Soil and Fertilizer Basics

Many problems that look like pest damage or bad luck are soil problems in disguise. Yields drop, leaves discolour, fruit sets badly, and the cause is structure, pH or a feeding habit rather than anything visible.

Texture, Structure and Why They Are Different

Texture is the proportion of sand, silt and clay in your soil, and it is fixed. Shifting clay to loam would take tonnes of sand per square metre, and part-way there you get concrete.

Structure is how those particles clump into crumbs, and how much pore space sits between them. It is changeable, and it is where your effort should go: water infiltrates instead of pooling, roots push through instead of stalling, air reaches the microbes.

So stop fixing your soil type and start building structure. Add organic matter, avoid working soil when wet, keep the surface covered, and walk on paths rather than beds; compaction undoes a season of improvement in an afternoon. Heavy clay rewards the effort, because once it has structure it beats sand in a dry summer.

A Five-Minute Soil Test You Can Do at Home

The ribbon test. Roll a walnut-sized lump of moist soil into a ribbon over the edge of your index finger. Sand will not ribbon and feels gritty; silt ribbons under 2.5 cm (1 in) and feels soapy; clay ribbons to 5 cm (2 in) or more and shines when rubbed.

The jar test. Fill a straight-sided jar one third with soil, top up with water, add a drop of washing-up liquid, shake for two minutes and leave it. Sand settles within a minute, silt over hours, clay over a day or two, giving rough percentages by band.

The drainage check. Dig a hole 30 cm (12 in) deep, fill it, let it drain, then refill and time the second drain. Under two hours is free-draining, four to eight normal, over twelve means drainage work or raised beds — one reason the raised bed planning guide exists.

For pH, buy a chemical test kit; the probe meters are close to useless.

pH: The Setting That Unlocks Everything Else

pH does not feed plants. It decides which nutrients are chemically available, so soil can be full of iron and still starve a plant.

Most vegetables want pH 6.0-7.0. Brassicas prefer the top of that range near 7.0, which also suppresses clubroot; potatoes prefer 5.5-6.0, because scab worsens as pH rises; blueberries want 4.5-5.5. Above pH 7.5 you see iron and manganese lock-out; below 5.5, phosphorus becomes unavailable.

Correcting it is slow. Lime raises pH: apply in autumn, expect the full effect a season later, and reckon on 250 g per square metre to lift a loam by half a point. Lowering pH is harder, and coffee grounds do not do it.

What N, P and K Actually Do

Nitrogen (N) drives leaf and stem growth. It is mobile in the plant, so shortage shows first on the oldest leaves as uniform yellowing, and mobile in soil, leaching away with rain each season.

Phosphorus (P) supports roots, flowering and fruit set. It barely moves in soil, so work it into the root zone rather than sprinkling it on top. Most gardens with a compost habit have plenty.

Potassium (K) governs water regulation, fruit quality and disease resistance. Fruiting crops are the heavy users, and shortage shows as scorched margins on older leaves.

Blossom end rot, meanwhile, is a calcium delivery failure caused by irregular watering rather than a deficiency — see watering schedule mistakes.

Organic Matter Is Not a Fertiliser

Compost typically analyses around 1-1-1, close to nothing next to a bottled feed at 10-5-10. What it does matters more long term: it feeds soil biology, improves aggregation so clay drains and sand holds water, and holds nutrients where roots reach them.

Use it as a mulch, 3-5 cm (1-2 in) on the surface each year rather than dug in. Well-rotted manure is stronger in nitrogen but must be genuinely old: fresh manure burns roots and can carry herbicide residues that deform tomatoes.

Container Soil Is a Different Problem

Beds are a buffered ecosystem; containers are a closed system on a short clock. Potting mixes carry a starter charge of fertiliser that runs out in four to six weeks, after which every watering flushes nutrients out of the drainage holes — hence the pots that look perfect in June and stall in July.

• Assume you are the entire nutrient supply from week six onward.
• Feed at half strength twice as often, not full strength half as often.
• Refresh the top third of the compost each spring, and never fill a container with garden soil, which compacts into a waterlogged brick.

Reading a Fertiliser Label

The three numbers are always N-P-K by percentage of weight, so a 10-5-10 is 10% nitrogen, 5% phosphate, 10% potash. A 5-5-5 is not weaker in balance, only in concentration. Then check three things.

• Slow release or soluble. Soluble feeds act within days and vanish within weeks; coated granules meter out over two to six months and suit pots.
• The nitrogen form. Nitrate is immediately available, while urea and ammonium need warmth and microbes, so they underperform in cold soil.
• The dilution rate, then halve it. A 6-3-6 liquid at 10 ml per 5 L is a weak but useful feed; at 20 ml per 5 L in a dry pot it scorches roots within a day.

A Simple Feeding Schedule

1. Early spring: spread 3-5 cm of compost across every bed.
2. At planting: a balanced granular feed for heavy feeders only — brassicas, squash, tomatoes.
3. Six weeks after planting: a nitrogen-leaning liquid feed for leaf crops.
4. At first flowering: switch fruiting crops to high-potassium feed every 10-14 days.
5. Late summer: stop feeding perennials so they harden before winter.

Leafy crops want nitrogen. Fruiting crops want nitrogen early, then potassium once flowers appear. Root crops want almost nothing; extra nitrogen gives magnificent carrot tops and forked roots.

Tracking which bed was fed when, and what grew there last year, is the part that quietly falls apart by midsummer. Vegarden keeps that history per bed if you would rather have it on record than in your head.

Signs You Are Overfeeding

Overfeeding is more common than underfeeding, and harder to spot: the early symptom looks like success.

• Dark, lush, oversized leaves with almost no flowers: nitrogen excess.
• Fruiting crops that grow upward and never set.
• A white crust on the soil or pot rim: salt build-up from feed.
• Brown, crisp leaf edges on a plant that is not dry: fertiliser burn.
• Soft, sappy growth covered in aphids, which target soft tissue.

To flush a container, water slowly with three times its volume and do not feed for two weeks. In beds, simply stop.

When a plant looks wrong, check water, then light, then pH, and only then reach for fertiliser. The bottle is rarely the first answer.