Soil, compost and what plants actually need

Attention in gardening goes to plants and products, and the growing medium determines most of the outcome.

What soil is

A mixture of mineral particles, organic matter, water, air and living organisms.

The mineral proportion determines the type: sand drains fast and holds little; clay holds water and nutrients and drains slowly; silt sits between.

Loam is a balance of all three and is what most plants prefer.

A simple test: wet a handful and try to roll it into a ball. Sandy soil will not hold together; clay rolls into a smooth ball and can be formed into a ring; loam holds together and breaks apart readily.

What improves any soil

The answer is the same regardless of type: organic matter.

In sandy soil it improves water and nutrient retention.

In clay it improves structure and drainage.

Added as a mulch on the surface, it is incorporated by worms and soil organisms without digging, which is what the no-dig approach relies on.

Sources: garden compost, well-rotted manure, leaf mould, spent mushroom compost, and composted bark.

The one consistent recommendation across almost all gardening advice is to add organic matter annually, and it is more effective than any fertiliser.

Making compost

Which is straightforward and frequently overcomplicated.

The requirement is a mixture of nitrogen-rich material — grass clippings, vegetable waste, coffee grounds — and carbon-rich material — dry leaves, cardboard, straw, woody prunings.

Roughly equal volumes, layered or mixed.

Plus air, which means turning occasionally or building in ventilation, and moisture, which means damp but not wet.

What not to add: cooked food, meat and dairy, which attract vermin in an open heap; perennial weed roots and seeding weeds; and diseased material.

It takes several months to a year depending on conditions and how often it is turned.

Compost for containers

A different product from garden compost.

Potting compost is formulated for drainage, structure and nutrient content in a confined volume, and garden soil in a pot compacts and drains poorly.

Peat-free formulations have improved substantially and are now the appropriate default, given the carbon and habitat consequences of peat extraction, and several jurisdictions are phasing out peat in horticulture.

They dry out differently from peat-based products, which requires some adjustment in watering.

Container compost is exhausted of nutrients within about six weeks, after which feeding is required.

Feeding

Where products are oversold.

The main nutrients are nitrogen, which drives leafy growth; phosphorus, for roots; and potassium, for flowers and fruit.

The proportions are stated on packaging, which is why a tomato feed is high in potassium and a lawn feed is high in nitrogen.

Plants in open ground with good soil generally need little or no feeding.

Plants in containers need regular feeding during the growing season.

Over-feeding causes soft growth prone to disease and pests, and in containers causes salt build-up.

Feeding a plant that is struggling for another reason — light, water, root problems — does not help and frequently makes things worse.

pH

Which matters for specific plants.

Most plants tolerate a range around neutral.

Ericaceous plants — rhododendrons, camellias, blueberries, heathers — require acid conditions and will not thrive in alkaline soil regardless of feeding, showing yellowing leaves as a symptom.

In alkaline areas these are better grown in containers of ericaceous compost than fought with in the ground.

A simple test kit establishes the position, and it varies within a garden.

Mulching

The practice with the widest benefit.

A layer of organic material on the soil surface suppresses weeds, retains moisture, moderates soil temperature, and gradually improves the soil beneath.

Applied to moist soil, several centimetres deep, kept clear of plant stems.

Autumn and spring are the usual times.

For most gardens it is the single most effective annual task, and it replaces a good deal of weeding and watering.

Improving soil rather than feeding plants

The distinction that changes results.

Fertiliser feeds a plant for a season; organic matter improves the soil permanently, and improved soil grows better plants with less intervention.

Which means the most valuable garden practice is adding organic matter annually — garden compost, well-rotted manure, leaf mould — as a surface mulch rather than dug in.

It improves drainage in heavy soil and water retention in light soil, which sounds contradictory and is not: both are consequences of better structure.

It feeds soil organisms, which do the work of making nutrients available.

And it suppresses weeds and reduces evaporation while doing so, which makes it the single highest-return job in a garden year.