Soils and Fertilization
Sweet potatoes grow best in loamy soils. The best soil types are well-drained, fine sandy, or clay loams. Light, loamy soils usually result in roots with better shapes than those grown in heavy or clay soils, which results in rough, irregular roots. High (more than two percent) organic soils also reduce production. Coarse, deep, sandy soils are generally low in fertility, subject to moisture stress, and require more irrigation and fertilizer to grow a good crop. Poor aeration caused by poor drainage decreases yields. With severely impeded drainage, insensitive cultivars can cause either souring (tissue breakdown of the storage roots) or water blisters (enlargement of lenticels on the periderm) if the drainage problem is less severe. Sweet potatoes will grow at a soil pH of 4.5 to 7.5, but 5.8 to 6.2 is optimal. To ensure that the soil is properly limed and fertilized, representative soil samples should be collected from each field. Careful sampling and testing are most important for fields being used for the first time or whenever little is known about recent liming and fertilization practices.
Agricultural lime, if needed and applied several months before planting, can effectively change the soil pH value. Lime can best be mixed by disking or chiseling before plowing or bedding. Use dolomitic lime if the soil’s magnesium level is low; other calcitic lime may be preferred if it is less expensive. In general, fields that have the proper soil characteristics and have not produced a crop of sweet potatoes in the last 2 years are preferred. Avoid fields that have been idle (grassy), seriously eroded, or have very high nematode populations. Fields with a history of pox should be avoided for 5 years. Before planting, determine which herbicide(s), if any, have been used on these fields for the past 2 seasons to be certain that there will be no herbicide carry-over problems.
Fertilizer is applied during 2-3 cultivations, under the row or banded preplant. Harvesting 1 ton of sweet potatoes removes 4 to 5 pounds nitrogen, 1.4 to 3 pounds phosphorus, and 7 to 11 pounds potassium from the soil. Sweet potatoes only need moderate amounts of nitrogen and phosphorus, but need significant amounts of potassium.
Fertilizer can be either banded or broadcast after transplanting. Application rates should be determined by a soil test. The general recommendation is 40 to 60 pounds of nitrogen per acre about 28 days after planting, 60 pounds per acre of phosphate at or shortly after planting, and 150 to 200 pounds of potash (50 pounds at or near planting and 150 pounds at layby). Beauregard requires even less nitrogen than other varieties. Low potassium reduces yields and increases the number of long, slender, malformed roots.
The following are fertilizer recommendations for the popular variety, Beauregard:
1ST CULTIVATION – (WITHIN 1 WEEK AFTER TRANSPLANTING)
**400 LBS./ACRE 00-15-25
** 400 LBS/ACRE 00-00-30
THIS BLEND IS MADE BY USING 1/2 MURIATE OF POTASH &
1/2 GYPSUM (GRANULAR)
3RD CULTIVATION – (28-30 DAYS AFTER TRANSPLANTING)
150-160 LBS/ACRE – 34% NITROGEN
MUST USE AMMONIUM NITRATE (PRILLED)
REGULAR AMMONIUM NITRATE WILL MELT IN JUNE
**MICRO NUTRIENTS ARE NECESSARY FOR PROPER GROWTH
BEAUREGARD SWEET POTATOES
Sweet potatoes also require more boron than many vegetables. On boron deficient soils, 0.5 pounds B per acre (5 pounds Borax or 2.6 pounds solubor) should be added to prevent a disorder called blister. This disorder is characterized by small, raised bumps on the root surfaces and plant stunting. Except for very susceptible cultivars, blister does not show up until sweet potatoes have been stored for several months.
Note: Covington sweet potatoes will use a different fertilizer program as it has a demand for higher Nitrogen in the range of 75 – 90 pounds per acre.