Almost every fertilizer is built around three nutrients that plants need in large amounts: nitrogen, phosphorus and potassium. Those are the three numbers printed on every bag, known as N-P-K. A smaller group of nutrients and a handful of trace metals fill out the recipe, and many soils already supply them. What a plant actually gets depends as much on a nutrient’s form and how fast it dissolves as on how much is in the bag.
How to Read the Three Numbers on a Fertilizer Bag
A label reading 18-4-10 means the bag is 18% nitrogen, 4% phosphorus and 10% potassium by weight.1Home & Garden Information Center. Reading A Fertilizer Label There’s a catch in the wording. The phosphorus figure is counted as phosphate and the potassium figure as potash (the common name for potassium fertilizer), not as the pure elements.2Home & Garden Information Center. Reading A Fertilizer Label So a 50-pound bag of 18-4-10 holds 9 pounds of nitrogen, 2 of phosphate and 5 of potash. The remaining weight is other nutrients, carrier material and filler that help spread the product evenly.
US labels use this compound method, while some other countries list pure elements.3Botanicare. What Makes Fertilizer? NPK and Key Ingredients To get pure phosphorus, multiply the middle number by 0.43, and for pure potassium multiply the last number by 0.83.4Botanicare. What Makes Fertilizer? NPK and Key Ingredients That means the 4 in 18-4-10 is under 2% actual phosphorus. A zero in a slot means that nutrient is absent, which is common in lawn products where phosphorus is restricted. A higher-number bag isn’t better, only more concentrated, so you use less of it. Equal numbers like 10-10-10 mean equal amounts of each nutrient, which isn’t necessarily what your soil needs.
To compare value fairly, first turn the first number into pounds of nitrogen: multiply the bag weight by the nitrogen percentage, such as 40 pounds times 0.20 for 8 pounds in a 20-percent bag. Then divide the price by those pounds to get a cost per pound of nitrogen. A cheap bag with low numbers can end up costing more per pound than a pricier, more concentrated one, so the sticker price alone misleads.
Nitrogen Feeds Leaves and Green Growth
Nitrogen is part of chlorophyll, the green pigment that captures light, as well as proteins, enzymes and DNA.5Cornell University. Competency Area 1: Basic Concepts of Plant Nutrition A plant with enough nitrogen grows fast and produces plenty of lush green leaves.6Mosaic Crop Nutrition. Nitrogen7Mosaic Crop Nutrition. Nitrogen
A nitrogen-starved plant stays small, grows slowly and turns pale green or yellowish from lack of the green pigment.8Mosaic Crop Nutrition. Nitrogen Too much does the opposite: leaves turn extremely dark green and the plant pushes leafy growth.9Cornell University. Competency Area 1: Basic Concepts of Plant Nutrition Tomatoes or peppers given heavy nitrogen can end up all leaves and few fruits.
In a home garden, a shortage often starts on the older, lower leaves, because the plant moves nitrogen to new growth. Those leaves fade while the top stays greener. Yellow spots, yellow edges or yellowing between veins usually point to a different cause. Waterlogged soil or damaged roots can also cause yellowing, so check watering before assuming the plant needs feeding. Lawns show a shortage as a dull, light green, and they usually green up within a couple of weeks after feeding.
High-nitrogen fertilizer suits lawns, leafy greens and young plants best. Once flowering or fruiting plants start setting buds, switch to something lower.
Phosphorus Powers Roots and Energy
Phosphorus is a building block of DNA, cell membranes and ATP, the molecule cells use to carry energy.10PubMed Central. Root architecture responses: in search of phosphate It moves very little through soil, which can limit growth even when the soil holds some.11PubMed Central. Root architecture responses: in search of phosphate When phosphorus is short, roots spread through the top layer of soil, where it tends to collect.12PubMed Central. Root architecture responses: in search of phosphate That’s why mixing phosphorus into the soil near the root zone at planting works better than scattering it on top later.
A shortage shows as slow growth and a dull or purplish look, especially in cold soil. New transplants and seedlings feel it most because their roots are small.
