Article 3 in Composting Series

Nitrogen the N in NPK

Most compound fertilisers will contain three elements essential for growth.

NPK which stands for:

An equally balanced fertiliser (ie Growmore) may be described as 5:5:5 – 5% Nitrogen, 5% Phosphorus and 5% Potassium. You may also see Potassium described as Potash

Nitrogen is used by the plant to produce leafy growth and formation of stems and branches. Plants most in need of nitrogen include grasses and leafy vegetables such as cabbage and spinach. Basically, the more leaf a plant produces, the higher its nitrogen requirement.

Although 78% of the atmosphere is nitrogen, most plants cannot utilise this. Plants in the bean family, legumes, have nodules on their roots where bacteria live that fix nitrogen from the air for use by the plant. They provide their own nitrogen fertiliser this way.

Symptoms of Nitrogen Deficiency in Plants –

You can tell if your plants need nitrogen when their growth is stunted with weak stems and they will have yellowed or discoloured leaves.

Application of Nitrogen

Nitrogenous fertilisers are quickly washed out of the soil by rain and need to be renewed annually. With crops that require a lot of nitrogen over a period of time, like cabbages, adding nitrogen incrementally through the growth period is the most efficient application method.

Phosphorus the P in NPK

Phosphorus is essential for seed germination and root development. It is needed particularly by young plants forming their root systems and by fruit and seed crops. Root vegetables such as carrots, swedes and turnips obviously need plentiful phosphorus to develop well.

Phosphorus the P in NPK

Phosphorus is essential for seed germination and root development. It is needed particularly by young plants forming their root systems and by fruit and seed crops. Root vegetables such as carrots, swedes and turnips obviously need plentiful phosphorus to develop well.

Symptoms of Phosphorus Deficiency in Plants – Without ample phosphorus you will see stunted growth, probably a purple tinge to leaves and low fruit yields

Without ample phosphorus you will see stunted growth, probably a purple tinge to leaves and low fruit yields.

Application of Phosphorus

Phosphates remain in the soil for two or three years after application so the amount in a general fertilizer is probably enough. Add just before planting or top dress during growth periods.

Potassium has the chemical symbol K from its Latin name kalium. It promotes flower and fruit production and is vital for maintaining growth and helping plants resist disease.

Potassium is the K in NPK

It’s used in the process of building starches and sugars so is needed in vegetables and fruits. Carrots, parsnips, potatoes, tomatoes and apples all need plenty of potassium to crop well.

Potassium encourages the plant to set flowers and develop fruit. It also controls the water content and transportation of other plant foods from roots to shoots.

Potassium is naturally found in wood ash which is where it its name potash is derived from.

 To recap potash is potassium and vice versa when discussing fertilisers.

Symptoms of Potassium Deficiency in Plants –

Plants that are short of potash will have low resistance to disease, scorching of leaves and poor fruit yield. Tomatoes will really show the effects of a shortage of potassium

Application of Potassium

Potash usually last for two or three years in the soil but for vegetable production (tomatoes, potatoes especially) additional will be required.

This can be applied as a liquid feed, either commercial or made from comfrey as comfrey tea, for tomatoes or a specially prepared fertiliser, high in potassium for potatoes.

The table below gives the N-P-K values for the popular fertilisers sold in the Hut on a Sunday morning.