NPK values for chilli plants

We humans are fussy eaters. Chillies are not. Chillies will take whatever meal you throw at them and, feast-or-famine, they will still grow, flower and fruit. Mostly.

However, the subtle science of feeding chilli plants well is a true art form that takes many a Summer to perfect. The variations of an ever-changing, seasonal menu for your chilli babies are as endless as Annabel Karmel recipes for troublesome toddlers. A good diet will hopefully produce a bounty of flavoursome, aromatic & glorious chillies to make your friends go oooh and ahhh. Much the same effect as when your young child selects an olive instead of a chicken nugget in front of an audience of NCT peers.

What does a chilli like to eat?

Let’s stick with the core ingredients for now: NPK, or Nitrogen, Phosphorus and Potassium to the uninitiated. Pretty much all feeds, fertilisers, composts, sprays, granules, enhancers, hydroponic potions contain these wonder components. As do most soils around the world. These macronutrients are the backbone of a good chilli plant diet. The most basic of fertilisers will provide an NPK ratio on its label. But just what are they and what do they do?

Nitrogen (N)

Why is Nitrogen necessary?

Think healthy leaves.

It is a key component of chlorophyll, the green stuff in plant cells. Chlorophyll uses sunlight to convert water and carbon dioxide into sugars as plant energy. Nitrogen is also a major component of protoplasm, the sappy contents of plants cells. Protoplasm is the vessel which holds trace minerals throughout the plant.

What does the correct supply of Nitrogen do to your plants? Healthy, green, lush plants. Strong stems and perky leaves. Speedy new shoots and flower bud formation.

What does the wrong balance of Nitrogen do to your plants? Too much Nitrogen will cause rapid growth, causing the plant to over commit. High Nitrogen feeds will inhibit flowering. Not enough Nitrogen and growth will be slow and the plant will end up stunted.

Traditional growers will add Nitrogen to their plants through blood meal, composted manure, composted coffee grounds. Blood meal is high in Nitrogen (12%).

Phosphorus (P)

Why is Phosphorus necessary?

It’s all about those roots and fruits baby!

Phosphorus is essential for plant cell division and development of new tissue. Think about when your chilli seedling is just getting going. All those new leaves, all that stem and root to grow, maintain and look after. Phosphorus is the ultimate support partner. It is in charge of seed development and as a result, timely flower formation, good sized and uniform fruit development and maturity.

What does the correct supply of Phosphorus do to your plants? Get the phosphorus levels right and you can expect strong early growth; plants reaching maturity earlier and roots showing through the bottom of the pot quickly. When potting on the first time, a good root system will be visible. It can also stimulate tillering, shoots springing from the bottom of the original stalk. making bushier plants.

What does the wrong balance of Phosphorus do to your plants? An excess of phosphorus in chilli plants can cause leaf issues, ending in leaf death. Too little Phosphorus and the plants may just not mature. In a few more detailed trials, it seems that less capsaicin and less fructose was found in plants that grew and fruited with lower levels of phosphorus. Not what is intended at all!

Bonemeal is high in Phosphorus (18%). Hoof and Horn is even higher (29%) Phospohrus is tricky to uptake, especially at low temperatures, low pH or in conditions with excessive iron in the soil.

Potassium (K)

Why is Potassium necessary?

Hearty growth and protection is the name of Potassium’s game.

Potassium regulates the closing and opening of stomata, and so can affect the amount of CO2 supply to the plant. CO2 is essential for photosynthesis and energy production. Low Potassium=low energy. Potassium plays a major role in the regulation of water movement within plants, in the roots and again the stomata. Essential transportation of nutrients, water and other minerals are also dependent on Potassium. Potassium is also a major player in cell wall thickening. This means that stems are stronger and all cells are less susceptible to disease.

Potassium has been proven to increase the likelihood of fruit setting in chillies. Not more flowers but definitely more fruit set.

What does the correct supply of Potassium do to your plants? Generally healthy plants with good tolerance to changes in environmental factors. Good drought and cold resistance in chillies. Allows fruit to develop and set well.

What does the wrong balance of Potassium do to your plants? A sad, stunted, yellowing plant will be on your hands if it is lacking in Potassium. More susceptible to drought, cold, pests as it will have thin cell walls. Less likely to grow quickly with strong stems. Waste products are not removed well, with yellowing of the leaves, ultimately ending in leaf drop. When seedlings first emerge they are vulnerable to damping off. Correct potassium levels at this stage can mean more seedings survive.

Potassium is linked to many growth enzymes in plants. Get the balance wrong and growth can be stunted. Too much Potassium may inhibit uptake of Nitrogen and Phosphorus as well.

Seaweed is a traditional plant treatment containing high Potassium. As is a more modern banana tea.

Is that it? Get the NPK value right and the chillies will be perfect?

