Understanding Fertiliser Labels: What Those Numbers Mean

Walk into any garden centre and you'll find shelves stacked with fertilisers, each bearing a string of numbers like 3:1:5 or 5:1:1. These numbers represent the NPK ratio - the percentage by weight of nitrogen (N), phosphorus (P), and potassium (K) in the product. Understanding these numbers takes the guesswork out of feeding your plants.

Nitrogen promotes lush, green leafy growth. High-nitrogen fertilisers (the first number is highest) are ideal for lawns, leafy vegetables, and plants where you want vigorous foliage. Phosphorus supports root development, flowering, and fruiting. Seedlings and transplants benefit from phosphorus-rich feeds. Potassium strengthens overall plant health, improves disease resistance, and enhances fruit quality. Flowering and fruiting plants need adequate potassium during their productive season.

Different plants have different nutritional needs. Vegetables are heavy feeders and benefit from balanced fertilisers applied every two to four weeks. Succulents and indigenous plants generally prefer low-phosphorus formulations, as many South African native plants have evolved in phosphorus-poor soils and can actually be harmed by excess phosphorus. Always follow the application rates on the label - more is not better and can burn roots or damage soil biology. When in doubt, start with half the recommended dose.