Anything that prolongs a healthy leaf canopy will increase the average tuber size.
Correct balanced nutrition is important for increasing potato tuber size. The nutrients nitrogen, phosphate, potassium, calcium, magnesium and manganese have all been shown to affect potato tuber size.
Nitrogen is important in fuelling growth and providing high yields. Nitrogen is largely needed during leaf formation and then for increasing tuber growth and size, when it ensures optimal photosynthate production in the leaves. Nitrogen fed at an early stage of crop development will help build the overall size of the leaf canopy, whereas at later stages of growth, nitrogen use helps maintain the greenness of the canopy and maximize yield.
However, this needs to be balanced as too much early nitrogen can lead to excessive vegetative growth at the expense of tuber formation and an excess supply of nitrogen at later stages of growth will keep the crop growing and can also result in a crop with many 'oversize' potatoes and place it under increased risk of blight infection. However, in hot, dry climates, where true yield potential and desired tuber size has not been met, additional nitrogen can maintain canopy growth and prolong bulking.
Thus, the right rate and timing of nitrogen is important in managing potato yields, tuber size and desired quality characteristics.
This series of trials from England demonstrates increasing rates of nitrogen resulted in an increase in tuber size and so an increase in overall yield.
Nitrogen form is also important. A balance of ammonium and nitrate is best used at planting but too much ammonium nitrogen is a disadvantage as it reduces root zone pH and thereby promotes rhizoctonia. During tuber initiation onwards through tuber bulking nitrate nitrogen has distinct advantages and is the preferred source.
Foliar phosphate, applied after tuber initiation increases tuber size and so increases tuber yields, however, foliar phosphate is not a substitute for soil applied phosphate and without adequate soil phosphate early season growth is sub-optimal.
These trials conducted independently in England show a consistent yield increase from applications of foliar phosphate after tuber initiation resulting in an increase in tuber size and so overall yield.
Potato plants absorb large quantities of potassium throughout the growing season and it is critical for high yields.
In five trials across three years on K rich volcanic soils, 120kg K2O/ha increased average yields by 10t/ha.
Calcium is a key component of cell walls, helping to build a strong structure and ensuring cell stability. It is critical during cell division and expansion, and is therefore essential prior to, and during, the rapid growth phase of tubers.
Magnesium is needed during tuber bulking and if supply is restricted tuber size and yield will both be reduced. Soils with a severe magnesium deficiency can reduce yields by up to 15%, in such cases regular applications of magnesium on an annual basis has achieved yield increases of 1 to 10% in trials.
Uptake of magnesium is dependent upon the balance with the other cations, especially potassium. High concentrations of potassium in the soil can induce magnesium deficiency. Similarly sufficient magnesium may be present in the soil yet dry soil conditions may restrict uptake, in both cases, foliar applications of magnesium can be very effective.