###### Abstract
Other important soil factors affecting heavy metal availability include: (1) clay content, and thus cation exchange capacity (CEC), as clays are negatively charged [19]; (2) time, which results in heavy metals gradually shifting into less-accessible soil pools as they move into clay interlayer regions and form inner-sphere surface complexes with clay lattices [19; 32]; (3) soil texture, wherein coarse-textured soils (greater sand content) are expected to have greater availability than fine-textured soils (greater clay content) [19]; (4) amount of heavy metal(s) present [33]; (5) metal ion speciation and, in particular, the concentration of free metal ions present [34; 35]; and (6) soil oxidation-reduction (redox) potential, particularly in flooded soils [19; 36]. Biochar is a carbon-rich organic material produced through pyrolysis of straw, manure, wood, and other agricultural wastes under a limited or no-oxygen environment [37; 38]. Interest in biochar has grown significantly in recent years as demand increases for low-cost, environmentally-friendly technologies to address soil and water contamination, and to increase soil health. Used in a variety of applications, research shows that biochar can play an important role in increasing soil C sequestration [39], reducing NO\({}_{2}\) emissions [40], and can decrease fertilizer needs and nutrient leaching [41]. It also has significant capacity for adsorption of both organic and inorganic compounds, including heavy metals [7; 42]. A recent meta-analysis of 74 scientific papers found that amendment with biochar resulted in overall decreases in soil bioavailable metal concentrations of Cd, Pb, and Cu by 52%, 46%, and 29%, respectively, and reductions in the average concentrations in plant tissues by 38%, 39%, and 25% [43]. Persisting in soil for centuries [39], biochar is highly recalcitrant due to its stable, aromatic structure [44] and has a large surface area and porosity [43; 45]. The total surface area correlates with sorption and retention of both nutrients and contaminants, while pore volume affects water availability and soil aeration [46]. Oxygen-containing functional groups, especially carboxyl groups, on biochar's surface result in a net negative charge, increasing soil cation exchange capacity (CEC) and adsorption capacity [7; 42]. Pyolysis temperature is one of the most important factors influencing the molecular structure, nutrient composition, and physicochemical properties of biochar [46; 47]. In general, surface area increases at greater pyrolysis temperatures [46; 47], as does pore volume [46]. Higher pyrolysis temperatures also produce greater ash and carbon content and a higher pH [46; 47], but lower N content [47]. The nutrient composition of biochar is also strongly influenced by its feedstock source [46]. A meta-data analysis of over 5400 peer-reviewed journal articles suggests that the greatest total N content, along with total P, K, Ca, and Mg, are found in biochar produced from manures/biosolids, whereas wood biochar typically has the lowest concentrations [46]. Compost, a relatively popular soil amendment among urban gardeners, consists of both readily decomposable materials and humic substances derived from raw organic materials, such as manure and plant biomass. Like biochar, negatively charged functional groups on its surface can help limit heavy metal availability [48]. Further, compost is rich in humic substances, which are responsible for many complex chemical reactions in soil due to their ability to interact with metal ions, minerals, oxides, hydroxides, and organic contaminants to form water-soluble and water-insoluble complexes [8; 19; 48]. Evidence suggests that humic substances like humic and fulvic acids enhance plant growth and stimulate nutrient uptake [49]. Compost has also been shown to reduce the available portion of heavy metals in both soil and plant tissue [50; 51; 52].