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  The ‘problem soil’ herein means the soil that has agricultural problems due to the soil’s unsuitable physical and chemical properties, or less suitable for cultivation, resulting in that crops are not able to grow and produce yields as normal. These soils always occur naturally, including saline soil, acid sulfate soil, sandy soil, organic soil, skeletal soil and shallow soil. Furthermore, it may also include areas with steep slope. If these lands are used for agricultural purpose, then it may cause some severe effects on the ecology and environment.

1. Acid sulfate soils
  This refers to the soil with very high acidity because it may currently have or used to have sulfuric acid, which is a consequence of the occurrence of pyrite mineral in the soil profile, and the amount of sulfuric acid formed is large enough to cause the changes of certain soil properties and to affect the growth of plants in that vicinity.
Acid sulfate soils in Thailand occur in the Southern part of the Central Plain, Eastern part and along the South Coast. In the areas with marine or brackish water sediment as parent materials, pyrite mineral may be formed. Upon oxidization of pyrite, a substance called jarosite will be obtained, which will finally release sulfuric acid to the soil. The distinguished characteristic of acid sulfate soil is the presence of the straw-yellow mottles in the subsoil, with strong acidity – a pH below 4.0.

Acid sulfate soils in Thailand can be divided into three categories based on the depth of jarosite found in the soil:
(1) Acid sulfate soil with jarosite found at a shallow level. Straw-yellow mottles are found
     not deeper than 50 cm from the soil surface. Examples are Ongkharak series, Chian
     Yai series, Muno series, etc.
(2) Acid sulfate soil with jarosite found at a moderate level. Straw-yellow mottles are found
     between 50-100 cm from the soil surface. They inclue Rangsit series, Don Mueang
     series and Sena series.
(3) Acid sulfate soil with jarosite found at a deep level. Straw-yellow mottles are found
     deeper than 100 cm from the soil surface. They include Ayutthaya series, Bang Khen
     series, Bang Nam Priao series, Maha Pho series and Tha Khwang series. 

The problem arisen from the acid sulfate soils.
  The strong acidity of an acid soil affects the availability of various nutrients such as nitrogen, phosphorus, potassium, sulfur, calcium and magnesium to the plants, resulting in the shortage of these elements in plants, so they cannot grow normally. In strongly acid soil, iron and aluminum may dissolve in the soil to the levels that are toxic to many crops as well as soil microorganisms. Water in an acid sulfate soil area is normally astringent (sour) and unsuitable for agriculture and consumption. In a fish pond, there might be the toxicity of hydrogen sulfide gas, carbon dioxide and organic acids.

Utilization guidelines
  Acid sulfate soil management for crop cultivation. It is important that the acidity of the soil must be controlled, so it will not increase. For soils with low acidity, a dilution method may be used, i.e. by keeping freshwater on the land for a long time, and drain it off before cultivation. Additional treatment is to select plants that tolerate soil acidity. For severe acid sulfate soils, applying calcium carbonate (CaCO3) materials such as marl, lime, crushed limestone or limestone dust to the surface soil at an appropriate rate can effectively reduce soil acidity. However, applying CaCO3, combined with washing with water and groundwater control, is the most complete and most effective method to use in areas where the soil is very strongly acid and has been abandoned for a long time.


2. Organic soils
  Organic soil or peat soil means a soil mixed with organic matter in the uppermost part of a soil profile, at a depth of 40 cm or thicker. This is mainly caused by the deposition of organic materials, especially deriving from the vegetation that grows naturally in an environment of a closed shallow basin, with water inundating for a long time, causing the rotting process to proceed slowly, making the organic soil layer thicker and thicker.

The problem of organic soil 
  An organic soil is the soil that has many plant parts as its component, mostly located in an inundated area. If the water is entirely drained out, the soil will subside, with a light density and the cultivated plants cannot stand upright, and the soil itself is sensitive to fire. The soil is mostly composed of organic materials; both being completely decomposed and partly decayed plant parts such as branches, stems or roots. The presence of this non-uniformly mixed organic material makes it difficult to plow. Besides, in the area of organic soil, there is often a layer of clay with potential to become acid. In such case, when the area is drained out to dryness, the result will be a strongly acid soil.

