Exchangeable NH4+-N in paddy soils is adsorbed onto cation
exchange sites in the soil or present in the soil solution. Although numerous
researchers have reported the existence of a relationship between the amount
of exchangeable NH4+-N in soil and rice growth, differences in the tiller
number and the amount of N in rice among different fields cannot be explained
by the amount of exchangeable NH4+-N in soils. On the other hand, it has
been shown in pot experiments that higher amounts of NH4+-N in the soil solution
led to a higher tiller number and a larger amount of N in rice. However,
little information is available on the relationship between the amount of
NH4+-N in the soil solution and tiller number under different field conditions.
The objective of this study was to clarify the relationship between the amount
of NH4+-N in the soil solution and the tiller number in different farm fields.
The results obtained were as follows: 1) The relationship between the amount
of NH4+-N in the soil solution and the tiller number at 20 days after transplanting
(DAT) was positive and could be represented by a linear equation. 2) There
was no relationship between the soil CEC and tiller number at 20 DAT. No
significant relationship between the soil CEC and the amount of NH4+-N in
the soil solution was detected. 3) The amount of positive charges in the
soil derived from Fe2+ was approximately 4.0–40.8 cmol(+) kg-1 dry soil,
and that of NH4+-N was approximately 0.15–0.82 cmol(+) kg-1 dry soil at 20
DAT, suggesting that ferrous iron occupied the cation adsorption sites in
soil, resulting in an increase in the amount of NH4+-N in the soil solution
.