Dry or liquid fertilisers, which are better for indoor growing?

Purely in theory, it makes no difference to our plants whether we supply nutrients in liquid, powder or granule form. However, this only applies if the amount and concentration of nutrients in the substrate or nutrient solution correspond to their requirements. Nevertheless, each type of fertiliser has its advantages and disadvantages. Let us look at what they are together.

The choice of the right type of fertiliser will always depend first and foremost on the feeding method of cultivation and the feeding programme you have chosen. In hydroponic and soilless cultivation, we use exclusively water-soluble fertilisers. These are suitable for plants placed in inert (soilless) substrates such as rockwool, perlite, expanded clay or vermiculite. The term “fertigation” has become established for this type of plant nutrition, arising from the combination of the words “fertilisation” and “irrigation”. In this cultivation method, plants obtain nutrients directly from the nutrient solution and are fertilised with every watering, while the hydroponic substrate itself contains no nutrients and serves only to anchor the plants and retain the nutrient solution around the roots. In some cases, the roots float submerged in a nutrient solution that is continuously oxygenated.

On the other hand, when cultivating plants in classic growing substrates (that is, those containing peat, tree bark, coconut fibres or compost), gardeners have far more options. Soil mixes have a greater ability to retain nutrients than hydroponic substrates, and therefore solid slow-release fertilisers, including organic variants, can be used. However, in highly intensive indoor cultivation it is better to use both types of fertiliser, because some nutrient elements such as nitrogen, phosphorus or potassium can be depleted from the soil very quickly.

Water-soluble fertilisers

Water-soluble fertilisers can be either liquid or dry (water-soluble powder), and they can be supplied to plants using hydroponic or irrigation growing systems. Water-soluble fertilisers may contain only one type of mineral salt (for example calcium nitrate or potassium phosphate), but more often they are mixtures of different mineral salts that contain several nutrient elements at once. The composition, concentration and method of preparing the nutrient solution then vary according to the specific feeding programme.

The feeding programme Green House Feeding includes a wide range of water-soluble mineral fertilisers and nutrition supplements in powder form, including specialised mixes for growing hybrids, varieties with a short flowering period and long-flowering cultivars. Nutrient solutions prepared from Green House Feeding fertilisers retain EC and pH for at least one week, and highly concentrated nutrient solutions can be stored for several months.

When using water-soluble fertilisers containing mineral salts, we must bear in mind that some mineral salts can react with one another and form precipitates, which not only reduce the effectiveness of the nutrient solution but also clog the nozzles and piping of irrigation systems. This most often happens when highly concentrated water-soluble fertilisers containing lime salts (for example calcium nitrate) are combined with those containing phosphates or sulphates. This is one of the reasons why manufacturers of these fertilisers often divide their formulations into two or three components. In general, the easiest way to avoid this problem is not to combine fertilisers from different feeding programmes and different brands.

Three-part fertilisers Terra Aquatica TriPart are among the most sought-after by professional growers, including prestigious scientific institutions and NASA. A huge advantage of this range is that all the nutrients needed throughout the plant life cycle are found in just three bottles. The TriPart formulation contains all primary and secondary nutrients and trace elements, and guarantees reliable results when growing specialised crops where taste, aroma and the production of essential oils and active compounds are important. In addition, the manufacturer offers a version for hard and soft water.

Many indoor growers prefer ready-made liquid fertilisers over powder ones because they are easy to measure and preparing the nutrient solution is simple. However, liquid fertiliser blends tend to be much more expensive than dry variants. Various factors play a role here, including transport and storage costs. In the end, this means that with dry water-soluble fertilisers you can prepare the same amount of nutrient solution at a significantly lower cost than with liquid ones.

Advantages of dry water-soluble fertilisers

• they reduce the cost of preparing nutrient solution
• they are easy to store
• longer shelf life

Solid slow-release fertilisers

Solid fertilisers can take the form of pellets, granules, tablets or powder and, unlike water-soluble ones, they cannot be used in hydroponics or for preparing nutrient solution, but are used only when growing in soil or horticultural substrates. These fertilisers are either mixed directly into the soil (when transplanting plants or with pre-fertilised growing soil mixes) or can be applied to the surface of the substrate.

Plagron Royalmix is a pre-fertilised horticultural substrate that contains fertiliser for approximately 6 weeks of plant life. Strongly pre-fertilised substrates are suitable only for older plants with a developed root system. For young cuttings or seedlings, such a substrate may be too “hot” and the plants could be burnt by fertiliser.

The main advantage of solid fertilisers is the possibility of gradual nutrient release into the soil. In the case of nutrient solution, nutrient elements are available to plants immediately after watering and only for a short time. By contrast, some solid slow-acting fertilisers can supply plants with nutrients for weeks or even months.

The basis of the feeding programme and range of organic fertilisers BioTabs are slow-release tablets that supply plants with nutrients and other beneficial substances. The basic tablets are then supplemented as needed with liquid and dry nutrition supplements, including organic PK boosters or compost teas.

Solid slow-release fertilisers allow the gardener to cover most of the plant’s nutrient needs already at planting time and save both time and money. This makes it possible to grow plants in a more ecological and sustainable way. On the other hand, when indoor growing nutrient-demanding plant species under powerful artificial lighting, it is often better to combine solid and liquid fertilisers. In some growth stages, plants may need a rapid supply of nutrients.

Fertilisers from the Dutch manufacturer Guanokalong are based on the use of fertile substrates and organic soil amendments in both liquid and solid form. This feeding programme is excellent for advanced indoor and outdoor growers, including those growing by the “living soil” method. You can learn more about Guanokalong in our article.

Advantages of solid slow-release fertilisers

• gradual and long-term nutrient release
• easy growing without the need to prepare nutrient solution
• the possibility of fully organic growing
• easy storage and shelf life