1970 Volume 39 Issue 2 Pages 99-106
Experiments were carried out to clarify the combined effects of various water regimes and fertilizer rates on the growth and fruiting of young Satsuma orange trees during two months, from July to August in 1969. Four and five year old plants grown in the Wagner′s pots (1/2000 are), filled with a loam soil, were used as materials.
At regular intervals, watering were done to maintain the appropriate levels of soil moisture by determining the water content in each plot at the depth of 10cm according to the volumetric method. The water suctions were calculated to express this force on the energy scale, pF and bar using soil moisture constants as shown in Table 2.
To make the plots with different osmotic suctions in the soil, the mixed fertilizers on various levels were supplied. Thereafter, separate fertilizers were done by determining the electrolytic conductivity of salts solution prepared from the saturated soil in each plot at regular intervals to maintain the definite osmotic suction. The osmotic suctions (P) in bar were estimated by the calculation from the values of the electrolytic conductivity (EC) of saturation extracts in mmhos/cm according to the following equation.
Total suction in the soil solution can be obtained as the sum of water suction and osmotic suction. The ranges of total, water and osmotic suction in the soil solution under different treatments are shown in Table 1.
Results obtained are as follows:
1. The trees reached their full growth when the total suction in soil solution did not exceed 2.5 bar as the sum of water suction of 0.5 bar and osmotic suction of 2.0 bar. Above its critical pressures, the trees showed a tendency to inhibit their growth. In this case it appeared that the water suction was more important than the osmotic one.
2. The suction in soil solution had the same effect on the fruit growth as described above. Index of fruit shape was also affected by the soil water suction, showing the tendency of increasing the ratio of vertical diameter to cross diameter with increasing the water suction. The full development of fruit color was shown in the plots with lower total suctions, especially the lowest osmotic suction, while incomplete coloring in plots of higher water suctions. Percent of fruit rind was high in the treatments with higher total suctions.
3. Soluble solids in fruit juice were higher in plots with total suctions below 3 bar than the ones in the other plots. On the contrary, citric acid was high in content in plots with higher total suctions and therefore tastes due to lowered sugaracid ratio showed bad quality.
4. Both water saturation deficit and diffusion pressure deficit as measures of internal water status in leaves showed the tendency of increasing their values with increasing water and osmotic suctions in soil solutions. Leaves exposured at higher values of water suctions accompanied by increased solute suctions in soils showed a marked drop in both photosynthesis and transpiration. The suctions in such conditions had the same effect on the viability of fine roots, which were going to rot in very bad conditions.
5. The content of nitrogen, phosphorous and potassium in leaves was high at higher osmotic suctions, while the case with calcium was the reverse.
In conclusion, to give the best possible growth and fruiting of Satsuma orange trees, it is important to keep the water suctions below 0.5 bar and the osmotic suctions below 2.0, in other word total suctions below 2.5 bar in the soil solution by adjusting watering and fertilizer rate during the summer. The upper critical range of 2.5-3.0 bar as the total suction in the soil solution may be shown from our experiments.