A large number of tubers per hectare will produce a crop of predominately small tubers, ideal for canning, salad or seed potatoes. A relatively low tuber number provides less competition per unit area and allows the crop's energies and resources to be used to produce larger potatoes for the fresh or processing markets.
Correct balanced nutrition is critical to influencing tuber numbers. Phosphate, Potassium, and Calcium have all been shown to have effects on tuber number.
Phosphate availability at tuber initiation is important to ensure maximum tuber set, especially if tuber numbers need to be increased for certain varieties, or where the market demands a large number of smaller tubers (e.g. seed production).
This study from Wales shows the effect of phosphorus on increasing total tuber numbers and overall yield.
While potatoes are very responsive to fresh phosphate, the economic optimum rate is often difficult to define. Rates will depend on soil type and soil test results. Where sufficient soil phosphate is not available for growth, foliar phosphate ensures rapid availability.
Study from Scotland showing the effect of foliar phosphorus on increasing total tuber numbers.
Applied just before tuber initiation foliar phosphate increases total tuber number.
Because phosphorus is relatively immobile in the soil it is important that fertilizer –P is placed close to the tuber, banding the fertilizer usually works better than broadcasting, especially on soils with the potential for very high phosphorus lock-up.
This chart shows the influence of soil pH on the availability of phosphorus due to precipitation by calcium at high pH and fixation by iron and aluminium at low pH.
Potassium form and rate applied can also influence the numbers of tubers which are set.
This study from shows the effect of potassium form on increasing total tuber numbers.
These trials demonstrate the yield increase due to potassium and also the benefit from potassium sulphate (sulphat of potash - SOP) rather than potassium chloride (muriate of potash - MOP).