Vegetable production: How to build soil fertility with organic fertiliser


Chemical fertilisers may be expensive, but they are needed for optimal vegetable production. A panel of experts from the South African Agricultural Research Council (ARC) has the following advice on how to build soil nutrient status with organic fertilisers.

Where funds are limited, use at least 25% of the recommended chemical fertiliser and apply organic fertilisers such as manure, compost and comfrey, to improve soil fertility. Remember though that most organic fertilisers break down slowly and large quantities are needed to achieve the same impact as chemical fertilisers.


Manure is a natural animal waste product that, if decomposed, can greatly improve the general condition of the soil.

Animals that consume lucerne, good-quality hay, silage and concentrates will produce better manure than animals that graze in the veld alone.

Animal bedding – especially soiled hay and straw – also contains many valuable nutrients, particularly nitrogen, which absorbs urine.

The quantity and type of nutrients present in manure depends on:

  • The type of animal or bird from which the manure is derived
  • The animal’s diet
  • Whether or not straw has been used as bedding
  • The way in which the manure was handled before being used

Poultry manure is usually the best, followed by sheep, horse, cattle and lastly pig manure. Since poultry manure contains large quantities of nitrogen, it should be used carefully to prevent it from burning crops. Apply lightly (150 to 200 g/m²) and work it well into the soil before planting the crop.

Liquid manure is usually applied to plants as top-dressing. It is easy to prepare and is beneficial for most vegetables.
Prepare it in this way:

  • Place a bucket of manure in a hessian sack.
  • Hang the sack in a drum of water.
  • Some of the manure will dissolve in the water.
  • After two weeks, fill a bucket or watering can to a quarter full of the dissolved manure and fill up with water.
  • This bucket full of liquid should be sufficient for 1 m².


  • Green manure, also called cover crops, are plants that are grown to benefit the soil rather than for consumption.
  • Green manure replaces and holds nutrients, improves the structure of the soil and increases its organic content.
  • Crops such as cowpeas, pigeon peas, Sunn hemp, clover, lupines, oats, babala (millet) and grazing rye are excellent green manure crops.
  • Green manure can be planted in areas that lay fallow before the next season.
  • It should be planted early enough to give four to six weeks of growth and it should be ploughed, disced or dug into the soil at least four to six weeks before the next crop is planted to give time for the green material to rot down.
  • It is best not to allow green manure to go to seed, as all types have the potential to become weeds.


  • Compost improves the soil structure by upping its water retention capacity as well as the availability of nutrients.
  • Compost is made by decomposing a variety of materials of plant and animal origin, such as plant debris, animal dung and chicken manure.
  • Organisms such as bacteria, fungi, earthworms, snails, insects and birds help to decompose the material and turn it to humus.


  • Seaweed is easy to gather at the coast.
  • Rinsing the seaweed well with water helps to remove any extra salt that could harm the plants.
  • The seaweed can then be used as mulch around the plants.
  • Dig it into the soil when the season is over and the crop has been harvested.
  • Seaweed can also be soaked in water for two to three months or until the water turns brackish-brown.
  • Mix it with equal quantities of fresh water and apply the dilution while watering, every one to two weeks.
  • Seaweed is rich in potassium and other trace elements, but is extremely low in nitrogen and phosphorus.
  • Adding fish emulsion to the dilution will balance the fertiliser and ensure that it contains all the essential elements.
  • Seaweed fertiliser can be used for soil treatment before planting. It can also be applied as a foliar spray, or it can be poured into the soil around the base of a plant.


  • Blood meal has a high nitrogen content and works relatively slowly but it has a long-lasting effect.
  • Bone-meal contains large amounts of phosphorus.
  • It’s a good idea to add a few bones and some blood to your compost heap.
  • If an animal is slaughtered, its blood can be collected, dried and added to the compost heap.


  • Comfrey leaves are a natural source of nitrogen, phosphorus and potassium.
  • Freshly cut comfrey leaves can be applied as:
  • Mulch to fruit and vegetables.
  • Fertiliser in planting holes.
  • The leaves break down rapidly and provide nutrients at the roots.
  • Liquid fertiliser (comfrey tea).
  • To make it, fill a barrel or dustbin halfway with fresh comfrey.
  • Add water, cover, and leave it to stand for three to six weeks.
  • This solution can be used at full strength or diluted to half strength.


  • Vermi-composting or vermiculture is the process of growing earthworms in a bin in order to turn organic waste into high-quality compost.
  • The earthworms feed on organic raw food waste such as vegetable peels, livestock manure and other plant material, leaving behind rich compost and a liquid fertiliser.
  • The excretion of earthworms is called castings (worm manure) and leachate.
  • Earthworm castings are excellent soil enhancers and leachate acts as a soil catalyst to help release locked nutrients and provide stimulants for plant growth and flowering.

Build your own worm farm

For the best results, earthworm castings and leachate should be produced in aerobic conditions.

  • Earthworm colonies are kept in containers.
  • The containers are made from plastic basins with holes in them (three holes about 5 cm above the base of the container) to facilitate the catchment of leachate.
  • Make a base of dry grass (straw) or shredded cardboard to a depth of 30 mm to 40 mm.
  • Start with a layer of soaked hay. Introduce worms, they will disappear into the soaked hay to avoid light.
  • Add farm waste (kraal manure, horse manure, goat manure or pig manure).
  • Chicken manure is not suitable, as it contains ammonia.
  • Add kitchen waste (such as potato peels, cabbage, lettuce, egg shells, teabags and lettuce) but do not add acid-rich foods (such as orange and pineapple peels, onions and tomatoes) as this will kill the worms.
  • The earthworms, which decompose the organic matter, require moisture, oxygen and cool temperatures.
  • Too much heat will kill them.


  • Don’t use poultry manure when planting a root crop
  • Don’t plant carrots in manure or compost-rich soil
  • Rain that falls on unprotected manure washes out many nutrients
  • Manure should be covered or worked into the soil as soon as possible

Also read:
How to test soil life and health
Earthworm tea gives soil life a kickstart
Maintaining soil health – Make your own compost

  • This article was compiled by Sunette Laurie, Erika van den Heever and Lulama Mkula from the South African Agricultural Research Council and first appeared in Farming SA.

share this