The soil was also supplied with
nitrogen (0.2 g N kg 1 soil as NH4NO3) and potassium
(0.125 g K kg 1 KCl) fertilizers. Polystyrene (12 cm in
diameter and 9 cm in height) pots containing 500 g of soil
were used in this experiment. Deionized water was added to
bring the soil water content to about 20%. After a 2-week
incubation period, four pre-germinated seeds of the two
vegetable crops were sown in each pot, 7 days after
emergence seedlings were thinned to two per pot. During
the experiment, pots were watered with deionized according
to water loss by weighing. The plants were grown in a
greenhouse with temperatures between 18 and 25 jC in
autumn in Beijing, China. Plants were harvested 2 months
after emergence.