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Nutrition

Nitrogen, Phosphate and potash

The Effects of Nitrogen

Raising the level of N application up to a certain point, increases tuber yields. Higher applications delay the development of tubers. These tubers end up with a watery texture and of low cooking quality. The foliage of these plants with excessive nitrogen is very susceptible to blight and also delay potato set and maturity of the potato.

The Effects of Phosphorus

Phosphorus encourages early rooting as well as later maturity, harder skins, and may reduce tuber blight. P can increase dry matter.

The effect of Potash

Potash increases tuber size, but excess K (even excess sulphate of potash) may slightly reduce dry matter content. The use of sulphate of potash instead of muriate of potash (chloride) will help to increase tuber dry matter.

Recommended Fertiliser Rates

Fertiliser application should be based on a recent soil test. The generally recommended range of fertiliser application for maincrop ptatoes is as follows:

Fertiliser should be applied to the seed bed. Potatoes poorly utilise phosphate due to its root structure so a well prepared seed bed is essential for good uptake.

Table 42: The nutrient requirement kg/ha (units/acre) for 15t/ha (6t/acre) dry matter main potatoes are:

Soil Fertility (P & K Index)N kg/ha (u/ac)P kg/ha (u/ac)K kg/ha (u/ac)
Poor (1) 170 (136) 125 (100) 305 (244)
Deficient (2) 145 (116) 100 (80) 245 (196)
Moderate (3) 120 (96) 75 (60) 185 (148)
High (4) 95 (76) 50 (40) 120 (96)

Note: Where soil P test is above 15 mg/l no fertiliser P is necessary.

Soil nutrition for salad potato production.

Adequate soil nutrition is key to producing high numbers of evenly sized tubers. Ideally soils should have pH of approximately 6.5 so that soil nitrogen, phosphorous and potassium are available to the plant.

Nitrogen in salad crops needs to be carefully managed, as salads are a relatively short term crop ie. 90 days nitrogen requirement is not generally as high as that of a maincrop. Excess applications of nitrogen can lead to problems with maturity and dessication. Table 1 below is taken from the Teagasc Nutrient advice book and shows the nitrogen required by salad potatoes.

Table 1; Nitrogen required for salad potatoes (Kgs/Ha)

Length of growing season

Variety

determinacy

group*

Soil N Index1

Index 1          Index 2         Index 3          Index 4

> 60 days

1

60-100

50-90

40-80

30-60

2

50-90

40-80

30-70

0-40

60 – 90 days

1

80-120

60-100

40-80

30-70

2

70-110

50-100

40-90

30-70

* Variety determinacy or haulm longevity is a measure the time from 50% emergence to haulm death where varieties in group 1 are short term longevity and those in group 4 are very long haulm longevity.

Phosphorous plays an important role in early plant growth, tuber formation and tuber maturity. Phosphorous is generally applied into the seed bed at planting however foliar P can be applied to increase the tuber numbers, especially in low P sites and also in cold springs.

Potassium regulates the amount of water in the plant, therefore, where there is insufficient potassium, crops will not use water efficiently. Potassium will also help crops to cope better with stress caused by drought, heat, waterlogging etc

Magnesium deficiency occasionally occurs in sandy soils or where high levels of potassium have been applied

Sulphur supply can be low in sandy soils, it is usually applied as sulphate of potash into the seed bed and is associated with increased dry matter.

Further details on potato nutrition can be found on the Teagasc website at:

https://www.teagasc.ie/media/website/publications/2016/soil-fertility-green.pdf

Details of general soil fertility can be found on the Teagasc website at:

https://www.teagasc.ie/crops/soil--soil-fertility/