A guide to feeding plants

This year as I have been writing 'how to grow' guides for the NMC, I have become increasingly baffled by all the different types of plant feed I was reading about - seaweed, bone meal, high potash, chicken manure pellets and more. I have always used tomato feed on everything I grow but this year have found myself questioning whether it is the right thing to do or whether the many types of feed available are some sort of trickery to get me to buy a lot of products I don't really need. In order to try and de-mystify myself and to help others I have put together this guide. 

All photos by Toni Abram.

Giant yellow sunflowers.

The science bit

All plants need three nutrients in particular.
Most fertilisers contain varying amounts of each of these nutrients. Tomato feed for instance has a high ratio of potassium to supplement the growth of fruit, whereas a general purpose fertiliser will have similar ratios of the three nutrients and smaller amounts of other nutrients that plants need to remain healthy such as calcium (Ca), sulphur (S) and magnesium (Mg), as well as zinc (Zn), iron (Fe), copper (Cu), boron (B) and manganese (Mn). You can see what ratios of nutrients are contained in any feed you buy on the packet.

Bottle of tomato feed showing the nutrients contained in it.

Why is it necessary to feed plants and when to feed them

Plants need feeding if they are:
  • showing signs of nutrient deficiency
  • producing a low amount of flowers or fruit
  • putting on less growth than expected (but appear otherwise healthy and are growing in suitable conditions). 
If you think a plant may be deficient in something, the simple plant deficiency guide below will help you identify what it is deficient in.

Image by
GrowREALFood.

When should you feed your plants

Plants in containers need regular feeding, as they only have the feed that you give them. Plants in beds and borders are able to use the resources found in garden soil and may not need feeding at all.

Feeding is usually done in spring or summer, during the growing season. Few plants need fertiliser in the winter months, even if they are winter flowering, although some organic fertilisers, such as fish blood and bone or poultry manure pellets, are slow to release their nutrients, so should be applied in late winter or early spring in preparation for the growing season.

Plants not to feed

  • Aromatic plants with a mediterranean provenance: these have deep root systems that seek out food and water so are often best left to their own devices
  • British wildflowers: prefer low nutrient soils
  • Leguminous plants (sweet peas, beans and garden peas): these make their own nitrogen through root nodules - if you apply lots of nitrogen you’ll produce lots of leaf at the expense of flowers. 

Do packeted fertilisers work on their own?

Good soil structure is also important for plant health, so fertilisers alone will not improve your plants, as a fertilisers efficiency relies on your soil having plenty of air gaps for root development and water dispersal. Garden compost, well rotted manure (either dug in or used as a mulch that’s pulled down by worms) is also vital for your plants. Garden centres sell sterilised well rotted manure, or you can learn how to make your own garden compost, here on the NMC Gardening Times website.

Make your own feed

If you don't want to buy feed, you can make this also.

Comfrey

You can make potassium feed by soaking comfrey leaves in water for a week to make comfrey tea. Comfrey can be found alongside canals and in town marshes.

Nettles

You can make nettle tea by soaking nettles in a bucket of water for two weeks. The resulting brew is high in nitrogen and other nutrients.  Remember not to add too much nitrogen to fruiting crops though as you may get lots of leaves and not many fruits.
Helpful hint: Want to make a liquid feed go further? Try spraying it on to the leaves of your crops using a hand spray.

Worm compost

Worm compost is rich in vitamins, minerals and  nutrients. It is good for container plants and for adding soil life to new compost.

How much should you feed your plants

How much to feed your plants depends on lots of variables - whether your plant is planted in the ground or a pot, the size of pot, what compost you are using, how big your plant is and how fast it is growing, all needs to be taken into account, together with what you are trying to achieve from feeding your plant e.g. better leaves, bigger flowers or more fruit.

Start slowly, too much feed can be as bad as too little but give it a go, observe the difference and learn from the results. 

Sources and further information

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