Utilizing wood ash as a liming agent for the improvement of soil health and growth of bermudagrass

Wood ash is a common by-product of industrial timber and timber product production. It has been used as a soil amendment to increase pH for centuries; however, much of the current industry sends this byproduct to landfills. In this study, we investigated the response of soil properties and bermudagrass (Cynodon dactylon) growth to the addition of ash as a soil amendment. A soil that was previously in agricultural use was amended with wood ash along with five other treatments (lime, fertilizer, lime + fertilizer, ash + fertilizer, and a control with no amendment) at the level of lime equivalency to raise the soil pH from 5.35 to 6.5. Soils were amended with liming treatments (lime and wood ash) and allowed to equilibrate for five months, during which they were planted with bermudagrass, and differences in plant growth and soil physical and chemical properties were monitored. Treatments with ash had the highest overall productivity, with a 2× higher yield than the control treatment in the first cutting, and the largest shifts in soil properties. In particular, there was increased soil carbon (~1% higher with ash amendment compared with other treatments), cation exchange capacity (~2× with ash amendment), and phosphorus and potassium concentrations in treatments with ash compared to non-ash treatments. This type of amendment is generally well-suited for intensively managed or degraded southeastern piedmont agricultural soils that would benefit from an increase in pH, nutrient concentrations, soil organic matter, or overall soil health. We found that using forest waste wood ash as a soil amendment can be an effective strategy to increase soil carbon sequestration, divert waste from landfills, and increase productivity.

Highlights

● Ash was an effective replacement for lime to increase soil pH (20% lime equivalence)

● Soils treated with ash and ash + fertilizer had ×2 the initial biomass yield and ground coverage compared with control (no treatment) and lime treatments

● Ash application improved nutrient retention in the soil (cation exchange capacity and total nutrient concentration at the end of the study)

● Ash application significantly reduced soil bulk density

1 Introduction

1.1 Production of ash as an industrial byproduct

Ash is a common waste product of several forest industry products, such as paper and pulp, and approximately 18.06 Mt of ash is produced in forestry operations globally each year (1). A significant proportion of this wood ash is directed to landfills as waste; some estimates suggest that this is as high as 80%–90% (2–4), which represents a significant cost to forest product producers. With increasing pressure to divert wood ash waste from landfills, research has focused on barriers to the use of wood ash as a soil amendment (5, 6). Some of the challenges to the adoption of wood ash as a soil amendment are that it is not a homogenous product, as it differs in chemical and physical properties based on the initial feedstock material (bark, wood chips, etc.) and burning temperature and duration (7–10), which makes it difficult to make generalized recommendations for its use. Other significant barriers to the use of ash as a soil amendment include logistical challenges associated with storing, transporting, and applying ash, the need to obtain appropriate permits, and the need for sufficient expertise to test and interpret wood ash and soil quality (11).

1.2 Ash use as a soil amendment

There has been a long history of humans adding ash to improve several soil properties, including Terra Preta del Indio soils (12–14). Ash made from virgin biomass is generally free from contaminants, making it relatively safe for use as a soil amendment (1). Ash can raise the soil pH (i.e., liming effect), and when produced with incomplete combustion, can act as a source of organic matter to the soil (15). As a liming agent, there is a wide range in the reported lime equivalence, which reflects the burn conditions and fuels that control the formation of carbonates in ash (16). Ash is frequently reported to change soil fertility, as it may have high concentrations of relevant soil nutrients, including calcium (Ca), phosphorus (P), magnesium (Mg), potassium (K), and other nutrients (17–19) and can increase cation exchange capacity (CEC) (20). Different wood ash products have different concentrations of each nutrient, and their persistence in different soils varies; therefore, they are not often recommended for regular, repeated applications as fertilizers without soil nutrient monitoring. Additionally, site-specific factors such as differences in soil types, vegetation, climate, and land use history also control the effects of ash on a given soil (21, 22), making it difficult to make widely generalizable application recommendations.

