Correction of very clayey or too calcareous or too sandy soils

Correction of very clayey soils in agriculture

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

The full effectiveness of manures can only be achieved on soils in good clay and calcium condition. Above 40% clay, the soil is compact and heavy, making it easier to retain water.

Soil that is too clayey can be corrected by adding river sand, which can be bought from a building materials dealer, and by using calcareous and humus-containing amendments.

There is no standard for sand amendments, but it is preferable to use neutral sands such as very fine Loire sand. Some sands can contain up to 50% limestone, which can be interesting when the soil is also poor in limestone. To increase the sand content of a soil, about 1 to 3 t/are of sand is needed, depending on the clay content.

The addition of sand is not enough to make a soil with too much clay lose its sticky and asphyxiating properties. The clay/sand mixture quickly turns into concrete. When limestone and humus are mixed with clay, the latter flocculates and takes on the consistency of sand, facilitating aeration and water infiltration (see article on CAH).

Too much limestone can be detrimental to crop soils.

Lime makes the father's fortune and the son's ruin, as the saying goes. This is the case when lime is used every year with a blindly estimated rate. Before correcting a soil that is too clayey with lime amendments, it is essential to know the pH of the soil using a laboratory test strip or a pH tester.

The volume of lime amendments should remain between 7 and 8. Too much lime will cancel out the solubilisation of phosphorus and certain trace elements such as boron, zinc and manganese. In the vegetable garden, slaked lime is more than enough. The amount of lime needed to correct an acid soil is 7.5 t/hectare for a compact clay soil when the pH is between 5.5 and 6. This dose drops to 5.5 t/hectare for an average soil composed mainly of sand and clay. In Provence, due to the nature of the soil, which is often rich in limestone, the addition of lime is often unnecessary. There are even many soils where it is the excess of limestone that is the problem.

In France, limestone amendments used in agriculture to improve the pH and structure of soils must comply with the NF U 44-001 standard, which covers 5 classes:

1) simple limestone amendments such as marl, chalk powders, maerl (debris from a seaweed rich in limestone).
2) Raw amendments rich in magnesium such as dolomite.
3) Fired amendments: calcium and magnesium limes.
4) Mixtures of lime with raw amendments.
5) Sugar factory defecation scum or fruit juice residues fixed by a milk of lime.

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, soil improvers containing quicklime or slaked lime are forbidden (regulation RCE 889-2008) on the pretext that they are not natural and that they require the use of fossil energy resources to produce them. Organic farmers are therefore depriving themselves of a particularly effective amendment to rapidly reduce the toxicity of aluminium ions in overly acidic soils. There are slower calcareous amendments consisting of larger particles that slowly break down in the soil. They 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 aeration, the penetration of water-borne nutrients and the rooting of plants. Agricultural lime enriches the soil with calcium and magnesium which are essential plant nutrients. Lime, by facilitating the formation of CAH, also promotes the absorption of fertilisers and improves yields.

Correct application of lime corrects excesses of copper, manganese, boron and zinc. Quicklime is used in arboriculture to destroy insects nesting in tree trunks.

Lime should not be used with organic amendments

Lime is not applied to a soil after spreading a nitrogen-rich organic fertiliser, as lime tends to neutralise the nitrogen. Lime is usually applied in the autumn after ploughing and sometimes quicklime, also known as tree white, is used. Quicklime is very corrosive and precautions must be taken when spreading it (goggles, gloves, protective clothing). Quicklime attacks all organic matter, including pathogenic fungi and bacteria and useful microflora. It is recommended to spread lime every 3 years, alternating with fertiliser applications.

Some crops like limestone amendments, others do not

The nature of the crop that the soil will support is also a factor in the dose of lime amendments. Potatoes prefer acidic soil, so lime should not be used for at least 12 months on clay soil before planting. Beans, on the other hand, do not tolerate acidity and appreciate lime amendments.

Where to find limestone amendments for cultivated soils?

For gardening purposes, limestone soil improvers (dry dolomite, granulated or powdered calcium carbonate, magnesium carbonate, crushed quicklime or slaked lime, limestone sand ....) with instructions for use are readily available in garden centres and on websites. 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 antiquity and obtained from pure limestone, and hydraulic lime obtained from limestone containing a little clay

Correction of soils that are too calcareous or too sandy.

Without laboratory analysis, it is not always easy to determine with the naked eye whether a soil contains too much limestone. Some observations can be confusing. The colour of a very calcareous soil is often whitish, but it can be yellowish if the soil also contains some clay and/or iron.

