Fertiliser Calculator

Fertiliser application rates are specified on the product packaging in grams per square metre (g/m²). Common UK lawn fertilisers recommend 35 to 70 g/m² depending on the product and season. Spring and summer feeds are nitrogen-rich to push growth, while autumn feeds carry more potassium for root strength and disease resistance heading into winter. Reading the label matters more than following any general rule, because the rate moves with the formulation rather than with the job.

The sum itself is area multiplied by application rate. A 100 m² lawn at 40 g/m² needs 4 kg of feed. A 20 kg bag covers 400 to 700 m² of lawn and costs £15 to £40 for general purpose or £25 to £60 for organic, so most gardens need one bag a season at most. Elsewhere the rates change. Hungry crops such as brassicas and leafy greens take 60 to 80 g/m², legumes and root crops need only 25 to 40 g/m², and borders fed with Growmore or fish, blood and bone want 70 to 140 g/m².

This calculator takes your lawn or garden area and the product's recommended application rate to calculate the total weight needed, along with the number of boxes or bags and the coverage they give. For liquid feeds, it converts to litres of diluted solution. Applying too much fertiliser risks scorching the grass or causing excessive soft growth that is vulnerable to disease, so always weigh the product rather than estimating by hand. Water the lawn before applying granular fertiliser and again lightly afterwards to help it reach the roots.

The three numbers on the bag are the percentages of nitrogen, phosphorus and potassium. A high-nitrogen blend such as 24-4-12 is aimed at lawns, promoting leaf growth and green colour. A balanced 10-10-10, or Growmore at 7-7-7, handles general garden use. High-potassium mixes like 4-4-22 go on tomatoes, peppers and fruit to improve flowers and fruiting, while a high-phosphorus 10-30-10 supports root development in root vegetables. Liquid feeds act faster but need reapplying more often, and a slow-release product such as Osmocote lasts 4 to 6 months from a single feed.

Organic feeds work more slowly and improve the soil as much as the plant. Fish, blood and bone is a balanced 5-5-5 at £6 to £15 for 3kg. Chicken manure pellets at 5-3-2 lean towards nitrogen and cost £8 to £20 for 8kg, with a smell that dissipates within 24 to 48 hours. Seaweed extract at £10 to £25 a litre brings trace minerals and acts as a biostimulant, and bone meal at 4-21-0 supplies phosphorus for autumn planting. Garden compost is roughly 1-1-1 and works best as a 50 to 100mm mulch, while comfrey tea makes a free high-potassium liquid feed.

Timing follows the season closely. March and April are for the spring feed, high in nitrogen on lawns and balanced in borders. May and June suit side-dressing vegetables, July and August call for high-potassium feeds on fruit and tomatoes, and heavy nitrogen is best avoided in late summer because the soft growth it encourages will not survive a frost. September and October are the moment for an autumn lawn feed, low in nitrogen and high in potassium, and from November to February nothing is needed because it simply runs off. Never apply to wet leaves or frozen ground.

Brown patches on a lawn are almost always nitrogen burn from feed that was spilled or laid on too thickly. Lush growth with no flowers or fruit points the same way, too much nitrogen set against too little phosphorus and potassium, and an algae bloom in a pond usually means fertiliser has run off into it. Nitrate Vulnerable Zones cover about 58% of England and restrict the timing and quantity of farm fertiliser, though domestic gardens carry no such regulation. A soil test every 2 to 3 years costs £20 to £40 through the RHS and tends to show the same result, that most UK gardens are overfed, especially with nitrogen.