High Carbon Wood Fly Ash as a Biochar Soil Amendment

High Carbon Wood Fly Ash as a Biochar Soil Amendment Preliminary Assessment Completed by Northern Tilth June 2010

Executive Summary Resource Management, Inc. (RMI) recycles wood ash from biomass plants throughout New England. The physical and chemical properties of the fly ash generated at one of these biomass plants are unique relative to the fly ash from the other biomass plants in this region. This particular fly ash has a significantly higher carbon content and lower bulk density than fly ash generated at other biomass plants. Practical applications of this fly ash, including use for odor control at biosolids composting facilities and for adsorbing VOCs in soil clean-up projects, have indicated that the carbon in this ash has char-like properties. In the summer of 2009, Northern Tilth initiated a soil blending trial with the high-carbon fly ash to compare the material to biochar in terms of its properties as a soil amendment. For this trial, the high-carbon fly ash was added to a slightly acidic sandy loam stripped topsoil at volumetric rates of 10:1, 5:1 and 2.5 soil to fly ash. The soil blends, including the stripped topsoil with no fly ash added (a control soil) were incubated for the equivalent of 1.5 growing seasons in northern New England and then analyzed for standard soil fertility parameters as well as soil carbon and a direct measurement of cation exchange capacity (CEC). Cucumbers and tomatoes were also grown in the soil blends and germination and early biomass production were quantified. Analysis of soil physical properties on the control soil and 5:1 blend indicated that the fly ash increased water-holding capacity and soil porosity. Results from the trial indicate that this high carbon fly ash does provide some of the same benefits to soil as those reported for biochar, including increasing stable soil carbon content, increasing CEC and improving crop yields. The 5 parts soil to 1 part fly ash provided the best balance of soil fertility properties and increased soil carbon content. The fly ash in this soil blend increased CEC by 6.4 meq/100-g soil and increased soil carbon by 1.6% over the control soil. This increase in soil carbon represents 21 tons of carbon sequestered in soil per acre if the blended soil were placed at a depth of 8 inches.

High Carbon Fly Ash as a Biochar Soil Amendment Preliminary Assessment By Northern Tilth June 2010 In the summer of 2009, Northern Tilth initiated a soil blending study for Resource Management, Inc. (RMI) with fly ash generated at a New England biomass plant. The biomass plant burns clean wood to produce electricity using a steam turbine. Fly ash collected from this plant s pollution control system has a significantly higher carbon content than the fly ash generated at typical wood-to-energy plants in the region. The organic matter content, as measured by loss on ignition (LOI) in this fly ash averages 60.8%, while the LOI in fly ash from other biomass plants in New England is typically lower than 30%. With the recent academic and agricultural interest in the use of biochar as a soil amendment and for carbon sequestration, RMI was interested in determining if this particular fly ash (high-carbon fly ash) has similar soil amendment properties as are currently attributed to biochar. Historically RMI has utilized this high-carbon fly ash to reduce malodors generated when composting biosolids, and to adsorb volatile organic compounds in contaminated soils. Based on the success in these applications, it appears that the carbon in this ash has similar physical properties to activated carbon or charcoal. In this study, the high-carbon fly ash was blended with a relatively low fertility, slightly acidic topsoil at three rates to determine the impact of the fly ash on basic soil fertility properties, soil carbon content, seed germination and early plant growth. A portion of the blended soils were incubated at a controlled temperature for a time that simulated 1.5 growing seasons in New England. Soil Blending Average nutrient and physical properties of this high-carbon fly ash are included in Table 1 below.

