(a)(i) Nitrogen is essential in protein syntheses. It encourages vegetative growth in plants and is responsible for the dark green colouration 'chlorophyll' of livea. Nitrogen facilitates in utilization of phosphorus and potassium. Also, It increases the succulence of leaves due to the increase in protein and water content and decreases in carbohydrate content. It is necessary for synthesis of plant enzymes and auxins.
(ii)(1) Phosphorus in plant nutrition helps in seed germination. (2) It is important in cell division and hastens the maturity of plants. (3) It improves the quality of plant produce and influences flowering and fruiting in plants. (4)1i improves the resistance of plants to diseases. (5) Phosphorus enhances ripening of fruits and assists in enzymatic reaction in plants. (6) It improvises palatability of forages and vegetables.
(iii) Potassium in plant nutrition is important in the synthesis of carbohydrates and proteins. It is also important in tuberisation in root crop. Potassium encourages root growth, regulates the rate of root respiration and helps in the production of well developed grains. It tends to have a balancing effect on nitrogen and phosphorous. Potassium influences the rate of transpiration. It helps in the development of still straw and hence reduces lodging. It neutralises organic acids in plants and enhances nitrate uptake from the soil.
(b) Factors that affect availability of nutrients to crops are: soil Ph, leaching, crop removal (e.g. harvesting, overgrazing, continues cropping), moisture, content of soil, burning, excess of other nutrients, organic matter content, soil type / texture, soil structure, oxidation and reduction and erosion