Mekelle University The School of Graduate Studies Faculty of ...

2.16 Earthworm Casts for Plant Growth and HealthPresence of worms increases plant growth and N uptake as opposed to unfertilized soil.In the 1980’s, at a research station in Rothamsted, earthworms were collected and put inbuckets of clean water, in batches of 250. A solution of 0.2% formaldehyde was spreadon the field to drive the worms out of their burrows. They were then rinsed in a secondbucket of clean water and spread at a rate of 250 worm’s m -2 over a landfill site cappedwith 15cm of clay subsoil, treated with domestic dried sewage solid at 1 kg m -2 andplanted with grass. A higher plant growth was observed in the presence of worms(Edwards & Bates, 1992). Earthworm casts were found to increase nutrient uptake inTomati (1994), including nitrogen and several ions, particularly Mg and K. When used inhorticulture, earthworm casts have a hormone-like effect. The biological effect of casts islinked to microbial metabolites that influence plant metabolism, growth and development(Tomati et al., 1997).Root biomass was slightly lower in the earthworms than in thebare soil treatment and N content of leaves was twice higher in the treatment withearthworms. This was only partially explained by earthworm mortality. N uptakeincreases in the presence of earthworms and is correlated (r = 0.85) with the increase inCO 2 production (Ruz – Jerez, 1992). Casts increase plant dry weight and N, P, Mg and Kuptake from the soil. The application of earthworm casts (0, 100, and 300 g per 3.5 kgsoil) increased the dry weight of soybean by 40 to 70%. The nitrogen absorbed by theplants from the soil increased to 30 to 50%. Phosphorous and potassium in the plant weretwice that of the control. The amount of organic matter, total nitrogen, phosphorous andpotassium in the soil also increased, as well as available phosphorous and potassium inthe soil (Lui et al., 1991).34