How do I read a fertiliser label?

Every fertiliser in the shop tells you exactly what it is on the back, in a format that has been standard for decades, and almost nobody reads it. Five minutes with the label distinguishes a product that will do what you want from an identical-looking one that will not, and it exposes the ones that are mostly water sold in a large bottle.

Safety

Step by step

1. Find the three numbers.Written as N-P-K, for example 7-7-7 or 4-3-8 or 20-5-10. They are percentages by weight: a 7-7-7 product is 7 per cent nitrogen, 7 per cent phosphate and 7 per cent potash. They do not add up to 100 — the rest is carrier, sulphur, calcium, water and the compounds the nutrients are bound into.
2. Know what each element drives.Nitrogen: leaf, stem and green growth. Phosphate: roots, establishment and early growth. Potash: flowers, fruit, disease resistance and ripening wood before winter. A high-nitrogen feed on a tomato gives you a magnificent plant and very few tomatoes.
3. Read the ratio, not the size.A 4-4-4 and a 12-12-12 are the same balanced fertiliser; the second is three times as concentrated so you use a third as much. What changes the effect on the plant is the proportion between the numbers. Anything roughly equal is 'balanced'; a much larger last number is a high-potash feed.
4. Look at the trace element list.Below the main three, usually in much smaller figures: magnesium, iron, manganese, boron, zinc, copper, molybdenum. A feed for acid-lovers or a good tomato feed will list chelated iron, which stays available at higher pH. A cheap general feed often lists nothing, which is fine in decent soil and a limitation in a container.
5. Find the application rate — this is the number that governs everything.For a dry feed, grams per square metre. For a liquid, a dilution such as millilitres per litre, or a capful per watering can. This is the only number that tells you how much to use, and it is where the difference between two products with the same NPK shows up.
6. Work out the cost per treated area.Pack size divided by the application rate gives the area or the number of cans it covers. Then divide the price by that. This is the only honest comparison, and it routinely reverses the apparent ranking on the shelf — concentrated products in small bottles are frequently better value than large bottles of dilute feed.
7. Check the form, because the same analysis behaves differently.Soluble powder and liquid concentrate: available immediately, gone within a week or two, leaches readily. Uncoated granules: dissolve over a few weeks. Coated controlled-release granules: meter themselves over three to six months, faster when warm and wet. Organic pellets and meals: released by soil biology over weeks to months, slower and less predictable.
8. Check whether it is organic or mineral.The label usually says. Mineral products deliver nutrients in a form the plant can use immediately. Organic products — bone meal, blood, fish, seaweed, poultry manure — have to be broken down by soil microbes first, which means they are slower, work poorly in cold soil, and are almost impossible to overdose to the point of scorch.
9. Note anything that changes where you can use it.Ericaceous feeds are acidifying and contain chelated iron. Lawn feeds are high in nitrogen and often contain a herbicide. Anything containing a weedkiller must not go on beds, and the label will say so prominently.

Tips

• Phosphate is expressed as P₂O₅ and potash as K₂O rather than as the elements themselves. That is a convention rather than a trick, and it does not change how you use the numbers — but it does mean the figures are not directly comparable to a soil test that reports elemental P and K.
• The analysis is a guaranteed minimum rather than an exact figure. Products generally contain at least what they say.
• 'Feeds up to 200 plants' on a bottle is a marketing figure with no standard behind it. The dilution rate and the bottle size are the real information.
• Tomato feed is simply a widely available high-potash liquid, usually around 4-3-8. It is not tomato-specific and it is the single most useful bottle in a garden.
• Slow-release granules mixed into compost at planting and a weekly liquid feed later in the season is what commercial growers do, and it is why their containers look better than yours in August.

Questions people ask