The Effects of Different Fertilizer Rates on the Yield of Sweet Potatoes

Variables

• Specific fertilizer yields are affected by the type of soil and how the soil has been treated historically.

  For instance, in Hubei, China, areas of sweet potato production have low soil fertility that have received unbalanced fertilizer in the past, resulting in very low potassium levels.

  Farmers in the Philippines find soil moisture and soil depletion to affect what fertilizer application rates are necessary, and whether or not green manures instead of chemical fertilizers can be used. A soil test for your location is a good idea.

Nitrogen

• Plants need nitrogen for growth, reproduction and photosynthesis. In most areas studied, nitrogen (N) was not as effective as other fertilizers (potassium or phosphorus) in increasing sweet potato yield.

  The Purdue University Department of Horticulture recommends that fertilizers with only moderate nitrogen be used for Indiana growers. An example of the NPK (nitrogen, phosphorus, potassium) ratio for sweet potatoes would be 5-10-10 applied at 3 pounds per 100 square feet. Purdue horticulturists note that too much nitrogen could result in excessive vine growth, misshapen tubers with cracked skin and poor storage qualities.

  Agriculturists in Ghana found that when the soil nitrogen level is 0, increasing nitrogen fertilization is beneficial to yield, but that above 30 kg/ha nitrogen application was detrimental to yield increase.

Phosphorus

• Phosphorus (P) is essential for energy transfer in plants and helps early growth and plant maturation, including flowering.

  Most studies have shown that phosphorus does not have a significant effect on increasing sweet potato yield. For instance, the Philippine study commented that "…at all levels of N and K used, addition of P did not result in any significant increase in yield."

  Researchers in Ghana showed that increasing the rate of phosphorus application does not increase sweet potato yield.

Potassium

• Potassium (K) contributes to early growth, water and nutrient use, protein production and improved resistance to disease. It has been shown to be an important fertilizer for sweet potatoes.

  In Hubei, China, all nine field trials conducted demonstrated that potassium greatly increased sweet potato yield. Potassium application rates varied from plot to plot, as did sources of potassium. Potassium chloride was more efficient than potassium sulfate in increasing yield. Using potash as a potassium source, the following applications were made in kilograms per hectare: 0, 75,150, 225 and 300. The optimum applications varied from site to site, with site 1 reporting 150 kg of potash per hectare, site 2 reporting 225 kg of potash per hectare, and site 3 reporting 300 kg of potash per hectare as optimum rates.

  In Ghana, potassium applications significantly improved sweet potato yield. Applications were made at NPK ratios of 30-30-0, 30-30-30 and 30-30-60, with increased yields in tons/ha of 8.75, 12.3 and 14.4, respectively. The FAO document that outlines this study comments that "[p]otassium appears to be the most important nutrient in the production of sweet potato."