With reference to agricultural soils, consider the following statements: 1. A high content of organic matter in soil drastically reduces its water holding

1. A. 1 and 2 only
2. B. 3 only
3. C. 1 and 3 only
4. D. 1, 2 and 3

Answer: B

Explanation

Organic matter affects both the chemical and physical properties of the soil and its overall health. Properties influenced by organic matter include: soil structure; moisture holding capacity; diversity and activity of soil organisms, both those that are beneficial and harmful to crop production; and nutrient availability. Increased levels of organic matter and associated soil fauna lead to greater pore space with the immediate result that water infiltrates more readily and can be held in the soil. The improved pore space is a consequence of the bioturbating activities of earthworms and other macro-organisms and channels left in the soil by decayed plant roots. Sulfur is one of three nutrients that are cycled between the soil, plant matter and the atmosphere. The sulfur cycle describes the movement of sulfur through the atmosphere, mineral and organic forms, and through living things. Although sulfur is primarily found in sedimentary rocks, it is particularly important to living things because it is a component of many proteins. Sulfur is released by weathering of rocks and minerals. Water, temperature and chemical reactions break down minerals releasing their component elements. Once sulfur is exposed to the air, it combines with oxygen, and becomes sulfate (SO4). Plants and microbes take up sulfate and convert it into organic compounds. As animals consume plants, the sulfur is moved through the food chain and released when organisms and plants die and decompose. Primary salinization occurs naturally where the soil parent material is rich in soluble salts, or in the presence of a shallow saline groundwater table. In arid and semiarid regions, where rainfall is insufficient to leach soluble salts from the soil, or where drainage is restricted, soils with high concentrations of salts (“salt-affected soils”) may be formed. Several geochemical processes can also result in salt-affected soil formation. When an excess of sodium is involved in the salinization process this is referred to as sodification. Secondary salinization occurs when significant amounts of water are provided by irrigation, with no adequate provision of drainage for the leaching and removal of salts, resulting in the soils becoming salty and unproductive. Salt-affected soils reduce both the ability of crops to take up water and the availability of micronutrients. They also concentrate ions toxic to plants and may degrade the soil structure.