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Soil & nutrients

Healthy plants grow on healthy soil — not the other way round. This chapter covers the four SFG nutrient categories, the three main plant nutrients (NPK) plus the trace elements, how to read deficiency symptoms on the leaves, the quiet world of soil life that does most of the work for you, and how to test your own soil without a lab.

The four SFG nutrient categories

Plants are grouped by how much they eat. Use these categories for crop rotation (don't follow a heavy feeder with another heavy feeder) and for placement (don't put a light feeder next to a hungry heavy feeder that will starve it).

Heavy Feeders

High nutrient demand. Need rich soil with plenty of compost. Examples: Tomato, cabbage, corn, cucumber, pumpkin. Top-dress with compost during the growing season.

Medium Feeders

Moderate nutrient needs. Grow well in soil that was heavily composted the previous year. Examples: Carrots, onions, beetroot, leeks, kohlrabi.

Light Feeders

Low nutrient demand. Thrive in lean soil without extra fertilizer. Examples: Herbs (thyme, rosemary, sage), lettuce, radishes, arugula. Too much nitrogen makes them leggy.

Nitrogen Fixers

These plants (beans, peas, clover) have symbiotic bacteria in their roots that convert atmospheric nitrogen into plant-available nitrates. Plant them before heavy feeders to naturally enrich the soil.

NPK & trace nutrients

Every bag of fertilizer carries three numbers — N-P-K — the three macronutrients plants need in large amounts. A fourth category, trace elements, is needed in tiny amounts but just as essential.

Nitrogen — the leaf builder

Drives leaf and shoot growth. Leafy crops (lettuce, spinach, cabbage) need plenty; fruiting crops need less (too much N = lots of leaves, few fruits). Best sources: compost, well-rotted manure, legumes, coffee grounds, liquid nettle feed.

Phosphorus — the root & flower builder

Supports root development, flowering and fruit set. Critical for seedlings and fruiting crops. Best sources: bone meal, rock phosphate, compost, bat guano. Phosphorus moves slowly in the soil — mix into the root zone, not just on top.

Potassium — the fruit ripener

Strengthens cell walls, improves disease resistance, boosts fruit quality and colour. Crucial for tomatoes, peppers, potatoes, berries. Best sources: wood ash (in moderation!), comfrey feed, banana peels, kelp meal.

Trace elements

Calcium, magnesium, sulfur, iron, zinc, manganese, boron, copper, molybdenum. Needed in small amounts but quickly limiting if missing. A varied compost from many sources usually covers them all.

Deficiency cheat sheet

Plants tell you what they're missing — learn to read the signs. Almost every deficiency starts on leaves, so check them weekly.

Nitrogen: yellow old leaves

Yellowing starts on the OLDEST leaves (bottom of the plant) and spreads upwards. Whole plant looks pale and stunted. Fix: liquid nettle feed, top-dress with compost.

Phosphorus: purple stems

Stems and leaf undersides turn reddish-purple, especially in young plants in cold soil. Growth is slow and hard. Fix: bone meal worked into the root zone, warmer soil.

Potassium: brown leaf edges

Leaf edges turn brown and scorched, starting on older leaves. Fruit is small and pale. Fix: wood ash (light sprinkling), comfrey tea, banana-peel compost.

Magnesium: yellow between green veins

Leaves yellow between still-green veins (interveinal chlorosis), on older leaves. Common on tomatoes and peppers. Fix: a dissolved teaspoon of Epsom salts per watering can, every 2 weeks.

Calcium: blossom-end rot

Tomatoes, peppers and courgettes develop black sunken spots at the blossom end of the fruit. Actually a watering issue — the plant can't move calcium around when watering is irregular. Fix: mulch, steady watering — not more fertilizer.

Iron: yellow new leaves

NEW leaves (top of plant) yellow while veins stay green. Opposite of magnesium. Often caused by soil that's too alkaline — iron is there but plants can't absorb it. Fix: lower pH slightly with coffee grounds, sulfur or an acidic mulch.

pH — the silent gatekeeper

The pH scale (0–14) measures how acidic or alkaline your soil is. Even a perfectly nutrient-rich soil can starve a plant if the pH locks up the nutrients. Most vegetables love pH 6.0–7.0 (slightly acidic to neutral).

  • pH < 5.5 (acidic) — nitrogen and phosphorus become unavailable. Raise with garden lime (50–100 g/m²).
  • pH 6.0–7.0 (ideal) — the sweet spot where almost all nutrients are maximally available to the plant.
  • pH > 7.5 (alkaline) — iron, manganese and phosphorus lock up. Lower with sulfur, pine-needle mulch, or acidic compost (coffee grounds).
  • Test yourself: a €10 soil pH probe from any garden centre or an old-school litmus strip with a sample of soil + distilled water. Repeat every 2–3 years.

    • Soil life — the invisible workforce

    A teaspoon of healthy garden soil contains more living organisms than there are humans on Earth. They do 80 % of the work that turns dead material into plant food. Keep them happy and your plants almost take care of themselves.

    Bacteria

    Billions per teaspoon. Break down fresh organic matter (grass clippings, kitchen scraps) into nutrients plants can absorb. They love warmth, moisture and carbon-rich material.

    Fungi

    Thread-like hyphae spread through soil breaking down tough materials (wood, leaves, roots). They prefer cooler, undisturbed soil — another reason not to dig an SFG bed.

    Earthworms

    The star gardeners. Aerate soil with their tunnels, pull organic matter underground, produce castings (worm poo) that are 5× richer in nutrients than surrounding soil. If you have worms, your soil is healthy.

    Mycorrhizal fungi

    Special fungi that form partnerships with plant roots — extending their effective reach by 10–100×. Plants trade sugar for water and nutrients. Chemical fertilizers and tilling destroy them; compost feeds them.

    Feed soil life by mulching, adding compost and AVOIDING: deep digging, bare soil, chemical fertilizers, and pesticides. SFG is naturally good for soil life because you never dig or walk on it.

    Organic vs. mineral fertilizers

    Organic (recommended)

    Compost, well-rotted manure, worm castings, bone meal, fish emulsion, liquid nettle/comfrey feed. Feed the soil life, which feeds the plants. Slow release, almost impossible to over-fertilize, build long-term fertility.

    Mineral (use sparingly)

    Synthetic NPK pellets, liquid tomato feeds, soluble powders. Feed the plant directly, bypass soil life. Fast acting, but easy to over-fertilize (salt burn), wash into groundwater, and long-term harm soil microbes. Only as an emergency intervention, not a routine tool.

    Simple home soil tests

    No lab, no complicated kit. A few minutes with household items tells you most of what you need to know.

  • Ribbon test (texture): squeeze a moist handful. Forms a long ribbon = clay-heavy. Breaks into crumbs = loamy (ideal). Falls apart = sandy.
  • Jar test (composition): shake a soil sample with water in a jar, let it settle 24 h. Sand sinks first, silt in the middle, clay on top. You'll see exactly what you have.
  • Worm count: dig a 30×30×30 cm hole, count the worms. Over 10 = excellent, 5–10 = good, under 5 = needs more compost.
  • pH strip: mix soil + distilled water 1:1, dip a pH strip. Adjust as described above.

    • If in doubt, always add more compost. In an SFG bed, compost solves almost every soil problem on its own: feeds plants, feeds microbes, fixes both sandy and heavy soils, balances pH slightly, adds mycorrhiza, retains water. Two handfuls per square per year keeps any bed thriving.