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

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.

Assorted apparatus used to measure plant-available water holding capacities.
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.

Figure of soil texture triangle.
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:

  1. 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).

    Figure showing mean increase in PAWC for 9 WA soils.
    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%.
  2. 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 mean changes in PAWC
    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.
  3. 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).


This article is also posted on AgClimate.net.


5 comments on "A New Method for Measuring Plant Available Water Capacity Helps Document Benefits of Biochar-Soil Mixtures"
  1. I and my spouse are currently making biochar from fruit tree prunings, lilac prunings and willow prunings and black walnuts.

    I have used the smaller barrel filled with biochar feedstock inverted upside down within a larger barrel that is then filled with scrap wood fuel around the outside of the smaller barrel and then lit. This works well unless the inner barrel develops air gaps sufficient that the could be biochar burns to ash. I also once the fire gts going good carefully put a lid and a section of stovepipe on top of he lager outside barrel aka 55 gallon drum. I puncture draft holes in the outside barrel around the bottom so in essence this makes a well drafted retort barrel. Within a few minutes the smoke from the stovepipe disappears into shimmering heatwaves. However this method is time consuming and labor intensive.

    We now use a method whereby our prunings aka (brush) and scrap wood is started in a small (about 3 ft. diameter circle) fire and as that flames up we just keep adding material until the pile of burned and burning material is what we estimate at about what will fill a (CHUB) barrel I guesstimate about 75 +/- pounds of finished charcoal. Once the large tongues of flame die down and short bluish flames and glowing embers remain we douse the fire and mix the material with a pitchfork until the pile is cooled and little steaming is evident.

    It takes a little practice but we have very little ash remaining and a CHUB barrel full of beautiful charcoal. I step on the big chunks with my boots and/or use a tamper commonly used to tamp gravel or soil to reduce particle size.

    We have a large backlog of black walnuts from our walnut trees, far more than I could eat or crack, and they simply become a fire hazard if not dealt with; so e are currently turning them into charcoal as well. I am sure the charred walnuts could be used for cooking or blacksmithing as well as biochar. (Why buy briquets of unknown composition when you can make your won). I am anticipating putting the biochar in a large leakproof container with water and compost teas and some soluble fertilizer prior to soil application until the biochar all sinks below the water surface so I can assume it is saturated. Then it, in my opinion, will not be hydrophobic once it is in the soil. The intent is to shorten the time frame until we see benefits from the biochar

    We only have our domestic well to irrigate our one acre with so keeping everything irrigated in August when it is hot and dry is a problem. I was thinking the application of biochar to the soil could help with yields because of microbial /biochar/ plant root interactions but hopeful it would also improve soil moisture holding capacity. It appears to me this study verifies that my hope is not unfounded. I already get amazing crop yield and quality but if this will help with keeping moisture in the vicinity of plant roots during times of low irrigation water availability the implications for small scale producers such as us are of (in my opinion) world wide positive potential. The best part is the biochar feedstock largely comes from our own property. Yes I could cut all the brush into firewood with an axe, and collect the walnuts for home heating as well, but that is time consuming compared to simply piling and burning the prunings and walnuts to make charcoal.

    I would like to be able to make better use of all the heat produced, however, other than simply sitting around enjoying the “campfire” as the last armload of brush and walnuts is burnt prior to dousing the fire.. Of course the Grandkids really enjoy making hot dogs and S’mores then tossing the cooking sticks into the fire so the heat is not entirely wasted.

    1. Hi James,

      Thank you for your interest and for sharing your experience with producing your own biochar. There are a few more posts in the works on other biochar work, so stay tuned…

  2. I want to buy a bio-char reactor for my 4+ Acre forest at home. Can you put me in touch with someone who can make me one and I could buy it from them. I live on Vashon island. I would like it to be larger larger than a 55 gallon drum hopefully at least 100 gallons.

  3. Boutique Biochars: Exploring Engineering Strategies to Increase Phosphate Adsorption | CSANR | Washington State University says:

    […] I explored work looking at the potential for biochar to draw down atmospheric carbon dioxide and increase water holding capacity in soils. Michael Aniayia (Figure 1) and his colleagues in the lab of Dr. Manuel Garcia-Perez at Washington […]

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