How to Choose a Fertiliser: A Complete Guide to Buying Plant Nutrients

There are dozens of brands and hundreds of different types of fertilisers on the market—in liquid, powder and solid forms, organic and mineral, for different growth stages and growing methods. How can you make sense of them all and avoid choosing the wrong one? We have put together an overview of the basic types of fertilisers, along with practical advice to help you choose the right nutrients for your plants.

Basic types of fertilisers by growing medium

There is a difference between fertilisers for growing in soil, coco, inert growing media or hydroponic systems. Before you buy fertiliser, decide  which growing medium and method you want to use for your plants. You can then choose the right type of nutrients to suit.

• Fertilisers for soil: Suitable for growing in open ground (in beds) or in pots.
• Fertilisers for coco growing media: Growing media made from coconut fibres do not contain nutrients themselves and require special fertilising. Coco growing media are generally used for indoor growing or in greenhouses.
• Fertilisers for hydroponics: Designed for preparing a nutrient solution for soilless systems or for growing in inert media such as rockwool.

You may also be interested in: A guide to growing media

Choosing fertiliser according to the growth stage

From germination to harvest, plants go through several growth stages, and their nutrient requirements change accordingly. During the first few weeks of life, they need little or no fertilising. During the vegetative growth stage, it is important to provide them with enough nitrogen, while during flowering it is advisable to use fertilisers with higher potassium and phosphorus content.

• Starter fertilisers-Start: They contain small amounts of nutrients and often also substances that promote root growth, as well as beneficial microorganisms such as mycorrhizal fungi and soil bacteria.
• Fertilisers for the growth stage-Grow: They contain higher amounts of nitrogen, which plants need to produce leaves and stems. Nitrogen fertilisers can be purely organic, mineral or combined. The nitrogen they contain may be in the form of nitrates (NO₃-) or ammonium (NH₄+), which affects how quickly it is absorbed and influences fluctuations in the pH of the growing medium.
• Flowering fertilisers-Bloom: They contain more phosphorus and potassium to support the formation of flowers and fruit and, like other types of fertilisers, may be organic, mineral or combined.
• Fertilisers for the pre-harvest period-Flush: Particularly when growing medicinal plants, some growers stop using fertilisers several weeks before harvest and replace them with plain water or special flushing fertilisers or finishers. The aim is to improve the aroma and flavour of the resulting product.

You may also be interested in: What are NPK fertilisers and what are they used for?

Organic vs. mineral fertilisers

Although plants do not care where the nutrients absorbed by their roots come from, the difference between organic and mineral fertilisers is significant when growing plants. Organic fertilisers are effective only in the presence of soil microorganisms, so they are suitable only for growing in soil-based (organic) growing media or in beds. Synthetic fertilisers contain mineral salts that plants can absorb directly from water, and they can be used for growing in coco, inert growing media and hydroponic systems.

• Organic fertilisers: Made from natural materials such as compost, manure, bone meal, guano, fish, seaweed or plant extracts. When using organic fertilisers, bear in mind that it takes some time for them to start working.
• Mineral fertilisers: They contain nutrients in the form of mineral salts, which are obtained from phosphates and ammonia, as well as by synthesising nitrogen from the air. They act immediately and carry a risk of over-fertilising. Always follow the feeding charts and measure the pH and EC of your watering solution.

You may also be interested in: Organic vs. synthetic fertilisers: Which are better for your plants?

Fertiliser form: liquid, solid or powder?

Fertilisers come in several forms—from fully liquid solutions to loose powders and solids with slow-release properties. Each form has its advantages; it depends on where and how you grow. Organic, mineral and combined fertilisers can all come in different forms.

• Liquid fertilisers: From a user’s point of view, concentrated fertiliser solutions for preparing watering solutions are the most convenient. They work quickly, are easy to use and have a long shelf life.
• Dry fertilisers: Water-soluble fertilisers in powder or granule form can be used to prepare a nutrient solution or applied directly to the growing medium (if designed for this purpose).
• Slow-release solid fertilisers: Slow-release fertilisers in pellet or granule form are suitable for outdoor growing because they leach from the soil more slowly and supply nutrients to plants gradually. One disadvantage is that they are more difficult to dose.

You may also be interested in: Dry vs. liquid fertilisers: Which are better for growing?

Complete nutrition: multi-part vs. single-component fertilisers

Multi-part fertilisers are commonly used, especially for hydroponic growing. They are mixed in specific proportions to create the ideal nutrient solution for a given situation (for example, more nitrogen during the growth stage or more phosphorus and potassium during flowering). Single-component fertilisers, on the other hand, contain a particular ratio of nutrients, generally for a specific growth stage (veg/flowering).

Nutritional supplements and stimulators

In addition to basic fertilisers, which contain the macro- and micronutrients essential for plant growth, growers can also find a range of nutritional supplements and growth stimulators in specialist shops. These additives are not usually essential for growing, but many growers use them to help their plants reach their full potential.

• Root stimulators: They speed up root formation and are used during germination, propagation from cuttings or when transplanting plants.
• Growth and flowering boosters: They stimulate plants to produce leaves or flowers.
• Vitalisers and enzymes: Various types of additives that protect plants from stress and support microbial life.

You may also be interested in: When and how to use root stimulators

You should now be ready to buy the right fertilisers for your plants. Before you go, though, allow us to share a few final tips.

Avoid combining brands: Different manufacturers use different formulations, which may not be compatible.

Use a pH and EC metre: Measuring the pH and EC of your nutrient solution or growing medium will give you a clearer idea of the nutrient content in the root zone.

Fertilise regularly: Plants benefit from a consistent and stable supply of nutrients. Do not wait until they show signs of nutrient deficiency before fertilising.

Need help choosing fertilisers, or just starting out with growing? You can find useful information for beginners and experienced growers on our Higarden blog, or get in touch with our sales team.