Plants need

To grow well, plants need a wide range of nutrients in various amounts, depending on the individual plant and its stage of growth. The three key plant nutrients usually derived from soil are nitrogen, phosphorus and potassium, while carbon, oxygen and hydrogen are absorbed from the air. Other vital soil nutrients include magnesium, calcium and sulphur. Gardeners can add nutrients by applying fertilisers (either artificial or naturally derived) to boost plant growth and improve flowering and fruiting.

• nitrogen (N) – for the production of
  Chlorophyll is a natural green pigment responsible for capturing light energy from the sun and converting it into chemical energy through the process of photosynthesis. It is found inside the cells of leaves and stems in structures called chloroplasts.

  chlorophyll (for photosynthesis) 
• phosphorus (P) – for root development 
• potassium (K) – for reproduction 

On fertiliser packaging, the ratio of these nutrients is always written as N:P:K, which you can think of as shoots: roots: fruits.

If you look at the label on a tomato fertiliser, you’ll see it contains a larger proportion of potassium (K), as this boosts flowering and fruiting.

• macronutrients such as N, P and K, which plants need in larger amounts  
• micronutrients or trace elements, such as iron (Fe), which are only needed in small amounts 

All the nutrients a plant needs can be found in the soil. Soil composition varies depending on factors such as the rock it’s formed from and the amount of carbon-containing organic matter present. As a general rule, sandy soil is lower in nutrients than clay soil. 

The way we grow plants in a garden breaks the natural nutrient recycling process, as we don’t generally leave plants to die and rot down into the soil. So, we make up for this nutrient shortfall by adding organic matter and fertiliser.

Organic matter is lower in nutrients than a man-made fertiliser, but it has wider soil benefits, such as improving moisture retention and drainage, and boosting soil micro-organisms. 

Roots explore the soil, seeking out water and mineral nutrients. They make dense networks and have a large absorbent surface area due to thousands of root hairs just behind their tips. Damage to these delicate root hairs hampers a plant’s ability to take up water and dissolved nutrients.

To extend their reach, plant roots have a symbiotic (mutually beneficial) relationship with mycorrhizal fungi. These webs of beneficial fungi live within the plant or in the soil. They help roots take up mineral nutrients more efficiently, acting as an extension to the root network. Heavily fertilised or manured soils support fewer of these fungi, as plants have less need for them. Applying fungicides to plants also lowers their numbers.

Thousands of hairs give roots a fuzzy appearance

The toadstools of fly agaric (Amanita muscria), a mycorrhizal fungus of birch trees

Trees and shrubs have surprisingly shallow but wide root systems, as they usually seek food and water in the upper, fertile layer of soil (top soil). So, when planting, make the hole only slightly deeper than the rootball, but three times as wide – see our guide to planting trees.

Plants in containers have a limited volume of soil for their roots to grow in. They need more feeding and watering than those in the open ground, as the extent of their root network and its ability to support the plant is restricted.

Soil minerals need to be soluble – dissolvable in water – so they can be absorbed by roots and transported around a plant to the cells that need them. If the soil is too dry, mineral nutrients may be present, but can’t be taken up by the plant as there’s not enough water to transport them.

As well as needing nutrients to be soluble, plants also need minerals to be in simple molecular forms. Luckily, microorganisms in the soil form a food web that helps to break down complex molecules. These organisms consume nitrogen, for example, and when they die or excrete, they release it back into the soil as nitrates, a form that plants can take up.  

Caring for your soil underpins your plants’ health. Organic matter (such as spent mushroom compost) supports soil microorganisms, which make nutrients available to plant roots. 

Liquid feeds can be seen as a first-aid treatment for a poorly plant, as the nutrients are already dissolved and in a form that plants can use quickly. The compounds in organic fertilisers (such as blood, fish and bonemeal), on the other hand, need to be broken down by soil organisms before they can be used. As these take longer to be absorbed, they are considered slow-release feeds.

Preparing a liquid fertiliser

Applying a slow-release fertiliser

Once mineral nutrients are dissolved in soil water, they move into root cells by osmosis – the natural movement of water molecules from an area of high concentration to an area of low concentration.  Sap - which is the dilute solution of mineral nutrients in water – moves across root tissue from cell to cell and up through xylem vessels (the pipework in plant stems). These mineral nutrients are then delivered to plant tissues for processing. 

Sap rises in late winter and early spring to deliver nutrients to buds in preparation for the new growing season. If pruned at that time of year, some plants (such as birches) can bleed sap heavily, losing all the goodness they’re pumping up to the branches.

It’s not just roots that can absorb nutrients – leaves can too. Foliar feeds are specially formulated liquid fertilisers that are sprayed directly onto leaves. They’re a useful way of applying micronutrients, and seaweed feeds are an especially rich source.

Plants need nutrients when they’re actively growing. The ‘growing season’ is the period when the light and temperature range is suitable for growth – in the UK this is generally spring to early autumn.

Spring is the start of the growing season in the UK

Plenty of plants get all the nutrients they need from the soil, so fertilising is often unnecessary. However, even if your soil contains all the necessary nutrients, they may be unavailable for plants to take up if the soil is dry or the nutrients are 'locked up'. 

Common examples of this include:

• Ericaceous (acid-loving) plants being unable to absorb enough nitrogen, iron and manganese for photosynthesis in an alkaline (chalky) soil
• Plants being overfed potassium (for example by too regular applications of liquid tomato fertiliser) and their roots prioritising its uptake over magnesium, resulting in a deficiency of the latter

Deficiency symptoms on rhododendron

Magnesium deficiency on a tomato leaf

The good news is that most plants recovery quickly if they are fed the correct fertiliser. Testing your soil can help you learn more about your soil's fertility and take actions to improve it, so you don't see deficiencies reocurring. 

Now you know more about how plants absorb nutrients, put this into practice to help your plants thrive:

• Don't feed plants if you don't need to - most plants can find all the nutrients they need in the soil. Generally only edible crops, container plants and those that flower prolifically, like summer bedding, need regular feeding. For poorly plants, a one-off feed with the correct fertiliser is often enough. See our guide to fertilisers
• Improve soils by digging in or mulching with organic matter each spring as the growing season starts. This not only boosts fertility but populations of soil microorganisms too. See our guide to using organic matter
• Take care when handling young plants and cover the rootballs of bare-root specimens to avoid damaging or dessicating the delicate root hairs responsible for water and nutrient absorption
• Keep containers well-watered during dry spells. Plants in pots are more susceptible to nutrient shortage as a result of drought, owing to their limited root run. See our guide to watering
• Look-up the correct pruning time for plants that are prone to bleeding to avoid wasting valuable sap. See our guide to bleeding from pruning cuts