Utilization guidelines
  In general, organic soil is classified as a problem soil, not recommended to use for growing economic crops. It should be left to maintain the natural ecosystem; therefore, a still undisturbed area should be reserved to maintain its natural forest condition. But for an area along the border of a swamp that has already been disturbed and turned to be beds and (small) canals, an acid soil might have occurred. In such case, lime or marl is required to reduce the acidity and the sour or salty water in the area has to be controlled, so it will not affect the growth of the plants.


3. Salt-affected soils
  ‘Salt-affected soil’ refers to a soil having too high the amount of salt dissolved in the soil solution that consequently affects the growth and productivity of plants. Generally, if the electric conductivity (EC) of a soil solution extracted from a water-saturated soil is higher than 2 dS/m the soil is considered saline.The general characteristics of salt-affected soils are similar to non-problem soils but contain more easily soluble salts than normal. The result from an EC measurement will tell us whether the soil is a salt-affected soil. However, we can observe the condition of the area and the types of vegetation growing there. The area with salt-affected soils would show a thin film of white salt especially in the dry season. However, as the distribution of salt is not uniform, each area may possess different salinity levels. In the saltiest area the condition may not allow any crop to grow on or there may be some salt-tolerant plants only. For the areas with low salinity there may be some plants, but their growth is not so well. Therefore, for such a plot of land one may see many empty patches or some salt crusts appearing on the surface in some spots.

The salt-affected soils in Thailand can be classified according to their origin into two types:
(1) The coastal saline soils. They are mostly found along the coast in the areas that the
       tide can still reach or previously inundated by the sea that had caused salt
       accumulation in the soil.
(2) The inland saline soils or salt-affected soils on the land. They are caused by the 
      decomposition of sedimentary rocks interspersed with salt bearing rock, or from the
      salt brought up by groundwater that is brackish or salty, or caused by water that 
      leached salt from other place and accumulated on the soil surface. The inland saline
      soils are found mostly in the Northeastern part of Thailand at low areas or at the
      shoulders of mounds, and occasionally found in some provinces of the Central part of
      the country.

The problem of saline soils
  Too much soluble salt in the soil can harm the plant growth because the plants can suffer the lack of water (dehydration) and from receiving excessive amounts of elements that are constituents of the salt that are accumulated in the soil, particularly sodium and chloride, making crop failure, crop yield reduction and low quality produces.

Utilization guidelines
(1) The management of coastal saline soil may be carried out in two ways: 
     (a) Management to suit the existing natural conditions, such as preservation or 
           rehabilitation of mangrove forest. For the area that has been transformed to be salt
           farm or shrimp farm, a preventive measure must be exercised so the salt water will
           not spread to other agricultural areas. 
      (b) Modification of natural conditions, such as building a dam to block the sea water so
            the area will become permanent farmland or raising the beds to grow crops and
            using water to wash the salt out and apply soil improving materials such as rice
            husk, compost and manure. If there is an acid soil layer beneath, the digging of the
            bed has to be very careful, not to dig the small canal down to the layer of acid soil.
            If the some part of the acid soil layer is dug up to fill the beds, the acidity of the soil
            must be corrected with lime or marl.

(2) Management of inland saline soil can be done by using locally available
      
technologies,e.g. washing the salt out of the soil with water. Other methods include:
      (a) Addition of organic matter to the soil by plowing in the green manure and organic
            fertilizers, 
      (b) Addition of soil amendments such as rice husk to improve the structure of the soil to
            be more friable and allow more water to seep and leach the salts down to a deeper 
            layer as well as to increase the soil fertility,
      (c) Covering the soil with waste materials, such as rice straw, to preserve soil moisture
            all the time, 
      (d) Transplanting older rice seedlings than usual or planting a higher number of
            seedlings than normal, and 
      (e) Select salt-tolerant crops such as acacias and eucalyptus. At the same time one
            must be careful when performing some activities that may cause the spreading of
            salt to other vicinities such as large salt mining, deforestation or construction of
            reservoirs in salt-accumulated areas.


