The effects of environmentally sound soil amendments on soil physical properties and on lettuce growth

The impact of predicted climate change on drought-sensitive soils in Flanders is expected to induce severe challenges in future agriculture. This impact could be countered by the incorporation of soil amendments to improve soil physical properties and thereby making the soil more resilient to climate change. The main goal of this research is to compare the effects of seven compost types and six ecological soil amendment types on soil physical properties and on the crop growth of lettuce (Lactuca sativa). The experiment is performed on two drought-sensitive soil types, sandy loam (SL) and loamy sand (LS). A pot experiment was carried out in the experiment greenhouses of ILVO – Ghent university. Soils were incorporated with seven types of composts; bio green compost (BGC), green compost (GC), fruit, vegetable and yard waste compost (FVYC), spent mushroom compost (SMC), vermicompost (VC), manure compost (MC), and bark compost (BC) and six ecological soil amendments; chitin (C), biosolids (BS), biochar (B), crushed alfalfa pellets (CAP), flax fiber (FF) and seaweed powder (SWP), and at various doses. The irrigation schedule adopted for the experiment was based on predicted climate data. Following lettuce harvesting and measurement of its yield, undisturbed soil cores were collected to measure soil water retention (SWR) and water droplet penetration time (WDPT). It was shown that all compost treatments in the LS soil significantly increased the volumetric water content at field capacity (θFC taken at -33 kPa), and permanent wilting point (θPWP taken at -1530 kPa). The soil moisture content in the SL soil improved significantly with BC, MC, FVYC, and VC treatments at -1 kPa, -10 kPa and field capacity, and the SMC and BC treatments increased the water content at the permanent wilting point by 30% compared to the control. The treatment with SWP 16 g/kg and FF 16 g/kg in LS soil under the dry spell (DS) water regime greatly improved water content at -1 kPa, field capacity and permanent wilting point. The treatments of SWP 16 g/kg, SWP 8 g/kg, FF 16 g/kg, FF 8 g/kg, and B 8 g/kg in the LS soil and SWP 16 g/kg, FF 16 g/kg, and FF 8 g/kg in the SL soil significantly increased the water content at the field capacity under the MDS water regime. Furthermore, the plant available water content (PAWC) significantly increased in SL soils treated with SWP 16 g/kg and FF 16 g/kg with the MDS water regime. The water droplet penetration time (WDPT) was significantly reduced by compost treatments in the SL soil but not in the LS soil. The treatments with the soil amendments showed a significant influence on the WDPT in LS soil, while no significant effects were obtained for SL soil. The dry weight of plants was significantly improved by all compost types except BC in both soil types. In contrast, the soil amendment treatments had no significant effect on the dry weight.