Different sources and production methods for ash can create significantly different end products. When the ash has a high proportion of charcoal and black ash, it can improve the soil in the same way as biochar (improved hydraulic conductivity and CEC and decreased bulk density) (23, 24). In contrast, higher combustion temperatures and longer burning durations result in ash with less retained carbon, higher mineral proportions, and lower total mass (9, 25).

In addition to driving shifts in soil chemistry, nutrition, and organic matter, wood ash applications have been demonstrated to significantly affect soil physical properties, including structure, porosity, and saturated hydraulic conductivity (26). In particular, wood ash applications have been tied to reduced water holding capacity and pore clogging, which reduces hydraulic conductivity (16, 26, 27); however, the effects of ash on soil hydraulic conductivity depend on both the qualities of the ash and soil texture.

1.3 Ash use to improve grass productivity

Wood ash has a long history of application in forest soils as a byproduct of the forest industry that can be used to improve soil fertility and raise pH (15, 16, 20, 21, 28, 29). The use of wood ash as a soil amendment for grasslands has received recent research focus and has been demonstrated to improve soil fertility and grass growth (30–32). In a study on acidic soil amended with wood ash, ryegrass (Lolium multiflorum) had significantly higher biomass yield and foliar N than the unamended soil (33), and another study on paiaguás grass in Brazil found significantly higher yield with wood ash amendment (34). There is also evidence of an altered nutrient profile of grasses and other plants growing in soils amended with wood ash, which is critical for forage grasses (18, 35, 36).

1.4 Project aims

This study aimed to systematically describe the effects of using wood ash, a waste product, as a soil amendment to improve establishment and growth of bermudagrass (Cynodon dactylon) and soil chemical and physical properties in a degraded Piedmont ecoregion soil. Bermudagrass was chosen as our study species because it is widely used as both a turf and forage grass in the Southeastern US. Southeastern Piedmont soils tend to be acidic and are commonly lower in nutrients and organic matter when extensively used for agriculture. Our aim was to understand the potential of wood ash as a low-cost soil amendment to improve soil pH, soil health, and bermudagrass yield. We hypothesized that wood ash would have a larger impact on soil health metrics than lime, fertilizer, or their combination on soil organic matter, CEC, and nutrient concentrations. We also hypothesized that any improvements in soil fertility and pH resulting from ash application would result in higher bermudagrass yield.

2 Methods

2.1 Site selection and experimental set up

The study site was outside the city of Royston, in Hart County, GA, which is located in the Piedmont physiographic region of the southeastern US. Royston, GA has a humid subtropical climate with a MAT of 18 °C. The site was previously a pimento pepper (Capsicum annum) field that has been managed as an unirrigated lawn for several decades. The site’s soils are classified as the Madison soil series, which is a fine, kaolinitic, thermic Typic Kanhapludult.

Study plots were established using a randomized block design, with five blocks set to account for the slope (Figure 1A). Each block consisted of one of the following treatments, which were randomly distributed within each block: control, lime, ash, fertilizer, lime + fertilizer, and ash + fertilizer. The lime and ash treatments were applied at equivalent lime values to the recommended lime dose (to raise soil pH to 6.5) after soil testing. The plots were 1.5 m × 1.5 m in size and delineated by a wooden box to prevent potential erosion or leaching of nutrients from upslope to downslope treatments (Figure 1B).

Figure 1

Figure 1. Experimental plots were laid out in five blocks with all six treatment categories (A). These blocks were arranged down the slope to account for differences in soil between them (B).

Soils were sampled during plot establishment on 23 October 2020, at 0 cm–10 cm depth using an auger. These soils were tested for nutrient content, pH, and lime requirement at the University of Georgia (UGA) soil-testing laboratory. There were no significant differences (p >0.05) based on ANOVA models between the treatments at the initial sampling point for pH, bulk density, carbon (C), nitrogen (N), CEC, or clay content. However, there were significant differences (p <0.05) between the blocks at the initial sampling point for pH (F = 2.866), bulk density (F = 3.562), C (F = 13.71), N (F = 15.1), and CEC (F = 14), but not for clay content (F = 0.445, p>0.05). Overall, the soils were acidic, with an average pH of 5.35, and had relatively low phosphorus concentrations (Table 1).

