Improve soil fertility organically

From Sarah's garden to yours

As gardeners, each season, we are encouraged to enrich our soil to replace the nutrients used by the plants, and feed the soil micro-communities to keep everything fertile and healthy for the benefit of the garden. There is no right or wrong when it comes to gardening choices, and both synthetic and organic fertilisers benefit the garden in their own way. The greatest harm can come from under or over fertilising.

There are plenty of organic options available to give the garden a pick-me-up, not in the sense of “certified organic” but derived from animal or plant-based sources. Each one comes with its own benefits and disadvantages and understanding what they can do will help you make the best decisions for your garden. 

Manure 

All animal manures contain the full set of NPK and trace elements, however, they are in low numbers, so you need to use generous amounts. An added bonus is that they also contain beneficial bacteria that is good for plants and soil. Manure from vegetarian animals is best and each provides its unique value to the garden. It is best to avoid manures from cats, dogs, and people due to the risk of pathogens and disease. Manures bought from garden centres are most likely considered aged.

The downside is that there may be weed seeds that have passed through the animal and could germinate in your garden. Another risk is the presence of weed killer residues unaffected by the digestive process that can cause deformities in plants rendering the garden useless for many seasons. It is important to check that fresh manure hasn’t come from fields sprayed with a broadleaf herbicide.

Hot manures 

These are rich in nutrients, low in moisture content and quick to decompose, but are too strong to be used fresh and will burn plant roots, inhibit seed formation, and cause stunting or even death. They should be aged to become ‘well-rotted’ to the point they look and smell more like soil than manure. 

• Horses – Requires at least six months to break down before use.
• Chickens – Quite strong and best added to the compost heap to break down before use.

Cold manures 

These are lower in nutrients than hot manure but on the plus side won’t harm the plants when used fresh.

• Cows – Considered the best farmyard manure, packed full of great nutrients, and breaks down into a lovely soil conditioner. 
• Sheep / goat / alpaca – A mild additive, although it is better to let it age a little rather than using fresh. It can be found in pellet form in most garden centres.
• Rabbit / Guinea pig – As rich as horse or cow manure but is safe to be used fresh directly onto the garden.
• Worms – ‘Castings’ or ‘wees’ are a great conditioner to help increase soil's water retention, improve soil aeration, and lock nutrients into the soil to prevent leaching. Castings also feed beneficial soil microorganisms. Wees is used to make worm tea, a liquid fertiliser.

Compost 

This is the most common addition to the garden and although it can be found at garden centres relatively cheaply, it is also easy to make at home with compost systems to suit every taste. It is good for the environment to keep nutrients within the garden by returning plant material produced in the garden back to the garden instead of sending it away as green waste.

Essentially, it is a combination of one part green garden waste (fresh lawn clippings, vegetable peelings, etc.) and two parts brown dry waste (hay, shredded newspaper and cardboard) that all rots down together in perfect harmony to give you a great soil conditioner. 

Compost becomes a source of food for the microbes in the soil. Once added to the soil it can take a further 5 years to completely break down and is slowly released as bio-available molecules that plants can absorb. It’s airy and spongy texture also helps improve soil structure and holds water. 

A 2 - 5 cm layer each year is enough to regenerate soil. You can leave it on the top or dig it in, it doesn’t really matter. On its own, compost can be too rich and plants grow better in a soil-based mix.

Blood and bone 

Another common additive. It is made of dried and ground blood and bone and sometimes includes fish, seaweed, and other ingredients. It is a good all round rich organic fertiliser that releases the nutrients slowly and improves the soil structure and increases earthworm and microbial activity. 

Fish

Fish are full of the nutrient’s plants love; they are high in NPK and a healthy range of trace elements. Bury fish frames about 30cm in the garden and dogs and cats digging them up. Adding to the compost may attract pests. They take 3 – 4 months to completely break down in the soil, depending on conditions. 

Seaweed

This is generally low in NPK but rich in trace elements. It benefits plants by improving resistance and protection from pests, drought and frosts and reduces transplant shock. It is a natural source of plant growth regulators, such as auxins, cytokinins, gibberellins and betaines, as well as alginates and minerals. It can stimulate plant and root growth and aids recovery from stress conditions. It has been used as a soil improver for centuries. If using fresh from the beach, harvest responsibly and rinse off the salt before adding to the garden.

Cover crops 

Also called Green Manures, these regenerate soil when beds would otherwise be empty. Sow thickly across bare soil and chop down and return to the soil before flowering. After 6 -8 weeks the organic material will be broken down and the nutrients returned to the soil.

The benefits of cover crops:

• Covers bare earth and out competes the weeds making the spring preparation easier.
• Prevents soil erosion and compaction caused by heavy winter rains.
• Prevents nutrients being leached from the soil in winter rains by being locked in the leaves.
• Their roots can penetrate deeply and improve the structure of the soil.
• They break down to release nutrients to enrich the soil and improve soil structure. 
• Gives a vibrant look and feel during the bleak and dismal off season.

Types of Cover crop:

• Mustard helps cleanse the soil of some soil borne pests and diseases. As a member of the brassica family don’t use before or after a brassica crop.
• Legumes help to restore a nitrogen balance through their nitrogen fixing capabilities and are good to use following large hungry crops. 
• Cereal crops like oats can add bulky material to the soil to help improve structure.
• Buckwheat can help improve the availability of phosphorus in the soil. 
• Daikon Radish breaks up hard compacted soil, with its enormous tap root. Leave in place to die to return the nutrients it pulled up from deep with the soil. But don’t let them flower. 
• A blend of several cover crops can work together for the benefit of the soil.

Lightning 

During a lightning storm, Nitrogen in the atmosphere is combined with oxygen and dissolves in the rain and falls to the ground in a form that plants can use.

There are also plenty of other things that can improve the soil including mushroom compost, leaf mould, wood ash (used very sparingly). Taking care with your soil will mean a better, healthier harvest at the end of the day and that is what we are after.