The sandy soils are those that are characterized by presenting a content of more than 70% of sand in the first hundred centimeters of depth. The clay content in these soils is less than 15%.
They are known as sandy and their characteristics vary between dry, temperate and humid zones. In general they are soils with little structure. They have a low content of organic matter and a low cation exchange capacity. They have excellent drainage, good aeration and low moisture retention.
They are distributed throughout the planet in different conditions of humidity and temperature. The most common crops are perennial species with low nutrient demand. These include rubber, cashew, cassava and particularly coconut.
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These soils can be formed by sands with different origins. Depending on this type of parent material, the physical and chemical properties of the soil may be different. Three types of sand sources are known:
They are the result of the prolonged wear of rocks rich in quartz. They can be made of granite, sandstone or quartzite. All have a deep layer of sand, being very poor in clay content and very drained..
They are deposited by the action of the wind, both in dunes or extended sheets of sand. The parent material can be rich in quartz or carbonates. The soils from these sands are common in hot and dry regions (deserts).
The means of transport of the parental material is water. They tend to be less eroded than other types of sands. In some cases they come from sediments deposited by rivers.
Sandy soils are classified into three types according to their parent material and environmental conditions. These are:
They are formed from aeolian sands (dunes). Soil formation is minimal until some type of vegetation is established. There is very little organic matter content and may be covered with clays, carbonates or gypsum..
They have a high permeability and a very low capacity to retain water. There is low biological activity.
They are formed mainly from alluvial sands from fluvial deposits of glacial origin. They can also be formed from lacustrine or marine sands as well as aeolian sands rich in quartz.
They can be very young originating from alluvial lacustrine sands or aeolian sands. Others are older soils originated by the wear of rocks (residual sands).
It refers to the attributes of the soil that are observed in the field. In sandy soils it varies according to the type.
Soils in dry areas are very poorly developed. The most superficial layer (horizon A) has very small sand particles and almost no content of organic matter. Immediately below this is a C horizon (rocky material).
For temperate zones, the shallowest horizon is quite thin. A thin layer of humus may be present. Other components such as iron and clays are very scarce.
Young tropical soils are similar to those in temperate zones. In the case of old tropical soils, there is a more developed organic matter horizon. Below this, there is an underdeveloped mineral layer and then a deep horizon of coarse sand.
The size of the particles that make up the soil can range from 0.05 - 2 mm in diameter. The bulk density (weight per volume of soil) is relatively high due to the high content of sand particles.
Porosity (percentage of soil volume not occupied by solids) is between 36-46%. However, in some tropical soils porosities of 28% have been found associated with the absence of gravel and sand. In other cases, percentages of 60% have been indicated when the soils have been cultivated.
The wide ranges of porosity are associated with the low clay content in these soils. This results in a low cohesion force between the particles..
On the other hand, soils have quite large pores. This characteristic gives them good aeration, fast drainage and a low moisture retention capacity..
In temperate and tropical regions the soils are highly leached (displacement of soluble particles by the action of water). Likewise, they are decalcified and have a low capacity to store bases..
On the other hand, organic matter is little decomposed. The organic carbon content is less than 1%. This combined with the low proportion of clays, makes its cation exchange capacity very low (less than 4 cmol (+) / kg).
Soils in dry regions are rich in bases. Leaching and descaling is moderate compared to other sandy soils.
The organic carbon content is less than 0.5%, but its cation exchange capacity is not very low. This is because the proportion of clay minerals (vermiculite and others) is higher than in other sandy soils..
Sandy soils have little moisture retention capacity. Due to the large pore size, much of the retained moisture is lost at just 100 kPa.
The available water capacity varies according to the size and distribution of the particles that make up the soil and the content of organic matter. Values can range from 3-4% to 15-17%.
The hydraulic conductivity of the soil is highly variable in relation to the density of the sand. It can be in a range between 300-30,000 cm / day.
Regarding the infiltration capacity of the water, it can be up to 250 times faster than in clay soils. It can be between 2.5-25 cm / hour.
In the sand and silt fraction of the soil, the main minerals are quartz and feldspars. Other components are micas and ferromagnesic minerals such as amphiboles, olivines and pyroxenes..
Other minerals such as zircon, magnetite, garnet and tourmaline have also been found among others..
The composition of the clay fraction is determined by the characteristics of the bedrock. Vermiculite, chlorite and kaolin may be present.
The arenosols are distributed throughout the planet. They occupy approximately 900 million hectares that correspond to 7% of the surface of the mainland.
Although they are more frequent in arid and semi-arid areas, they can occur in almost all types of climate. The distribution range can go from very arid to very humid sites. Likewise, temperatures can be from very high to very low and can be associated with any type of vegetation..
Soils formed by aeolian sands occupy a large area of central Africa, such as the sands of the Kalahari. In this continent we also find the Sahara desert.
Almost all of central and western Australia is made up of sandy soils. They are also common in various areas of China.
Sandy soils have some limitations for agriculture, due to their low moisture retention capacity and nutrient content.
One of the factors to take into account for the development of crops in these soils is the topography. Sandy soils with slopes greater than 12% are used for conservation purposes and some forest plantations.
In some areas of Southeast Asia rice is planted with irrigation replacement, even though these are not the best conditions for cultivation. Padi rice is grown in West Africa.
However, the crops that grow best on these soils are some perennials. Among these we have rubber, pepper and cashew. Likewise, casuarina and pine can be grown when properly irrigated..
The largest plantations on these soils are coconut. Some root and tuber crops are grown under these conditions for ease of harvest. The most common species is cassava (Manihot esculenta) for its tolerance to low levels of nutrients.
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