Table 1

Table 1. Soil pH and nutrient levels before amendment (n = 30, standard error in parentheses).

After testing the soils, lime and ash (at a rate of 20 tons/hectare) were applied in December 2020 to raise the soil pH to a target value of 6.5. Lime and ash were hand incorporated to a depth of 10 cm in the soil. Plots were weeded and sampled for soil on 19 March 2021, to test for an increase in pH levels. The plots were then weeded, fertilized, and seeded with raw unhulled bermudagrass donated by Pennington Seed, Inc. on 6 May 2021. Seeds were applied at the Pennington recommended rate of 145 kg/ha, and fertilizer was applied at the recommended rate of 112 kg/ha for summer growth (37). Plots were watered to supplement rainfall to receive a minimum of 2.5 cm of water per week from 6 May 2021, through the final sampling in September 2021.

2.2 Ash characterization

Wood ash was obtained from Huber Engineered Woods, LLC, and analyzed prior to application (Table 2). The ash was obtained by combusting loblolly pine (P. taeda) waste (bark, wood scraps, and sawdust) in a furnace. The furnace reached peak temperatures of 790 °C, but inefficient heat transfer within the furnace led to lower temperatures at the bottom, particularly when wet wood pieces were loaded into the furnace. The ash constituted a heterogeneous mixture of charcoal and black and white ash and contained a wide range of particle sizes (mean of 35% by mass were particles >250 μm, 20% of particles were >1 mm, and the majority of particles were finer than 250 μm). The ash pH was measured in a 0.01 M CaCl₂ solution in a 1:1 ratio, and the ash was extracted with a Mehlich I solution for nutrients and heavy metals and to determine the CEC. The ash had a relatively high mean pH (10.31) and CEC (123.17), and the nutrient content varied considerably between the ash subsamples (Table 2). Furthermore, the equilibrium lime buffer capacity, as determined by the UGA soil test laboratory, was used to calculate the lime requirement for each soil to raise the soil pH by one pH unit. Additional testing for barium was conducted using inductively coupled plasma–optical emission spectrometry (Table 2) at the Agricultural and Environmental Services Laboratory at UGA.

Table 2

Table 2. Chemical properties and nutrient concentrations in wood ash.

2.3 Soil property responses

Soil samples were collected monthly during the growing season (May–September) using a hand auger to a depth of 10 cm. One sample per plot was collected at each time point to minimize the effects of sampling disturbances on the plots. These soils were analyzed for pH in a 1:5 w/w CaCl₂ solution. Soils collected at the conclusion of the field season (September 2021) were further analyzed for total C and N, CEC, and nutrient content. In addition, intact core samples were collected for bulk density measurements.

2.4 Plant growth responses

After planting, plots were monitored monthly for grass growth by photographing to determine percent ground cover (four times during the growth season until ground cover was 100%). Biomass was determined by collecting grass from 10 cm square clip strips (triplicate samples per plot, monthly during the growing season). Biomass was oven dried at 60 °C after collection and weighed to determine dry biomass. The final two collections of biomass samples were analyzed for nutrient concentrations at the Waters Agricultural Laboratory in Camilla, Georgia, USA.

2.5 Statistical approach

The pH response variable, along with later growing season soil N and P concentrations (July and September), foliar C, Mg, Ca, and Fe, and ground cover were not normally distributed according to the Shapiro–Wilk normality test (p <0.05), and these data could not be transformed to fit the assumptions of normality with this test. Therefore, the Kruskal–Wallis rank sum test was used to assess differences in treatment, and the Friedman test was used to test differences over time. A pairwise Wilcoxon rank-sum test was used as a post-hoc analysis to test significant differences between the treatment groups. The bulk density, initial soil P and N, biomass yield, and foliar N, phosphorus (P), and potassium (K) data met the assumptions of normality and were analyzed using ANOVA and Tukey’s Honest Significant Difference post-hoc test to assess statistical differences. All statistical significance was assessed at p <0.05. Data analyses and figure generation were performed in RStudio (Version 1.2.1335).

