Heap and spade
In most gardens the compost heap is smaller than this!

Organic gardeners aim to feed the soil, and let a healthy soil take care of the plants. The best way to add fertility to your garden is to make compost, but in most gardens there is never enough compost to go around and there will be times when supplementary feeding is needed.

An obvious choice is to find a source of animal manure, but this is becoming increasingly difficult – many farmers and stables have realised that their waste products are a valuable resource. And if the animals are not organically raised then there are issues with chemical contamination.

If you do use manure then allow it to compost fully, in a separate heap, for three to six months before using it. Not only does this stabilize the nutrients (so that they’re not washed out by rain), but it also helps to reduce any contamination. Well-rotted manure can be dug into soil or applied to the surface as a mulch. It’s usually possible to buy well-rotted manure and ready-made compost in bags from garden centres.

Cultivating a comfrey patch in your garden can add considerably to its fertility. Comfrey draws up nutrients from the subsoil, and stores them in leaves. Comfrey is a vigorous plant and can be cut several times each year. The leaves are used as a compost activator, as a mulch or to make a liquid feed.

The standard method for making a liquid feed is to cover comfrey leaves in water and leave them for several weeks to decompose. The resulting brew is then watered down before use. However, there’s a real problem with this method – the liquid feeds it produces reek.

A better method is to allow the comfrey leaves to decompose without added water. A bucket with a tap is ideal for this, as it allows you to drain off the finished liquid; the remains of the comfrey leaves can go on the compost heap. This method is almost odourless. The resulting liquid can be diluted 15:1 with water before use.

Comfrey liquid is an ideal food for flowers and for fruiting vegetables such as tomatoes – rich in potassium and phosphorous, but low in nitrogen that would promote leafy growth.

There is a good range of commercially available organic fertilizers, although you may need to find an online retailer if your local garden centre doesn’t stock them. Many products use seaweed because it contains many trace nutrients, making it a balanced feed for plants that promotes strong growth. Seaweed meal is slow acting, dug in before planting. Seaweed extract is a great tonic or booster, used as a liquid or foliar feed.

Pelleted organic poultry manure is also widely available. This is potent stuff, very rich in nitrogen, and should be used sparingly.

N, P & K

  • Commercial fertilizers will have their NPK content listed on the label. Choose the right fertilizer for the job, and apply it at the rate suggested on the packaging.
  • Nitrogen (N) is the main nutrient involved in leafy growth. A fertilizer that is high in nitrogen can be used on leafy vegetables and to promote rapid growth in spring. Organic sources include poultry manure and hoof & horn. Too much nitrogen can encourage soft plant growth that is susceptible to pests and diseases.
  • Phosphate (P) is needed for good root growth. Organic sources include bonemeal and rock phosphate, but phosphate-only fertilizers are generally only applied to correct a known deficiency.
  • Potassium (K) promotes fruiting and flowering and organic products are generally made from plant sources. On its own potassium is best applied at the beginning of the season.

This article first appeared in Country Gardener in July 2008.