The main factors limiting plant growth on fly ash deposits are excessive B; high pH; high salinity; and lack of N and, to some extent, P. In addition, indurated layers of ash produced by compaction and pozzolanic action may inhibit the normal growth of the root system. Agricultural and horticultural crops have been classified on the basis of tolerance to fly ash. According to Hodgson and Buckley , Chenopodiaceae are all highly tolerant to fly ash, and Leguminosae, Cruciferae, and Graminae species show considerable variation in their capacity to tolerate fly ash. It is reported that grouping species in relation to their ash tolerance is similar to grouping them on the basis of B requirements. Although the ability of plants to grow on fly ash indicates mainly their tolerance to B excess, the role of soluble salts, unstable crumb structure and macro- and micronutrient deficiencies should also be considered.
On the basis of data from British fly ashes, Hodgson and Townsend  proposed certain B concentration ranges for plant growth on fly ashes. Specifically, they proposed that concentration of hot water-extractable B at levels less than 4 mg kg-1 be considered as nontoxic; 4 to 10 mg kg-1 as slightly toxic; 11 to 20 mg kg-1 as moderately toxic; 21 to 30 mg kg-1 as toxic; and greater than 30 mg kg-1 as highly toxic. Also, Townsend and Gillham  suggested that hot water-extractable B levels in fly ashes higher than 20 mg kg-1 are probably toxic to most agricultural crops; more sensitive crops such as barley (Hordeum vulgare), peas (Pisum sativum), and beans (Phaseolous vulgaris) could show symptoms at values as low as 7 mg kg-1. However, such data do not take into account the long-term B release characteristics of ash, especially unweathered fly ashes, and the genetic characteristics of plant species, concerning their behavior to B excess. For example, an Australian fly ash containing 3 mg kg-1 hot water-extractable B — a level considered nontoxic — resulted in B toxicity and reduced yields of French beans and Rhodes grass (Chloris gavana) .
As Section 1.2.1 mentioned, weathering of fly ash reduces B content. However, the time needed for reducing B concentration in fly ash at acceptable levels for plant growth, under natural conditions, varies widely. Some reports in the literature suggest a period from a few to several years of fly ash aging, depending on the initial B content of fly ash and the climate. Jones and Lewis , for example, reported that in a fly ash stock pile, B content of some recently deposited fly ash was 216 mg kg-1 and that of 25-year-old ashes was 4.3 mg kg-1. Boron content of wild white clover (Trifolium repens) grown on ash decreased with increasing ash aging and became similar to that of the soil control in the 25-year-old ash.
According to Townsend and Gillham , weathering of fly ash with moderately high initial B content for 4 years could reduce B to acceptable levels. Burns and Collier  developed a simulation model to predict the period required to leach B from the top 30 cm of a coal ash deposit in the U.K. This model predicted that B concentration would decrease at acceptable levels for most crops after 5 to 15 years. According to Nass et al. , B concentration in grasses grown on 20-and 30-year-old fly ash deposits was lower than that obtained for plants grown on fresh fly ashes, but higher than the values considered normal for these specific species.
In general, plant species grown on fly ash deposits weathered for certain years attained better growth and absorbed B at levels probably high but nontoxic. The growth of species in relatively weathered or fresh fly ash was limited by B toxicity, among other factors. However, the opposite cannot be precluded, depending on the fly ash B content and the plant species.
Several cases of different species grown well on weathered fly ash deposits are reported in the literature. It was found that B concentration of white sweet clover grown on a coal fly ash containing 7.5 mg B kg-1 was slightly higher (51 mg kg-1) than that grown on a soil containing 5.2 mg B kg-1 (45 mg kg-1) . In a study with grasses and legumes growing on four soil-capped ash landfills in New York, Weinstein et al.  found elevated levels of B in most of the plants, with generally higher levels in the legumes than in grasses. To explain this difference, they hypothesized that the deeper rooted legumes had probably penetrated the underlying ash deposit.
On the other hand, Woodbury et al.  found that B concentrations of several plant species, i.e., bird's foot trefoil (Lotus corniculatus); red clover (Trifolium pratense), timothy (Phleum pratense), orchard grass (Dactylis glomerata), velvet grass (Halcus lanatus); and several fescues (Festuca spp.) grown on a soil-capped ash landfill ranged at levels similar to the soil control (29 to 53 mg kg-1 in the legumes and 2 to 11 mg kg-1 in the grasses). Conversely, growth of cucumber (Cucumis sativus), which is considered a semitolerant crop to B toxicity, was suppressed in a weathered acidic fly ash, although the ash had a low B content (total B 28 mg kg-1 and water-extractable B 3.7 mg kg-1). After 8 weeks, all plants exhibited certain toxicity symptoms and plant biomass was significantly less compared to the control soil. It was concluded that B toxicity was the primary cause for plant growth suppression because B concentrations in ash grown plants were tenfold of the control and exceeded the B toxicity threshold for cucumber (>300 mg kg-1) .