3 Results

3.1 Ash characterization

The ash had a pH of 10.31 and a lime equivalence of 20%, and highly variable concentrations of plant-relevant nutrients (Table 1). The ash also has a relatively high CEC and increases the retention of essential cation nutrients (including Ca⁺, K⁺, Mg²⁺, and NH₄⁺) for plant growth. It is important to note that the high variability in the nutrient concentration of the ash is reflective of variations in the quality of the ash due to variations in burn conditions and original source wood.

3.2 Changes in soil properties

During the initial sampling, there were no differences in pH between the different plots, and there was significant variability in the measured pH across the duration of the study (Figure 2, Friedman test, X² = 79.1). After amendment with lime and ash, treatment was a significant predictor of soil pH (Kruskal–Wallis rank sum test, X² = 77.727), and the pH in the ash treatment was significantly higher than in all the other treatments, except for ash + fertilizer and lime + fertilizer (Pairwise Wilcoxon rank sum test). The lime treatment was significantly lower than the ash and ash + fertilizer treatments but significantly higher than the control and fertilizer treatments (p <0.05). The lime and lime + fertilizer treatments were not significantly different from each other, but the pH of the lime + fertilizer treatment was not significantly lower than that of the ash or ash + fertilizer treatments (p >0.05). The pH of the control and fertilizer treatments was not significantly different from each other (p >0.05) and was significantly lower than that of all other treatments (p <0.05).

Figure 2

Figure 2. Time series of soil pH during the study period. The pH measurements were made in CaCl₂ solution with n = 5. Error bars represent the standard error, and the dashed line indicates the time of amendment with either wood ash or lime. Fertilizer was added on the May sampling date after sampling.

At the conclusion of the study (September 2021), the soils treated with ash had the highest increase in soil pH compared to those treated with lime. The variability in pH increased with the ash treatment, likely due to variability in the ash properties (Table 1), variation in initial soil properties, and variations in reaction completion within the soil matrix.

The addition of wood ash can reduce the bulk density of soil. We measured the bulk density after amending the soils and at the conclusion of this study (Figure 3). The two ash treatments had lower bulk density than the other treatments, but it should be noted that the bulk density was generally low after the initial tillage, which also reduced the bulk density. However, the further reduction in bulk density with the addition of ash suggests that it may be an effective amendment for reducing bulk density without tillage.

Figure 3

Figure 3. Soil bulk density was measured using a ring from 0 cm to 5 cm of soil (n=5) at the conclusion of the study. Error bars represent the standard error, and letters indicate significant differences based on a Tukey’s HSD test.

Based on ANOVA, there was a significant effect of treatment on the soil bulk density at the conclusion of the study (F = 4.79, p <0.05). The Tukey HSD test indicated that the ash + fertilizer treatment resulted in a significantly lower pH than the fertilizer, lime, and lime + fertilizer treatments. The effect of ash treatment was not significantly different from that of the other treatments.

At the initial timepoint of the study, there were no significant differences in CEC between the different treatments (Figure 4); however, at the final sampling, the treatments had a significant effect on CEC (X² = 17.76, p <0.05). The treatments with fertilizer, control, and lime + fertilizer was not significantly different from each other but were different from the ash and ash + fertilizer treatments. The lime treatment was not significantly different from any of the other treatments, and the ash and ash + fertilizer treatments were not significantly different from each other.

Figure 4

Figure 4. Soil cation exchange capacity was measured at three separate time points during the study: the initial soil sampling point (October 2020), mid-growing season (July 2021), and at the conclusion of the study (September 2021). The dashed line represents the amendment with wood ash. Error bars represent the standard error with n = 5.

The soil available nutrients changed throughout the duration of the study (Figure 5). After amendment with NPK fertilizer, only P and K increased in the treatments with ash. This is related to the high P concentration in the ash and its relatively high CEC. The highest P concentration in the soils was measured at the conclusion of the study, which might indicate that there was some breakdown or desorption of the ash that released bound P from the ash into the soil matrix.