The adverse effects on crops of soils too rich in limestone.

When a soil is too rich in limestone, it prevents plants from absorbing iron. Some plants suffer from chronic chlorosis. Soil that is too rich in limestone can be deficient in magnesium and potassium. Magnesium is an essential element for plants; for example, it is a component of chlorophyll. Soils that are too calcareous often have an excess of carbonate of lime which, when combined with a little clay, produces a substrate that is as compact as a clay soil.

Some examples of clay amendments to improve cultivated soils

Adding clay to improve the texture of a soil that is too calcareous is the most effective solution, especially since clay is essential for producing colloidal complexes with humus (see the article: Clay-humus complexes). Clays also have the advantage of including cations such as potassium and magnesium between their sheets to form real food reserves for plants.

For sandy soils, the addition of organic matter will not be enough to restructure the soil and produce a stable carbon reserve. Clay and limestone must also be added if these are also absent. There are no standards for clay amendments, but they are not easy to find in garden centres for agricultural use. These amendments are also intended to correct peaty and sandy-silty soils.

Clay occurs naturally at the bottom of some lakes, ponds and irrigation channels. This clay becomes available when these water reserves are emptied. Marsh clay below the topsoil can also be used if it is not of a silty type. To increase the clay content to 1%, 0.90 to 1.5 t/are of marl containing about 50% clay is needed.

Beware that harvested clays may contain a lot of limestone or other elements that are not always favourable to crops. Beware of abandoned clay deposits that may contain toxic substances such as pollutants.

The ball test is a way of approximating the clay content of a marl. After mixing a sample of marl sufficiently moist to form a compact ball (moisture content comparable to that of clays used by potters), an attempt is made to fold the ball into a circle. A few small cracks before breaking the ball into a semi-circle indicate that the clay content is in the order of 15-20%. If there are cracks at ¾ of the closure before the flange breaks, the clay content is estimated at 20-30%. The completion of a complete ring without it being broken indicates that the clay content is above 30%. The result of this more or less reliable test depends on the preparation of the clay and certain impurities it may contain. Insufficient moisture, superficial mixing leaving small cracks and/or the presence of tiny organic fibres will distort the conclusion.

Where to find clay to improve cultivated soils?

There is a map of France of outcropping or subcropping clay zones likely to produce shrinkage and swelling which can be consulted by clicking here. Clay formations are prioritised according to the intensity of soil shrinkage that can affect buildings. These risk areas for buildings also contain clay marl (a mixture of carbonates and clay minerals) which produce the same effects. It should be noted that clayey marls are often used by farmers as soil improvers because of the high clay content (35-65%). For the authorisation of extraction, see the owners of the plots and the local regulations.

Commercially available clays can be found at all prices and in different colours depending on the minerals they contain, the most expensive being used in cosmetics. Some clays are used in arboriculture, fish farming, in the manufacture of cat litter, as cooked (bricks) or raw (plaster) construction materials... Clays associated with fibres are used as construction materials. They often contain some fine sand, barley or flax straw, lime and are recyclable and compostable. In principle, cob should have a maximum clay content of 30%, beyond which the mortar cracks when drying. In Provence, a type of clay used in construction as a single-layer plaster can be purchased here - price 18.41 € incl. VAT per 25 kg bag, November 2016) :

C'EST TOUT VERT Route de Dragignan Usine Emphoux 83690 SALERNES Tel : 04.94.70.11.05 http://www.cest-tout-vert.com/

Some websites offer bentonite or montmorillonite clay that can be used in agriculture and spread at a rate of 4 to 8 kg per 10 m2, accessible by clicking here.

Tillage and clay losses.

Clay is not eternal. It can undergo movement depending on the texture, structure and chemistry of the soil. If a continuous pore system exists in the soil, percolating water can transport clay particles downwards. This movement accelerates as the soil shrinks and cracks during dry seasons followed by rainfall. Clay accumulates when the movement of water stops due to an obstacle. The clay particles can then redeposit deep down to form thin layers of clay called clays. Clay particles can also be destroyed by chemical processes involving hydrogen ions with the release of aluminium and silica. In this case, the pH of the soil below the topsoil is often lower. It is therefore necessary to add clay from time to time, depending on the extent of losses observed by laboratory analysis. Ploughing, because it turns the soil, is also a very effective way of limiting clay losses.

1) CLAY SORBENTS: THE MINERALOGY, PROCESSING AND APPLICATIONS - Haydn H. MURRAY - Indiana University, Department of Geological Sciences, 1001 E. Tenth Street, Bloomington, Indiana 47405 USA