Timing of application of fertilisers
Nitrogen: Potatoes use large amounts of N, frequently more than the total applied as fertiliser. Normally all N is applied to the seedbed at planting but in cases where leaching by rainfall is likely (sandy soils etc) there are often benefits from applying half in the seedbed and half at tuber initiation. On heavier soils in temperate areas the benefits from splitting N application are less predictable.
Phosphorous: Many potato production fields have need for P applications since this nutrient is especially important for early plant development and rapid tuber growth. Phosphorous is generally best applied in spring in the seedbed.
Potassium: Where large amounts (e.g. > 400 kg/ha K2O) are to be applied, they are probably better split. Agricultural field practice suggests that 6-8 weeks between dressings in temperate conditions is adequate. On all except the lightest or most K-deficient soils, there appears to be no disadvantage in autumn application of K fertilizer followed by spring potato planting.
Inadequate supplies of K can result in decreased yields and quality of the tuber. There are rare reports that heavy applications KCl can result in lower dry matter percentage (specific gravity) in tubers compared with K2SO4 or KMgSO4. This problem is not common and can be overcome with proper K management.
Micronutrient deficiencies are usually most efficiently corrected by foliar application, but a good general NPK fertiliser with micronutrients is also a very good choice and all nutrients will then be applied at the same time.
Preferred nutrient forms
Nitrogen: Usually as readily soluble materials (Ammonium and Nitrate Nitrogen)
Phosphorous: Generally as materials containing a high proportion of water-soluble phosphate applied at, or immediately prior to planting.
Potassium: Application as Potassium Sulphate rather than Chloride (KCl) provides some small benefits in the form of relative increases in tuber dry matter (specific gravity) and is most appropriate to potatoes for processing.
The Chloride, however, through its effects on reducing tuber dry matter, has beneficial effects in reducing tuber bruising at harvest, other factors being unchanged. Large dressings of KCl at planting will have a deterious effect on crop emergence in dry soils.
Magnesium: Can be applied as a foliar spray where deficiency symptoms occur. More often it would be applied in a Mg containing NPK fertiliser or, as a routine on inherently Mg-deficient soils, in Mg-containing lime.
One of the most typical potato NPKs is 12-12-17sop+2MgO. Sometimes growers prefer to use it with micronutrients, while other growers apply these nutrients as a foliar spray.