Table 1. Fly Ash Analysis Parameters reported on a dry High Carbon Typical weight basis* Fly Ash Fly Ash** ph 11.9 12.3 12.0 12.5 Solids (%) 61.3 58.4 Loss on Ignition (%) 60.8 27.5 Total Nitrogen (%) 0.022 0.093 Estimated C:N 1600:1 170:1 Calcium (%) 8.7 17.3 Potassium (%) 2.7 5.2 Magnesium (%) 0.70 1.6 Phosphorous (%) 0.69 1.2 Bulk Density (g/cm 3 ) 0.29 0.64 *Based on the average of eight analyses from 2007-2008, with the exception of nitrogen species which were analyzed only one time. **Average data from four other wood-to-energy biomass plants in New Hampshire and Vermont over the same time period. The nutrient analysis indicates that the high-carbon fly ash is a high-ph material with a significant amount of base cations (calcium, potassium and magnesium), although a lower concentration of these cations and calcium carbonate equivalence than found in typical fly ash from other New England biomass plants. The bulk density of the high-carbon material is low, relative to the other sources of fly ash, and the LOI in the high-carbon fly ash, as stated above is relatively high. Trial blends of soil and high-carbon ash were designed to have a wide final ph range to help determine rates at which the ash would add significant amounts of carbon and/or char without inducing adverse soil fertility impacts related to high ph soils. The trial blends and estimated lime addition associated with the fly ash in the blends are listed in Table 2. % Soil (by volume) Table 2. Soil/Ash Blends % Fly ash (by volume) Estimated lime addition (tons/acre)* Soil:Ash ratio (by volume) Control (no ash) 100 0 0 10:1 90.9% 9.1% 4.6 5:1 83.3% 16.7% 8.4 2.5:1 71.4% 28.6% 14.3 *based on calcium carbonate equivalent of wood ash and hypothetical 8 depth of topsoil. Immediately following topsoil blending a sub-sample of each treatment was collected and shipped to the University of Maine s Soil Testing Service for analysis of common soil fertility parameters and for a direct measurement of High Carbon Fly Ash as a Biochar Soil Amendment Page 2

% Soil Carbon cation exchange capacity (CEC) and total soil carbon. These time=zero soil carbon results are included in Figure 1 below. At the same time, approximately 250 ml of each topsoil blend were placed in 1-liter mason jars and incubated in the dark at room temperature (between 78 and 82 0 F) for 130 days. The jar lids were slightly loose on the jar to allow for some air exchange with the air outside of the jars. Additionally, the jars were placed on their side to allow for a maximum amount of air exchange with the soil within the jars. The incubation jars were checked weekly for moisture levels, and additional water was added when the moisture level was less than ½ of field capacity. Based on the average incubation temperature of 80 0 F, this eighteen-week study represented 3900 growing degree days (GDD). As a comparison, the average growing season for southern New Hampshire consists of approximately 2600 GDD. At the end of the incubation period, sub-samples were collected from each of the treatments and shipped to the University of Maine s Soil Testing Service for the same parameters analyzed at time=zero. Figure 1. Total carbon in soil/ash blends 5 4 Soil Carbon before and after soil incubation before after 3 2 1 0 control 10:1 5:1 2.5:1 Volumetric Blending Ratio (control soil : wood ash) Soil Carbon Content Soil carbon content increased in the soil blends in proportion to the amount of fly ash added to the soil. Comparing the soil carbon content before and after soil incubation, it is clear that the carbon content did not change considerably High Carbon Fly Ash as a Biochar Soil Amendment Page 3

