Sequential effects of spent coffee grounds on soil physical properties

Abstract

Spent coffee grounds are a bio-residue studied as soil organic amendment and it has been proven that it has short-term effects on soil physical properties. However, its sequential effects on the cultivation of clayey soils are little studied. Therefore, an in vitro experiment was carried out to evaluate the effect of increasing doses (1%, 2%, 2.5%, 5%, 7.5%, 10%, 12.5% and 15%) of spent coffee grounds on the physical properties of a clayey soil in the Spanish Mediterranean area which is rich in smectites. The addition of spent coffee grounds increased water retention at −33 and −1500 kPa proportionally to the added amounts, but the increase in the wilting point was much larger than the field water capacity, decreasing the plant available water content. A non-linear influence on the aggregate size is demonstrated. It increased total porosity and consequently reduced soil bulk density. This fact was reflected in the stereomicroscopy images where an increase in the pores analysed with image analysis was observed. Furthermore, SEM images corroborate that spent coffee grounds act intensely in the short-term due to the interaction between their particles and those of clay. The 5% dose acted as a threshold dose from which the greatest effects on soil physical properties occur. In general, the use of SCG as an organic amendment is a good sustainable solution because it supposes a reuse of this bio-residue (15 million tons per year), an increase in soil organic carbon (SCG contains ≈ 50% carbon) and an improvement of the soil physical and chemical properties.