Although potatoes can grow very well below pH 6.0 it is necessary to maintain the soil at a pH suitable for the other crops in the rotation. The pH levels to aim for in mineral soils are about 6.3 for oats and 6.5 for most cereals. Lime can take years for full reaction with soil, and therefore needs to be applied in good time. Common Scab is caused by the organism streptomyces scabies which is endemic in soils, and infection can be exacerbated by fresh applications of Lime. Therefore lime should not be applied within the two years preceding a potato crop because of the increased risk of common scab. At least 4 years should intervene between liming and sowing a seed potato crop.
The main effect if nitrogen is to increase canopy size and prolong its duration. Yield increases from higher nitrogen rates will only result if harvest is delayed to take advantage of the extended canopy duration. Furthermore harvesting prior to canopy maturity may result in reduced dry matter/specific gravity and after cooking blackening in some cultivars. Where sowing date is delayed, nitrogen rates should be reduced accordingly. Thus, where tuber quality is important it is essential not to use high amounts of N in the event of a restricted growing season. The maximum N rates permitted at sowing time according to SI 610 are shown in Table 1.
Potato cultivars differ in their nitrogen requirement and there is no correct standard rate of N for all cultivars. Indeterminate deep rooting varieties such as Cara are effective nitrogen scavengers and may require lower application rates in certain circumstances.
For early, second early and late maturing cultivars high nitrogen delays maturity therefore it is important to avoid the over-use.
Phosphorus and Potassium
Potatoes are very responsive to P and K and it is necessary to apply these nutrients even at Index 4 (Table 1). All the P should be applied at sowing time but some of the K can be applied the previous autumn. The use of potassium sulphate instead of potassium chloride gives tubers of higher dry matter content but they are more susceptible to internal bruising.
The most efficient method of application of fertilizers to potatoes is by placing it in bands about 50 mm to the side and 50 mm below the level of the seed.
|Table 1. N, P and K broadcast rates for potatoes1 (kg/ha)|
|Soil N, P,K Index||N||P||K|
|1. Where all the fertiliser is band-placed or broadcast on the open drills before planting by hand, the recommended rates can be reduced by 20%.|
|2. When soil P test is above 15mg/l, no fertiliser P is necessary.|
Magnesium deficiency is occasionally encountered with potatoes. High K applications in conjunction with high soil K can reduce the availability of soil Mg. Apply 90 kg/ha of Mg for a soil Mg Index of 1 or 45 kg/ha at Index 2 (check soil Mg levels as per soil Mg index)
Potatoes are tolerant of relatively low soil pH values compared with other crops except in soils with high easily reducible Mn concentrations. In such soils, Mn toxicity can occur