Feeding chiles: nutrients by stage

Chiles are not heavy feeders: Clemson Extension says peppers need moderate amounts of fertilizer.⁶ Most of the extra feeding comes once the plants start to flower. Every pepper guide used here warns against overfeeding with nitrogen, which gives a leafy bush and few pods. Start with a soil test and good soil (the Soil chapter covers pH, compost and container mixes), then feed to match what the plant is doing.

What N, P and K do

Label numbers

A 10-10-10 fertilizer is 10% nitrogen, 10% phosphate and 10% potash by weight³

Seedlings

Half-strength soluble complete fertilizer, once a week⁵

First side-dressing

About one week after flowering begins²

Potting compost

Its nutrients last about 6–8 weeks, then pots need feeding¹

Water in fruit

At least 5 cm (2 in) a week in dry spells, especially after fruit set⁷

Plants need 17 nutrients. They get carbon, hydrogen and oxygen from air and water, and the other 14 from the soil. The three primary nutrients are nitrogen (N), phosphorus (P) and potassium (K); calcium, magnesium and sulfur are secondary nutrients; and boron, iron, manganese, zinc and a few others are micronutrients, needed only in tiny amounts.⁴

• Nitrogen drives green, leafy growth.¹ It is easily washed out of soil by rain and out of potting compost by watering.¹
• Phosphorus supports healthy root and shoot growth.¹ For chiles, NMSU notes it matters for good root development and fruit production.²
• Potassium (potash) supports flowering, fruiting and general hardiness.¹

Plants don’t use the three in equal amounts: they need more nitrogen and potassium than phosphorus.³ Most garden soils already hold enough magnesium, calcium and sulfur.¹ Nebraska Extension notes that nitrogen is often the only nutrient actually limiting growth, which is why a soil test is worth doing before you buy anything.⁴

Feeding by stage

The timings below count from transplanting and first flowers, not calendar dates, so they work in either hemisphere. In the Southern Hemisphere the season simply starts after your own last spring frost.

Seedlings

Seedlings need only a light feed: Utah State University suggests feeding transplants weekly with a half-strength soluble complete fertilizer until they go into the garden.⁵ A complete fertilizer is one that contains all three of N, P and K.³

Planting out and vegetative growth

Before planting, work in whatever your soil test calls for. Clemson Extension suggests broadcasting pre-plant fertilizer and working it into the top 15 cm (6 in) of soil so it is available when the plants go in.⁶ The Chile Pepper Institute recommends a balanced fertilizer such as 5-10-5 or 10-10-10, and going easy on the nitrogen.⁷

Unless your soil test shows a need for phosphorus, the University of Minnesota advises a low- or no-phosphorus fertilizer, noting that many soils in its region already have enough.⁹ NMSU gives a light nitrogen application before planting to help plants get off to a good start, and saves the main nitrogen side-dressing for flowering.²

Flowering and fruiting

This is when chiles benefit most from extra feeding, usually as a side-dressing: a small amount of fertilizer placed in the soil beside the row rather than on the plant. Timings differ by source:

• NMSU side-dresses with nitrogen one week after flowering begins, in a shallow trench 2.5–5 cm (1–2 in) deep and 10–13 cm (4–5 in) to the side of the plants, then covers it and waters immediately. More nitrogen can follow later in the season if the plants need it.²
• Clemson side-dresses with calcium nitrate three to four weeks after planting and again three to four weeks after that, keeping the fertilizer 10–15 cm (4–6 in) away from the plants to avoid burning the roots.⁶
• Utah State side-dresses 4 and 8 weeks after transplanting, placing the fertilizer 15 cm (6 in) to the side of the plant and watering it in.⁵

They count from different starting points: NMSU from the first flowers,² Clemson from planting⁶ and Utah State from transplanting.⁵ Clemson and Utah State each give two side-dressings; NMSU gives one, adding more only if the plants need it. Follow whichever matches how you track your plants. Utah State warns that overfeeding peppers causes excessive foliage and delays fruit set and maturity.⁵

Plants in pots are different, because potting compost runs out of nutrients after about 6–8 weeks.¹ From then on they need regular liquid feeding. The RHS notes that crops in containers may need extra potassium-rich feeding, such as a tomato feed; potassium is the nutrient that supports flowering and fruiting.¹

Calcium, magnesium and blossom-end rot

Blossom-end rot starts as a sunken spot at the flower end of the fruit (opposite the stalk) that spreads and turns black. It is caused by a calcium shortage in the young, rapidly expanding fruit.⁸ That rarely means the soil lacks calcium: the RHS says it is very rare for soils or potting media to be short of it.¹ The usual problem is getting calcium into the fruit.

