Abstract
A field experiment was conducted in order to investigate the effect of nitrogen on the yields of Napier grass. The grass was grown on a calcareous soil, which is one of the typical soils and called "Jagaru" in Okinawa, for two years from 1976 to 1977. Nitrogen was applied at four levels, namely 0, 30, 60 and 90kg per 10 are per year. The obtained results were summerized as follows ; 1. The grass showed remarkable increase in the yield with the rate of nitrogen up to 60kg in both years. Little difference was recognized in the yield between the nitrogen rates of 60 and 90kg. The grass produced more than 30 tons fresh yield or more than 4.5 tons dry yield per 10 are per year over 60kg of nitrogen applied. 2. The rates of nitrogen did not affect on the number of tillers, but increased on the weight of a tiller. 3. The effect of nitrogen on the grass height was recognized. The grass grew more than 200cm on the plots of 60 and 90kg of nitrogen in the season with high temperature. The number of leaves per tiller was affected by the rate of nitrogen. More than 9 leaves developed per tiller one the plots of 60 and 90kg of nitrogen in the season with high temperature. Also, by the nitrogen rate affected was the leaf area per leaf. The leaf area showed more than 250cm^2 on the plots of 60 and 90kg of nitrogen in the season with high temperature. On the other hand the leaf weigth ratio was highest with the lowest rate of nitrogen and it decreased with the increased rate of nitrogen. 4. With the rate of nitrogen the leaf area index (LAI) increased and showed more than 9 on the plots of 60 and 90kg nitrogen at each cutting except the 4th cutting in 1976 and the 5th cutting in 1977. 5. The net assimilation rate (NAR) ranged from 15 to 50g・m^<-2>・week^<-1> and higher values were obtained from the plots 60 and 90kg of nitrogen in all cuttings. Both of crop growth rate (CGR) and the optimum LAI were strongly affected by the nitrogen application. The maximum CGR at 60 and 90kg of nitrogen yielded more than 200g・m^<-2>・week^<-1> from April to September. The optimum LAI was more than 7 in the plots of 60 and 90kg of nitrogen at each cutting except the 5th cutting of 1977. From the results obtained here the most appropriate amount of nitrogen was considered to be around 60kg per 10 are per year for higher yield of Napier grass on this specific "Jagaru" soil experimented.