4. Sandy soils
  Sandy soil means the soil that its upper part is sandy or sandy loam, with at least 50 cm thickness, but most sandy soils are thicker than 100 cm from the soil surface. The soil particles do not stick together. The drainage is too fast; that makes the soil less able to hold water. Its ability to absorb plant nutrients is low, making the fertility of the soil to be low. Plants are vulnerable to suffer water shortage during dry spells. Sand grains tend to become compacted under the plow layer, becoming prone to erosion. Soils at certain locations have compacted organic layer, making the seepage of water into the soil and penetration of plant roots even more difficult.Sandy soil is usually caused by the deposition of coarse sediments or sandy sediments on the coast; it can be found in both lowland and upland areas.  
Their details are as follows:
  Sandy soils in upland areas. They are found along the beaches, coastal dunes, or on undulating terrains up to the foothill slope. Underlying rocks are course textured; soil texture is sand with great thickness and very quick drainage. The soils have very low water holding capacity and are prone to erosion, as soil particles do not bind together. They are used mostly for growing field crops such as cassava and pineapple.Sandy soils in lowland basins. They are usually found between beach ridges or coastal dunes or on the plains close to sandstone mountains. The drainage of these soils is poor or rather poor, making the area wet or submerged in short periods after heavy rain events. Some areas are used for growing rice and some for field crops such as sugarcane and jute; but certain places are abandoned or left as a natural grassland.Besides, in some areas such as old sandy beach or coastal dunes, especially in the east and southern coasts, we may find sandy soils with organic subsoil layers that have specific feature, i.e. the upper layer is white sand, but when going lower it becomes a reddish brown compacted sandy layer caused by the coagulation of iron compounds and organic matter. During the dry season the compact layer is very dry and hard that the plant roots cannot penetrate. But in rainy season, the soil is wet and muddy. Most of these areas are ‘samed’ (Melaleuca leucadrendon) forest, beach forests, or some areas are planted to coconut and cashew nut, etc.

The problem of sandy soil
(1) Surface soil erodes easily because soil particles bind together rather loosely; it is a
      serious problem in upland and undulating areas. The problem becomes severe in
      mountainous areas that plants are grown without suitable soil and water conservation
      measures. It also causes many consequent problems; such deterioration of the land
      causes sedimentation in streams, rivers, storage dams and irrigation reservoirs, with
      repetitive drought-flood events.
(2) Low fertility soils: they are often due to having low contents of organic matter,
      potassium and phosphorus, all being essential to the plant growth. The exchange
      capacity for plant nutrients of the soil is very low. Therefore, when fertilizers are put into
      the soil, they tend to be lost easily. The plant’s response to fertilizer application is poor.
(3)  In soils with very coarse sand component, soil pores are large. When it rains, the water
       will flow through the soil quickly while the land can absorb only a small amount of 
       water. The crops are easily prone to water scarcity. But if the soil is fine sandy and is
       in a lowland area, it may result in soil compaction, with poor drainage and poor air
       movement; they are major obstacles to the penetration of plant roots.

Utilization guidelines
  The soil should be improved to increase its fertility by adding organic matter to the soil in various forms such as plant residues, animal carcasses, compost, farmyard manure, organic fertilizers, to increase the aggregation of soil particles and eventually better soil structure and increase the ability of the soil to absorb water and plant nutrients, reduce the compaction of the soil under the plow layer, which will also reduce soil erosion. Then use chemical and organic fertilizers appropriately to enrich the soil with sufficient amounts of plant nutrients for the requirement of crops. Preserve water to use during dry spells and manage to have efficient soil and water conservation system.


5. Shallow soils or skeletal soils 
  Shallow soil or skeletal soil means the soil with layers of dense laterite, gravel, rock debris, marl or the laterite layer can be found shallower than 50 cm from the soil surface, which impede the penetration of plant roots and tillage operation. In addition, the shallow soil or skeletal soil has less amount of soil that plants can grow on; it has less ability to absorb water, adsorb plant nutrients. As a result, plants cannot grow as well as they should, and will give low yields.

