施肥量がバレイショの根の生長におよぼす影響についての研究はきわめて少なく, またその結果が一致していない. そこで, 施肥量の異なる (窒素, リン酸, 加里のそれぞれについて 0, 75, 150 および 300 kg/ha) 圃場条件下で, 根乾物重(根重), 根長, 分枝根長/根長比(分枝根長比), 根長/根重比 および根長当り根呼吸速度を, 萌芽後28日目と56日目に調査した. 根の採取は鉄製のモノリス枠 (幅5 cm, 長さ30 cm, 深さ30 cm) を用いて行ない, 根呼吸速度は前報^と同様の手法で調査した. 分枝根長比および根長/根重比に対する処理の影響は概して小さかった(第3表, 第2図). このため, 根重と根長の処理に対する反応はほぼ類似し, 施肥量が少ないと根重および根長の増加が抑制され, また根の枯死が早まった(第2表, 第4表). さらに, 根長当り根呼吸速度にも処理間で有意な差異が認められた. 両調査時期とも多肥区ほど高い値を示したが, 特に56日目でその差異が顕著であった(第5表). 以上のことから, 施肥量の増加にともない根の養分吸収能力が増加すること, これは開花始め以前では主として根量の差異に, またこれ以降では主として単位根量当りの根活力の差異に起因することが示唆された(第3図). Knowledge of the fertilizer effect on the roots of potato plants is surprisingly limited and not definitive. In order to obtain more detailed information, root dry weight (DW), root length, root morphology and root activity were investigated under field conditions at different fertilizer rates (0, 75, 150 and 300 kg/ha in each component of N, P_2O_5 and K_2O). The roots were sampled 28 days and 56 days after sprouting with a monolith of 5 cm in width, 30 cm in length and 30 cm in depth. Root activity was identified by root respiration rate per unit root length (ULRR). The ratios of lateral root length to total (nodal+lateral) root length and of root length to root DW were relatively constant regardless of the fertilizer rates. The root length showed similar changes such as root DW, depending on the fertilizer rates. The increases of root DW and root length were suspended and the decay of the roots started earlier at the lower fertilized plot. In addition, ULRR increased significantly with the increase of the fertilizer rates at both sampling times. Its differences were much larger at 56 Days. These results suggested that an increase of the fertilizer rates would result in the increased nutrient absorption ability of the roots especially after the initial flowering stage, depending mostly on the differences in root activity per unit root mass.