What are NPK fertilisers and what are they used for

For beginning gardeners and growers, it can be difficult to choose the right fertiliser range from the seemingly confusing selection on shop shelves. Unsuitable plant nutrition can often jeopardise all your growing efforts. We will explain how NPK fertilisers work, what information you can find on the label, and how to choose the right product for your grow room, greenhouse or garden.

When growing plants, most gardeners use fertilisers to supply their plants with nutrients. This is because in most gardening methods, from indoor cultivation under artificial lighting to outdoor gardens, the substrate becomes depleted after a certain time and no longer contains enough nutrients for healthy plant development.

What are NPK nutrients

All plants need a total of 17 nutrient elements to live, and apart from three of them (oxygen, carbon and hydrogen), they obtain all of them from the soil, growing medium or nutrient solution. Roots absorb 14 elements in total, which we further divide according to how important they are from the point of view of plant nutrition.

• Basic (macrobiogenic) nutrients (nitrogen, phosphorus, potassium, sulphur, calcium, magnesium) are consumed by plants in the largest quantities and their deficiency causes serious growth disorders or death.
• Micronutrients (chlorine, iron, boron, manganese, zinc, copper, molybdenum, cobalt) are required only in minimal amounts, but a lack of micronutrients can also cause major damage to plants.

In practice, we most often encounter the need to supply plants regularly with three basic nutrients through fertilisers – nitrogen (N), phosphorus (P) and potassium (K).

The importance of NPK for plant development and health

The consumption of NPK nutrients varies in plants depending on their age, species, climatic conditions and cultivation method. You should take all the above-mentioned factors into account before you start fertilising. It is important to use fertilisers that contain NPK nutrients in the appropriate amount and ratio. If you do not, your plants may either suffer from nutrient deficiencies or you may over-fertilise them and literally burn them with an excess of mineral salts in the substrate.

• Nitrogen (N): Plants need nitrogen especially when forming chlorophyll, the green pigment essential for the process of photosynthesis, but it is also part of plant amino acids and proteins. A nitrogen deficiency is shown by yellowing leaves and slowed growth. Plants absorb nitrogen best in the form of nitrates dissolved in water. Recommended fertilisers for supplying nitrogen are organic fish emulsion or fertilisers for vegetative growth.
• Phosphorus (P): Phosphorus is important for the reproductive phases of plants and the formation of cell structures. A phosphorus deficiency is shown by darkening leaves and smaller flowers. Plants absorb it best in the form of inorganic phosphates dissolved in water. Recommended fertilisers for supplying phosphorus are bat guano and fertilisers designed to support flowering.
• Potassium (K): Potassium is key to flower formation and increases plant resistance to disease. A potassium deficiency is shown by burnt leaf edges and slowed growth. Plants absorb it best in the form of potassium salts dissolved in water. Recommended fertilisers are fertilisers for supporting flowering, PK boosters and palm ash.

Choosing an NPK fertiliser

On the packaging of every fertiliser you will find several basic pieces of information that will help you determine exactly what the formula is suitable for and what it contains.

• A trio of numbers separated by hyphens (for example 2-1-2) indicates the content of the basic nutrients nitrogen (N), phosphorus (P) and potassium (K). The order of the elements is fixed (N-P-K) and the numbers represent the percentage share of the given element in the total volume of the fertiliser.
• A detailed chemical analysis usually specifies the product composition more closely, lists other nutrient elements in the fertiliser and states their minimum content in the product.
• A list of the salts from which the nutrients contained in the fertiliser were obtained. It contains useful information for those who want to combine fertilisers from different manufacturers. Some forms of mineral salts cannot simply be mixed without precipitating. This is also why some fertilisers have multiple components in separate bottles.
• Dosage tables tell you how often and in what quantity to apply the fertiliser. If you are just starting to grow, it is best to follow the manufacturer’s instructions as closely as possible and not mix different brands.

Also read: Which are better for indoor growing, dry or liquid fertilisers?

How to choose the right NPK fertiliser according to the growth stage

Annual plants go through two basic phases during the growing season: vegetative (growth) and generative (flowering). As they gradually move from one to the other, their nutritional requirements change. In general, during the first part of life in vegetative growth, when plants form new roots, leaves and stems, they need the most nitrogen. During flowering, their phosphorus consumption then increases. Potassium is usually absorbed from the soil throughout the entire vegetative cycle in balanced amounts.

Fertiliser manufacturers include dosage tables with their products, which tell you exactly when and in what amount to fertilise. However, these are only recommendations, and an experienced grower should know the other factors affecting nutrient consumption, such as the growing medium used, temperature and other climatic conditions, the method and frequency of watering, or the specifics of the cultivar.

• Fertilisers for vegetative growth
• Fertilisers for the flowering phase
• Fertilisers for growth and flowering

Also read: Growth fertilisers: What are they for and how to use them