Well, yes. And, no. Getting the NPK values balanced correctly for the plant’s needs at key points in the growing season is a huge, important task. BUT there are other factors too though. Other macronutrients (Sulphur, Magnesium and Calcium), micronutrients (Iron, Boron, Chlorine, Manganese, Zinc, Copper, Molybdenum and Nickel), micro-organisms, micro rhizomes, humidity, pH, pests, light levels, leaf quantity and size, pests, electronic conductivity of water, air flow and music choice.

The factors are myriad. Plenty more Googling to be done.

Homegrown Jalapeno chillies, the first of the season
Homegrown jalapeños, the first of the season

The science of feeding your chillies is in your hands. Maybe that weekly slosh of tomato feed needs rethinking?

The quest to find the best soil

Just which compost to choose for our precious chillies?

Loam. Peat. Coir. Grit. Vermiculite. Sand. Clay. Silt. Perlite. Manure. Compost. Sand. So much to consider.

The options are a little overwhelming but let’s dig down to what we know about chilli plants to see if we can come up with our perfect potting medium.

Cultural roots

Considering the geographical heritage of our chilli plants should hopefully give some guidance to the type of soil required for perfect chilli production. Chillies originated from Mexico. They gradually became mainstream as part of the Columbian Exchange in the 15th & 16th Centuries. A large percentage of the soil in Mexico is shallow to medium depth, dry, free draining, not layered or super structured, contains gravel, often fertile, pH leaning towards acid: 5.5-6.5. (Food Agricultural Organisation for the United Nations state that Leptosol, Regosol and Calcisol make up nearly 65% of the soil in Mexico if you want the scientific terms) Replicating this soil composition seems like a good starting point for creating the perfect chilli compost.

Back here in the UK

However, where chillies come from is not the only factor in soil choice. Surely where they are actually going to grow plays a part too. The UK offers a dizzying range of soil types, some of which would be incredibly inhospitable to a poor little chilli plant. Our Hampshire soil is about 6 inches top soil and then solid chalk. Not sure what our chilli amigos would say to that. A slightly acid, sandy loam in the South Hams of Devon would be perfect for growing chillies in the ground. With that in mind, we will be planting in pots.

Our UK weather is such that too much organic matter, such as peat or manure, silt or clay could easily become water logged and get cold. No one likes a soggy cold bottom, especially not chilli plants.

The potting soil in question will have two main functions for our plants: 1. to provide a habitat for our chilli roots to do their thing. 2. To be a vessel for the chemical requirements for great plant growth and chilli production. Let’s tackle each function separately.

A cosy home

We want the roots to have plenty of space to stretch out. Soil is typically 50% solids (mineral and organic) and 50% spaces, about half of which is occupied by water and soluble and suspended nutrients. A light, airy soil, with small particles would match the requirements. The roots can work their way through the gaps.

We will be gradually moving our chillies into progressively bigger pots. They sulk for ages if the pot size goes up too quickly. The indicator that they need to move home is when their roots poke through the bottom. Watering from the bottom should avoid compaction (which leads to a reduction of spaces within the soil available for root growth & essential gas, water and nutrient storage). Perhaps a layer of Horticultural grit in the base of each pot would avoid aforementioned sulky soggy bottom too.

Adding Perlite to the mix is another way to allow the soil mix to stay loose, encourage root growth and water drainage. No rotten roots here.

Feed the need

No great nutrient supply is needed from the soil solids as we will be feeding with Chilli Focus regularly. The organic components of the soil provided by the garden compost will ensure a good supply of microorganisms to exist alongside our chilli plants and work their bio magic. No doubt a few extra seeds will germinate as well but that is all part of the fun of doing things yourself.

We do not want too much green Nitrogen releasing organic matter. We hope to encourage good all round plant health and then fabulous flowering and fruiting. Extra Nitrogen may cause bushy green plants that forget to flower.

The seedlings are currently receiving a weekly feed of 5ml per litre of Chilli Focus. This will increase to 10mls per litre once a week when the plants have been pinched out and show some signs of flowers forming. At the height of flowering and fruiting the plants will have two feeds each week.

Vermiculite is another ingredient to consider. A natural product that is a good addition to any soil type. Added to clay soils it allows aeration and flow through the soil, reducing water logging and stunted or bound root growth. In sandy soils it soaks up water allowing retention where there would be very little otherwise. Good access to water, slowly released, means good access to those soluble and suspended nutrients. Sounds like Vermiculite is definitely going into our chilli mix.

So what does this all mean?

It’s looking like a hand mixed potting medium of 3/10 loam top soil, 2/10 coir, 2/10 organic homegrown well rotted compost, 1/10 grit for the bottom of the pot, 1/10 vermiculite to hold on to water and nutrients between watering and feeds, 1/10 perlite for free drainage within the soil. Probably very similar to John Inness No 2!

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