and after reduction of nitrate with Devarda's alloy for water samples
(ISO 5553-84). The pH was measured in a soil/water suspension (ISO
10390-2005). The NNH4
+ and NNO3
− contents were determined using
the Kjeldahl method. The quantities of NNH4
+ and NNO3
− lost in the runoff
water and leachate were measured in taking account the volume of each
sampled solution. The total K content was determined after extraction
with H2SO4 and HClO4 using a Flame photometer (TCVN 4053-81).
The total P content was determined using a spectrophotometer after extraction
with H2SO4 and HClO4. Soil moisture was continuously measured
(every 2 h), as the soil water potential (kPa) with tensiometers
placed between 20 and 30 cm deep in the soil. In this study, only the average
soil moisture over 10 day periods during cultivation was considered
for analysis.