Cassava (Manihot esculenta Crantz) is well adapted to low fertility soils and tolerates long periods of low soil moisture. Although the area dedicated to cassava production has decreased dramatically in recent years, it is still traditionally planted in the central mountainous regions and in well-drained Oxisols in the northwestern part of Puerto Rico. As with other root crops, there is an increasing tendency to switch production to the southern semiarid coast of Puerto Rico. In this region soils are fertile with high cation exchange capacity, rainfall is low, and farmers commonly utilize microirrigation and fertigation for crop production. Fertilizer recommendations for Cassava production are based on studies conducted decades ago in low pH, low fertility Ultisols without microirrigation. A cassava fertilizer-production trial (cultivar 'Serrallés') was established at the Lajas Agricultural Experiment Substation during 2002 in a Fraternidad clay (Typic Haplusterts). The conventional recommended fertilization rate (275-50-250-60 kg/ha of Ν, P2O5, K20, and MgO, respectively) was compared with the same rate via fertigation. Three additional fertigation rates were tested which were a reduced Ν treatment (100- 50-250 kg/ha, N, P205> K20), reduced Κ treatment (100-50-125 kg/ha, N, P205, K20), and no Κ (100-50-0 kg/ha, N, P205, K20). Magnesium application was eliminated because of the high-soil test value observed in this soil. Fertigations were performed monthly with urea, phosphoric acid, and potassium chloride as the Ν, P, and Κ sources, respectively, whereas the conventional treatment was split applied at four and six months after planting. Mean total yields, average root weight, and number of roots/ha obtained seven months after planting were 7,229 kg/ha, 0.17 kg/root, and 42,929 roots/ha, respectively. There were significant differences among treatments. Reduced fertilizer applications can be recommended for cassava produced in these soils.