Fertilisers – How and when should we use them ?

Now we all look forward to the heady days of summer when our plants are flourishing, and our gardens are at their peak.

But, the question is, “How do we ensure plants reach their full potential and give an excellent show?”

At this time of year, one aspect to consider helping these plants well on their way is to ensure they have enough nutrients for a good root system, lush foliage together with all important, abundant flowers.  

Soil that is in good condition, rarely suffers from lack of nutrients.  Good condition can be maintained through regular mulching with organic matter, such as composted garden waste (another topic in itself!!!).  This will ensure your soil has a good water holding capacity, whilst not being too wet.  Most often poor growth of plants is due to waterlogging or drought.  Incorporation of organic matter to improve soil condition also means that the nutrients already present in the soil are readily available for the plants. 

However, when plants are grown in containers, need a boost or are suffering from a specific nutrient deficiency, fertilisers can be useful.

When there are so many different fertilisers available to provide these nutrients, it can sometimes feel like a bit of a minefield; deciding what to use, when to use it, how to apply it and how much.

Fertilisers – What are they?

Most fertilisers are based on the three main plant nutrients 

Nitrogen (N) – For leafy green growth

Phosphorus (P) – For healthy shoot and root growth

Potassium (K) – For flowering, fruiting and general hardiness

Product packaging gives the ratios which are quoted in N:P:K

A balance fertiliser for example will have a ratio of 20:20:20 whereas a fertiliser high in potassium may have a ratio of 10:12:24.

How to choose

Fertilisers can be available in organic form derived from plant and animal sources or inorganic which are synthetic, artificial forms of plant nutrients or naturally occurring mined minerals.

Both can be found as:

  • Compound fertilisers - containing a mixture of two or more nutrients
  • Straight fertilisers – contain only one or mainly one nutrient
  • Controlled release fertilisers – usually granules of inorganic fertiliser coated with a porous material that allows the nutrients to leach into the soil. Tends to release nutrients more slowly in cooler weather and faster in warmer weather.
  • Slow release fertilisers – they break down slowly, relying on soil organisms to assist in the release of nutrients into the soil. Usually organic.

Your choice will depend on the plant you wish to feed. A straight fertiliser may be ideal for treating a plant showing symptoms of nutrient deficiency. Whereas a controlled release fertiliser could be the perfect treatment for a plant in a pot where the nutrients in the compost have been exhausted.

How to Apply and when

The product packaging will suggest how much to use and the appropriate method of application.  Application methods may include:

  • Top dressing – usually carried out in spring at the beginning of the growing season, an application to the soil surface around the plant. Avoid leaf contact and over application.  Beware, twice as much as recommended will not lead to twice as much growth but may harm the plant!
  • Base dressing – Mixing fertiliser with soil or compost before planting, often appropriate for planting shrubs or trees
  • Watering on – Liquid or soluble granules mixed with water, perfect for annuals in pots or hanging baskets throughout the growing season
  • Foliar feeds – Application of diluted fertiliser directly to the leaves as and when required. Young leaves allow for quick and effective absorption.  Be careful not to apply in direct sunlight to avoid scorching of leaves.

Hopefully this simplifies the wealth of information available about fertilisers and helps you on your way to a beautifully floriferous and productive garden!!!

Further information about mulching and fertiliser is available from the Royal Horticultural Society website.