Healthy topsoil is vital for any garden, and if your soil isn’t in good condition fertility can plummet – and fast. The magic ingredient that keeps soil healthy is the addition of organic material every year, which helps it hold onto moisture and nutrients. Healthy soil is rich, crumbly and full of life, and absolutely essential for successful organic gardening.
The most important element in your plot isn’t the plants. It isn’t the animals either, nor the water, nor the climate: it’s the soil. Good old common-or-garden topsoil.
If you think of soil as an inert substance to plant things in, think again. Healthy soil is a bustling ecosystem of plants, animals, fungi and bacteria, with enough biodiversity to make a rainforest blush. It holds nutrients and water well, so plants that grow in it are healthy and suffer less from diseases and pests than plants grown in poor soil. And the magic ingredient that fuels this ecosystem, keeping soil and plants vibrant and healthy, is compost.
Picture Bob, who’s new to growing. He’s given a half-sized allotment plot with soil in great condition, and he buys a selection of seeds and gets planting – but he doesn’t know about feeding the soil. The first year is fantastic. He gets loads of fruit and veg, and hardly sees any pests. The second year is OK, but yields aren’t so good and he has one or two problems with pests and diseases. The third year yields are disappointing, and the plants are clearly in trouble: the level of soil in the beds has fallen too, and it seems to dry out really quickly. Bob starts applying stronger fertiliser the following year, but it just makes the pest situation worse, which is bad news for neighbouring plots. So what’s gone wrong with Bob’s plot?
What makes soil fertile?
Deciduous woodland usually has deep, fertile soil. The reason for this is that it’s constantly being top-dressed with leaves, animal dung and other organic material. Over time this rots down and is gradually incorporated into the soil, where the nutrients are recycled and become available for plants again. More importantly, the soil becomes enriched with a complex mixture of stable organic compounds, which are collectively called humus. Humus helps the soil hold onto moisture and nutrients. It also helps the soil form a crumb-like structure with plenty of air spaces, so it bulks up – just like a sponge.
Bob’s topsoil has ‘slumped’, which means that the humus content has fallen year after year. The structure has collapsed, leaving it less able to hold moisture and nutrients. As a consequence Bob’s plants are perpetually short of nutrients, which makes them prone to pests and diseases: adding a nitrogenous fertiliser will produce lots of dramatic-looking sappy growth that’s just perfect for sap-sucking nasties like greenfly and whitefly, which will introduce fungal and bacterial infections. Bob will end up spraying with insecticides and fungicides just to keep on top of this imbalance, so it’s bad news all round.
Building soil fertility on your plot
The soil in a vegetable garden isn’t getting this natural top dressing of organic material, so if we want it to stay in good condition we have to add it ourselves. This is done by giving it an annual dressing of compost or manure each autumn. Traditionally this is dug in to a spade’s depth, but if you practice no-dig gardening you can simply leave it on the surface for the worms to pull down over the winter. As the compost is incorporated into the soil it breaks down leaving stable humus behind, and by spring the soil will be in great shape to hold onto a top-dressing of organic fertiliser such as blood, fish and bone meal or calcified seaweed.
If your soil is looking anything other than rich and crumbly with plenty of worms, the chances are that it’s short of humus. Poor soil can’t be improved overnight, but adding a barrowload of organic compost (or well rotted manure) per five square meters (50 square feet) annually will make a difference that has to be seen to be believed – whatever your soil type. Over a whole vegetable plot this adds up to a whole lot of compost – which is why we should all be very interested in learning to make the stuff ourselves. More soon!