Calcium travels in water, pulled up the plant as leaves lose moisture. Fast-growing leaves lose far more water than fruit, so they get most of the calcium, and the fruit loses out whenever water runs short.⁸ Utah State lists the triggers as poor water management, excessive nitrogen, root pruning and drought stress.⁵ The fixes follow from that:

• Water evenly. Keep the soil uniformly moist but not saturated, and don’t let it dry out between waterings.⁶ The Chile Pepper Institute recommends at least 5 cm (2 in) of water a week in dry spells, especially after fruit set.⁷ Mulch helps hold moisture steady.⁵
• Don’t overdo nitrogen. Lush leafy growth pulls calcium away from the fruit.⁸
• Don’t rely on calcium sprays. UF/IFAS says foliar calcium is not likely to correct or prevent blossom-end rot, because calcium doesn’t move from the leaves down into the fruit.⁸
• Test before adding calcium. A soil test before planting tells you whether the soil has enough.⁶

The first fruits on a plant are the most prone.⁸ The Problems chapter covers blossom-end rot in more depth.

Magnesium shortage shows as yellowing between the leaf veins, with older leaves browning and dropping early. It is most likely on light, sandy soils, and heavy use of high-potassium feeds can cause it, because plants take up potassium in preference to magnesium.¹ Too much potassium feed can also make calcium problems worse,¹ so feed potash to need, not by habit.

Reading a fertilizer label

Every fertilizer label carries three numbers, the guaranteed analysis or grade. They give the percentage by weight of nitrogen (N), phosphorus expressed as phosphate (P₂O₅), and potassium expressed as potash (K₂O), always in that order.³ The rest of the bag is inert carrier material that helps you spread it.³

• Complete: contains all three, such as 18-4-10.³
• Balanced: equal amounts of all three, such as 10-10-10.³
• Incomplete: one or more is zero, such as 24-0-11 when a soil test shows you already have enough phosphorus.³ Calcium nitrate (15.5-0-0) is a nitrogen-only example used for side-dressing peppers.⁶

Labels also list the nitrogen sources. In the US, by law, a fertilizer sold as slow-release must have at least a third (33%) of its nitrogen in slow-release form, shown as water-insoluble nitrogen (WIN) or slowly available nitrogen.³ Secondary nutrients and micronutrients are listed too, but adding them only helps if your soil is short of them; too many micronutrients can damage plants.³ Outside the US, check what your local labelling rules mean by slow-release.

Organic and synthetic fertilizers

Both work. Synthetic (inorganic) fertilizers are usually fast-release and water-soluble, though some are coated or pelletized to release slowly. They deliver nutrients quickly, are easy to measure because their content is standardized, and are cheaper, but they are easier to overapply and can leach into ground or surface water when misused.⁴

Organic fertilizers such as manure, blood meal, bone meal and fish emulsion are typically slow-release: soil microbes have to break them down before plants can use the nutrients, which can take days or weeks. They cost more per unit of nutrient, but they add organic matter that improves soil structure.⁴

NMSU describes two organic approaches for chiles. Liquid feeds such as fish emulsion or compost tea are available immediately but leach quickly, so they need frequent applications. Slow-release organic granules can be scattered around established plants; they won’t burn them, and they supply micronutrients as well as N, P and K.²

Two food-safety points with animal products:

• Use well-rotted manure or compost, never fresh manure, which may carry harmful bacteria.⁹
• Nebraska Extension advises at least 90 days between the last animal-based fertilizer and harvest for crops like chiles whose edible part doesn’t touch the soil, or 120 days if it does.⁴

You can also combine the two and use the strengths of each.⁴

Spotting deficiencies

Read leaf colour with care. Drought, waterlogging, low light and poor establishment can all look like deficiencies, so rule those out first.¹ Where a symptom appears matters: nitrogen and phosphorus shortages show on older leaves first, and iron shortage on the youngest.¹

Too much nitrogen

Every pepper source here warns about this. NMSU warns that overstimulating plants with nitrogen gives excessive vegetative growth at the expense of fruit.² The University of Minnesota describes the result as bushy, leafy plants that are slow to bear.⁹ Clemson puts it plainly: lots of foliage with few blooms or fruit means too much nitrogen.⁶

It also causes other problems:

• Blossom-end rot. Rapid leaf growth during fruit set pulls calcium toward the leaves and away from the fruit.⁸
• Salt stress. Excess fertilizer raises soluble salts in the soil, which makes it harder for roots to take up water.⁸
• Toxicity. Severe overapplication shows as dark green leaves and stunted growth, and surplus nitrogen washes out of the soil as a pollutant.¹

Manure counts too: UF/IFAS notes that high nitrogen from heavy manure use causes the same leafy growth as chemical fertilizer.⁸ If your plants are already too leafy, stop feeding nitrogen and keep watering evenly. Next time, keep pre-planting nitrogen light and side-dress about a week after the first flowers open.²