WAEC Agricultural Science 2025 Theory — Question 2

Model answer

(a) Ways nutrients are depleted from the soil (any four): - Leaching: encourages percolation of mineral salts from the topsoil to the subsoil, making nutrients unavailable to plants. - Plant uptake/removal: nutrients removed by harvested crops or grazing animals are permanently lost from the soil. - Bush burning: causes volatilization of nutrients; humus and microorganisms are also depleted. - Oxidation and reduction reactions: result in the loss of organic minerals and gaseous elements from the soil. - Soil erosion: fertile topsoil is carried away by wind or water, removing most of the available nutrients. - Continuous cropping: without replenishing lost nutrients leads to nutrient depletion. (b)(i) Advantages of applying green manure (any three): - Adds primary nutrients such as nitrogen and phosphorus/improves soil fertility - Improves soil aeration - Helps bind soil particles together - Adds organic matter to the soil - Increases the population of soil microorganisms - Helps check soil erosion - Improves soil structure - Helps fix nitrogen in the soil - Encourages microbial decomposition/enhances microbial activities - Reduces soil temperature (ii) Disadvantages of applying green manure (any three): - May lead to weed infestation - Requires much labour/labour-intensive - Could lead to pest infestation - Takes a long time to prepare - Weather dependent (c) Reasons deforestation should be discouraged (any four): - Prevents global warming/climate change/greenhouse gas emissions - Prevents desert encroachment/desertification - Maintains balance in the ecosystem/prevents destruction of the ecosystem - Checks soil erosion - Prevents drought - Prevents extinction of forest trees/plants/animals and protects biodiversity - Prevents flooding of the environment