Soil Amendments for Vegetables

No matter what type of soil you have it can benefit from organic amendments: compost can improve the structure and drainage properties of soil containing too much sand or clay. Lime can make acid soil more alkaline. Even good loam needs to have nutrients replaced each year that are used up by growing plants.

The primary amendments for the organic garden are lime, compost, green manures, animal manures, and organic fertilisers.

Determining What Your Soil Needs

A soil test can check your soil’s mineral content and acidity. Test your soil before adding nutrients, particularly if you’re starting a new garden or if your plants aren’t thriving. In general, testing every two or three years is sufficient.

Soil can be tested with a home test kit from the garden centre or by a soil testing lab. Be sure to test samples from several areas of your garden, as results may differ. Testing in the autumn will avoid the spring rush and allow you to lime in the autumn, if necessary.

Raising pH Levels

In a cool, damp climate such the UK, soil tends to be acidic, as rain washes away the calcium. Lime, which is ground limestone (calcium carbonate), is added to raise the pH in acid soils. Dolomitic lime, which also contains magnesium, is best: apply about 6 pounds (3 kg) of lime every 100 square yards (metres) for average loam. For sandy loam use 4 pounds (2 kg), for heavy clay use 8 pounds (4 kg). Beware of adding too much lime. Also, don’t lime where you plan to plant tomatoes or potatoes the following spring.

Wood ash from a woodstove or fireplace (but not coal or charcoal) can be used in place of lime. Wood ash also adds potassium and some phosphorus, and usually has about a 0‐1‐3 (N‐P‐K) ratio.

What is Compost?

Composting is the conversion of organic material into humus. Humus is organic matter that is converted by microorganisms into a state that can’t be broken down further. Composting occurs in nature without any human intervention ‐ all organic matter eventually decomposes. The gardener’s goal is to speed up the process.

Why Use Compost?

Compost is the best way for organic gardeners to add beneficial minerals, nutrients, and organisms to their gardens. It also improves water retention and soil structure, and prevents erosion.

How Can I Make Compost?

Compost can be made in a simple pile, or in any number of commercial or home‐made bins. One of the easiest bins is made from 48‐inch‐wide (1.2m) hardware cloth with 1/2‐ to 1‐inch mesh (12‐25mm), formed in a circle.

Compost is made from kitchen scraps (vegetables and fruits), grass clippings, and plant debris. Do not compost meat, fish, dairy products, or pet waste. Weeds can be added if they haven’t gone to seed, but beware of invasive or perennial weeds. Autumn leaves decompose slowly: shred or add sparingly (or compost separately for leaf mould).

Composting tips:

  • Make compost bins about one yard (metre) wide to create the optimal internal temperature.
  • Alternate brown (dried) and green (fresh) matter.
  • Cut material into small pieces.
  • Keep the pile moist but not wet.
  • Turn compost to aerate and speed up decomposition.
  • Compost is ready for use in 2 to 12 months when it looks like dark, crumbly soil.

If you can’t make (enough) compost, you can purchase it from a garden centre. Worm composting is another good option.

Green Manures

Green manures are crops grown for the purpose of digging them into the garden, thus adding nutrients directly to the soil. They also act as cover crops, preventing soil erosion and loss of nutrients due to run‐off. Green manures are often planted in the late summer or early autumn and dug in the following spring (before they flower and reseed).

Legumes such as peas and beans can be grown both as crops and as green manures. They have the added bonus of “fixing” nitrogen: bacteria that live in nodules on their roots take nitrogen from the air and convert it into a form that plants can absorb. The peas or beans are harvested first, then the plants are dug under. Winter tares are also a good nitrogen‐fixing green manure.

Green manures that don’t fix nitrogen include winter rye and winter wheat. Plant them in late summer and they’ll come up in early spring. Dig under before planting late spring crops. Mustard is also a good green manure – it’s quick growing and good at keeping out weeds.


Well‐rotted manures from organically‐raised cows, horses, and chickens can add nutrients to garden soil. Be sure not to apply manure and lime at the same time, which will create ammonia gas and waste beneficial nitrogen.

It’s possible to have perfectly healthy soil without the addition of animal materials. Recent E. coli outbreaks traced to commercially‐produced vegetables have led many organic gardeners to avoid the use of animal manures.

Organic Fertilisers

If you don’t have enough compost, or your soil needs a lot of a particular nutrient, organic fertilisers may be your best bet.

Organic fertilisers include plant‐based products such as seaweed meal, and animal‐based products such as blood, fish, and bone meal. In the UK, the Soil Association does not recommend using seaweed or peat as soil amendments for ecological reasons.

Healthy soil means healthy plants – and more abundant and nutritious vegetables!

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