Garden doctor: compacted soil

Frances Tophill explains how to help plants that struggle in hard soil

Soil is a magical substance that, if properly cared for, holds the key to thriving plants. But it can also be a real stumbling block that trips us and prevents our gardens from reaching their full potential. To get the basics of your soil right it helps to understand two terms: ‘texture’, which is the percentage of silt, sand and clay particles in your soil; and ‘structure’, meaning how those particles are held together. One of the most troublesome problems with soil is compaction, which occurs when the soil particles lock together in an impenetrable, rock-like formation. This is a problem with structure and is caused by pressure on the soil that, over time, compresses it. It may be that people have walked on that piece of ground over several years, or that vehicles have driven on it. It could simply have been badly waterlogged and never cultivated properly to bring it back to health. The good news is that spotting compaction is easy: water won’t penetrate, pooling on the surface, and digging is really difficult. You’ll also notice your plants look sickly and small. They’ll be having difficulty breaking through the tough ground caused by compacted soil, will be unable to access adequate water and may become root-bound. In extreme cases, compaction eliminates vital air pockets in the soil and plants will die. Be aware that some soil may look absolutely fine on the surface but be compacted below, often at a spade’s depth, where over the years you’ve never dug deeper than that. This is just as crippling for plants as compaction on the surface. Improving the structure To remedy the problem, start by puncturing the soil with a fork and then wait for rain, as the water will help to soften the ground ready for digging. Next, dig the soil with a spade, fork or rotavator. Make sure you dig right down to break any compaction in the lower layers. You should be aiming for a soil structure that has a crumb formation. This comprises small fragments of soil, not dusty, but held together in little clumps, which are easy to dig and have plenty of air pockets between them. With some compacted soils, it may be possible to simply dig or fork the ground over to break up the large lumps, but with other, more serious or long-term cases, a little more effort might be needed. Be reassured though, that your effort with soil will be rewarded. While digging, you can incorporate ingredients that will help your soil avoid compaction in the future. Add drainage material, such as washed gravel or sand, to help prevent waterlogging (which leads to compaction). You can also add organic matter (such as well-rotted garden compost) which is the glue that holds the individual particles of clay, silt and sand together in your crumb structure. Remember that once the problem is solved, you should avoid walking on your soil, especially if it’s wet or the soil is clay. The relationship between healthy soil and healthy plants is crucial. Given the right conditions underfoot (or under-root), plants can flourish with minimal input from us. A little exertion in the beginning will pay great dividends for the future.

NEXT MONTH: Mark Lane looks at growing plants in windy locations and discovers that some not only survive but thrive in such conditions