When do plants need high nitrogen? Leaf vs fruit

By Joe Barrington, founder of Dr Forest · Published June 2026 · Updated 6 November 2026

Quick answer. Plants need a high nitrogen fertiliser while they build leaves and stems, or when the lower leaves go pale and growth stalls. Leafy veg and brassicas want it most. It's any feed whose first NPK number, nitrogen, sits well ahead of the other two, such as dried blood, hoof and horn, or a 13% N plant protein feed. All three of those count as a natural nitrogen fertiliser, since the nitrogen comes from animal or plant protein. Stop when fruiting crops flower, or you get soft, sappy growth and fewer flowers.

If you already know your plants want nitrogen and just need a high nitrogen fertiliser, my range is here.

I reach for nitrogen in two situations. If a plant is yellowing at all, that's often a sign it's short of nitrogen. I'll also use it when growth has gone a bit stagnant. I only really want to be putting it on in sizeable doses in the first half of a plant's life. After that I taper off, because it affects crop quality. Overdo it and you get sappy growth, and you get pest and disease pressure.

When people overfeed nitrogen, what I see is sappy, leggy growth.

What is a high nitrogen fertiliser?

Every bag shows three numbers, the NPK: the percentages of nitrogen (N), phosphate (P) and potash (K), in that order (more on NPK meaning). The further the first number pulls ahead, the more the feed is built for leaf and stem. You'll also see it sold as a nitrogen rich fertiliser or high nitrogen plant food.

Nitrogen goes into proteins and chlorophyll, which is why a hungry, leafy plant given some shows deeper colour and bigger leaves within a couple of weeks.

What you'll find on the shelf, strongest first:

• Urea, 46% nitrogen, and sulphate of ammonia, 21%. Both synthetic.
• Hoof and horn, and dried blood, at around 12 to 13%. The usual organic options, both from animals.
• Plant protein feeds at a similar strength. The High Nitrogen Fertiliser 13% N I sell carries its nitrogen as amino acids and proteins drawn from fermented plant sugars (molasses). It's plant-based and packed in Stockport.
• Liquid nitrogen feeds for the watering can, such as Liquid Nitrogen Fertiliser 10% N in the Dr Forest range.
• Alfalfa meal, at about 2.5%, is a gentle slow feed rather than a high nitrogen feed.

When your plants need nitrogen

Demand runs high while a plant builds leaves and stems, then drops as it flowers and fruits. Feed with that curve:

• Leafy veg and brassicas, such as lettuce, spinach, chard, cabbage, kale and sprouts. They turn nitrogen straight into bigger, greener leaves.
• Anything that has paled or sat still early in the season.
• Sweetcorn, courgettes, squash and pumpkins while they're making leaves. Switch courgettes and squash to a balanced feed once they flower.

How to spot nitrogen deficiency

A plant short of nitrogen moves it from old leaves to new ones. So the lowest leaves turn pale, then evenly yellow, while the top stays greener. Growth slows and stems go thin. Yellowing on new growth, or patchy yellow between the veins, is usually something else: the guide to yellow tomato leaves helps tell them apart.

When to hold off

Once a plant has its leafy framework and starts setting buds, heavy nitrogen pushes soft, sappy growth that flops in wind and rain, and it holds back flowers and fruit. Ease off:

• At first flower on tomatoes, peppers, chillies and aubergines. They need nitrogen only while they build their first leaves, then a high-potash feed such as my Tomato Feed 3-4-6.
• On peas and beans after the early dose. Their root nodules fix much of their own nitrogen from the air, so one or two light feeds before flowering, at the fruiting-crop rate below, is plenty. It is one reason crop rotation puts hungry brassicas in the bed after peas and beans.
• Late in the season. Nitrogen in late summer pushes soft growth that won't harden before the first frosts.
• On roses and flowering shrubs after one spring feed. Then move them to a balanced or high-potash feed, or a slow alfalfa feed.

Seaweed carries too little nitrogen to help here. I explain why seaweed feed is low in nitrogen in its own guide.

Organic vs synthetic nitrogen

Synthetic straights such as urea and ammonium nitrate are soluble salts. They act fast in the root zone, wash through easily, and urea can lose nitrogen to the air as ammonia [1]. They add no carbon for soil life and aren't allowed in organic growing. At the other end sits farmyard manure, bulky and full of carbon but too low in nitrogen to count as a high nitrogen feed.