during incubation. Instead of going through a rapid phase of decomposition within the first field season, as would be the case with the fresh organic matter in un-composted animal manures, biosolids and wood-based paper mill residuals, the small decrease in soil carbon content during the incubation indicates that the carbon provided by the fly ash is relatively stable. By comparison, in topsoils manufactured with papermill residuals as the organic matter component, approximately 60 70% of the organic matter is oxidized within the equivalent of one full growing season in similar incubation conditions. The stability of the carbon indicates that the physical and chemical attributes contributed by the fly ash, such as increased CEC and water-holding capacity, will be long-lasting in the soil. This stability also indicates that this carbon represents long-term carbon sequestration in the soil environment. The carbon attributed to the fly ash remaining in the soil after the incubation period equates to 10, 21 and 37 tons per acre for the 10:1, 5:1 and 2.5:1 blends, respectively if contained in an 8 depth of soil. If carbon sequestration were to be determined based on a change in practice, in this case, relative to the carbon that would be added to soil from the ash in typical biomass plants, the amount sequestered could be calculated based on the difference in LOI between the fly ashes. Using the ratio of the LOI from the high-carbon fly ash compared to the LOI from the average of the four other plants listed above, the additional amount sequestered by the biomass combustion process and resulting fly ash would be 5.5, 11.6, and 20.4 tons per acre for the 10:1, 5:1 and 2.5:1 blends, respectively. Soil Fertility Soil fertility parameters from the soils collected after incubation are included in Tables 3, 4 and 5 below. Table 3. Soil Blend Fertility Results Soil Blends Soil Fertility Results Summary ph P K Mg Ca CEC* Soluble Salts Soil/Ash Blends Mg/kg (plant available) meq/100g mmhos/com Control Soil 5.3 12.2 138 120 978 3.7 0.49 10 Soil:1Ash 7.1 23.4 518 276 3546 7.2 0.67 5 Soil:1 Ash 7.7 40.4 1204 474 6526 10.1 0.89 2.5 Soil: 1 Ash 7.9 71 2606 880 12008 11.0 0.82 *based on direct measurement of CEC using strontium chloride for cation displacement Most soil fertility parameters increased with increasing rates of fly ash addition to the soil. Even at the lowest ash rate, exchangeable acidity in this initially slightly acidic soil was eliminated. Soil ph, plant-available High Carbon Fly Ash as a Biochar Soil Amendment Page 4

phosphorus, potassium, magnesium and calcium increased with increasing rates of fly ash to the soil blend. Comparing the total amount of phosphorus added in the fly ash to the increase of plant-available soil phosphorus over the control soil for the three soil/ash blends, an average of 17.2% of the total phosphorus in the fly ash became plant-available. Soluble salt content also increased with increasing fly ash addition, but even at the highest rate of ash addition in this study, the soluble salt content did not reach a level that would be inhibitory to seed germination or plant growth. CEC on standard soil fertility tests is estimated, and errors in these estimates can be exacerbated when high amounts of base cations (potassium, magnesium, and/or calcium) are added to the soil in the form of alkaline amendments. Because of the high rates of fly ash added in these soil blends, the U Maine Soil Testing Service was asked to quantify CEC by direct measurement. After determining that using a mono-valent cation (ammonium) as the displacement cation was not appropriate for the direct measurement of the CEC contributed by the activated carbon in the fly ash, the lab tried the CEC measurement using barium as the displacement cation. This too, was deemed unsuitable, because of the potential error associated with the barium chloride stripping sulfate from the wood ash instead of displacing cations on he cation exchange sites. Finally, the lab used strontium (added as strontium chloride) as a di-valent cation to get an accurate direct measurement of CEC in the ash-amended soils. Soil CEC increased with increasing addition of fly ash, ranging from 3.7 meg/100-g soil in the control to 11.0 meq/100-g soil in the highest rate of fly ash addition (28.6% ash by volume). In natural, un-amended soils, this tripling of CEC would be equivalent to changing the soil texture from a loamy sand, or sandy loam to a much finer textured soil, such as a silt loam, and would dramatically increase the ability of the soil to store cations for uptake by plants. Results from the direct measurement of CEC using ammonium chloride indicated no trend in CEC changes with increasing fly ash rates suggesting that the increase in CEC using strontium chloride was not likely due to ph-dependent charges. Table 4. Soil Blend Base Cation Balance and Plant-Available Nitrogen Soil/Ash Blends %K* %Mg %Ca NO3 NH4 Total PAN Mg/kg (dry wt. basis) Control Soil 3.3 12.4 52.7 25 2 27 10 Soil:1Ash 6.2 10.8 83.0 11 1 12 5 Soil:1 Ash 7.8 10.0 82.2 9 1 10 2.5 Soil: 1 Ash 9.0 9.9 81.1 8 1 9 *Based on charge equivalent of extractable base cations and acidity High Carbon Fly Ash as a Biochar Soil Amendment Page 5

