Soils Q&A

Although we usually notice problems with plants manifesting themselves above ground, the root cause is often at the roots!

Plants with unhappy root systems become weak, and are more vulnerable to pests and diseases that merely finish the job.

Soils first need the right mix of sand, silt and clay particles to form the best environment for roots to colonise.

Overly sandy soils rapidly dry out and become lifeless, while clay soils are difficult to work and can easily become waterlogged and oxygen deficient. The ideal for most garden plants is a loam, which means roughly two parts sand and silt plus one part clay.

Most plants also require a relatively neutral pH (soil acidity) to thrive. If your soil is particularly acid or alkaline, plants may appear to struggle despite good care being given.

Different soil types – how do I improve sandy, clay, hydro-phobic?

If your soil is overly sandy, you can permanently improve it by adding a small amount of clay, usually up to around 10% by volume.

However this must be very well incorporated, so it is often not practical to do without the use of mechanical cultivators.

Biochar makes a great addition to sand as it has a very high surface area, and therefore holds on to water and nutrients that would otherwise be leached out, while providing habitat for soil life. Biochar is worked into approximately the top 20cm of soil and can last hundreds or thousands of years, so it is also a near-permanent amendment.

A small amount of sand added to clay soils can worsen the soil, so it is important to add at least 50-100% by volume, again mixing this in very well. Gypsum is often used to help break up heavy clays, but it is only useful if you have a dispersive soil. In these soils, high levels of sodium cause clay particles to separate when wet, destroying soil structure and eliminating air from the soil.

To test your clay soil take several pea-sized pieces of well-dried soil sample and place in a shallow tray of rainwater or distilled water. Dispersive soils will form a milky halo around the soil pieces over 24 hours – or in as little as ten minutes for strongly dispersive soils. If you do have a dispersive clay, up to 500g of gypsum per square metre can be applied to the surface or worked into the topsoil to remedy the problem.

Almost all soils can be improved simply by the addition of organic matter.

Incorporating compost, green manures or aged animal manure will increase drainage in heavy (clay) soils and improve nutrient holding in light (sandy) soils. Avoid using fresh manure as it can burn plant roots, and don't incorporate any dry, woody materials into the soil because they will starve plants of nitrogen; these things should be composted first instead, or applied as a mulch layer.

I heard someone say that soils were living, what does this mean?

Five percent or more of healthy soil is made up of microscopic organisms, known as the ‘soil food web’. These are good bacteria, fungi and larger organisms that collectively break down the organic matter in the soil. The smaller creatures are a food source for the larger ones, and their waste products are food for plants. If you';ve ever made a compost pile and watched heat rise from it on a cool day, this heat is generated by the activity of the millions of microorganisms breaking down the contents of the pile, and the same process happens slowly in ordinary garden soil too, with the right conditions.

The really exciting soil research of the last 20 years is showing that not only do these microorganisms produce good waste products, many of them also attach to plant roots directly to provide water and nutrients in exchange for plant sugars. This means the plants can obtain nutrients from much further away than just the span of their root systems.

Minimising soil disturbance, using only organic based fertilisers and avoiding chemical sprays will help to stimulate a healthy soil food web, creating the best possible environment for thriving plant roots.

I was told a good garden soil was almost 50% air, could you explain why please?

Just like humans, plants need to breathe! Although the green parts of the plant perform photosynthesis, breaking down carbon dioxide into oxygen, the leaves and roots also ‘respire’ which converts that stored food into energy just like in our own bodies – and this needs oxygen.

Good soil is very well aerated, with many tiny air spaces between the soil particles containing oxygen for these growing roots. The effect of over-compaction and waterlogging in soil is identical: starving plant roots of oxygen so that they cannot utilise their stored energy for critical life-maintaining processes.

This often happens in potted plants after several years as potting mix breaks down and compacts, leaving soil pores clogged. In undisturbed garden soil, a healthy soil food web supports organisms like worms that improve aeration by the tunnels they create while moving about.

Compaction is one reason why digging over your vegetable beds seasonally can have an initially good result, though over several years this will create other problems. It is also why many vegetable gardeners try not to walk on their soil, using ‘double reach’ style beds that are fully accessible without standing on.

Why is humus important and how do I create it?

I think humus is best described as the stable end product of organic matter breakdown. It is a black, crumbly material that is not well understood chemically, but known to be highly beneficial to plant growth by storing water and nutrients as well as improving the soil structure. The blackness of very rich soils often indicates the high level of humus contained within them.

Making your own compost is the traditional way to generate humus, whether it is from a large compost pile, worm bin or other technique.

Mulching replicates the way that nature creates and preserves humus, by forming a layer of organic matter on top of the soil that protects from erosion and slowly decomposes. Humus can last a very long time in the right conditions but it is destroyed by overexposure to oxygen, so try to keep topsoil moist and avoid regular digging.