When we begin to cultivate plants, we are growing more intensively than mother nature had intended, and this presents problems.  If we continue to crop our soil, the organic matter it contains slowly declines as the microbes in the soil decompose it.  Every time we remove a crop from the soil, we also remove the nutrients with those crops, and our soil can become slowly depleted.  So we have to make sure that firstly, we maintain organic matter, and secondly, we maintain the supply of nutrients.

This page aims to provide most of the information you need to achieve and maintain healthy soil.  I have included notes on making compost, which is a valuable addition to any vegetable garden.  There is a section outlining the different types of manure available to Gardeners, and their benefits.  Also, there is a section on planting cover crops.
Soil pH is the measure of the acidity (sourness) or alkalinity (sweetness) of a soil.  A simple numerical scale is used to express pH.  The scale goes from 0.0 To 14.0, with 0.0 being most acid, and 14.0 being most alkaline.  The value, 7.0 is neutral, ie; neither acid or alkaline.
Soil pH is important because it influences several soil factors affecting plant growth, such as soil bacteria, nutrient leaching (loss), nutrient availability, toxic elements, and soil structure.  Bacterial activity that releases nitrogen from organic matter and certain fertilizers is particularly affected by soil pH, because bacteria operate best in the pH range of 5.5 to 7.0.  Plant nutrients leach out of soils with a pH below 5.0 much more rapidly than from soils with values between 5.0 and 7.5.  Plant nutrients are generally most available to plants in the pH range 5.5 to 6.5.  Aluminium may become toxic to plant growth in certain soils with a pH below 5.0.  The structure of the soil, especially of clay, is affected by pH.  In the optimum pH range (5.5 to 7.0) clay soils are granular and are easily worked, whereas if the soil pH is either extremely acid or extremely alkaline, clay soil tends to become sticky and difficult to cultivate.
A pH determination (soil test) will tell whether your soil will produce good plant growth or whether it will need to be treated to adjust the pH level.  For most plants, the optimum pH range is from 5.5 to 7.0, but some plants will grow in more acid soil, whilst others prefer a more alkaline level.

The pH is not an indication of fertility, but it does affect the availability of fertilizer nutrients.  A soil may contain adequate nutrients yet growth may be limited by a very unfavorable pH.  Likewise, Builder's Sand, which is virtually devoid of nutrients, may have an optimum pH for plant growth.
Normally, lime or dolomite is used to increase the pH, or ‘sweeten’ the soil.  Lime contains mainly calcium carbonate and dolomite contains both calcium carbonate and magnesium carbonate.  Ground limestone and dolomite are less likely to ‘burn’ plant roots than hydrated lime, and are therefore recommended for home use.  The amount of these materials necessary to change the pH will depend on the soil.  The greater the amount of organic matter or clay in a soil, the more lime or dolomite will be required to change the pH.
If a soil is tested as too alkaline, you will need to determine if this is due to recent application of lime or whether it is due to an inherent characteristic of the soil.  It is quite difficult, if not impossible, to change appreciably the pH of naturally alkaline soil by use of acid-forming materials.  If a high pH is due to applied lime or other alkaline additives; ammonium sulphate, sulphur or similar, acid-forming materials can be applied.  When using sulphur to lower the pH, it is recommended that not more than 450g (1lb) of sulphur per 100m2  (sq ft) should be used in any one application.  It is important to remember that repeat applications of sulphur should not be made any more frequently than once every 8 weeks.  Sulphur oxidizes in the soil and mixes with water to form a strong acid that can burn the roots of plants, and therefore, should be used with caution.
Soil organic matter includes the following; Fresh organic plant material that we add to the soil.  The decomposing remains of spent plants or kitchen scraps for example.  When it has thoroughly decomposed and formed what we call humus, that's soil organic matter.
It contains nitrogen.  That nitrogen is going to be liberated and supply nitrate for plants to grow on.  It also greatly improves the structure of the soil.
There are many sources of organic matter.  Firstly, we can bury our kitchen waste in our soil, or make it into compost.  We can also bury our crop residues as well, or make it into compost.  Sawdust is a good source of organic matter, but it takes a lot of nitrogen to break it down, so it should be composted beforehand.  Many people like to dig their crop residues straight into the soil, letting the composting take place there.  The earthworms, beetles, and microbes of all kinds contained within the soil will do the decomposition for you.  When they decompose it, they excrete ammonia, which gets converted to nitrate by soil bacteria.  Ammonium and nitrate are the two forms of nitrogen that all plants that cannot fix their own nitrogen from the air require from the soil if they are to grow.  That is why it is vital to keep organic matter levels up in your soil.

You may well find that digging in all your crop residues, and growing green manures, may still not be enough to maintain your soil in optimum condition.  Therefore, you will find it beneficial to add compost.  There are now many different varieties readily available from your local Garden Centre, including composts made
from garden waste, composted bark and sawdust.   You can also use composted poultry manure and spent mushroom compost.
To achieve that lovely friable condition, you will find that, in addition to green manures and crop residues, you will need to put on about 5 cm of compost over the whole planting bed.  That means a cubic metre will cover around 20 square metres of soil.  You will know when to add it, as your soil will have become sticky and/or lumpy and difficult to dig and make seedbeds.  So that is the time to get lots of organic matter into the soil.  Whilst there are many things we add to improve our soil, there is one problem; they can all differ very much in their chemical composition.  So although they improve the physical condition of our soil, they can claim very different amounts of nutrients.  Some will supply a lot of nitrogen, others very little.  Some will be rich in potassium, others not so rich.
Poultry manure for example, is very rich compared with most.  This is because bird faeces and urine is mixed together inside the bird and expelled as a combined product.  It is a very rich material so will need composting with sawdust or straw before use.  Whereas if you just collect horse, cow or sheep manure without the urine, there are valuable nutrients missing.  Because in the urine of mammals like us, nearly all the potassium we excrete, and 75% of the nitrogen we excrete is in our urine as urea.  75% of the sulphur we excrete is in our urine as sulphite.  So urine is a very valuable liquid fertiliser (Hence why many people advocate weeing in their compost bin!).  Sheep pellets straight from the shearing shed with the urine mixed in is also a rich source of organic matter and nutrients.  Many people use a mushroom compost made with poultry manure because of the lime it contains.
Legumes are nature's way of building up organic matter in the soil.  But we did not know until the late 1800s why legumes were so important, and it is because they can fix their own nitrogen from the air.  The reason they can do it is, if you dig up a clover plant (an invaluable legume) for example, you will see, scattered over the root system, little nodules about the size of pinheads.  Those nodules are filled with thousands of bacteria which fix nitrogen from the air.

Clovers have been vital in agriculture of countries such as New Zealand, who have depended on them in their pastures to provide lots of nitrogen and food for their animals.  In some places they are beginning to grow clovers between their rows of vines to supply not only nitrogen and organic matter, but also to attract the hoverfly, which controls aphids.
We can put legumes to use in our gardening too.  We can grow peas, beans, clovers or lupins to build up nitrogen and organic matter.  Some people grow red winter-growing clover as a green manure, sowing it directly into the soil and raking the seed in.  So it is worth remembering that there is an important place for legumes like lupins and clovers in our gardens, for us to dig in and add organic matter and nitrogen to our soils.  Many gardeners are now discovering this benefit
and recommending it to others.