Teacher B is correct. The thesis states the main idea of the entire essay. Each paragraph should serve the purpose of supporting and/or developing the essay's thesis.
Often, the writer will develop multiple supporting points to achieve this aim. Each body paragraph will develop a different idea which supports the main idea, or thesis of the essay. These smaller ideas ideally will also have a statement within the paragraph that sums them up. Teacher A may have been thinking of the topic sentence, which should reflect the main idea of the paragraph it's in.
It helps in understanding which crop should be cultivated on a particular land. It also helps in gaining information about how agriculture can be done in poorer soils. It gives information about the nutrient quality of the soil. This avoids wastage of capital on the cultivation of unsuitable crops. It also helps in controlling pests and insects.
