2.4. Mechanical and hydraulic properties
The sinkage and penetration resistances were measured after
puddling and after harvesting for the second rice in 2011 by using a
penetrometer (SC-900, Spectrum Technologies, Inc.) as described in
Kirchhof et al. (2000). The tensile strength was measured by using
soil cores sampled after three drainages during the rice growth
period. The penetrometer was equipped with a metal plate
(30402.80mm) for measuring sinkage resistance and with a
cone (12.83mm in diameter) for measuring penetration resistance.
The penetrometer was connected with an automatic recording
system to record and store the data of resistance force with depth.Measurements were repeated at 15 sites randomly distributed in
each plot. The readings of resistance force were used to calculate
sinkage resistance by dividing the plate surface area and to calculate
penetration resistance by dividing the maximum area of the cone.
The readings of 15 replicates were averaged for each depth. The
measurement using penetrometer can only be carried out without
interference of crop. Therefore, thismethod was not applied during
the rice growth period.