- TYPES OF BRACKISH SOILS
Brackish Soils, or as they are also called Alkaline or Saline Soils, occur widely in arid climatic conditions, in other words in areas of very low rainfall. It is as well to remember that one third of the land surface of the earth is classed as arid or semi‐arid. The Brackish or Alkaline soils have a number of characteristics which make them unsuitable for agricultural purposes unless treated, and these can be summarized as follows:
Brackish Soils contain large amounts of sodium salts dissolved in the soil water:
- Sodium Chloride (NaCl) which is Common Salt
- Sodium Sulphate (Na2SO4) also called Glauber Salt
- Sodium Carbonate (Na2CO3) which is Washing Soda
Because there is so much Sodium (Na) in the soil water, the clay particles in the soil attract the Sodium ions rather than the much more desirable Calcium ions (Ca”), causing the formation of what is known as a Sodium Clay. This is a very sticky and unworkable clay, one on which it is very difficult to get a seed‐bed and a good tilth. It is the same as the clay which is formed when a soil is flooded with sea water, and for the same reason there is too much Sodium in the soil.
Because the soil water is, in fact, a strong solution of salt, the effect on plants trying to grow in this soil is very bad. As you know from your Botany lectures; in the process called‐Osmosis, water passes through a semi‐permeable membrane (e.g. the cell wall of plant cells) from a weak salt solution to a stronger salt or sugar solution: see the diagram below.
Figure 1: Cell wall of a plant cell, water moving out the plant cell into a salty solution through
In this case, the weak solution is in the plant cells and the strong solution is the salty soil water. Water flows out of the plant cells into the salt solution and the plant cells become dehydrated. The correct term for this is plasmolysed. The result is that the plant dies from lack of water in the cells and tissues.
Brackish Soils are divided into two types alkaline, or black brackish soils and saline or white brackish soils.
ALKALINE BRACKISH (BLACK BRACKISH)
These soils have the following characteristics:
- These soils contain large amounts of Sodium Carbonate, and are very alkaline, with a pH from 8,5 up to the maximum of 10.
- This very high pH causes plant cells to become Plasmolysed, which affects the availability of nutrients to the plant and affects the activity of the soil micro‐organisms. These function best at a pH of 5,5 to 6,5 and cease to function altogether at high pHs.
- The clay particles in the soil form Sodium Clay, and the soil loses its Soil Structure. The soil becomes very compact, the pore spaces are reduced, and the soil does not absorb water easily. The water simply runs off the soil surface.
- Alkaline Brackish Soils are totally unsuited to all plant growth, and therefore are of no value as agricultural soils.
SALINE BRACKISH (WHITE BRACKISH)
- These soils contain excess Sodium Chloride and Sodium Sulphate, and are alkaline, but the pH is not above 8,5.
- The main problem with these soils is that the high salt concentration causes plasmolysis of the plant cells. Some Calcareous Soils are Saline Brackish Soils.
- The clay in such a soil is a sodium clay, but the Soil Structure is good because the Sodium Chloride will have flocculated the particles, which will be aggregated into crumbs.
- Where the soil to a depth of 6 meters contains more than 0,55% of salt, the soil is unsuitable for all crops.
Where the soil to a depth of 6 meters contains 0,35% to 0,55% of salt, the soil will grow crops which are very tolerant to alkaline conditions. Such crops are: Mango’s, Sugar Beet, Barley, Sorghum, Cotton, Lucerne, Fig Trees and Olives.
Where soil to a depth of 6 meters contains from 0,15% to 0,35% of salt, the soil will grow most crops except Peach Trees and Orange Trees, provided that irrigation is thorough and excess evaporation is avoided.
Where the soil to a depth of 6 meters contains less than 0,15% of salt all crops can be grown, provided that the farmer is careful with his irrigation and tillage.
The amount of salt present in a soil can be obtained by Chemical analysis. It is interesting to note that a concentration of 0,1% of salt to a depth of 6 meters in the soil is equal to 32 tones of salt per hectare.
One of the problems which can occur in this type of soil is that Salt Pans can be formed. These are areas of high salt concentration that are formed below the surface. The topsoil and subsoil may be free of salt with the total salt content to a depth of 6 metres being less than 0,2%, but a Pan with a very high salt content may exist at, for instance, 2 metres below the surface.
