Agricultural Science 2020 Jamb Examination

Sulphur plays a critical role in plant metabolism because it is a key component of amino acids such as cysteine and methionine, which are the building blocks of proteins. In addition, sulphur is involved in the synthesis of certain plant hormones and vitamins that regulate growth and development. Without adequate sulphur, plants show stunted growth, yellowing of young leaves, and reduced protein content, making it indispensable in agriculture.

Soil is composed of four main components: mineral matter, organic matter, water, and air. These combine to support plant growth. Texture, on the other hand, is not a component but rather a property that describes the relative proportion of sand, silt, and clay in the soil. It influences water retention, aeration, and fertility but is not a direct constituent.

The initial weathering of rocks is largely driven by temperature changes. Expansion occurs when rocks heat up during the day, and contraction occurs when they cool at night. This repeated stress weakens the rock structure and results in cracks and fissures. While ice, wind, and human activities also contribute to weathering, temperature variation is the primary natural cause of the first visible cracks.

The pH scale ranges from 0 to 14, with 7 being neutral. Soils with a pH below 7 are acidic, and those above 7 are alkaline. A soil with pH between 6.1 and 6.9 falls just below neutral, which is categorised as slightly acidic. This range is generally suitable for most crops because it allows optimal nutrient availability.

Leguminous crops such as clover and alfalfa fix nitrogen in the soil through symbiotic bacteria in their root nodules. This nitrogen is used to synthesise amino acids, which are the building blocks of proteins. Therefore, legumes increase the protein content of pastures, making them especially valuable in livestock feeding systems.

Centrosema, stylosanthes, and giant star grass are well-established pasture crops used for feeding livestock. Mucuna, although a legume, is primarily cultivated as a cover crop or green manure to improve soil fertility rather than being used widely as pasture. Hence, it is not normally categorised as a pasture crop.

Stem borers are major pests of maize. They tunnel into the stem, feeding on the internal tissues, which weakens the plant and causes hollowness. This damage reduces nutrient flow, results in poor grain filling, and often causes lodging, where the plant falls over. Other animals like rodents or grasscutters may feed on maize, but they do not cause the characteristic hollowness in stems.

Quartz is a highly abundant mineral in the Earth's crust and is present in nearly all crystalline rocks. It is a primary constituent of granite and sandstone, giving these rocks their hardness and durability. Biotite and mica are also minerals found in rocks but not as universally dominant as quartz. Granite is itself a rock, not a mineral.

A soil pH of 2 is extremely low on the pH scale, indicating a very high concentration of hydrogen ions. Such soils are classified as highly acidic. They are generally unsuitable for most crops unless treated with lime to raise the pH and reduce acidity.

In regions with high rainfall, the constant percolation of water through the soil profile leads to leaching, a process where essential nutrients such as nitrogen, potassium, and calcium are washed away. Even fertile soils lose productivity under these conditions because nutrients are unavailable to plants, making leaching the major factor responsible for poor yield in such environments.