Study of calendula and Gaillardia growth in two composts prepared from agroindustrial wastes.

Two composts prepared from agroindustrial wastes were assayed as substrates: C1 from brewing waste (yeast and malt) plus lemon tree pruning and C2 from the solid fraction of olive mill wastewater plus olive leaves. Sixteen substrates were prepared by combining each compost with Sphagnum peat or a Commercial Substrate (CS) in different proportions. The nutrients (N and K) provided by the composts, which acted as slow-release fertilizers, influenced especially the development of calendula, although the physical and physicochemical properties such as total pore space and Electrical Conductivity (EC) were also relevant. On the other hand, in the salt-sensitive Gaillardia hybrid, EC and chloride concentration were the main factors influencing growth. The best results were found in substrates prepared by mixing C1 at up to 75% with peat, or at up to 50% with CS, or by mixing C2 at up to 50% with peat or CS, for calendula. For salt-sensitive species such as Gaillardia, adequate substrates for plant development were found for C1 at up to 50% with peat or CS, but the use of C2 should be limited to 25% in mixtures with peat or CS. Therefore, composts of agroindustrial origin such as these can be used as an alternative to peat and CSs for growing ornamental plants, provided the mixture contains at least 25% peat or CS.