Figure 5

Figure 5. Soil nutrient testing (n = 5) included phosphorus and potassium at four time points during this study: pre-planting in winter (October 2020), immediately pre-planting (May 2021), mid-season growth (July 2021), and conclusion of the study (September 2021). Error bars represent the standard error, and the dashed vertical line indicates the time of ash amendment.

Prior to amendment (October 2020) and at the beginning of the growing season (May 2021, Figure 5), there were no significant differences in P or N (p >0.05, ANOVA) between any of the treatment groups. However, in the mid-growing season sampling (July 2021), there were significant differences between the treatment groups. The ash, ash + fertilizer, and lime + fertilizer treatments had significantly higher P concentrations than either the control or lime groups, and the ash + fertilizer and lime + fertilizer treatments also had significantly higher soil P than the fertilizer alone. This indicates the significance of soil pH in increasing the available P in these soils. At the mid-growing season sampling point, the ash and ash + fertilizer treatments had significantly (p <0.05, Wilcoxon) higher soil K concentrations than any of the other treatments and were not significantly different from one another (p >0.05, Wilcoxon). K in the ash may become more available through this study, as it is a readily soluble cation. At the conclusion of the study, soil P and K concentrations were significantly higher (p <0.05, Wilcoxon) than those in any of the other treatments, and they were not statistically different from each other.

At the conclusion of the growing season, soils were analyzed for total C and N (Figure 6), and there were no significant differences between the treatment groups for either C (X² = 6.69, p >0.05) or N (X² = 2.55, p >0.05). However, the treatments with ash had approximately 1% more carbon than the other treatments.

Figure 6

Figure 6. The mean soil carbon and nitrogen were measured at the conclusion of the study. Error bars represent the standard error. There were no significant differences in total carbon or nitrogen between the treatment groups.

3.3 Bermudagrass growth response to treatment

Bermudagrass growth was rapid after planting, and it reached 100% ground cover by July 2021 (two months post-planting; Figure 7). The ground cover in the treatments with lime, ash, and fertilizer was 150% higher than that in the treatment with lime only and the control. However, there were no significant differences in treatments across the monitored period for ground cover (X² = 5.9076, p >0.05). Each time point significantly differed from the others (X² = 75.226, Wilcoxon, p <0.05).

Figure 7

Figure 7. Time series of the fraction of vegetation cover in plots (1 = 100%). Mean cover measurements (n = 5) were determined from photographs of individual plots, with error bars representing the standard error. The vegetation cover at the first two time points was a mix of weedy cover, and the dashed vertical line indicates when the plots were weeded (taken to 0% vegetation) and planted with bermudagrass.

Growth was also monitored through bermudagrass yield across three cuttings (Figure 8), and there were significant differences based on the treatment (ANOVA, F = 7.338, p <0.05). The control had a significantly lower yield (TukeyHSD, p <0.05) than all other treatments except for the lime addition, and the ash + fertilizer treatment was significantly higher than both the control and lime treatments. The two highest yields were found in the treatments with ash, and they did not significantly differ (p >0.05). In the first cutting, the yield from the ash treatments was approximately 2× higher than that of the control, and by the third cutting, this gap narrowed to approximately 1.4×.

Figure 8

Figure 8. Time series of vegetation growth determined by three individual cuttings of biomass. Cuttings were made to the ground surface in triplicate per plot and pooled together before drying (n = 5).

3.4 Foliar nutrient concentrations

Foliar tissues were collected during the peak growing season (August) and late growing season (September) and analyzed for nutrient content (Table 3). There were significant differences in the foliar tissue nutrient concentrations. The control treatment foliar tissue had significantly higher N concentration than the lime + fertilizer, fertilizer, and ash + fertilizer treatments (p <0.05; ANOVA), and there were no significant effects of time on foliar N concentration (p >0.05; ANOVA). There were no significant differences in foliar C concentrations between the treatments or over time (p >0.05; Kruskal–Wallis). For foliar tissue P, the September sampling date had significantly higher concentrations than August (p <0.05; ANOVA). The ash and ash + fertilizer treatments had significantly higher foliar P than any of the other treatments (p <0.05; ANOVA), and the fertilizer and fertilizer + lime treatments had significantly higher foliar P than either the control or lime treatments (p<0.05; ANOVA).

