Can Wood Ashes of Commonly Planted Tree Species in Ghana be Applied as Fertilizers

The fertilizer values of ashes from commonly used woody species in the tropics are not well-known. To determine whether ashes of Theobroma cacao, Persea americana, Cola nitida, Mangifera indica, Citrus sinensis, Prunus dulcis, and Senna siamea are suitable fertilizers, we studied the contents of total and plant-available elements. Ashes of dry wood samples burnt at 500 °C were digested in Aqua-regia and Mehlich-3 extractants to determine the contents of total and plant-available elements, respectively, with inductively coupled plasma-mass spectrometry. Ash yields ranged from 3.1 to 7.1% with pH (H2O) of 7.8–12.9. The ashes contained high P, Ca, K, Mg, S, Fe, Na, Mn, Cu, and Zn contents, with significant effects among the species. Total element content ranged from (g kg−1) 2–49 P, 19–218 Ca, 64–325 K, 48–95 Mg, 0.4–16 Fe, 3.6–29 Na, (mg kg−1) 143–798 Mn, 52–81 Cu, and 188–312 Zn. Available plant elements ranged from (mg kg−1) 22–512 P, 37,491–88,303 K, 2948–19,713 Ca, 13,719–29,001 Mg, 75–168 Mn, 40–74 Cu, 44–91 Zn, 1.5–383 Fe, and 836–6937 Na. The releasability (proportion of plant-available elements from the total) ranged from 0.08 to 92.8%. The relatively high releasability (25.4–80.4%) of Ca, K, Mg, Mn, Cu, and Zn in the ashes of the over-all species can supplement low nutrient availability for plants. The low releasability of P (1.1%) of the overall species explains its persistence in soils applied with ashes. The high yield of the ashes and pH support their ideal use, especially for very acidic soils. Ashes of the studied species are suitable fertilizers for soils according to the availability and releasability of macro(P, Ca, K, Mg S) and microelements (Mn, Cu, Zn, Fe, Na). Total and plant-available Ni, Pb, As, and Cd contents in the ashes were below permissible limits in soils. Thus, the studied ashes are especially suitable for soils with high acidity and low fertility.