Influence of Planting Density on the Root Growth and Yield of Sweet Potato : Ipomoea batatas Lam.

Influences of planting density on the root growth and the yield of sweet potato (Ipomoea batatas Lam.) were studied. Two main crops of sweet potato cultivars Koganesengan and Norin No.1 and the two levels of planting density (35 and 70 cm X 100-cm spacing) were examined. Sampling was done at 50,100 and 150 days after planting (DAP). The non-tuberous roots including rootlet (<2mm in diameter), medium root (>2mm in diameter) and tuberous root (>1cm in diameter) also were investigated to a depth of 45 cm under ground. LAI increased progressively from early stage until the 100 DAP in both the cultivars at the higher densities and then greatly declined until the 150 DAP. Our observation fixed that, cool day and night temperatures, which prevailed during autumn caused leaf losses. The differences between low and high densities in top-DW per area (m^2) in both the cultivars were evident. Increasing of the plant densities obviously shows that the top-DW and tuberous roots-DW per m^2 in both cultivars increased. Many of the non-tuberous roots were distributed at the shallow layer (0-20 cm) and the loss of activity of roots which occurred after 50 DAP caused the soil hardness by soil compaction. Furthermore, most of the tuberous roots were formed within a mound of each ridge at the shallow layer. Based on these observations, we may conclude that the roots of two sweet potato cultivars were tested by a remarkably deep distribution of root system (non-tuberous roots) at the shallow and deeper layers concerning the soil equally covering a wide range of environmental condition