Understanding macronutrients for crop growth

While modern technologies are advancing and helping drive progression in the agricultural sector, fundamentals such as understanding the role of macronutrients remain vital.

Here, we delve into what these nutrients are and why they are so essential for farmers in the UK.

What are macronutrients?

Macronutrients are the primary nutrients that plants require in large amounts for their growth and development.

They play a crucial role in various biochemical processes, energy production and overall health of the crops.

The big three: Nitrogen, phosphorus, potassiumThe big three: Nitrogen, phosphorus, potassium

Nitrogen (N):

Key for leaf and stem growth, nitrogen is fundamental to protein synthesis in plants, as it is a major component of amino acids.

Nitrogen also plays a critical role in chlorophyll production. This is important as chlorophyll produces sugars which the crop uses to grow and store reserves in the seeds or tubers, which can then be harvested.

Some crops, such as leafy vegetables and cereals, require high amounts of nitrogen, so when looking ahead at applying nutrients to your crops, it is key to take this into account.

Yellowing of older leaves is the most common sign of nitrogen deficiencies, as plants transport the available nitrogen to the younger leaves. Stunted growth and poor yield are also typical deficiency symptoms.

Phosphorus (P):

Phosphorus is essential for energy transfer and storage within a plant. It also aids root development, flowering, and fruiting. Root crops, such as potatoes and grains like wheat, usually have a high demand for phosphorus.

An easily identifiable sign of a phosphorus shortage, especially in younger plants, is leaves that have a dark green hue with a purple undertone. This process starts in the oldest leaves first.

Plants lacking in phosphorus may also have poor root growth, resulting in the crop maturing late.

Potassium (K):

Vital for physiological processes, potassium plays a crucial part in photosynthesis, protein synthesis, and enzyme activation.

Potassium also regulates the opening and closing of stomata, a pore on the leaves, affecting water regulation.

Crops like potatoes are potassium-loving and a deficiency in this nutrient might manifest as brown scorching and curling of leaf tips. Black spots may also appear on the leaves.

Secondary macronutrientsSecondary macronutrients

While not as regularly discussed as the big three, the following macronutrients are equally important for certain phases of plant growth:

Calcium (Ca):

Calcium helps in cell formation, growth of new roots and shoots, as well as neutralising cell acids. All of these are particularly crucial for crops such as apples, carrots, and brassicas.

A lack of calcium can result in distorted growth and dark spots on fruits. Young leaves may appear distorted or might die at the tips.

Magnesium (Mg):

Magnesium sits at the heart of the chlorophyll molecule, as it is essential for photosynthesis and aiding enzyme activation.

Moreover, magnesium deficiencies can be seen in older leaves which display interveinal chlorosis.  This is where the areas between the veins turn red, purple, or brown, while the veins remain green.

Sulphur (S):

Sulphur is a structural component of amino acids and is involved in enzyme and vitamin synthesis while aiding in disease resistance.

When crops lack sulphur, younger leaves turn pale yellow, similar to nitrogen deficiency. However, with sulphur, it is the younger leaves that show symptoms first.

Considerations for UK farmers

The climate and soil types across the UK vary significantly. Farmers should consider where their farm is located and how it could be affected when considering what additional nutrients crops may require.

Understanding macronutrients for crop growth Soil testing:

Before sowing any crop, it is important to carry out a soil test to understand existing nutrient levels. This helps in determining the right amount and type of fertiliser to be used.

Further information is available here on why to soil test, and how to soil test.

Localised recommendations:

Certain regions may have specific nutrient requirements or deficiencies.

For example, the South and East of England have chalk and limestone soils that typically have good levels of calcium but can lack magnesium. Northern Scotland and areas of Ireland have mainly peat soils that are highly organic and retain moisture well, but are often low in nutrients like phosphorus and potassium.

It is therefore beneficial for farmers to be aware of their local soil type and adjust fertiliser usage accordingly.

Weather patterns:

The weather can be unpredictable and where possible should be considered. Excessive rain might wash away nutrients, while drought can hinder their uptake.

Areas of the UK that experience high rainfall, such as the West of Scotland or the North West of England and Wales, can face nutrient leaching. This is where essential nutrients are washed away from the topsoil. This can particularly affect mobile nutrients like nitrogen and potassium.

Keeping an eye on the weather forecast and adjusting irrigation and fertiliser application can help mitigate these challenges.


Understanding the critical roles of macronutrients in crop growth and health, while recognising deficiency symptoms helps farmers to proactively manage crop nutrition.

For optimum crop growth, it is essential to ensure a balanced supply of all macronutrients, tailored to the specific needs of the soil and the crop being grown.

Regular soil testing, combined with an understanding of the crop’s nutrient requirements, will put farmers on the path to high yields and healthier produce.

Related articles