Lime and liming
Lime is strictly defined as a white caustic alkaline substance—quicklime; chemically, calcium oxide (CaO); made by heating chalk or limestone. But in gardening, lime means any calcium-containing material which is capable of correcting soil acidity.
Why lime is used
The main reason for using lime is to reduce the acidity of a soil that is acid or, in other words, to sweeten the soil. Few plants will grow well in a very acid soil mainly because their intake of plant foods is reduced; phosphates, in particular, get ‘locked up’ in acid soils. There is often a shortage of calcium in very acid soils.
Aluminum and manganese, on the other hand, are often released in such large amounts that they can poison many plants. Tomatoes, beans and brassicas are particularly sensitive in this respect.
Lime encourages soil life
The bacteria that convert ammonium salts to nitrates —one of the steps necessary before nitrogen-bearing foods can be used by the plant—are almost or completely inoperative in very acid soils. The organism which is responsible for fixing nitrogen in the roots of peas, beans and other leguminous plants operates most favorably when the soil is well limed, which is the reason why peas do not thrive in really acid soils. But rhododendrons would be very sickly or even die in a soil limed for peas. Earthworms, too, thrive in well-limed soils; they make channels in soil improving the drainage of clay soils and compacted lawns (we know that wormcasts are unsightly but the good that worms do in improving drainage outweighs their harm).
Lime improves tilth
Many clay soils, when limed regularly, become more porous and allow rain to drain away quicker, thus allowing you to get on to the ground earlier in the spring. It is possible to cure a really sticky clay soil by liming, but not all clays will respond since some are naturally limey. Lime has very little effect on the tilth of sandy and loamy soils.
Lime controls some diseases and pests
Clubroot disease of brassicas flourishes in acid soils and can usually be controlled by liming, but it takes two or three years before it works fully. Slugs, leather-jackets or wireworms and several other soil pests are discouraged by liming.
How to tell whether soil needs lime
The presence of spurrey, sheep’s sorrel, corn marigold and other weeds that thrive in acid soils often indicates the need for lime, but these weeds are not very reliable indicators since they continue to grow for some time in soils that have been limed. If you see rhododendrons and blue hydrangeas growing really well in nearby gardens it is fairly safe to assume that your soil is naturally acid. But the only reliable method of finding out whether a soil is acid or alkaline is to carry out a soil test for lime. An old-fashioned way of telling whether a soil was limey was by pouring some dilute hydrochloric acid on to the soil to see whether it fizzed but this does not help very much; if a soil is rich in calcium carbonate it will react with the added acid and carbon dioxide gas which causes the fizzing. Lack of fizzing, however, is not a reliable indicator that lime is needed and, of course, this test gives no idea as to the amount of lime required to correct acidity in an acid soil.
How to test for lime
The simplest do-it-yourself method is to buy some indicator papers from your garden supply center and then take a sample of soil. If the soil is dry, moisten it well with water (distilled water if possible) in a saucer but do not make it runny. After half an hour place a 2cm strip of test paper so that half of it lies on top of the wet soil and the other half against the side of the saucer. After 5 minutes compare the color of the paper with the color panels on the chart, which is sold with the papers.
And while we are talking of old-fashioned but useful techniques, let us have a quick look at a method of determining the texture rather than the acidity or alkalinity of your garden soil. Take a sample of your soil, just a spoonful, and shake this up well in a glass or other clear vessel of water. Then set it aside to settle. If you have reason to believe that your soil differs in different parts of your garden then carry out the same test with more than one sample taken from other places.
After a time you will see that a number of definite layers have appeared. The stones are on the floor or base of the vessel and on top of these is a layer of sand. Loam having a proportion of sand forms the next layer. The clay content, light and powdery, will dissolve and do little more than color the water, remaining in suspension for a very long time. Finally the humus will tend to float on the top of the water, or if the shaking has been vigorous, perhaps some will have sunk to form the top layer.
By this simple means you can make a very fair assessment of your soil’s capabilities and requirements.
For peaty soils and those that are naturally rich in humus you will have to increase the amount of lime even more. In fact, it is virtually impossible to correct the acidity of some of them; a few of the fenland soils are so acid that even after enormous quantities of lime have been applied, the soil still shows an acid reaction the following year. So, all you can hope to do is to correct the worst of the acidity. Fortunately such soils are rare. If you feel that the whole business of determining lime requirement is too complicated and you know that your soil is acid, a good general rule is to apply 0.3kg of hydrated lime per sq m (sq yd) on sandy or loamy soils, and 0.8kg per sq m (sq yd) on clay or peaty soils. On the other hand, you may be a precision gardener and will, therefore, want to know more exactly how much lime is required. For you, there is a special lime requirement test kit which gives more accurate guidance, as to the amount of lime needed to correct the acidity of your garden soil or possibly to raise the pH value of your potting compost for a particular plant.
