Effects of phosphorus fertilization on Citrus unshiu MARC. I

Abstract

In order to clarify the behavior of phosphorus in the Satsuma orange (Citrus unshiu MARC.) trees, 32P was applied to the trees growing in the field or in pots, and its translocation and distribution were traced. 1. In the field experiment, 20 years old trees growing on the gravelly loam soil were used. NH4 H2 32PO4 solution was injected in the soil or into the trunk by the different methods shown in Fig. 1 on February 20. It was found that more than 10 mc. Of 32P per tree was necessary to obtain reliable data 3 months after the application. The results showed that the absorbed phosphorus accumulated in the branches on the treated side, while few or non of it was detected on the non-treated side. More counts ware read in the upper part of the tree than in the lower part. Difference in the counts was hardly found between new and old leaves. Radioactivity could not be detected in any fruit (Tables 1 and 2). 2. In the pot experiment, the root system was divided into two groups, and each group was enclosed in a plastic film with the soil. 32P as NH4 H2PO4 solution was applied to the one of them, and its distribution in the various portions of the tree was measured directly by the counter one day, one week, two weeks, and one month after the application (Fig. 2). A large amount of absorbed phosphorus was found in the leaves and branches on the treated side, while on the non-treated side no count was read one day after the application, the counts increased progressively up to two weeks, and a trace was found one month after the application (Table 3). 3. In the third experiment, a comparison was made of the site of fertilizer application on the absorption of phosphorus by the tree on a terraced grove (Fig. 3). There was no difference in the absorbing rate between the hill-side dressing and the valley side dressing. This may be caused by tangling of the root system (Table 4 and 5). 4. The fourth experiment was on the comparison of the dressing methods of fertilizers. The sheet dressing method, in which fertilizers were applied all over the soil under the tree crown, was superior to the crown-skirt dressing method, in which fertilizers were applied as a band around the periphery of the crown, in relation to the absorption rate of applied phosphorus (Fig. 4, Table 6).