Every crop grows using three main nutrients, these are nitrogen, phosphorus, and potassium. Levels of these three in soil and fertilisers are often in a ratio. Understanding this ratio is crucial to producing high yielding crops, increasing crop health, and preventing the environmental impacts from the over-application of fertiliser.

The required level of nutrients needed for optimal growth depends mostly on the crop. For example, potato plants require higher levels of potassium compared to grains like wheat or barley.

This guide will compare the nutrient requirements for different groups of crops as well as discussing the importance of maintaining a healthy soil environment to provide optimal growing conditions.

Before adding fertiliser

To achieve the optimal soil conditions, first a test on the soil should be performed. A soil test will give information about the pH, nutrient levels, and organic matter present. These can all be interpreted to give an overview of the quality of the soil. The results of this test will influence how much fertiliser will be applied.

The importance of pH

Most plants prefer a pH of around 6.5, a slightly acidic environment. The uptake of most nutrients is highest at this pH. A lower pH would cause less nitrogen to be absorbed and a higher pH would cause less potassium to be absorbed. If the soil test shows a pH that is above 6.5, nitrogen fertiliser application will lower it. However, it is more common that the soil is too acidic, caused by the repetitive application of nitrogen fertiliser. A liming agent is an alkaline substance, such as calcium hydroxide, that can be used to increase the pH back to optimal levels.

Why does organic matter, matter?

Organic matter in the term given to decomposing material that is being broken down in the soil. This could come from dead plants or animals, or from manure being applied. Soil with low levels of organic matter is either sandy or clay-like, and they do not retain water or nutrients very well. Peat soil is the best type, containing 20% or more of organic matter. Organic matter is crucial for the crops as it helps to retain the nutrients and water for the crop to use when it needs and provide some nutrients itself. Lower qualities of soil typically require more fertiliser.

Soil nutrients and fertiliser use

The most important piece of information that a soil test provides is the level of nutrients that the soil contains. Knowing the current levels of nutrients in the soil is important in deciding on the right fertiliser. Fertilisers are usually labelled with an NPK ratio, representing nitrogen (N), phosphorus (P) and potassium (K).

Too much fertiliser may lead to run-off into local water systems and vaporisation into the air. Too little fertiliser may reduce the health of the crops and lead to more biowaste.

Some fertilisers may contain other nutrients such as calcium, sulphur, or magnesium. High levels of other nutrients could be toxic to crops so they should only be applied if the soil test shows a deficiency of them.

A final important aspect of fertiliser is how the nutrients are termed. Fertilisers may display the phosphate or potash value rather than phosphorus or potassium. To avoid under or over fertilisation, the terminology that the fertiliser uses is important as this guide uses phosphorus as potassium. To convert phosphate (P₂O₅) to phosphorus (P) multiply by 0.436, to convert potash (K₂O) to potassium (K) multiply by 0.83.

The guide

To decide how much of each nutrient is needed, a guide for different crops is required. This guide is an overview of the fertiliser manual (RB209) provided by the UK government. Please consult the manual and take advice from a member of the FACTS group if you require further information about fertiliser use for crops.

The crop groups that this guide covers are grains, oilseeds, legumes, brassica vegetables, soft fruit, root vegetables and other vegetables. The units for all the following values are kg/hectare and represent the total amount of nutrients in the soil required. For specific values which consider soil type and current nutrition levels, please refer to the RB209 manual.

Phosphorus and potassium requirements

Phosphorus and potassium are generally the same for each category. For example, most grains require around 50kg of phosphorus and around 125kg of potassium per hectare. Below is a table covering all the groups and their approximate phosphorus and potassium levels. Any exceptions will be mentioned when discussing each group.

Nitrogen requirements

Grain crops include wheat, rye, oat, and triticale. Nitrogen varies widely, with winter varieties needing around 60 kg more than the spring varieties:

• Winter wheat requires 280 kg
• Winter barley requires 220 kg
• Winter oats require 190 kg.

Rye and triticale are both consistent:

• Rye requires 150 kg
• Triticale requires 280 kg

Oil seed crops include rapeseed, sunflower, and linseed.

• Winter rapeseed requires 220 kg, whilst the spring variety only requires 150 kg
• Linseed requires 100 kg
• Sunflower requires 50 kg of nitrogen and requires 30 kg less of phosphorus and potassium than the other oil seeds

Legume crops include peas, broad beans, and runner beans. Legumes require little/no nitrogen in the soil as they form a symbiotic relationship with the nitrogen fixing bacteria, rhizobium, in the soil. This allows legumes to use the nitrogen in the air as their source. After the legumes harvest, the soil will continue to contain high nitrogen levels so less nitrogen fertiliser is required.

The brassica family of vegetable includes sprouts, cabbage, collard, cauliflower and calabrese.

• Sprouts require 330 kg
• Winter cabbage requires 325 kg, with the spring variety requiring 240 kg
• Winter collards require 210 kg, with the spring variety requiring 320 kg
• Winter cauliflower requires 100 kg, with the summer variety requiring 290 kg
• Calabrese requires 235 kg

Soft fruit includes blackcurrant, gooseberry, raspberry, blackberry, strawberry, and redcurrant.

• Blackcurrant requires 160 kg
• Gooseberry, raspberry, blackberry, and redcurrant require 120 kg
• Strawberry requires 60 kg

Root vegetables include beetroot, swede, turnip, parsnip, and carrot.

• Beetroot requires 290 kg
• Swede requires 135 kg
• Turnip and parsnip require 170 kg
• Carrot requires 100 kg

Other vegetables such as onion, leek, lettuce, sweetcorn, radish, pumpkin, courgette, and celery did not fit in the above categories. The nitrogen, phosphorus and potassium values are shown in the table below.

Summary

As covered in this overview, it is crucial to use the correct amount of fertiliser that is specific for the crop and for the field. This can be calculated using this guide after a soil test has been performed. Fertiliser may have harmful impact on the local soil, water, and air environment so the application of them in the UK should follow the guidance covered in the ‘Protecting our Water, Soil and Air’ report from DEFRA.