Soil test laboratories carry out a special lime requirement test by means of electrically operated pH meters and buffer solutions; this is the most reliable method.
Dangers of overliming
Too much lime can be as bad as too little. Overliming may reduce the plants’ intake of iron, causing yellowing of leaves. Deficiencies or iron and boron are also common in over-limed soils in the garden. Alkaline conditions produced by liming favor the disease fungus responsible for scab on potatoes.
The danger of overliming is greatest in sandy soils so it is wise to add lime little and often to sandy soils, but there is rather less need to worry with clays and even loams.
Forms of lime Hydrated lime and carbonate of lime are the two forms most commonly listed in garden cataloges.
Hydrated lime comes from quicklime that has been treated (slaked) with water and is known chemically as calcium hydroxide. It is often sold under brand names.
It is a very fine powder which mixes well with soil particles and being slightly soluble in water is a most effective liming material where speedy benefits are wanted. It is an alkaline substance and is caustic and, therefore, likely to burn foliage if it blows on to plants during spreading.
When mixed with soil, hydrated lime combines with carbon dioxide and turns into calcium carbonate; this is the fate of all forms of applied liming materials.
Carbonate of lime or ‘garden’ lime as it is often called, is limestone or chalk that has been crushed to a gritty powder. Most garden limes are ground to pass through a 3mm 1/4(in) sieve.
The rate at which carbonate of lime works in the soil depends on how finely ground it is. Even the finest particles produced by grinding are not so fine as those produced chemically during the production of hydrated lime, but nevertheless they work quickly. The coarser particles act as a reserve and are longer lasting. Carbonate of lime does not burn plant foliage and is more pleasant to handle than hydrated lime; it is the best form to use in seed and potting composts.
Although insoluble in pure water, it does dissolve in soil water, forming calcium icarbonate from which the calcium portion can be taken up by the clay and humus of the soil; some is also absorbed by plant roots, worms and other organisms.
This form of lime is less concentrated than hydrated lime and so you need
larger amounts to reduce the acidity of an acid soil; but if you buy it in packs of 25kg or over, it is cheaper than hydrated lime.
It is important to know that the acid-neutralizing value of any form of lime is expressed in terms of its content of calcium oxide (CaO). Carbonate of lime contains about 50 percent CaO, and hydrated lime from 60 to 70 percent CaO.
Other materials used in liming. Marl is a clay-rich in lime. This is obtained in many parts of the country from beds that are sufficiently near to the surface to be worked economically as in the new red sandstone formations in the north and west midlands, or the shell marls which occur in Norfolk. It is of particular value in sandy or peaty soils not only for its lime content but also for the clay which gives ‘body’ to this type of soil. Marling is an ancient practice which is still carried out in areas subject to severe wind erosion; the clay part helps to bind sand grains together and prevent them from blowing away.
Waste materials from the sugar beet, paper, tanning and cement industries often contain calcium carbonate and make useful liming materials. Some may be wet, lumpy and difficult to handle, but if available, nearly all can be a useful source of lime.
Oyster and other sea shells are mostly calcium carbonate. When free from salt and ground finely they make useful liming materials.
Slags, which are waste materials from iron and phosphorus manufacture, contain calcium and magnesium silicates that are capable of reducing soil acidity.
Dolomite lime is a natural form of calcium and magnesium carbonate which supplies two elements important for plant growth, calcium and magnesium, and which also provides the necessary neutralizing effects. It is a useful form of lime to use on acid soils that are low in magnesium and is widely used in soil-less composts for potted and container grown plants.
Spreading lime Lime is not just magic out of a bag. To work properly it must be mixed thoroughly with the top-soil layers. To begin with the lime must be spread over the soil surface by hand from a bucket br a fertilizer distributor if the soil is firm and even. But do not dig it in because digging often shifts the surface lying lime simply into another layer below the soil surface.
It is best spread on a finely broken surface and then stirred about with the soil, using a hand or mechanical cultivator; when the soil is on the dry side, you cannot expect minute particles of lime to mix properly with clods.
When, in the first year after liming, results are disappointing—and this does sometimes occur—the cause is frequently the length of time needed for the dressing to become reasonably mixed into the top soil layer. If the lime fails to penetrate, seeds may be sown in what is locally and temporarily a too alkaline strip of soil. But if it has been dug some way under, seedlings may be trying to grow in a thin layer of very acid soil and plants will grow poorly until the root system reaches the buried lime layer.
When to lime If your soil is very acid, the sooner lime is applied the better—as soon as the ground becomes vacant. Autumn dressing is often recommended so that rain can wash it into the soil.