Growth and yield responses of sweet potato (ipomoea batatas) to time of potassium fertilzer application

Two field experiments were conducted at the Michael Okpara University of Agriculture, Umudike research farm in 2005 and 2006 wet seasons to determine the growth and yield responses of two sweet potato (Ipomoea batatas L. Lam) varieties viz TIS 87/0087 and TIS 8164 to time of potassium (K) fertilizer application in a humid forest zone of south eastern Nigeria. In 2005 cropping season, the experiment was a 2 x 4 factorial laid out in a randomized complete block design with four replications. Potassium (K) treatments comprised no K application, 100 kg K ha-1 at 4 weeks after planting (WAP), 100 kg K ha-1 at 8 WAP, 50 K ha-1 at 4 WAP + 50 K ha-1 at 8 WAP. Time of K application had no significant effect on the growth attributes of the varieties, except in tuber dry weight and total dry matter plant-1 at 16 WAP where it was higher with split K application at 4 and 8 WAP in TIS 87/0087. TIS 87/0087 was significantly (P < 0.05) higher in total tuber yield when 100 kg K ha-1 was applied at 4 WAP and also with split application at 50 kg K ha-1 at 4 WAP + 50 kg K ha-1 at 8 WAP. In 2006 cropping season, a 2 x 8 factorial in randomized complete block design with three replications was conducted at the same location as in 2005. The treatments comprised of no K application, 100 kg K ha-1 at 2 WAP, 100 kg K ha-1 at 4WAP, 100 kg K ha-1 at 6 WAP, 100 kg K ha-1 at 8 WAP, 50 at 2 WAP + 50 kg K ha-1 at 6 WAP, 50 kg K ha-1 at 2 WAP + 50 kg K ha-1 at 8 WAP and 50 kg K ha-1 at 4 WAP + 50 kg K ha-1 at 8 WAP. Tuber and total dry weights were highest when K was split applied at 4 + 8 WAP but lowest when it was applied whole at 8 WAP. TIS 87/0087 was significantly (P < 0.05) higher in tuber yield ha-1 when K was split applied at 2 WAP + 6 WAP or 50 kg K ha-1 at 4 WAP + 50 kg K ha-1 at 8 WAP. TIS 87/0087 which is a higher yielding variety is recommended for the study area with split application of K at 2 WAP + 6 WAP or at 4 WAP + 8 WAP.KEY WORDS: Sweet potato, Ipomoea batatas, K, time of K application.