An organic nitrogen fertiliser holds its nitrogen in protein. Plants can take up some amino acids straight through their roots [2], but most of the nitrogen has to be broken down by soil microbes first. That can still be quick: in a lab test at 10 to 25 °C, blood meal, feather meal, fish powder and seabird guano released 47% to 60% of their organic nitrogen within two weeks [3]. I give the same rough figure for my High Nitrogen Fertiliser 13% N: about half available in the first fortnight, the rest over the following weeks.

Organic doesn't stop leaching, mind. A 2024 review of 39 years of studies found no overall difference in nitrate leaching between organic and synthetic feeds, and coarse, sandy soils lost the most [4]. Give the plant what it needs now, and split doses on light soil.

How much to use

A 13% feed is concentrated. At 40 g per square metre it puts down about 5 g of nitrogen; getting the same 5 g from a balanced 4-4-4 would take about 130 g.

Scatter evenly on moist soil during active growth, keep granules off leaves, stems and crowns, and water in. One application feeds for around six weeks, so reapply no more often than every five to six weeks. Rates for High Nitrogen Fertiliser 13% N:

Use	Rate	Frequency
Base dressing, before sowing or planting	30 to 50 g/m²	Once, at bed prep
Heavy leafy feeders: brassicas, leeks, sweetcorn, squash	40 to 55 g/m²	Every 5 to 6 weeks in growth
Salad and moderate crops: lettuce, beetroot tops, leafy herbs	25 to 40 g/m²	Every 5 to 6 weeks
Fruiting crops: tomatoes, peppers, beans and peas, early only	20 to 30 g/m²	Once or twice, before flower
Roses, flowering shrubs	20 to 30 g/m²	Once, in spring
Pots and containers	2 to 4 g/L of compost	At potting, or a light top-up every 6 weeks

A rounded handful is roughly 30 to 40 g, and a level tablespoon about 15 to 20 g. On light, sandy soils, split the feeds and keep to the lower end. Weigh the first few applications, because doubling up doesn't double the crop.

Once leafy growth is strong and well coloured, stop the nitrogen. A balanced feed such as my All Purpose 4-4-4 (it contains bone meal) covers most of the garden from there, and fruiting crops move to Tomato Feed 3-4-6. Lawns have their own timetable in the high nitrogen lawn feed guide. For which crops want a nitrogen lead, see how to grow 194 plants.

Frequently asked questions

What is the best high nitrogen fertiliser for the garden?

For most gardens, an organic feed of around 12 to 13% nitrogen, like those in my nitrogen fertiliser range. It greens plants up within a couple of weeks and is less likely to scorch roots than urea.

When should I use a high nitrogen feed?

Use it at the leaf-and-stem stage, and on plants with pale lower leaves early in the season. Ease off at bud set.

What are the signs of nitrogen hunger in plants?

Pale or yellow lower leaves first, with slow growth and thin stems. Yellowing on top leaves or between the veins is usually something else.

Can too much nitrogen harm plants?

Yes: too much nitrogen gives soft, floppy growth and fewer flowers and fruit. A heavy dose on dry soil can scorch roots, so keep to the rates and water in.

What about lawns and tomatoes?

Lawns have their own rates and timing, set out in the high nitrogen lawn feed guide. Feed tomatoes nitrogen only while they build leaves, then switch to Tomato Feed 3-4-6 from the first truss.

Joe Barrington, founder of Dr Forest. I formulate and blend my own Dr Forest feeds, and stock bought-in products alongside them.

Sources

1. Cameron, K. C., Di, H. J. and Moir, J. L. (2013). Nitrogen losses from the soil/plant system: a review. Annals of Applied Biology, 162(2): 145 to 173. https://doi.org/10.1111/aab.12014
2. Näsholm, T., Kielland, K. and Ganeteg, U. (2009). Uptake of organic nitrogen by plants. New Phytologist, 182(1): 31 to 48. https://doi.org/10.1111/j.1469-8137.2008.02751.x
3. Hartz, T. K. and Johnstone, P. R. (2006). Nitrogen availability from high-nitrogen-containing organic fertilizers. HortTechnology, 16(1): 39 to 42. https://doi.org/10.21273/horttech.16.1.0039
4. Hina, N. S. (2024). Global meta-analysis of nitrate leaching vulnerability in synthetic and organic fertilizers over the past four decades. Water, 16(3): 457. https://doi.org/10.3390/w16030457