Because of the market desire for larger tubers, the amount of P fertilizer is greater for table-stock and fry-stock potatoes than for chip-stock and seed-stock. Many soils used in potato production release only small amounts of P for plant uptake during the growing season. Soil pH has a major effect on P availability; the optimal pH range is 6.5 to 7.5. Below pH 6.5, availability decreases rapidly. From pH 7.5 to 8.5, availability decreases gradually; then availability very rapidly increases up to pH 8.75. A second optimum to P availability is reached at pH 8.75 and remains stable until above pH 9.
Table 1 shows the effects of P levels on potato tuber yields (US#1 grade), dry matter content and sugar content of tubers. For table-stock, P fertilization is applied to increase the proportion of tubers 10 to 11½ oz weight (70-80 count carton). For fry-stock, P fertilization increase tuber length and the proportion of tubers greater than 10 oz and 16 oz for French frying. Excessive P will not injure potatoes. No differences were reported between super-phosphate, di-ammonium phosphate or mono-ammonium phosphate. Table 2 provides broadcast phosphorus (P) fertilizer recommendations for potatoes. As with other crops, soil tests provide the most accurate way to determine the supplemental P fertilizer needs.
DEFICIENCY and EXCESS SYMPTOMS (Tables 3 and 4):
Phosphorus is essential for early plant growth. Since P is immobile in the soil, it can be all added as starter but it needs to be in the root zone. It is placed most efficiently as a band lateral to the seed piece during planting; this raises uptake and lowers P fixation (tying up in soil). P-deficient plants produce smaller tubers having slightly lower dry matter content, and tending to be over-mature at harvest.
Phosphorus deficiency easily occurs in calcareous soils due to increased P fixation. Leaves do not expand normally resulting in a crinkly appearance and a cup shape. Leaf margins roll upward and the degree of leaf-roll increases with the severity of the deficiency. They are a darker green and may have brown margins ('scorched'). Lower leaves drop. Severe deficiency can cause stunting and the stems to be spindly. There is a decrease in the root mass, and fewer and shorter stolons are formed. Tubers tend to have rusty-brown spots radially scattered inside. P-deficient plants produce very small tubers having higher sugar and slightly lower dry matter contents, and tending to be overly mature at harvest. Tubers are disease susceptible.
There are no negative effects directly associated with excessive P, but, especially in alkaline soils, too much P can lower the uptake and use of iron and zinc.