Agricultural Science 2021 Jamb Examination

The A-horizon, also known as the topsoil, contains the highest amount of organic matter and nutrients that support microbial life. It is rich in humus and decomposed materials, providing a favourable environment for bacteria, fungi, and other microorganisms.
This makes the A-horizon the most biologically active part of the soil profile.

Silt particles range from 0.02mm to 0.002mm in diameter, making them intermediate between sand and clay in size. This gives silt soil a smooth, flour-like texture when dry and slightly soapy when wet.
Clay particles are smaller, while sand particles are larger, hence silt fits this specific size range.

The 'feeling' method is the simplest and quickest way to determine soil texture on the field. By rubbing moist soil between the fingers, one can estimate whether it is sandy, silty, or clayey based on its smoothness or grittiness.
This tactile method requires no equipment and gives an instant practical result.

Given that urea contains 46% nitrogen, the amount of nitrogen in a 25 kg bag can be calculated as:\[ \text{Nitrogen} = \dfrac{46}{100} \times 25 = 11.5 \; \text{kg} \]Therefore, the nitrogen content in a 25 kg bag of urea fertilizer is 11.5 kg.

A fertile soil must contain all the essential plant nutrients—macro and micro elements—in suitable proportions to support healthy plant growth.
While good drainage and aeration are important, fertility specifically refers to the nutrient richness and chemical balance that enable crops to yield optimally.

Crop rotation helps in maintaining soil fertility, controlling pests and diseases, and reducing erosion. However, it does not directly add humus to the soil.
Humus addition results mainly from organic matter decomposition and not from the practice of rotating crops, making option C the exception.

On steep slopes, sprinkler irrigation is the most suitable method because it evenly distributes water over the surface without causing erosion or runoff.
Flood and furrow irrigation are unsuitable for steep areas since they can lead to soil loss and water wastage due to uneven distribution.

In subsistence agriculture, human labour remains the primary source of power because the scale of operation is small and tools are manually operated.
Farmers rely mainly on their physical strength rather than machines or animals due to limited resources and small land areas.

Electricity provides a steady and reliable source of power for stationary farm equipment such as water pumps, grinders, and processing machines.
It allows continuous operation and efficiency, unlike manual or animal power, which are limited in capacity and endurance.

The tractor serves as the main source of mechanical power on the farm and is designed to couple with various implements such as ploughs, harrows, and planters.
It provides versatility, allowing the same machine to perform multiple agricultural operations by attaching different implements.