Do lignite-derived organic amendments improve early-stage pasture growth and key soil biological and physicochemical properties?

Abstract

Commercial products derived from lignite (brown coal), sold mainly as humate preparations, are widely promoted as plant growth stimulants leading to higher crop yields. These products are also claimed to improve key indicators of soil health including soil pH and microbial biomass. In a glasshouse setting, we investigated the effect of six lignite-derived amendments applied at the manufacturer’s recommended rate on the early-stage growth of two pasture species, lucerne (Medicago sativa L.) and ryegrass (Lolium multiflorum Lam.). We used two soil types common to south-eastern Australia, and following an 8-week growing period, assessed soil pH, microbial biomass carbon and mycorrhizal colonisation as key indicators of soil health. We hypothesised that humic acid (HA) and macronutrients derived from the products would positively influence pasture growth and soil health indicators. Although significant growth effects were observed in response to some products, the effects were inconsistent across pasture and soil types. Treatment effects on tissue nutrient accumulation were rare, with the exception of increased potassium in ryegrass in one soil amended with raw brown coal, and decreased nitrogen in lucerne in the same soil amended with a granulated, slow-release humate product. Further, we found no consistent trends in mycorrhizal colonisation or microbial biomass carbon in response to individual treatments. Given the variable responses of the plant species and soil types to the amendments used here, we emphasise the need for further mechanistic studies to help understand how these amendments can be used to greatest effect.