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Soils: fertility

Fertility rites

 Plants require at least 16 nutrient elements for growth. These include carbon, hydrogen and oxygen, which are obtained from the air or water and make up most of the plant tissue. They also need macronutrients – nitrogen, phosphorous, potassium, calcium, magnesium and sulphur – in relatively large quantities; as well as micronutrients – boron, zinc, molybdenum, manganese, iron, copper, and chlorine.

plants growing in greenhouse

Plants require at least 16 nutrient elements for growth.

The macro- and micronutrients are absorbed from the soil through the plants’ roots. However, they are not always present in large enough quantities to satisfy a crop’s requirements, in which case their levels must be boosted, and different strategies can be employed that will do the job. However, any plans to improve fertility should give priority to nutrient recycling within the garden, while reducing the need for brought-in materials.


Bulky organic materials

Bulky organic materials can be used as mulches or worked into soils to increase their levels of organic matter. They also play a valuable role in supplying plant nutrients, though their concentrations vary widely. Peat, for example, is low in nutrients and is almost useless as a nutrient source. Similarly, fibrous materials such as sawdust should be approached with caution since they use nitrogen from the soil as they decompose, thereby depleting the amount available for plants. Garden composts and animal manures are in a different class, and when compared to other bulky materials, they are nutrient rich. Nevertheless, they are still a poor source of fertility in comparison to the more concentrated proprietary fertilisers, and need to be used in large amounts to supply a crop’s nutrient needs.

Wheelbarrow with bulky organic material

Bulky organic material destined for the compost heap.


• Animal manures

Animal manures are made up of faeces and urine (except for poultry manure, which has no urine), plus the straw or other material used as bedding. The nutrient value of manure varies greatly, and depends on the animal species from which it came; the type and quantity of bedding material used; and the handling and storage conditions.

Whatever the source, animal manures should be managed in a way that makes full use of their potential. They should be applied only after they have been properly composted or well-rotted. The nutrients are highly soluble, and leaching losses must be prevented by covering heaps stacked outdoors with plastic or another inert, impermeable material. Manures from chickens are especially rich in nitrogen, so should be mixed with straw and used more sparingly than those from sheep, cattle and horses.

Rotted animal manure heap

Animal manure must be properly composted before it is used.

• Garden compost

 Composting is a process that breaks down plant and animal matter into a dark, crumbly material. It recycles kitchen, garden and animal wastes, reducing the need for importing soil improvers and fertility.

Successful composting establishes a moist and well-aerated environment where bacteria, fungi and other organisms can thrive. Wet soft green material high in nitrogen (including raw kitchen wastes, fresh plants and grass clippings) should be mixed with dry material high in carbon (such as dead leaves and straw). With the right combination of ingredients, composting will start spontaneously, without the need for further intervention.

  grass clippings  Compost heap with wooden sides

Grass clippings are high in nitrogen.

Not everything, however, can be composted. Meat and fish, dog and cat faeces, and coal ash, for example, should be left out. Disposable nappies are also a no-go area. Perennial weeds can go in, but only after they have been killed off – otherwise they may start growing in the compost.

When it comes to composting, there are enough tried-and-true techniques to keep even the most demanding gardener happy:

Hot composting. Gardeners needing a quick fix of compost can try hot composting. The waste materials are composted at one go, initially heating up to temperatures hot to the touch before eventually cooling down. The material should be turned to generate more heat and hurry the process along. Most weed seeds and diseases are killed by the high temperatures generated during the process, though material infected with persistent diseases, such as onion white rot, may survive so should be excluded from the compost.

Cold composting. Cold composting is a protracted affair that can take many years to complete. Wastes are added to the pile as they become available, making it a practical approach when there is not enough material to compost at one time. Cold composts may or may not heat up, and because of this uncertainty, there is no way of knowing if diseases organisms brought-in on garden plants have been killed. Weed seeds, too, may survive the composting process, so only seedless weeds should be added to the compost heap. Plastic garden bins, shaped like barrels, provide a neat and tidy way to cold compost garden and kitchen wastes.

  compost heaps plastic compost bins

Cold composting: traditional heap and plastic garden bins.

Worm composting. Worm composting uses tiger worms found naturally in compost and manure heaps. Collected from existing heaps, the worms are then confined to containers that provide them with a cool, moist environment. Commercially-made plastic wormeries are available but something as simple as a wooden box will do. The worms thrive on a varied diet that includes vegetable peelings, annual weeds and even paper. Though they are not overly fussy about what they eat, they are modest eaters and need only a little food at a time.

worms in compost bin Worms thrive on a varied diet.

Concentrated fertilisers

Concentrated fertilisers contain high levels of the macronutrients nitrogen, phosphorous and potassium, either individually or in some combination, and often contain some micronutrients. They are most valuable as a quick fix when the nutrient levels of the soil are low, and are used in either solid or liquid forms.

• Solid fertilisers

Solid fertilisers are usually mixed into the soil before sowing or transplanting. Alternatively, they can be scattered on the soil surface next to growing plants as a top dressing to give them an extra boost of nutrients – rain or tap water falling on the soil will dissolve the nutrients and carry them through the soil to the roots for absorption by the plants.

