The correct answer is Gypsum. Key Points Gypsum is used to solve the problem of salinity in the soil. The primary cause of man-made salinization is the salt brought in with irrigation water. All irrigation water derived from rivers or groundwater, however 'sweet', contains salts that remain behind in the soil after the water has evaporated. The secondary cause of salinization is waterlogging in irrigated land. Irrigation causes changes to the natural water balance of irrigated lands. Important Points The distinguishing characteristic of saline soils from an agricultural standpoint is that they contain sufficient neutral soluble salts to adversely affect the growth of most crop plants. Soluble salts most commonly present are the chlorides and sulphates of sodium, calcium and magnesium. Nitrates may be present in appreciable quantities only rarely. When leached with low saltwater, some saline soils tend to disperse resulting in low permeability to water and air, particularly when the soils are heavy clays. Excess soil salinity causes poor and spotty stands of crops, uneven and stunted growth and poor yields, the extent depending on the degree of salinity. The primary effect of excess salinity is that it renders less water available to plants although some are still present in the root zone. Additional Information Methods commonly adopted or proposed to accomplish the reclamation of saline soils: Scraping: Removing the salts that have accumulated on the soil surface by mechanical means. Although this method might temporarily improve crop growth, the ultimate disposal of salts still poses a major problem. Flushing: Washing away the surface accumulated salts by flushing water over the surface is sometimes used to desalinize soils having surface salt crusts. Because the amount of salts that can be flushed from the soil is rather small, this method does not have much practical significance. Leaching is most often accomplished by ponding fresh water on the soil surface and allowing it to infiltrate. Leaching is effective when the salty drainage water is discharged through subsurface drains that carry the leached salts out of the area under reclamation. Leaching should preferably be done when the soil moisture content is low and the groundwater table is deep. Gypsum is the most common amendment used to correct saline-sodic or sodic soils that have no calcium source such as gypsum or free carbonates.