Water-holding capacity and plant growth in compost-based substrates modified with polyacrylamide, guar gum or bentonite

Humectant agents are used to improve seed germination and survival in hard environments through their effect on water-holding capacity. In this work, we have evaluated the combined effect of substrate composition and humectant agents on the water-holding capacity and growth of Lolium multiflorum in different mixtures based on municipal solid waste composts. A commercial polyacrylamide, bentonite or guar gum have been added to mixtures of two municipal solid waste composts with either manure vermicompost or pine bark compost. Water-holding capacity of the substrates and plant production after five weeks in the greenhouse were evaluated. Only bentonite increased water retention of the substrates, likely because the compost-based substrates already presented high water retention. None of the humectant agents had significant effects on the growth of Lolium multiflorum. The use of bentonite can be recommended for increasing water-holding capacity of organic substrates in this case, but given the overall small effects of the humectant agents, an adequate choice of the organic materials could suffice for the formulation of adequate plant substrates in this case.