Clay Addition to Sandy Soil-Influence of Clay Type and Size on Nutrient Availability in Sandy Soils Amended with Residues Differing in C/N ratio

Addition of clay-rich subsoil to sandy soil results in heterogeneous soil with clay peds (2-mm) or finely ground (< 2 mm) clay soil (FG), which may affect the nutrient availability. The aim of this study was to assess the effect of clay soil particle size (FG or peds) and properties on nutrient availability and organic C binding in sandy soil after addition of residues with low (young kikuyu grass, KG) or high (faba bean, FB) C/N ratio. Two clay soils with high and low smectite percentage, clay and exchangeable Fe and Al were added to a sandy soil at a rate of 20% (weight/weight) either as FG or peds. Over 45 d, available N and P as well as microbial biomass N and P concentrations and cumulative respiration were greater in soils with residues of KG than FB. For soils with KG residues, clay addition increased available N and initial microbial biomass C and N concentrations, but decreased cumulative respiration and P availability compared to sandy soil without clay. Differences in measured parameters between clay type and size were inconsistent and varied with time except the increase in total organic C in the < 53 mu m fraction during the experiment, which was greater for soils with FG than with peds. We concluded that the high exchangeable Fe and Al concentrations in the low-smectite clay soil can compensate a lower clay concentration and proportion of smectite with respect to binding of organic matter and nutrients.