Many gardens don’t need extra phosphorus at all. Years of fertilizer and manure have built it up in many garden and lawn soils. A soil test through a local cooperative extension office tells you whether phosphorus is low, adequate or high, so you can skip it if it isn’t needed. If a test does show a shortage, bone meal and starter fertilizers are the common phosphorus-rich choices, worked in near the roots. Adding more than the test calls for won’t produce extra blooms, and the surplus can wash off into streams and ponds.
Potassium Controls Water and Toughness
Leaves have microscopic pores that let in carbon dioxide and let water escape.13Cropaia. Water-Smart Farming: The Essential Role of Potassium Potassium moving into the cells that ring each pore draws water in, swelling them so the pore opens.14Cropaia. Water-Smart Farming: The Essential Role of Potassium In drought, potassium is pumped out, the cells shrink and the pores close to save water.15Cropaia. Water-Smart Farming: The Essential Role of Potassium
Potassium is also the most plentiful mineral in plant cell fluid, and it helps cells stay firm during dry spells.16Cropaia. Water-Smart Farming: The Essential Role of Potassium It activates enzymes and helps plants make sugars more efficiently as well.17Cropaia. Water-Smart Farming: The Essential Role of Potassium
A shortage shows up as brown, scorched edges and tips on older leaves. Plants wilt faster on hot days and may have weak stems, and fruit quality and flavor can drop.
Because of its role in toughness, fall lawn and shrub feeds often lean on potassium for winter hardiness. Wood ash supplies potassium too, but it also raises soil alkalinity, so use it lightly.
Calcium, Magnesium, Sulfur and Trace Nutrients
Calcium is needed to form cell walls and for normal cell division.18Mosaic Crop Nutrition. Secondary Nutrients It helps roots absorb other nutrients and improves disease resistance.19Mosaic Crop Nutrition. Secondary Nutrients Liming soil supplies calcium, and gypsum adds it without changing soil acidity.20Mosaic Crop Nutrition. Secondary Nutrients Blossom-end rot, the dark sunken patch on a tomato’s bottom, is linked to calcium not reaching the fruit, often because of uneven watering rather than too little calcium in the soil.
Each chlorophyll molecule is built around magnesium, making it essential for capturing sunlight.21Mosaic Crop Nutrition. Secondary Nutrients Magnesium also helps plants take up phosphorus.22Mosaic Crop Nutrition. Secondary Nutrients If soil lacks magnesium, dolomitic lime or potassium-magnesium sulfate can supply it.23Mosaic Crop Nutrition. Secondary Nutrients A shortage in growing plants can be fixed with watered-in soluble magnesium or a leaf spray, though applying it before planting is best.24Mosaic Crop Nutrition. Secondary Nutrients
Sulfur is in every living cell and is needed to build certain proteins and to turn nitrogen into usable form.25Mosaic Crop Nutrition. Secondary Nutrients Elemental sulfur must be converted by soil bacteria before plants can use it, while sulfates work faster.26Mosaic Crop Nutrition. Secondary Nutrients
Iron, manganese, zinc and copper easily lock up in soil, so plants use them poorly.27EDIS. Understanding and Applying Chelated Fertilizers Effectively Based on Soil pH Chelated fertilizers, which wrap these metals in a protective molecule, keep them available.28EDIS. Understanding and Applying Chelated Fertilizers Effectively Based on Soil pH On a label, chelated iron is listed as Fe with a chelate name like EDDHA or EDTA. EDDHA stays effective in alkaline soil where others fail.
The pattern of yellowing offers clues. Magnesium shortage shows as yellowing between the veins of older leaves, with the veins staying green. Iron shortage creates a similar look on the newest leaves, and it is common in alkaline soil where iron locks up. Old versus new leaves is a useful hint but not proof, since other problems can look alike.
