Correct plant nutrition is essential to the health and longevity of your houseplants.  If you look in gardening books, or consult the web sites of a lot of garden writers, you might think that plant nutrition was on a par with the dark arts as far as mystery and complexity is concerned.  However, it really is not that complicated - especially for indoor plants - and we have some very simple guidelines to help keep your plants healthy and attractive.

Unlike pesticides, there are few regulations relating to the use of fertilizers.  However, handling concentrated fertilizers can pose some hazards – some are irritating to the skin or eyes, so make sure that you use any recommended PPE and ensure that you follow any health and safety recommendations on the product label.

A brief history of plant nutrition

Since ancient times, farmers have known that plants grew better when the soil they were grown in was treated with additives such as manure, or when crops were rotated to improve the fertility of the land. The ancient Chinese knew of the value of green manures and the great Roman agricultural writers also recommended the use of manures and crop rotation to improve the fertility of soils.  

However, no-one knew why these practices worked.

The middle part of the nineteenth century saw the beginnings of scientific investigations to understand what plants need to grow and the best ways of giving them nutrients.  This coincided with the development and use of manufactured fertilizers, formulated with known quantities of plant nutrients. 

The most important research in this area was started by two men with huge beards -  Sir John Bennet Lawes (1) and his business partner Sir Joseph Henry Gilbert (2) -  at Rothamsted, in Hertfordshire. Their early experiments, which started in 1843, are still running, and are the longest running continuous scientific investigations in the world.

What is fertilizer?

Fertilizers shouldn’t really be considered as food (even though we talk about feeding, which is a bit confusing).  This is because plants are not green animals, and the way that plants work is not analogous to the way that animals operate.  Fertilizers provide the simple chemicals that plants need to drive their physiological and biochemical processes – with the rare exception of some carnivorous plants, there is no digestive system.

Potting composts are pretty inert and have very little innate nutrition, although most have some fertilizers added to them when they are manufactured.  All the nutrients that your plants need have to be delivered through the addition of fertilizers.

Indoor plants are sometimes wrongly fertilized – usually too much, but sometimes not enough.  Remember, it is always easier to correct under use of fertilizer than over use.

What is in fertilizer?

Fertilizers contain two categories of plant nutrients.

Macronutrients are the elements that are required in significant quantities.  These are used to build proteins in plant tissues.

The essential macronutrients are:

  • Nitrogen (N), for healthy green leaves and stems;
  • Phosphorus (P), for healthy root growth;
  • Potassium (K), for better flowering and colour

A good fertilizer will contain a balanced ratio of these elements, as well as essential micro-nutrients, or trace elements.  

All fertilizers show an N:P:K ratio on their label.  This shows the concentrations of each of the macro nutrients. For houseplants, look for a range of concentrations between 3% and 10% for each of the main nutrients (e.g. 5:5:5 as shown here, or 9:3:6).

Micronutrients, also called trace elements, are required in minuscule quantities (trace amounts), but are essential for the healthy functioning of many physiological and biochemical processes in the plant. Many micronutrients are toxic to plants at high doses.

Good fertilizers will contain micronutrients such as

  • Magnesium (Mg) 
  • Iron (Fe)
  • Manganese (Mn)
  • Copper (Cu)
  • Boron (B)
  • Molybdenum (Mo)

For the nutrients to get into the plant, they must be water soluble.  The nutrients are dissolved in soil water and taken up by the plant roots.  Too much fertilizer, and the soil water becomes saline and the root tissues die.  This means that even if there is enough water, the roots cannot take it in and the plant suffers from wilting and drought symptoms.

In garden soil, its acidity and the presence of minerals such as lime or clay can have a dramatic impact on the availability of some nutrients. Some fertilizer components can be chemically tightly bound to minerals in the soil making them totally unavailable to the plant.  Indoor plants are grown in special potting compost, and all foli8 plants are supplied planted in a high quality compost.

How much fertilizer should I use?

Houseplants also do not usually need much fertilizer.  This is due to relatively low light levels and dry atmospheres found indoors.  Also, new plants probably already have enough fertilizer in the compost to sustain a plant for several months. Thereafter they will require supplementary feeds to maintain health.

The amount of fertilizer needed by a plant is dictated by a number of factors, such as size, species and the environment.  