In addition, several woody species have been tested in fly ash deposits. In certain cases, the deeper rooted system was proved to be the main factor preventing B accumulation at levels higher than those considered toxic, although the ash was enriched in B. For example, Polpulus robusta and Picea sitchensis grown on two ash deposits containing 14 and 100 mg B kg-1, respectively, did not exhibit severe B toxicity symptoms. This was attributed to the fact that B concentration at depths below 30 to 40 cm was low . Similarly, eight woody species — European black alder (Alnus glutinosa); sweet birch (Betula lenta); sycamore (Platanus occidentalis); sawtooth oak (Quercus acutissima); cherry olive (Elaeagnus multiflora); autumn olive (Elaeagnus umbellata); silky dogwood (Cornus amomum); and gray dogwood (Cornus racemosa) — grown on a strongly acidic fly ash did not show B toxicity symptoms, although plant B levels were elevated .
Carlson and Adriano  reported similar results for sweet gum (Liquidambar styraciflua) and sycamore growing on a 20-year-old fly ash wet basin (pH = 5.6). Trees growing on the ash attained better growth, even though they had higher trace element concentrations in comparison to the control soil. As far as B is concerned, elevated concentrations were observed in trees growing on the ash (three times the control); however, in almost all cases, B levels in the foliage were below 100 mg kg-1. The differences between fly ash-wet basin and soil observed in foliar B concentrations were attributed to differences in B substrate concentrations.
The deleterious effects of fly ash, especially the unweathered, on plant growth due to high B content can be avoided by weathering or by mixing the fly ash with an inert medium. Holliday et al.  conducted a pot experiment with oat (Avena vulgare) grown on an inert medium mixed with a strongly alkaline fresh fly ash (pH = 9.0) and on the same ash treated with acid (pH = 6.4), at rates up to 100%. They found that B toxicity symptoms were obvious in the plants grown in the inert medium with fresh ash > 6% and acid-treated ash > 25%. At these rates, B concentrations in the biomass were 260 and 310 mg kg-1 for the untreated and the treated fly ash, respectively.
Similar research was carried out by Townsend and Gillham  by means of a small-scale field experiment with cereals, grasses, legumes, and other crops, using initially fresh and weathered fly ash, amended or not with silt. Among the species studied, red clover; white clover and Lucerne; timothy; cocksfoot and ryegrass (Lolium perenne); and potatoes (Solanum tuberosum) have been grown well for a number of years, apart from failures in the early years on the new ash plots (with no silt), attributable to excess B. The improvement of all crops' performance with years was attributed to a decrease of B content of the ash by weathering.
Aitken and Bell  studied the uptake of B by French beans and Rhodes grass grown on alkaline fly ash-sand mixtures in a pot experiment. The five fly ashes used were untreated, leached, or adjusted to pH 6.5 and subsequently leached and mixed with sand at rates of 5 and 10% w/w. The untreated ashes resulted in lower yields than the leached and pH adjusted and subsequently leached for both species; this was attributed mainly to B toxicity because plant B uptake and hot water-extractable B were higher in the untreated ashes. For both levels of ash addition and both species, B concentrations and uptake decreased in the order: untreated ash > leached ash > pH-adjusted and leached ash.
Although B phytotoxicity is a possible risk for plants grown on unweathered fly ashes, especially those having high B content, the opposite is also true as stated by Nass et al. . For 6 years, they studied the growth of a mixture of grasses (Lolium perenne, Festuca rubra commutata, and Phleum pratense) and white clover, in lysimeters filled with three fresh fly ashes that differed in their reaction (alkaline, neutral, and acidic). Boron concentrations in plant tissues were always below 33 mg kg-1 and only concentrations higher than 50 mg kg-1 would cause B toxicity.
Some cases of detrimental effects on seedling growth in fly ash, due to trace elements, are also reported in the literature. Excessive levels of certain trace elements, including B, impaired the growth of lettuce (Lactuca sativa) seedlings on a British neutral fly ash . In addition, Shulka and Mishra  attributed the deleterious effects on the seedlings of corn (Zea mays) and soybean grown in nutrient solution amended with 2.5% Indian alkaline fly ash to B or heavy metals.
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