Table 3

Table 3. Mean nutrient elemental concentrations for foliar tissues with standard error in parentheses (n=5).

There were significant effects of sampling date (F(1) = 54.04, p <0.05) and treatment group (F(5) = 4.53, p <0.05) on foliar K concentrations, where the August sampling date was significantly higher than the September date, and the lime + fertilizer treatment was significantly lower in foliar K than the ash, ash + fertilizer, and control treatments.

There were no significant effects of sample date (X²(1) = 0.36968, p >0.05) and treatment group (X²(5) = 6.4192, p >0.05) on foliar Mg concentrations. There were significant differences in foliar calcium (Ca) between sampling dates (X²(1) = 5.6443, p <0.05) and treatment groups (X²(5) = 34.81, p <0.05). The ash + fertilizer treatment had significantly lower foliar Ca than all other treatments, except the fertilizer treatment (p <0.05, Wilcoxon). The lime and lime + fertilizer treatments had the highest foliar Ca concentrations, which were significantly different from those of the other treatments, except for the control (p <0.05, Wilcoxon). None of the foliar tissues significantly differed in Fe concentration based on sampling time (X²(1) = 0.12594, p >0.05) or treatment group (X²(5) = 9.1883, p >0.05).

4 Discussion

4.1 Wood ash as a soil liming agent

Overall, soils amended with ash and lime had equivalent and expected increases in soil pH, although the lime treatment without fertilizer did not have as much increase in soil pH as the other lime and ash treatments (Figure 2). This variability in soil pH with ash amendment is likely due to the variability in the ash product and the diversity of forms of calcium salts found in wood ash (9). Heterogeneity in wood ash is widely reported, and this is typically a function of the source of the woody material, temperature and duration of burning, and temperature control of the furnace (9, 25, 38, 39).

Wood ash contributed to increased soil nutrient supply, although the concentrations of these nutrients in the wood ash were variable (Table 2), most likely due to variable combustion conditions within the furnace where it was produced. The concentrations of Ca, K, Mg, and P in the wood ash were significantly higher than those of the initial soil nutrients. While barium contamination can be a cause for concern, the barium concentrations were found to be below many reported soil concentrations, including those of contaminated urban soils. In combination with its low mobility in soil and the time needed for sufficient ash breakdown for barium to become plant-available, it should pose no threat to human health (40) or plant production (41).

The wood ash used in this study had a somewhat lower calcium carbonate equivalent (CCE, 0.2) than what might be considered a median CCE (~0.5) across many types of wood ash (4). However, wood ash has a higher CCE than biochar, which is becoming a popular soil amendment for improving soil carbon storage and fertility (42–45). The different forms of Ca salts found in wood ash can lead to faster liming effects than agricultural lime (9), although this was not observed in the timing of soil pH shifts in this study. Additionally, the liming reaction of wood ash depends on the initial soil pH (46). Although this study was conducted over a single growing season, there may be differences in the longevity of the soil effects between wood ash and other treatments, as the C portion of the ash may not begin to significantly breakdown until approximately 6 months or longer in the soil (43), which could alter the pH further depending on what portions of the ash become solubilized.

4.2 Shifts in soil properties

In addition to modifying soil pH, ash treatments significantly altered other important soil properties, including CEC, bulk density, organic matter, and nutrient availability. In some soils, these may be critical additional benefits for improving soil health when the soil pH is overly acidic, certain nutrients are deficient, or organic matter is too low (21, 47). Soil organic matter is a significant soil health indicator, because of its major role in controlling soil functioning, including supporting increased microbial biodiversity, increased water retention, increased nutrient recycling, increased CEC, decreased bulk density, and increased aggregation, etc. (48–50). We observed a 1% increase in soil C following ash addition, which, although not statistically significant, likely drove changes in several soil physical (e.g., decreased bulk density) and chemical changes that led to the highest bermudagrass productivity (% ground cover and yield) in the soils amended with ash. Increases in soil C after biochar amendment have also been reported in pasture grassland (51). Other studies with wood ash have reported minor increases or decreases, or no significant change, in soil carbon and soil microbial biomass and functioning (e.g., respiration, enzyme activity, and microbial communities), which may reflect pre-amendment soil properties (particularly soil C concentration and size and diversity of the microbial community) and the variation in different wood ash materials (52–54).