Popular among organic gardeners are the so-called ‘natural’ fertilisers that are usually of animal origin. A particularly favoured product is made from composted and dehydrated chicken manure compressed into pellets, making it easy to apply. Though a bit old fashioned, dried blood (high in nitrogen) and bone meal (a source of nitrogen and phosphorous) have proven their worth over the years and are still readily available in nurseries and garden centres. Other than the dried blood, the natural fertilisers are generally slow to break-down and release their nutrients.

bonemeal fertiliser for growing vegetables blood meal for growing vegetables

Bone meal is a source of nitrogen and phosphorous; dried blood is high in nitrogen.

  chicken manure pellets  chicken manure fertiliser Chicken manure can be bought from garden centres

Chicken manure pellets

In contrast to the natural fertilisers are the chemical equivalents manufactured into powdered or granular formulations that are convenient to use.  They include the proprietary brand Growmore®, as well as the more generic potassium sulphate (a particularly good source of potassium) and ammonium nitrate (used for its high level of nitrogen).

Growmore fertiliser for growing vegetables  Growmore fertiliser for growing vegetables

The proprietary brand Growmore® can be bought at most garden centres.

 • Liquid feeds

 Liquid feeds are used to boost the growth of transplants, pot-grown plants and nutrient-stressed plants already established in the garden. Though proprietary materials are readily available in garden centres and catalogues, DIY versions are easy to make. All that is needed is a large tea bag made from a porous material like jute. The bag is filled either with animal manure, comfrey or wild nettles, then steeped in a barrel of water or other large container.

Comfrey is a particular favourite among the natural liquid feed brigade, probably because a comfrey crop is so undemanding. They are easy to propagate from offsets or cuttings taken from the roots. Comfrey is, however, like a bad reputation – easy to get, but hard to get rid of – and the patch where it grows should be permanently given over to its cultivation.

Comfrey makes a good liquid fertilser  liquid comfrey fertilser for growing vegetables

Comfrey makes a good liquid feed.

Proprietary liquid feeds are also available as either natural or chemical forms. They can be bought as a concentrated liquid that then needs to be diluted, or in a solid form that is dissolved in water before use. Miraclegrow® is an example of a liquid feed that is bought in granular form and applied, as a crop is watered, through an attachment to the hosepipe.

fertilisers at a garden centre

Miraclegrow® sold at Groves Nursery, Dorset

Green manuring

 Green manures are plants cultivated specifically for improving the soil. As they grow, they smother weeds, protect soil structure from the ravaging effects of rain and sop up nutrients that would otherwise be leached out. Fibrous rooted crops such as grazing rye and ryegrass can promote soil structure, while legumes like clovers contribute much-needed nitrogen. When still young green manures are worked into the soil, where they decay and add to the pool of nutrients and organic matter already there.

green manure in old sweet corn crop

Green manure planted into sweet corn crop.

Not all is good news with green manures, and crops recently incorporated into the soil will physically interfere with the preparation of the fine seedbed needed for seed sowing and germination. In addition to this physical meddling, green manure residues can also release chemicals that inhibit seed germination. Both problems can be overcome by waiting. In a vegetable plot, however, the wait can be shortened if transplants are used – they are better adapted to coarsely-prepared soils while being less susceptible to the toxic substances given off by the decomposing green manures.

Clover green manures increase the nitrogen content in a soil. Clovers, like all legume crops, establish a symbiotic relationship with Rhizobium bacteria, which take up residence in nodules formed on the roots. The clovers contribute high-energy carbohydrates to the bacteria, which reciprocate the favour by passing on the nitrogen that they have extracted from the air. This nitrogen is used by the clovers for their own growth, and is available to other plants when they decompose in the soil.

clover seedlings for green manure  White clover green manure

White clover green manure at the seedling stage and in flower.

Rhizobium nodule on a french bean root  Close up of rhizobium nodules

Rhizobium nodule on a legume root; close up of rhizobium nodules

Non-legume green manures such as ryegrass, grazing rye, phacelia and mustard do not form symbiotic relationships with Rhizobium bacteria. They are still valuable as a green manure, however, and they have a role to play in well-designed rotations.

Some precautions must be exercised, especially with brassica green manures, such as mustard. Brassicas are particularly attractive to pest and diseases and their use in the garden should follow a strict rotation, whether they are a green manure or a vegetable crop.

Down the drain

Good drainage is a sign of a healthy soil. It means that plant roots are well-aerated and are getting the oxygen they need to support strong, vigorously-growing plants.

Poorly-drained soils become water logged and must be amended to make them more amenable to growing crops. If a soil has a hard pan holding up water, drainage can be improved by breaking up the pan through deep digging. Sometimes a clay subsoil can hinder water movement down the profile, and working organic matter into the clay may open it up. Using raised beds and promoting a stable granular structure in the topsoil also improve drainage. If all else fails, then a drainage system made of rubble-filled trenches or special drain pipe can be put in. 

Poorly ftsinrf soils become waterlogged

A poorly drained soil

What goes around comes around

Rotation is a system of production that avoids year-after-year cropping of related plants on the same piece of ground. It is effective in controlling the build-up of pests and diseases. Rotations are usually based around plant families and should be planned to allow the longest period possible between growing crops of the same family. Plant families that need particular care are alliums (onions, shallots and leeks) and brassicas (cabbages, radishes, oriental leaves, etc.).

Rotations, however, are not just about controlling pests and diseases. Properly designed, they should include legumes to fix nitrogen, fibrous rooted crops to improve structure, and green manure crops that increase organic matter levels. In addition, leafy crops such as brassicas and potatoes compete well with annual weeds, and may be grown to clean up the ground for subsequent, less competitive crops such as carrots and onions.

There is no one definitive rotation that suits all gardens. The best anyone can hope to do is follow the principles, observe the results and adjust the cropping patterns as necessary.


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© Michael Michaud