Post-planting and emergence
Apply nitrogen every week at a constant rate for both winter and summer planted crops. It is not necessary to vary the nitrogen application rate with crop stage. The recommended total rate of nitrogen is 490kg/ha for summer planted crops and 570kg/ha for winter planted crops, which is lower than the total that most producers apply.
More nitrogen may be required if winter rainfall is higher than usual. The concentration of nitrogen in irrigation water should also be considered, as significant quantities of nitrogen can be applied to the crop from irrigation water, particularly during hot weather. More information can be found at Applying nutrients to vegetables via irrigation.
Potassium should be also be applied weekly at a constant rate. Total rates of 522kg/ha for summer plantings and 602kg/ha for winter plantings are adequate for maximum yield on new land sites.
On old land sites, 450kg/ha is adequate in most cases, throughout the year.
Sulphur is an essential element required by all crops. Sulphur applied in the trace elements and with phosphorus and potassium fertilisers is usually adequate to meet crop needs. If a low sulphur-containing phosphorus fertiliser is used (DAP for example), apply gypsum to supply sulphur if it is not provided in another form such as potassium sulphate.
A total of 20 to 30kg/ha of magnesium or 200 to 300kg/ha of magnesium sulphate (Epsom salts) is normally adequate.
Calcium is an essential nutrient for plant growth. As with sulphur, crops often get their calcium requirement through other fertilisers such as superphosphate or amendments such as lime. In some areas, significant quantities of calcium can be applied through the irrigation water.
Calcium should be applied if disorders such as hollow heart, brown centre or fleck are prevalent. This can be applied pre-planting as gypsum at one tonne/ha or post-planting by using calcium nitrate as a nitrogen source for some of the post-emergence applications up to a total of 100kg Ca/ha.
A summary of the suggested fertiliser program for application of N, K, Mg and Ca after planting and emergence of potatoes on sands of low N and K fertility is provided in the tables.
Urea often contains a contaminant called biuret which is a by-product of its manufacture. Urea with a high biuret content has been shown to damage crops, notably germinating seed and foliage if applied after planting. To minimise this problem, only use urea with a low biuret (<0.25%) content.