Obtaining commercial humic products from uncomposted manures: previous acid hydrolysis to enhance yields

Purpose In Saudi Arabia, owing to the abundant livestock,manures are a source of commercial humic amendments.Climate (hot weather plus scarcity of water sources)makes composting very difficult and compulsory to obtainthese humic amendments without any previous compostingstep. A substantial fraction of manures, cellulosic in nature,are unextractable by alkaline solutions. Our aim is to verifywhether a sequential combination of acid hydrolysis andKOH extraction could result in higher yields and betterquality of the humic extract.Methods Acid hydrolysis was applied to manures either(1) in a single step, with HCl or H2SO4, or (2) in two steps(Saeman method). In both cases, acid hydrolysis was followedby extraction with hot KOH.Results Acid hydrolysis before KOH extraction increasestotal extracted organic matter. If acid hydrolysis is performed,the proportion of humic fractions in the KOHextract alone is strongly increased, reaching very highvalues (up to 80 % of C in the KOH extract after Saemanacid hydrolysis). Acid hydrolysis enhances some opticalindicators of humification (E4E6, aromaticity, EBTZ ratio)in the KOH extract. Nevertheless, because increasing acidconcentration enhances carbohydrate extraction butdecreases that of compounds other than carbohydrates, theoverall result may be a drop in the proportion of humicfractions in the whole extracted matter (acid hydrolysate? KOH extract).Conclusions Acid hydrolysis improves yields of commercialhumic extracts from manures. However, care mustbe taken to avoid a decrease in the richness of humicfractions in the obtained products.