In semi-arid tropics, sugarcane yield and quality are affected by deficiencies in soil nutrients, including potassium and calcium. We examined the effects of two different potassium fertilizers, a traditional muriate of potash (MOP) and polyhalite (which contains potassium and calcium), on sugarcane growth, yield, and quality. Experimental treatments compared a control 0 kg K ha−1 (T1) to potassium applied as MOP only at 80 kg K ha−1 (T2) and at 120 kg K ha−1 (T3), and potassium applied as an equal split of MOP and polyhalite at 80 kg K ha−1 (T4) and at 120 kg K ha−1 (T5). Relative to the control the potassium-enhanced treatments had improved rates of key growth parameters, and of cane yields, which were 4.4, 6.2, 8.2, and 9.9% higher in T2, T3, T4,, and T5, respectively, than in T1. Regardless of fertilizer used, potassium applied at 80 kg K ha−1 achieved the highest sugar purity and commercial cane sugar content. All potassium fertilizer treatments had reduced (although non-significant) incidences of three key sugarcane insect pests. The economic benefits of polyhalite were reduced due to its higher cost relative to MOP. Combining MOP and polyhalite equally to achieve an application rate of 80 kg K ha−1 is recommended to enhance sugarcane growth and yield.