Fast-Release, Slow-Release and Organic Fertilizers
Water-soluble synthetic fertilizers release nitrogen rapidly.29Land Degradation & Development. Comparison of slow‐release nitrogen yield from organic soil amendments and chemical fertilizers and implications for regeneration of disturbed sites Slow-release synthetic formulas, which are designed to feed over weeks or months, release it steadily over a longer time.30Land Degradation & Development. Comparison of slow‐release nitrogen yield from organic soil amendments and chemical fertilizers and implications for regeneration of disturbed sites Slow-release pellets work through coatings that control how water reaches and dissolves the nutrients.31PubMed Central. Controlled Release Fertilizers: A Review on Coating Materials and Mechanism of Release Organic blends and compost release a quick first burst followed by a slower phase.32Land Degradation & Development. Comparison of slow‐release nitrogen yield from organic soil amendments and chemical fertilizers and implications for regeneration of disturbed sites Soil life breaks organic material down, and warm, moist soil speeds release compared with cool weather.33Land Degradation & Development. Comparison of slow‐release nitrogen yield from organic soil amendments and chemical fertilizers and implications for regeneration of disturbed sites
Soluble feeds, mixed in water, suit container plants and quick fixes for yellowing, but they wash out fast and need repeating every few weeks. Slow-release granules suit busy gardeners because one application lasts much longer, and they lower the chance of burning plants. Organic products feed soil life and improve soil over time, but their nutrient numbers are lower, so you apply more material for the same nutrient. Cool spring soil slows them further, so expect a delay.
Fertilizer burn happens when too much soluble salt pulls water from roots, browning leaf edges. Watering in after applying and following label rates prevents it.
Where the Ingredients Come From
Most nitrogen fertilizer starts as ammonia, made by combining nitrogen from the air with hydrogen over an iron catalyst.34Wikipedia. Haber process Plants can’t use nitrogen from air because its two atoms are bound tightly together, so factories break that bond with heat, pressure and the catalyst.
Ammonia is turned into ammonium nitrate by making nitric acid and neutralizing it with more ammonia.35Energy. Exergy-based estimation and comparison of urea and ammonium nitrate production efficiency and environmental impact Urea is made by combining ammonia with carbon dioxide.36Energy. Exergy-based estimation and comparison of urea and ammonium nitrate production efficiency and environmental impact Newer factories are testing ammonia made with renewable hydrogen, though it only pays at very large scale.37Current Opinion in Green and Sustainable Chemistry. Renaissance of ammonia synthesis for sustainable production of energy and fertilizers
Superphosphate is made by treating insoluble phosphate rock with sulfuric acid, turning it into a form plants can use.38New Zealand Institute of Chemistry. The Manufacture of Sulfuric Acid and Superphosphate Morocco holds about 77% of the world’s phosphate rock reserves.39Resources, Conservation and Recycling. The future distribution and production of global phosphate rock reserves
Potash comes from ancient evaporated seabeds, often mined in shafts as deep as 4,400 feet.40Wikipedia. Potash The potassium chloride is separated from table salt and clay and dried.41Wikipedia. Potash Muriate of potash is another name for potassium chloride on ingredient lists. It dissolves readily in water, so plants can use it quickly.42Wikipedia. Potash
Before factories, growers relied on seabird guano from Peru, which was the main US fertilizer from 1840 to 1870 until deposits ran out. Bones, fish scrap and wood ash were common early fertilizers too, and many organic products still use similar ingredients today.
Why More Fertilizer Isn’t Better
Heavy nitrogen use combined with heavy watering leads to nitrate washing down through soil and nitrous oxide gas escaping into the air.43Geoderma. Assessing impacts of nitrogen management on nitrous oxide emissions and nitrate leaching from greenhouse vegetable systems using a biogeochemical model Phosphorus in runoff feeds algae growth in lakes and streams.44Journal of Environmental Quality. Agricultural Phosphorus and Eutrophication: A Symposium Overview
Farms use variable-rate equipment to put only what each part of a field needs.45USDA Natural Resources Conservation Service. Reduce risks of nutrient loss to surface water by utilizing precision agriculture technologies This improves how much nutrient the crop uses and cuts losses.46International Journal of Agricultural and Biological Engineering. Variable rate fertilizer application technology for nutrient management: A review
Granules left on a sidewalk or driveway are washed by rain straight into storm drains, which often lead to local waterways without any treatment. Fertilizing just before heavy rain does the same on a lawn. Sweep stray granules back onto the grass and time applications for dry stretches. Splitting nitrogen into smaller doses through the season helps too, and phosphorus-free lawn products make sense unless a test shows need.