therefore has great application potential as a screening tool in future research and in monitoring changes in PAWC over time.

The data collected from the 72 combinations of soil and biochar led to the following conclusions regarding the effects of biochar amendments on the PAWC of soils:

  • Biochar did increase the PAWC of soils, though the increase in PAWC was not linearly proportional to the amount of biochar added. The addition of 0.5% and 2.0% biochar carbon (by weight) increased PAWC by 2.7 and 3.3%, respectively, averaged across all biochar-soil combinations. These application rates are approximately equal to biochar amendment rates of 5 and 20 tons of carbon per acre when mixed to the 15-cm plow depth. In other words, 80% of the maximum PAWC benefit observed was obtained with addition of only 0.5% biochar carbon; applying four times as much carbon did not yield a proportional increase in terms of PAWC (Figure 3).

    Curve showing an almost linear increase in PAWC as biochar additions increase, plateauing around 5 tons biochar C/acre

    Figure 3. Mean increase in PAWC for nine Washington soils observed using the centrifuge method and expressed as a function of the nominal rate of biochar application per acre assuming 1 acre of soil 6 inches deep weighs 1000 tons. Error bars represent 1 standard deviation. Least significant difference (P < 0.05) in PAWC increase between the two addition rates for a given soil is 0.18%.

  • Soil texture and mineralogy have a large impact on the degree to which biochar increases PAWC (Figure 4), with sandy soils, in general, receiving proportionally greater benefit from the higher biochar application; and

    Bar graph showing pairs of bars for different biochar rates. Each bar shows change in PAWC, each pair represents each soil type

    Figure 4. Mean changes in plant-available water-holding capacity (PAWC) as a function of soil type (as shown in Figure 2) and biochar amendment rate. Error bars represent 1 standard deviation. Different letters above error bars indicate significant (P=0.05) differences among means. Means having error bars without letters are not significantly different from means labeled with a, b, or c. LSD (least significant difference) = 0.78 weight %. Source: Amonette et al., 2019.

  • Inter-particle effects (caused by interactions between biochar and soil particles) are the largest contributor to the overall impact of biochar on PAWC. The exact mechanisms at play were not part of this study but could include creation of new void spaces between biochar and soil particles or increasing the proportion of hydrophilic to hydrophobic surfaces in the biochar-soil mixture. Amonette and his colleagues found that the increase in PAWC in soil-biochar mixtures could not be explained by the internal porosity of biochar alone, but instead was explained by the interaction of biochar and soil particles. Averaged across soil-biochar combinations, 86% and 62% of change in PAWC was attributable to these inter-particle effects for the 0.5% and 2.0% biochar application rates, respectively.

One important take home message is that biochar benefits do not necessarily increase proportionally with application rate. Figuring out the particular “sweet spot” to achieve the most economical application rate will be specific to a particular soil-biochar-crop combination. The dominance of the inter-particle effects in PAWC increases in this study was fascinating and begs for more research. This and future work will get us closer to what is considered the holy grail of biochar application: being able to target a particular biochar to a particular soil type to solve a particular issue – in this case, improving water-holding capacity.

For more detail, see the brief project report (13 pages, Chapter 8 in Hills et al. 2019) or the longer technical report (Amonette et al., 2019: 35 pages).


Amonette, J.E., M. Flury, J. Zhang. 2019. A Rapid Test for Plant-Available Water-Holding Capacity in Soil-Biochar Mixtures. 35 pp.

Hills, K., M. Garcia-Perez, J.E. Amonette, M. Brady, T. Jobson, D. Collins, D. Gang, E. Bronstad, M. Flury, S. Seefeldt, C.O. Stöckle, M. Ayiania, A. Berim, W. Hoashi-Erhardt, N. Khosravi, S. Haghighi Mood, R. Nelson, Y.J. Milan, N. Pickering, N. Stacey, A.H. Tanzil, J. Zhang, B. Saari, and G. Yorgey. Advancing Organics Management in Washington State: The Waste to Fuels Technology Partnership, 2017-2019 Biennium. 2019. Publication 19-07-027. Solid Waste Management Program, Washington Department of Ecology, Olympia, WA.

This article is also posted on the CSANR Perspectives on Sustainability blog.

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