Figure 2: Formation of a salt pan below the soil surface
- ORIGIN OF BRACKISH SOILS
In coastal areas, sea winds can carry salt particles inland which are deposited and build up in the soil in areas of low rainfall. Example: Areas of the North West coast of the Cape Province in South Africa.
Soils which have been flooded by sea water or soils which have been reclaimed from the sea bed are brackish. Such reclamation work is regularly carried out along the coast in Holland in Europe.
Brackish soils can be produced from the weathering of rocks as large quantities of soluble salts are formed during the weathering process. As we know from earlier Soil Science Lectures, Igneous Rocks such as Feldspars produce Potassium, Sodium and Calcium (Lime). Sedimentary rocks are also rich in soluble salts which can accumulate when the rocks weather to form soil. Furthermore, it is interesting to note that the formation of one hectare of soil 30 cm deep requires the removal of over 500 tones of soluble salts. In Humid regions this material is washed out of the soil by rain and carried down to the sea, which is known to be gradually becoming more and more salty. In dry regions, the soluble material remains in the soil and can form Brackish Soils.
The use for irrigation purposes, of water with a high salt content can produce Brackish Soils very quickly. Where such alkaline water is used on badly drained soils, Evaporation and Transpiration can remove the water, leaving the salt behind in the topsoil. Brackish Soils have been produced in this way in certain areas of South Africa. In parts of Egypt, continuous irrigation since the Nile dams were built is causing similar problems. In many areas, repeated irrigation: is causing a gradual rise in the ph of the soils being irrigated.
The following gives the total soluble salt content for irrigation water:
- Water containing Less than 0, 05 % of soluble salts: suitable for all irrigation.
- Water containing 0, 05 % to 0, and 15 % of soluble salts: cannot be used under conditions of poor drainage and high evaporation.
- Water containing 0,15 % to 0,25 % of soluble salts: of very doubtful value for irrigation
- Water containing over 0,25 % of soluble salts: too alkaline for irrigation
Water which contains 0, 1% of soluble salts, if used to apply 100 mm of irrigation water to a soil, will deposit one tonne of soluble salts per hectare. If these accumulate, because of poor drainage and high evaporation, the soil will rapidly become Brackish.
3. RECLAMATION OF BRACKISH SOILS
Brackish soils can be reclaimed by removing the soluble salts and improving the soil structure.
The soluble salts can be washed out by flooding the land with suitable water which will wash the salts to below root depth. This requires firstly, water which is not Alkaline and secondly, good drainage, although this is usually lacking in Brackish Soils. Without good drainage, matters can easily become worse. However, drainage of the soil can be helped by incorporating fresh organic matter into the soil and by planting with crops such as Sweet Clover, “Kweek” Grasses, Rhodes Grass, Barley, Rice and Wheat. Land left to dry out between floodings will form cracks which also helps the drainage problem.
Another thing which greatly assists the reclamation process is to apply gypsum (Calcium Sulphate) to the soil before flooding. This replaces the Sodium Ions with Calcium Ions on the Clay particles, and makes the soil less sticky and easier to work. Up to 40 tones per hectare may be required for heavy Clay and this should be worked into the topsoil with a cultivator.
Agricultural Lime (Calcium Carbonate) cannot be used for this purpose because it will not dissolve in water in very alkaline conditions. Other agents which can be used for this purpose are Sulphur and Sulphuric Acid, although this is a very dangerous substance for farm labour to work with.
The object of this treatment with Gypsum is to neutralize the top few inches of the soil to allow seeds to germinate and the seedlings to grow. Once brackish tolerant plants are established, reclamation is sped up by the Carbon Dioxide given off by the plant roots. The Carbon Dioxide joins up with Water in the soil to form Carbonic Acid which, being an acid, helps to neutralize the alkalinity in the topsoil.
Simply putting a fairly thick layer of organic matter on the soil surface (this process is known as mulching) also helps the reclamation process by preventing evaporation from the soil surface.
To improve patches of Brackish soil in an otherwise normal land, apply large amounts of organic manure (e.g. kraal, pig or poultry manure) and also work Gypsum into the patches at the beginning of the rainy season.