The stability of the carbon added to the soil from the fly ash indicates that this carbon is not readily oxidized by soil microbes, and consequently, even with the high estimated C:N ratio of the organic matter added by the fly ash, nitrogen immobilization is not likely to be significant. Plant-available nitrogen (PAN) levels did decrease with increasing application rates, but in practical applications, these lower levels of PAN could be compensated for with a relatively modest increase in nitrogen fertilizer application rates. For instance, with the highest application rate of fly ash in this experiment (2.5 parts soil to 1 part fly ash by volume) an additional 35#/acre of nitrogen would compensate for the lower PAN levels. Adding alkaline agents to soil in excess of liming recommendations has two potentially negative impacts to soil fertility. First, if one cation (typically calcium) predominates in the alkaline agent, an imbalance of cations in the soil may result. Second, if the soil ph, after alkaline addition, is greater than 7.5, there is a possibility that plant-available phosphorus and micro-nutrient levels, including copper, manganese, boron, iron and zinc could become deficient. Relative to calcitic or high magnesium lime, the fly ash contains a balance of potassium, calcium and magnesium, that makes a cation imbalance from high application rates less likely than would be the case from over-liming. In the soil blends in these trials the lowest application rate of the fly ash increased the charge equivalent ratio of potassium to both magnesium and calcium, but not to levels that would result in an imbalance in these three cations in soil. As the rate of fly ash addition to soil increased, the ratio of the base cations changed very little. Based on the soil fertility results from these blends, it appears that application rates of the fly ash in excess of a soil s liming recommendation are not likely to cause a base cation imbalance that would result in a negative impact to plant growth. Table 5. Soil Blend Micronutrient Content Soil/Ash Blends S Zn Mn Fe Cu B Mg/kg (plant available) Control Soil 60 0.3 3.6 19.6 0.21 0.3 10 Soil:1Ash 74 0.8 6.2 7.1 0.08 0.7 5 Soil:1 Ash 95 1.8 13.3 7 0.13 1.4 2.5 Soil: 1 Ash 142 4.1 26.9 6.2 0.13 2.5 The ph in both the 5:1 and 2.5:1 soil:ash blends were greater than 7.5 (they were 7.7 and 7.9, respectively). In these blends, the phosphorus added to the soil from the fly ash more than compensated for any potential decrease in phosphorus availability related to the alkaline ph. As stated above, the High Carbon Fly Ash as a Biochar Soil Amendment Page 6

approximately 17% of the total phosphorus added from the fly ash became plant-available during the incubation period. There was some notable variation in plant-available sulfur and micronutrient content between the soils. For sulfur, zinc, manganese and boron, concentrations increased with increasing application rates of the fly ash. For iron and copper, concentrations decreased with increasing application rates, presumably due to lower availability with the higher ph. In the case of iron, the concentrations were optimum or higher even at the highest application rate of the wood ash. For copper, the control soil was already below optimum, and the addition of the fly ash further decreased copper availability. Soil Physical Properties (Soil Tilth) Samples of the incubated soils from the control and the 5:1 soil:fly ash blend were sent to Hummel & Co. laboratory for analysis of water-holding capacity and porosity. Soil/Ash Blends Table 6. Porosity and Water-holding Capacity Bulk Density (g/cm 3 ) Total Porosity (%) Aeration Porosity (%) Volumetric Water Content (% by volume) 0.06 bars 0.1 bars 3 bars Control Soil 1.60 40.2 23.4 31.4 25.7 16.7 5 Soil:1 Ash 1.45 45.1 25.9 40.3 37.0 19.2 Results from the physical properties analysis indicate that the addition of the fly ash to the soil lowered bulk density and increased both soil porosity and water holding capacity. The decrease in bulk density and subsequent increase in porosity provides for better air exchange with the atmosphere and improves the access of roots to oxygen. For a sandy loam soil, such as the one used in this study, field capacity (the point at which macropores are drained of water and capillary pores are saturated) is typically close to 0.1 bar. These results suggest that the 5:1 blend holds approximately 44% more water than the control at conditions close to field capacity. Additionally, the fact there is greater water content from 0.06 bars and 0.1 bars indicate that the 5:1 blend releases water over a wider range of moisture conditions than does the control soil. Tomato and Cucumber Growth On June 22, 2009, sub-samples of the control and blended soils were placed in plant pots to check germination and early growth of cucumbers and tomatoes. The cucumbers and tomatoes were placed in 1.5 x 1.5 x 2 deep cells (18 for each treatment and each plant type). One seed was placed in each cell. The pots were placed under timed grow lights and the soil temperatures were maintained between 65 and 70 o F. High Carbon Fly Ash as a Biochar Soil Amendment Page 7

