Discussion
4.1. Impact of organic fertilization on soil chemical properties, plant growth and yield
Our study considered one cultivation cycle of maize–tomato–maize throughout a one-year period. Despite similar total soil nutrient (N, P and K) contents in all treatments, plant growth and yield were highest in the control treatment where only mineral fertilizer was applied. The above-ground biomass of the first maize planting was similar for all treatments, suggesting that organic fertilizer does show potential as a good substitute for mineral fertilizer. However, despite similar biomasses between control and vermicompost treatments throughout the experiment, the plants grew less when compost was applied before the tomato and the second maize cultivation. In addition, although the above-ground biomasses were similar between the mineral and vermicompost treatments, vermicompost had a decreasing effect on plant yield over time. Plant yield was initially (i.e. the first maize cultivation) similar between the mineral fertilizer and vermicompost treatments. The utilization of vermicompost then led to an intermediate yield for tomato which was between that of the mineral and compost treatments. The yields from the final maize planting were similar between the compost and vermicompost treatments. These results are consistent with the similar chemical properties in compost and vermicompost treated soils compared to the control at the end of the experiment and question the long term interest of using vermicompost instead of compost for the sustainable management of soil fertility.