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Phosphorus (P) is a vital soil macronutrient required by plants for optimum growth and development. However, its availability is limited because of fixation. The phosphorus fixation reaction is pH dependent. In acid soils, the predominance of aluminium (Al) and iron (Fe) oxides in both crystalline and amorphous forms reduces the solubility of soil inorganic P through fixation on positively charged surfaces and formation of insoluble Al and Fe precipitates. In alkaline soils, P readily reacts with calcium (Ca) to form sparingly soluble calcium phosphates. As a result, a large proportion of applied P may become chemically bound, whereas only a small fraction of soil P remains in the soil solution and available for plant uptake. To date, there is little information available on the use of charcoal with a highly negative charge and wood ash with high alkalinity to minimise P fixation in acid soils. Thus, this study examined the potential of the combined use of charcoal and wood ash to unlock P fixation in acid soils. Numerous studies have been conducted to identify effective approaches to improve P availability through the use of different types of soil amendments, regardless of whether P is organically or inorganically present. For example, to mitigate P fixation in acid soils, amendments such as compost and zeolite are used to reduce P sorption sites. These amendments have also been used to increase P uptake and crop productivity in P deficient acid soils by reducing soil acidity and the toxicity of Al and Fe. It is believed that long-term application of charcoal and sago bark ash can positively change the physical and chemical properties of soils. These improvements do not only reduce P fixation in acid soils, but they also promote an effective utilisation of nutrients through timely release of nutrients for maximum crop production. [ABSTRACT FROM AUTHOR]

Soil acidity is an important soil factor affecting crop growth and development. This ultimately limits crop productivity and the profitability of farmers. Soil acidity increases the toxicity of Al, Fe, H, and Mn. The abundance of Al and Fe ions in weathered soils has been implicated in P fixation. To date, limited research has attempted to unravel the use of charcoal with the incorporation of sago (Metroxylon sagu) bark ash to reduce P fixation. Therefore, an incubation study was conducted in the Soil Science Laboratory of Universiti Putra Malaysia Bintulu Sarawak Campus, Malaysia for 90 days to determine the optimum amounts of charcoal and sago bark ash that could be used to improve the P availability of a mineral acidic soil. Charcoal and sago bark ash rates varied by 25%, whereas Egypt rock phosphate (ERP) rate was fixed at 100% of the recommendation rate. Soil available P was determined using the Mehlich 1 method, soil total P was extracted using the aqua regia method, and inorganic P was fractionated using the sequential extraction method based on its relative solubility. Other selected soil chemical properties were determined using standard procedures. The results reveal that co-application of charcoal, regardless of rate, substantially increased soil total carbon. In addition, application of 75% sago bark ash increased soil pH and at the same time, it reduced exchangeable acidity, Al3+, and Fe2+. Additionally, amending acidic soils with both charcoal and sago bark ash positively enhanced the availability of K, Ca, Mg, and Na. Although there was no significant improvement in soil Mehlich-P with or without charcoal and sago bark ash, the application of these amendments altered inorganic P fractions in the soil. Calcium-bound phosphorus was more pronounced compared with Al-P and Fe-P for the soil with ERP, charcoal, and sago bark ash. The findings of this study suggest that as soil pH decreases, P fixation by Al and Fe can be minimised using charcoal and sago bark ash. This is because of the alkalinity of sago bark ash and the high affinity of charcoal for Al and Fe ions to impede Al and Fe hydrolysis to produce more H+. Thus, the optimum rates of charcoal and sago bark ash to increase P availability are 75% sago bark ash with 75%, 50%, and 25% charcoal because these rates significantly reduced soil exchangeable acidity, Al3+, and Fe2+. [ABSTRACT FROM AUTHOR]