Increases in soil CEC after ash amendment have been commonly reported (21) and are often due to changes in mineral surface charges, dissociation constants, increases in soil organic matter, or a shift in pH (55). We found that wood ash amendment significantly increased soil CEC from <10 meq/100 g to approximately 20 meq/100 g. Increasing soil CEC is a common management aim to increase nutrient retention and reduce nutrient leaching, although a CEC can lead to the need to apply more lime to achieve soil pH effects and increase the required dose of herbicide (56).

The significant increase in soil nutrients observed in this study has been commonly reported in other soils amended with wood ash (8, 46, 57). The significant increase in soil P concentration (Figure 5) likely drove the related increases in foliar P concentrations. Previous studies have suggested several mechanisms for increasing P availability with ash and biochar amendments: the release of organically bound P, increase in soil pH, microbial functioning, and sorption of P on the surfaces of charcoal, soil, or ash (58, 59). Due to these dynamic soil processes, it is unlikely that increases in soil nutrients would persist over longer periods, as mobile nutrients are leached from the soil or taken up by plants and removed at harvest (8, 60).

4.3 Shifts in productivity and foliar chemistry

In soils amended with ash, we found improved bermudagrass establishment (in fraction ground cover, Figure 7) and yield (Figure 8) throughout the course of this study. Bermudagrass growth may have been limited by several soil factors (e.g., pH, CEC, bulk density, and organic matter) that were improved by ash amendment. Foliar production is typically positively correlated with increasing soil P and K availabilities (18). Previous research on the use of wood ash to improve grass production has been mixed, with some studies finding up to 100% increases in pasture productivity, as in Ferreiro et al. (19), and others finding no differences (61), even when comparing wood ash and lime on bermudagrass grown in soils that were not fertilized with P (62). Some of these differences in bermudagrass response to ash application likely result from differences in pre-amendment soil pH and nutrient status, where grasses that benefit from higher pH respond more when pre-amendment (or control soil) pH is lower than recommended.

Shifts in foliar nutrient concentrations reflected shifts in soil nutrients. Increases in soil Ca have been reported after soils were amended with ash or biochar (42). In this study, the concentration of Ca in agricultural lime was comparatively much higher (2–3×) than that in wood ash, resulting in higher concentrations of Ca in the foliar tissues from plots amended with lime. We also found increased foliar P concentrations in the ash and fertilizer treatments, but not in the lime treatment, which reflects the lower available P in the soil at our site. The two ash treatments had significantly higher soil P levels than the other treatments, and they had the highest foliar P concentrations. The delay in P availability after amendment may be due to its relatively low solubility in wood ash and the need for some breakdown of the ash and desorption from soil surfaces to become plant-available (57, 58).

4.4 Application in the field

Wood ash has been used in forests, agricultural, and other managed lands for centuries; however, its potential to improve soils is still being explored. More recent work has focused on biochar, which has become widely popular in agricultural settings and is widely available for both bulk and small-scale use (44, 45). However, the use of wood ash as a soil amendment may be recommended for soils with different limitations, as wood ash tends to be higher in phosphorus than biochar and has a higher liming capacity (i.e., 20% lime equivalence vs. 3%–5%). Depending on the site history, this may be more or less beneficial, as P enrichment is common in agricultural soils with a history of excess litter, manure, or fertilizer application (63–66). There is also a concern regarding the potential of P runoff and downstream eutrophication, which can be mitigated by tilling the ash into the soil, establishing vegetation, and inoculating the soil with helpful microbes (59).

