the upper layer was filled with 1 L of soil 1 or soil 2. At the beginning of
May, 20 seeds of Spinacia oleracea L. were sown in each pot. The
pots were placed on fibre cloth on top of the ground (Fig. 1b) and
protected against rain with a plastic roof (Fig. 1a). Thus, irrigation
was the only controlled parameter and each pot was irrigated three
times a day with an automatic drip irrigation system.