Many studies have been conducted to assess challenges faced by smallholder crop producers in South Africa but few have focused on agronomic constraints. This study is a review of agronomic constraints faced by smallholder farmers under irrigation and dryland farming in South Africa. Constraints include choice of cultivars, planting dates and densities, tillage operations, water and fertilizer management and cropping patterns. Water availability is a major constraint under dryland farming and studies showed that the impacts can be very severe. Interaction of a variety of constraints under irrigation resulted in below average yields. Strategies for increasing productivity in dryland agriculture include capturing more water and allowing it to infiltrate to the root zone and the use of available water more efficiently. Yields under irrigation can improve when micronutrients are blended with macronutrients in relatively affordable blends. Use of green manure as an alternative fertilizer can also improve yields. Soil and water management technologies that improve soil fertility and productivity were as important as those that prevent soil erosion and water loss. It was recommended that practices such as supplementary irrigation and rainwater harvesting technologies take priority in efforts to address dryland water problems. As for farmers with access to irrigation water, practices that deal with improvement in planting dates and populations and water and fertilizer management can have positive impacts on crop yields. Research focused on smallholder agriculture should also start focusing more on water productivity and improving agronomic practices.