Studies on Fertilization in Established Stands (IV)

This work was carried out to find the suitable period for the fertilization of Sugi stands. The fertilization was done in several growing phases of sugi forest trees. The fertilizer effeciency was determined by the chemical analysis of needles and the measurement of growth. The fertilizer was broadcasted in the forests in Jan., Apr., Jun., Aug., and Nov., from 1963 to 1965. Each fertilizer application contained 300kg of N, 160kg of P₂O₅, and 160kg of K₂O per hectare. The measurements were taken during the four growing periods.<br> 1) At the end of the four year test period, the fertilizer produced a 2_??_-68 cubic meter per hectare increase in stem volume when compared to the control plot. Especially, in the plot fertilized in Jun., the largest growth increase of 68m³/ha was observed. A high efficiency of fertilizers is remarkable in such a high density forest under natural thinning.<br> 2) The concentration of potassium and phosphorus in the needles did not change significantly after each fertilization, but the increase in nitrogen concentration was consistent.<br> 3) The increase in the nitrogen concentration in the needles after each fertilization appeared soon in the growing period and a little later in the dormant period. In spite of the difference in fertilizing periods, the nitrogen concentration was approximately at the same level in all needles taken from the six test plots. The high nitrogen concentration persisted until the next year.<br> 4) Judging from the fact that the nitrogen concentration of the needles in the plot fertilized consecutively in April 1964 and April 1965 was not different from that of nitrogen concentration of needles in the plot fertilized only in April 1964, the amount of nitrogen in the latter case is supposed to be sufficient for the stand. These tests indicated the value of 2% nitrogen content in the current year needles is a sufficient growth indicator in established Sugi stands in the sample forest.<br> 5) The results obtained from the increase in volume growth and in nitrogen content of the needles gave us some significant information as to the best fertilization period. The suitable period for fertilization in the established Sugi stand is wide, because good effeciency was given whenever fertilization was done. In the early summer, at the end of the elongation of the spring buds, the fertilizer efficiency was especially good.<br> 6) The total volume growth varies annually in direct relation to the climate of the season. The effect of fertilization on volume growth seems to be greater in the year after the initial application. The greatest volume increase was seen in the third year with a possible fertilization effect in the fourth year. The fertilizer efficiency indicates that fertilization should be carried out at least four years prior to the final cutting.<br> 7) The volume increase of the stems by fertilization is largest in the more dominant trees. The trees which had the largest response showed a particular increase of growth in the diameter of the stems in the crown and butt portion.