WHAT ARE SOFT SOILS
Soft soil is generally referred as weak compressible soils. The definitions of soft soils have been made significantly through a systematic research and geotechnical experience over the years. By definition! soft soils are low strength! high compressibility and very sensitive. 
Soil that consist of silts! sand or gravel are primarily the results of physical and mild chemical Weathering process and retain much of the chemical structure of their parents rock. However, this is not the case with clay soils because they have experienced extensive chemical weathering and change into a new material quite different from the parent rocks. As a result! the engineering properties and behavior of clays also are quite different from other soils. 
Loam is soil composed mostly of sand and silt, and a smaller amount of clay (about 40%-40%-20% concentration, respectively).These proportions can vary to a degree, however, and result in different types of loam soils: sandy loam, silty loam, clay loam, sandy clay loam, silty clay loam, and loam. In the USDA textural classification triangle, the only soil that is not predominantly sand, silt, nor clay is called "loam". Loam soils generally contain more nutrients, moisture, and humus than sandy soils, have better drainage and infiltration of water and air than silty soils, and are easier to till than clay soils. The different types of loam soils each have slightly different characteristics, with some draining liquids more efficiently than others. For food production, a loam soil containing a small amount of organic material is considered ideal. The soil's texture, especially its ability to retain nutrients and water are crucial. 
In general soils are poor in nutrients and organic matter. Alkalinity, salinity, sodality,
poor water retention capacity and water- logging are all common features. 
The following fertility problems with different types of soils are:
Soils are sandy, Soils are poor in nitrogen & phosphorus. Soils are poor in organic matter. Soils have a very low structural stability. Soils such as clay soils have a poor water retention, absorbability and capacity Soils tend to form hard pans some inherent to the clay soil, others formed by repeated ploughing at the same depth. 
Water-logging in clay soil is common although rainfall is limited. The water content of some soils may change tremendously over short periods from water-logging to hard
crusting on the surface. Rainfall is low, erratic or falls in heavy storms, Direct sunlight is
intense and temperatures are high. Land is completely bare after the harvest season. 
Sodic soils (clay) are poorly drained and tend to crust. Sodic soils respond to continued use of good irrigation water,good irrigation methods, and good cropping practices. Sodic soils are often reclaimed by adding CaSo4 soil amendment. 
Soils with high levels of exchangeable sodium (Na) and low levels of total salts are called sodic soils. Sodic soils impact plant growth by: 1.) Specific toxicity to sodium sensitive plants; 2.) Nutrient deficiencies or imbalances; 3.) High pH; and 4.) Dispersion of soil particles that causes poor physical condition of the soil.
Sodic soils tend to develop poor structure and drainage over time because sodium ions on clay particles cause the soil particles to deflocculate, or disperse. Sodic soils are hard and cloddy when dry and tend to crust. Water intake is usually poor with sodic soils, especially those high in silt and clay. Poor plant growth and germination are also common. The soil’s pH is usually high, often above 9.0, and plant nutritional imbalances may occur. A soil pH above 8.4 typically indicates that a sodium problem exists. The term ?alkali? is often used to describe soils that are high in salt but sometimes people use the term to mean high pH and at other times to mean high sodium. 
Sometimes a specific ion can have toxic reactions in certain plants. Sodic soils can cause specific ion toxicity in sensitive crops such as potatoes, beans and woody plants such as vines and stone fruits. High sodium levels compete with calcium, magnesium, and potassium for uptake by plant roots. Therefore, excess sodium can prompt deficiencies of other cations (positively charged nutrients). High levels of other cations (calcium, magnesium, potassium) can also cause imbalances and induce nutrient deficiencies. 
CaSO4 Refined CaSO4 in the anhydrite form (no water) is 29.4 percent calcium (Ca) and 23.5 percent sulfur (S). Usually, CaSO4 has water associated in the molecular structure (CaSO4?2H2O) and is approximately 23.3 percent Ca and 18.5 percent S. CaSO4 usually has OTHER IMPURITIES so grades are approximately 22 percent Ca and 17 percent S. CaSO4 is sparingly soluble . CaSO4 is the neutral salt of a strong acid and strong base and does not increase or decrease acidity. Dissolving CaSO4 in water or soil results in the following reaction: CaSO4?2H2O = Ca2+ + SO42- + 2H2O. It adds calcium ions (Ca2+) and sulfate ions (SO42-), but does not add or take away hydrogen ions (H+). Therefore, it does not act as a liming or acidifying material. The Ca2+ ions simply interact with exchange sites in soil and sulfate remains dissolved in soil water.
Negatively charged soil clay particles can be bound together into clumps or aggregates by positively charged molecules (cations). The formation of stable soil aggregates,a process called flocculation, encourages water infiltration and drainage, and prevents surface soil crusting. Flocculation is promoted by high levels of salinity (which may not be conducive to plant growth) and by the presence of cations that are strong flocculators. The dominant soil cations in medium to high pH soils are the monovalent cations (one positive charge per mole cule) sodium (Na +) and potassium (K+),and the divalent cations (two chargesper molecule) magnesium (Mg 2+) and calcium (Ca 2+ ). In highly acidic soils the trivalent aluminum cation (Al +3) may be present.