Correction of soils that are very clayey, too calcareous or too sandy

A balanced content of clay, sand and limestone is essential for good garden soil

The full effectiveness of manure can only be achieved on land with good clay and calcium content. Above 40% clay, the soil is compact and heavy, facilitating water retention.

Soil that is too clayey can be corrected by adding river sand, which can be purchased from a building materials supplier, as well as limestone and humus amendments

There are no standards governing sandy amendments, but it is preferable to use neutral sands such as very fine Loire sand. Some sands may contain up to 50% limestone, which can be beneficial when the soil is also low in limestone. To increase the sand content of a soil, approximately 1 to 3 tonnes per acre of sand is required, depending on the clay content

An excessive amount of limestone can be harmful to cultivated soil

Lime makes the father’s fortune and the son’s ruin, says a proverb. This is the case when lime is used every year at a rate estimated blindly. Before correcting overly clayey soil with limestone amendments, it is essential to know the pH of the soil using laboratory test strips or a pH tester.

The volume of limestone amendments must remain between 7 and 8. An excess of lime cancels out the solubilisation of phosphorus and certain trace elements such as boron, zinc and manganese. In the vegetable garden, slaked lime is more than sufficient. The amount of lime required to correct acidic soil is 7.5 tonnes/hectare for compact clay soil when the pH is between 5.5 and 6. This dose drops to 5.5 tonnes per hectare for average soil composed mainly of sand and clay. In Provence, due to the nature of the soil, which is often rich in limestone, adding lime is often unnecessary. There are even many areas where excess limestone is a problem.

Limestone amendments used in agriculture to improve soil pH and structure must comply with French standard NF U 44-001, which comprises five classes:

1) Simple limestone amendments such as marl, chalk powder, and maerl (debris from a limestone-rich seaweed).
2) Raw amendments rich in magnesium such as dolomite.
3) Cooked amendments: calcium and magnesium lime.
4) Mixtures of lime with raw amendments.
5) Sugar refinery scum or fruit juice residues fixed with lime milk.

The remarkable properties of lime in agriculture

Soil that is too clayey and acidic can be improved by adding slaked lime, which also has the advantage of quickly loosening the soil (a). In organic farming, amendments containing quicklime or slaked lime are prohibited (Regulation EEC 889-2008) on the grounds that they are not natural and require the use of fossil fuels to produce them. Organic farmers therefore deprive themselves of a particularly effective amendment for rapidly reducing the toxicity of aluminium ions in overly acidic soils. There are slower-acting limestone amendments consisting of larger particles that break down slowly in the soil.

These are mainly intended to stabilise the soil structure over time after an initial correction with lime. Lime has long been known to improve the structural stability of the soil, facilitating its aeration, the penetration of water-borne nutrients, and plant rooting. Agricultural lime enriches the soil with calcium and magnesium, which are essential nutrients for plants. By facilitating the formation of CAHs, lime also promotes fertiliser absorption and improves yields.

The correct amount of lime corrects excess copper, manganese, boron and zinc. Quicklime is used in arboriculture to destroy insects that nest in tree trunks.

Lime should not be used with organic fertilisers

Soil should not be limed after spreading nitrogen-rich organic fertiliser, as lime tends to neutralise nitrogen. Lime is generally spread in autumn after ploughing, and quicklime, also known as agricultural whitewash, is sometimes used. Quicklime is highly corrosive and precautions must be taken when spreading it (goggles, gloves, protective clothing). Quicklime attacks all organic matter, including pathogenic fungi and bacteria and beneficial microflora. It is recommended that lime be spread every three years, alternating with fertiliser applications.

Some crops benefit from limestone amendments, while others do not

The type of crop that will be grown on the soil also influences the amount of limestone amendments required. Potatoes prefer acidic soil, which means that lime cannot be used on clay soil for at least 12 months before planting. Beans, on the other hand, do not tolerate acidity well and benefit from limestone amendments.

Where can I find lime amendments for cultivated soil?

For gardening, lime amendments (dry dolomite, calcium carbonate in granules or powder form, magnesium carbonate, crushed quicklime or slaked lime, limestone sand, etc.) are readily available in garden centres and online, with instructions for use. Advice on use is available on websites. For example, for those who wish to spread dolomite, the quantity varies from 15 to 20 kg per 100 m².

a)  There are two types of lime obtained from hydrated quicklime: air lime, known since ancient times and obtained from pure limestone, and hydraulic lime, obtained from limestone containing a small amount of clay.