Uncomposted Sheep and Cow Manures as a Source of Commercial Humic Amendments: An Exploratory Study

Abstract

Liquid humic amendments are widely used in many countries as an enhancer of crop production (particularly root growth). While leonardite is the main substrate for obtaining these products, there is growing concern about obtaining them from plant-derived residual materials, rich in lignin and polyphenolics, thus making possible a residue valorization. Composting is a necessary step for converting these residues into a source of humic substances. In arid countries, obtaining humic products from sources other than leonardite faces two main challenges: (i) often such countries are devoid of forested areas or tree crops capable of supplying lignin-rich residues in large amounts and (ii) water is scarce (or very expensive if obtained by desalination) and therefore composting on an industrial scale is very difficult. Thus it is essential to find alternative sources capable of yielding humic or humic-like compounds without any previous composting. Here we present the main results obtained using sheep and cow manure, two materials widely available in the Kingdom of Saudi Arabia (KSA). Both were extracted using potassium hydroxide (KOH) at a variety of concentrations (from 0.25 to 2 M), times (from 20 to 120 min), temperatures (from 100 to 150 °C), and pressures. Sheep manure, much easier to extract than cow manure, appears the more promising as a source of commercial humic amendments. No lengthy extraction times are needed to obtain alkaline extracts; 20 min yields only slightly less humic-like matter than 2 h. The relative abundance of humic-like substances in the extract tends to decrease with time, but some optical indicators of humification (E4/E6 ratio, absorption at 280 nm, ratios of absorption at 253/203 and 253/220 nm) tend to increase. Polysaccharides, the dominant form of organic matter in the original manure, strongly resist alkaline extraction: their prior hydrolysis may be necessary to increase the yields of humic-like matter from the studied manures.