(a) State three ways by which nitrogen is added to soil in the Nitrogen cycle. (b) List five ways by which nitrogen is lost from the soil. (c) State the imp...

(a) Three ways by which nitrogen is added to the soil:

1. Biological nitrogen fixation by leguminous plants (Rhizobium bacteria in root nodules) and free-living soil bacteria.
2. Application of nitrogenous fertilizers and organic manures, and decay of dead plant and animal remains.
3. Fixation of atmospheric nitrogen by lightning, which brings it down in rain as nitrates.

(b) Five ways by which nitrogen is lost from the soil:

1. Leaching of soluble nitrates down beyond the root zone.
2. Denitrification, in which bacteria convert nitrates back to free nitrogen gas.
3. Removal (uptake) of nitrogen by crops that are harvested and taken off the land.
4. Volatilisation of ammonia into the air, especially from surface manures and fertilizers.
5. Loss through soil erosion and by burning of vegetation and crop residues.

(c) Importance of the nitrogen cycle: It ensures a continuous natural supply and recycling of nitrogen, keeping the soil fertile so that plants can obtain the nitrates needed to make proteins for growth. It maintains a balance between the nitrogen used up by plants and micro-organisms and the nitrogen returned to the soil and atmosphere.

(d) Three processes leading to formation of nitrate from organic matter:

1. Ammonification: Decomposing bacteria and fungi break down proteins in dead plant and animal matter and release ammonia (ammonium compounds).
2. Nitritation: Nitrifying bacteria (Nitrosomonas) oxidise the ammonium compounds to nitrites.
3. Nitratation: Nitrifying bacteria (Nitrobacter) further oxidise the nitrites to nitrates, the form absorbed by plants. (Nitritation and nitratation together make up nitrification.)