🧪 Fertilizer Calculators

Understanding NPK: Nitrogen, Phosphorus, and Potassium

Every fertiliser bag displays three numbers separated by hyphens — the NPK ratio. These represent the percentage by weight of nitrogen (N), phosphorus (P₂O₅), and potassium (K₂O) in the product. A 10-5-5 fertiliser contains 10% nitrogen, 5% phosphorus pentoxide, and 5% potassium oxide. The remaining 80% is filler material (carrier) that aids even application. Nitrogen drives leafy, vegetative growth and gives plants their dark green colour — it is the nutrient most rapidly depleted from soil. Phosphorus supports root development, flowering, and fruiting. Potassium (potash) strengthens cell walls, improves disease resistance, and regulates water uptake. A high-N fertiliser suits lawns and leafy vegetables; a high-P formula suits flowering plants and root crops; a balanced formula suits general-purpose garden use.

How Much Fertiliser to Apply

Fertiliser rates are typically expressed as kilograms of actual nitrogen per 100 m² (or pounds per 1,000 sq ft). To calculate how much of a specific product to apply, divide the desired nitrogen rate by the product's nitrogen percentage. For example, to apply 3 kg of nitrogen per 100 m² using a 15% N fertiliser: 3 ÷ 0.15 = 20 kg of fertiliser per 100 m². Over-application is a common and costly mistake — excess nitrogen leaches into groundwater and can burn plant roots through osmotic stress (fertiliser burn). Always follow the manufacturer's recommended rate and split applications across the season rather than applying the full annual dose at once.

Organic vs. Synthetic Fertilisers

Synthetic fertilisers deliver nutrients in immediately plant-available forms (ammonium nitrate, urea, superphosphate) and act quickly, but they do not improve soil structure and can acidify soil over time. Organic fertilisers (compost, bone meal, blood meal, fish emulsion) release nutrients slowly as soil microbes break them down, improving soil biology and structure in the process. The nutrient content of organic products is lower and more variable than synthetics — compost typically runs 1-0.5-1 NPK. Slow-release granular fertilisers (polymer-coated urea) bridge the gap, providing steady release over 3–6 months. For lawn and vegetable garden maintenance, a combination approach — building soil health with compost and supplementing with targeted synthetic feeds — gives the best results.

Soil pH and Nutrient Availability

Fertiliser effectiveness depends heavily on soil pH, which affects which nutrients are in plant-available forms. At pH 6.0–7.0, all major and most minor nutrients are well available — this is the target range for most vegetables and lawns. Below pH 6.0 (acidic soil), phosphorus becomes locked up as iron and aluminium phosphates; manganese and aluminium become soluble at toxic levels. Above pH 7.5 (alkaline soil), iron, manganese, boron, copper, and zinc become unavailable, causing deficiency symptoms even when present in the soil. Lime (calcium carbonate) raises pH; sulphur or acidifying fertilisers lower it. Always test soil pH before adjusting — a £5 pH test kit can save significantly on unnecessary amendments and explain persistent nutrient deficiency symptoms.