Potatoes grow best in loose, well-drained loam soil. Heavy clay soil retains too much moisture and tends to become hard as it dries, which can eventually slow water absorption and make it difficult for tubers to grow. Sandy soil drains too quickly, which can result in drought stress unless you water often. Garden soil with a pH between 5.0 and 5.5 provides for the healthiest potatoes and minimizes potato scab disease. A soil test, either through a professional testing service or with a home testing kit, determines the pH and provides a recommendation for amendment amounts for lowering the pH and providing the correct fertility.
Organic matter, such as well-aged compost, improves soil quality. Working 2 to 4 inches of compost into the top 6 to 8 inches of soil also improves moisture retention so the soil doesn’t dry out quickly but still drains excess water well. Compost contains some nutrients, which can further feed the potatoes and help with abundant and healthy growth. Good loam soil may only need a 2-inch amendment, but adding 4 inches or more to clay soil can help loosen it and prevent compaction.
Fertilizer supplies any nutrients lacking in the soil. A 5-10-5 or 10-10-5 blend provides ample nutrition when applied at the rate of 1½ pounds per 50 square feet of bed before you plant. The main nutrients required by potatoes are nitrogen, phosphorus and potassium, or N-P-K. The first number on the fertilizer label is the percentage of nitrogen, which is followed by the phosphorus and potassium percentages. A second application 1 pound of fertilizer per 50-foot row, made after the potatoes reach a 4 to 6 inch height, provides the remainder of the nutrients needed. Sprinkle this fertilizer on the soil about 6 inches away from the plants and water immediately so it soaks in.
Proper soil moisture ensures healthy growth, but overly moist conditions can result in rot problems. Good soil amended with compost or organic matter retains moisture well, but it may still dry out during hot or dry weather. Deep irrigation prevents the soil from drying out too quickly, so provide about 1 inch of water weekly so it can penetrate at least 6 inches into the soil. Covering the soil with 2 to 4 inches of straw mulch further retains soil moisture, while also protecting potatoes that form near the soil surface from sun exposure, which can ruin them.