The shallow soil can be divided into four categories:
 (1) Shallow soils with poor drainage are those found in lowland. Drainage in these soils is
       rather poor, often waterlogged in rainy season. Most soils consist of a large amount of
       laterite gravels. There may be a soft laterite layer in the subsoil. Some areas are used
       as rice field while some are under scrub forest (Fig. A).

 (2) Shallow soils with laterite or gravels with good water drainage. They are found along
       the undulating areas or on the hills. They often contain a large amount of laterite or
       conglomerate, starting from the soil surface down to deeper layers. In some areas,
       gravels or laterite boulders may scatter on the soil surface (Fig. B).

 (3) The shallow soils mixed with stones with good drainage. They are normally found in
       the undulating areas or on the hills. There are big numbers of large and small pieces
       of rock debris mixed in the soil. In certain places some weathered rock or hard rock
       can be found mixed with rubble or stones of various sizes scattering widely on the soil
       surface (Fig. C).

 (4) The shallow soils mixed with marl. They are found in a flat to undulating areas or on
       the hills. At a depth of 20-50 cm, one may find white carbonate grains or clods that are
       the compounds of calcium or magnesium carbonate mixed in the soil. This type of soil
       is classified as a soil with high fertility but with one disadvantage that the soil is very
       alkaline in reaction, which is a limitation for the plants that do not like alkalinity, e.g.
       pineapple (Fig. D).

The problem of shallow soil
  Shallow soils are unsuitable for cultivation because there is a layer that hinders the growth of plants. The amount of ‘soil’ material is small because there are large amounts of course materials mixed in the soil. The soil aggregation is poor, being sensitive to erosion. The soils are not fertile; they contain less amounts of plant nutrients and can hold only small amount of water. The subsoil is very compact so the plant roots can penetrate with difficulty, making the spread of plant roots not uniform. Plants cannot grow normally, so there is a good chance that large trees will fall down easily.

Utilization guidelines
  Using these areas needs careful management. If one will farm the shallow soil areas, the topsoil thickness should be thicker than 25 cm and should not contain too large amounts of pebbles or lateritic materials mixed in the soil and with slightly sloping terrain. The soil should be improved by plowing green manure crops under the soil as well as applying compost or farmyard manure. Drought-resistant plants with shallow root systems should be planted. If fruit trees are to be planted, planting holes should be wider and deeper than usual so that roots can grow well. Soil improvement can be done by putting topsoil without pebbles or gravels into the holes or simply carrying good soil material from elsewhere. After that, the compost or manure as well as chemical fertilizers are applied to meet the requirement of the planted crops. After that the soil surface should be covered to preserve soil moisture and should arrange an efficient irrigation system such as drip irrigation. For shallow soil with shallow bedrock, the area should be developed as pasture or leave it as a natural forest.


6. Slope complex
  The slope complex refers to the mountainous areas with 35% slopes or greater. Most of them are improperly used and they lack good management, resulting in soil erosion and soil degradation quickly. The nature and properties of soils found on areas with steep slopes can be a lot different depending on different factors of soil formation. On such sloping area, one may find from shallow soils to deep soils. The soil texture can vary from sandy to brown clay to red clay. The soil reaction can vary from acidic to alkaline. The soil fertility can also vary from low to high. There may also be stones or rock fragments mixed in the soil or solid rocks protrude out of the soil surface. 

Utilization guidelines
  If it is necessary to use these areas for cultivation, there must be some good measures to prevent soil erosion. There are two major factors, i.e. reduction of the impact of falling raindrops to hit soil surface and to slow down the speed of runoff through the soil surface. The soil should be tilled as little as possible, only to maintain soil lumps not to break apart and washed away easily. Soil and water conservation systems should be established, such as contour cultivation, field terraces and bench terraces. Planting vetiver grass across the slope to prevent soil erosion is also an efficient soil conservation measure.

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