(a) The functions of nitrogen in crop plants are: (i) It improves growth and reproduction because it is a constituent of protein and protoplasm (ii) It improves the growth of the shoot system (iii) it assists the leaves to have a deep green colour especially in vegetables (iv) It increases the size of grains in cereal crops (v) It promotes the uptake of potassium and phosphorus from the soil.
(b)(i) The effects of excess soil nitrogen on plants are: (1) It causes excessive vegetative growth (2) It delays flowering, fruiting and ripening i.e it leads to reduction in reproductive growth (3) It weakens plant stems (4) It causes lodging, especially in cereals (5) It may decrease resistance in grains and thereby lower the quality of the grains (6) It enhances disease attack in crops.
(ii) The effects of nitrogen deficiency on plants are: (1) It leads to general yellowing of aerial plant parts (2) It leads to stunted growth (3) It leads to premature dropping of leaves (4) It causes poor flowering or fruiting (5) It makes stems to become thin and chaffy, especially in grain crops:
(c) Apart from lightning, nitrogen is also added to the soil through: (i) decay of organic matter (ii) application of inorganic fertilizers (iii) symbiotic nitrogen fixation by microbes in the root nodules of leguminous plants as a result of crop rotation (iv) nitrogen fixation by free living bacteria in the soil.
(d) The soil micro nutrients are: boron, iron, manganese, cobalt, copper, molybdenum, zinc and chlorine.