Weight (g/plant - fresh wt.) Cucumber germination was mostly complete within four days, and tomatoes were starting to germinate at that time. After seven days germination of the cucumbers and tomatoes were as follows: Table 7. Cucumber and Tomato Germination Soil/Ash Blends Cucumbers Tomatoes # of seeds % # of seeds % germination germinated germination germinated Control Soil 18 100% 18 100% 10 Soil:1Ash 18 100% 18 100% 5 Soil:1 Ash 18 100% 17 94% 2.5 Soil: 1 Ash 18 100% 15 83% Germination was 100% for the control and all three soil blends for the cucumbers, indicating that there was no inhibitory effect on germination from the application of even the highest rates of the fly ash. For the tomatoes, there was some indication that the highest rates may have had negatively impacted germination. The cucumbers and tomatoes were harvested and weighed five weeks after seeding. The plants were cut approximately at the soil surface and weighed (above-ground biomass only). Figure 2. Biomass production for cucumbers and tomatoes Cucumber and Tomato growth five weeks after seeding 3 2 cucumbers tomatoes 1 0 control 10:1 5:1 2.5:1 Volumetric Blending Ratio (control soil : wood ash) High Carbon Fly Ash as a Biochar Soil Amendment Page 8

The fly ash increased biomass production for both the cucumbers and tomatoes relative to the control soil. The five parts soil: one part fly ash (5:1) had the highest fresh weight per plant for both cucumbers and tomatoes, although the differences in weight per plant were marginal between the three fly ash-amended soils. Increased biomass production is likely the result of increased phosphorus and potassium availability and CEC and lower acidity in the fly-ash amended soils relative to the control soil. Clearly the lower nitrogen availability in the fly ash-amended soils did not negatively impact biomass production in the early stages of cucumber or tomato growth. All plant pots were watered on the same schedule, and it is possible that improved water-holding capacity in the fly-ash amended soils accounted for some of the increased biomass production. Figure 3. Cucumber growth after four weeks. 2.5 parts soil: 1 part fly ash 5 parts soil: 1 part fly ash 10 parts soil: 1 part fly ash Control Soil Summary Results of this bench-scale trial indicate that this high-carbon fly ash provides many of the major attributes that have been attributed to biochar as a soil amendment, including increased CEC, increased soil fertility, increasing stable carbon, increasing carbon sequestration in soils, and improved soil tilth. The fly ash is an alkaline material with a relatively high calcium carbonate equivalence, and, as expected, soil ph increased with increasing application rates. However, even at the highest rates used in this study, which was calculated to provide approximately 14.3 tons of lime per acre to the control soil, adverse impacts to soil fertility associated with alkaline soils were not evident in this trial. Results of this trial suggest that in order to maximize benefits to soil fertility, soil carbon and biomass production, while minimizing the potential for adverse impacts from excessive fly ash applications, the optimum blend is five parts soil to one part fly ash by volume (5:1). Although the ph of this soil blend was slightly alkaline (7.7), this blend provided the highest biomass High Carbon Fly Ash as a Biochar Soil Amendment Page 9

production for both cucumbers and tomatoes and represented an increase of 1.6% soil carbon and 6.4 meq/100-g soil CEC over the control soil. While this bench scale trial is limited in scope, the data collected and results compiled suggest that the use of the high-carbon wood ash as a soil amendment will impact soil in a manner similar to those attributes found in biochar amended soils. Contact Information: Shelagh Connelly, President Resource Management, Inc. (RMI) RMI manages and distributes high-carbon ash in the northeastern US 1171 NH Route 175 Holderness, NH 03245 603-536-8900 Shelagh.connelly@rmirecycles.com www.rmirecycles.com Andrew Carpenter, Soil Scientist (report author) Northern Tilth Andrew@northerntilth.com 207-338-5500 High Carbon Fly Ash as a Biochar Soil Amendment Page 10