FERTILISATION

Highbush blueberry has modest feeding requirements compared with other fruit bearing trees and bushes. Proper fertilisation, however, is crucial to successful cultivation as blueberry cannot extract nutrients from deeper, richer layers of soil due to its shallow root system. A deficiency of nutrients can lead to various symptoms such as yellowing of the leaves, poor or even stunted growth, and poor, low quality yields, and can render the bushes susceptible to infections. Cultivating blueberry means treating soils with fertilisers. Fertilisers dissolved in water (fertigation) can also be applied. This is a more precise and easier to control method that uses less fertiliser. Applying fertilisers to the leaves is used to supplement nitrogen (fertilising with urea) and micro-components. Elements which are easily rinsed out (like nitrogen) should be added to the soil several times a year, whereas those which remain in the soil need only be topped up every 2-4 years.

Fertilising with Nitrogen

Ammonium sulphate, saltpeter and urea are some of the nitrogenous fertilisers available. It is recommended that bushes be fertilised twice per year with approx. 25 kg per ha. This should first be done in early spring, once the ground frosts have vanished, and again in late June. Fertilising too late the second time can result in bushes being frozen in winter.
Nitrogen fertilisation needs to be adjusted to both the type of soil and its fertility. Nitrogen is an element which is very easily rinsed out by rains and watering. Sawdust mulch reduces the amount of nitrogen available to plants during their first few years of having been planted. The amount of nitrogen should therefore be increased by as much as 100% if need be.

Blueberry responds to nitrogen deficiency in a very visible manner. This manifests itself as chlorosis, mainly on the oldest leaves, as a reddening of young growth, as growth stagnation and poor crops.
Too much nitrogen can result in intensive growth and plants freezing as a result of entering their rest phase too late.

Fertilising with Potassium

Potassium sulphate makes the best potassium fertiliser as it does not contain any chlorides. Potassium salt and other fertilisers containing potassium in chloride form (Amofoska, Polifoska, Potafoska) are applied in autumn. Their use in spring can result in significant chlorine damage to the root system. It is recommended that the soil be fertilised with potassium before the bushes are planted. Then the soil is fertilized since the third year of plantation by use of ca 50-70kg of potassium per 1ha.

The symptoms of potassium deficiency are dying sprout tips, stunted leaf growth, necrosis and brown spots on leaves, and lower fruit yields.

Fertilising with Phosphorus

Phosphorus fertilisers include ammonium phosphate, which is applied in spring, and superphosphate which is applied in autumn and before planting bushes, as is the case with potassium. Phosphorus should be applied in a dose of approx. 20-25 kg per ha. Phosphorus, however, is not crucial to blueberry fertilisation and need only be used in small quantities, if at all. Phosporous is usually a component of multi-element fertilisers.
Phosporous deficiency is only manifested in extreme cases of very low quantities in soils and even then its symptoms are slight. The leaves shrink although they retain their green colour.

Fertilising with Magnesium

Magnesium fertilisers include magnesium suplhate, which is applied in spring. Plants in light soils need magnesium in a single dose of approx. 40-80 kg per ha. Magnesium deficiency can manifest itself during summer droughts or intensive potassium fertilisation as chlorosis of older leaves between the nerves, which remain green.

Fertilisers should be selected carefully after considering the fertility and type of soil in which the bushes are planted (mineral elements are rinsed out more easily from permeable soils) and evaluating their growth and fruiting. Recommended fertiliser doses should not be exceeded as American blueberry is susceptible to an excess of nutrients.
Applying balanced, multi-element fertilisers is the easiest way to fertilise Amerian blueberry. It is additionally recommended that nitrogenous fertilisers be applied in the case of nitrogen deficiency symptoms. Fertiliser should be poured evenly over the cultivated area, along the width of the roots. Applying concentrated portions of fertiliser directly under a plant should be avoided.