Although wood ash in this study was tilled into the soil, this may not be a practical approach for forestry or even some agronomic practices, where widespread tillage is not practical or feasible. In many forestry and agricultural settings, ash is wetted and spread onto land using a traditional spreader or is hand-applied for research studies (67, 68). However, land conversion (e.g., an intensively managed cropland converted pasture or forest land) may present an opportunity to improve the soil by tilling in an amendment to improve productivity and soil health.

4.5 Future research and perspectives

A challenge of this study was the high application rate of ash to achieve an equivalent increase in soil pH as the lime treatment. Although ash application did improve several soil properties, applying 20 tons/ha is likely unrealistic for many farmers, foresters, or other land managers because of the costs of transport and application. In a recent study, a similar dose even showed negative effects on the growth of Pinus banksiana (69), and another study found negative effects of a wood ash dose of 20 tons/ha on Zakova serrata (28). In contrast, a study performed in acidic and low P soil in Brazil found that a wood ash dose of 16 tons/ha–20 tons/ha produced the greatest yield for Paiaguás grass (31). Although this research was conducted on a single acidic Ultisol with a long history of agricultural use, this soil is widely found across the Piedmont ecoregion of the Southeastern US. Future research should consider other southeastern soil types.

Additionally, future research should seek to make specific recommendations on relevant wood ash doses based on soil properties and grasses of interest. Different grasses have different specified nutritional and chemical needs from the soil, which can be modified with different wood ashes, as their properties can vary significantly (17). Managers who plan to apply wood ash as a soil amendment should also consider pre-amendment soil properties, including pH, nutrient concentrations, and hydrological parameters. Future research should also focus on the impact of wood ash amendment on the long-term soil nutrient, chemical, and biological functioning of soil.

A major challenge for agricultural soil managers is to increase soil carbon storage to mitigate some of the effects of ongoing anthropogenic climate change (70). The high C wood ash in this study approximately doubled the storage of C in the top 0 cm–10 cm of these soils in the first growing season. While biochar is receiving much research attention for its ability to increase soil C, other organic soil amendments, particularly those that also increase soil nutrient status and can be cheaply obtained, such as wood ash, can also be considered for their potential to sequester C (70).

5 Conclusions

Wood ash is widely considered a waste product of many industries, and with the increasing demand for landfills, some communities are seeking other uses for ash to divert waste from landfills. The application of wood ash to soil can be an effective way to use a waste product as a liming agent and increase soil nutrient concentrations, improve soil physical properties, improve soil health, and potentially sequester C. Some challenges to the use of wood ash are its heterogeneity in chemical properties, resulting from differences in source materials and combustion intensity. We found that wood ash strongly outperformed the other soil treatments in terms of biomass yield, soil organic matter, CEC, bulk density, and soil and foliar P concentrations. Additionally, we found a 2× difference in initial growth with the ash and ash + fertilizer treatments compared with the control and lime treatments. This project supports numerous other published studies that indicate that ash can serve as an effective soil amendment for poorly drained, acidic, and nutrient-poor soils and improve both agricultural and forest production.

Data availability statement

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found below: https://osf.io/ca2r3/?view_only=513065700a26433e80c96091be04aa48.

Author contributions

RA: Supervision, Methodology, Visualization, Conceptualization, Project administration, Formal Analysis, Investigation, Data curation, Resources, Funding acquisition, Writing – original draft, Writing – review & editing. JF: Data curation, Investigation, Writing – review & editing, Visualization, Formal Analysis.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This study received funding from Huber Engineered Woods, INC. The funder was not involved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication.

Acknowledgments

We would also like to acknowledge members of the Abney laboratory for assistance with fieldwork for this project, Ali Moss and Courtney Scott. We also acknowledge the Warnell School of Forestry and Natural Resources and staff for their additional logistical support.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

Any alternative text (alt text) provided alongside figures in this article has been generated by Frontiers with the support of artificial intelligence and reasonable efforts have been made to ensure accuracy, including review by the authors wherever possible. If you identify any issues, please contact us.

Publisher’s note

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