You might think that the natural origins of a plant might give some indication of its fertilizer requirements, but in most circumstances you can forget about this. House plants are not in their natural surroundings, so do not have the same requirements.  For most houseplants, whether it comes from jungles of Peru or the African savannahs is pretty much irrelevant as far as fertilizer use is concerned.

The indoor environment is, however, important.  If the environment encourages active growth, then more fertilizer will be needed.  However, there is almost a direct relationship between active growth environments and water requirements, which makes most fertilizer applications very easy.

As a rule of thumb, water-use and fertilizer-use are usually directly proportionate.  The more water the plant uses, the more fertilizer is required.  This makes fertilizing easy when the fertilizer is added to the water – the more water you apply, the more fertilizer you will apply, meeting the plants watering and fertilizing needs simultaneously. 

You may wish to use a certified organic product.  These may be more expensive than conventional products, but they may also be more environmentally sustainable.  Nutritionally, however, there is no difference – a nitrate ion is the same whether it got to the plant via an animal’s digestive system, a clover root nodule or a fertilizer factory in Germany.

Two simple method of giving the right amount of fertilizer

Here are two methods of giving fertilizer that are tried and tested on millions of indoor plants each year all around the world by commercial interior plant companies.  You can’t use both methods on the same plants, so choose whichever suits your preferences, and stick with it - your life will be easier and your plants will reward you with excellent health.

Method 1: give your houseplants a small dose of a dilute liquid fertilizer every time you water your plant. This means that you don’t have to remember the last time your plants were given fertilizer.

This method of feeding guarantees that fertilizer is given at a rate the plant requires throughout the year. During more active periods of growth, plants will normally require more water and fertilizer, so if the fertilizer is given with the water, any increase in water will automatically give the plants an increase in fertilizer.

Another benefit of this method of giving fertilizer is that it will fit in with your preferred watering schedule. If you water plants frequently, you will tend to give less at each watering than if you water less frequently. Give the plants the water they need and the fertilizer will be given at just the right rate every time.

My preference is to use a balanced concentrated liquid fertilizer - many are available from garden centres, on-line or even from supermarkets.  Look for a balanced ratio of nutrients and only add a tiny amount.  If your fertilizer is mainly used for garden plants (which are actively growing and are in pots with drainage holes), use it at about 10% of the recommended rate.  

If you are using a fertilizer formulated especially for houseplants (usually recommended for use only in spring and summer), then I would use it at about 25% of the recommended rate.

Remember, some plants are more sensitive to excess fertilizers than others (e.g. Dracaenas) and it is always possible to add a little extra if you think your plants are under fed (if the leaves look a little washed out, for example).

Method 2: feed plants once a year in spring 

This method gives plants a measured amount of controlled release fertilizer ONCE A YEAR. This means giving your plants fertilizer in the spring of every year and at no other time.  The first weekend in April is a good time. 

Controlled release fertilizers come as small round pellets, or tablets (made up of those pellets glued together).  They must be buried in the soil to the depth of the roots, not scattered on the surface. This is essential because the fertilizer only works in the presence of moisture and the soil in the root zone should be permanently moist (not soaking wet, though).  Controlled release fertilizer tablets (or granules) left too near the surface will not release their nutrients evenly, and not at all if you water from the bottom up.

The easiest way to get the fertilizer to the depth you require is to use a stick to make a hole the right depth, put the fertilizer in the hole and use the probe to push it down into the soil.

There is no need to use controlled release fertilizer on plants that you have bought after April until the following year, as there will be enough residual fertilizer in the soil to last until the following spring.

Small plants need only 2g a year, and the large plants we provide will manage with approx 5g - 7g a year.  Look for macronutrient concentrations between 8% and 16%, e.g. 15:10:12.

Muck, magic and muddy water

Over the many years that I have been working with indoor plants, I have been sent samples of all sorts of plant feeds and supplements to test.  Almost without exception, they have proved to be pointless.

Always use a properly-formulated fertilizer with a good balance of nutrients.  Don’t be tempted by mystery potions of “hormones”, “natural extracts” and “stimulants” no matter how convincing the labels – sometimes these are no more effective than a dose of muddy water.

by Kenneth Freeman
11 March 2021

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