For minimum sulfur requirement for potato production, irrigation water may provide some or all of the needs. The water may be tested for S content. In sandy, low organic soils as used in most potato production in the Northern States, a soil test will help determine S deficiencies. S is most available in soils with pH greater than 6.0; availability gradually decreases as pH decreases below this level. Table 1 gives S fertilizer rates to avoid deficiency based on soil S levels.
There are no market pressures on S application as there are with N, P and K fertilization. However, there may be disease pressures affecting the desired amount of S fertilization (Pavlista, A.D. 1993. Control of Common Scab with Sulfur and Ammonium Sulfate. Spudman 31(8):13,32,34). There is evidence suggesting that higher amounts of sulfur applied in-furrow can substantially decrease tuber infection by common scab and black scurf. Common scab is especially important in the table-stock and chip-stock markets and somewhat important in the other two major markets. Black scurf is especially important in the table-stock and seed-stock markets. The best form of sulfur to apply is ammonium sulfate (AS) placed in the furrow at planting. Table 2 gives the effect of AS and the equivalent S on these two soil-borne diseases.
Nutrients: Deficiency and Excess Symptoms
The minimum sulfur requirement for potato production is usually satisfied by irrigation water. The water may be tested for S content. In sandy, low organic soils as used in most potato production in the Northern States, a soil test will help determine S deficiencies. To avoid S deficiency symptoms, 25 lb S/acre is sufficient even in the absence of S in the soil or water. S may be added in many forms (Table 3).
Sulfur deficiency is rare. There is a general yellowing of leaves and leaflets exhibit a slight upward roll. This yellowing is first observed with young leaves and is uniform on affected leaves.
There are no negative effects associated with excessive S.