30. (i) What are the soluble nitrogen compounds absorbed by the roots of plants?   31. (ii) What kind of plants directly absorb nitrogen from the atmosphere?   32. (iii) What term is applied to the conversion of atmospheric nitrogen to useful compounds of nitrogen?   33. (iv) Compounds such as ammonium nitrate, urea and super-phosphate are used to replace nitrogen and other elements lost from the soil as a result of cultivation. What is the common name given to these compounds?   34. (v) What insoluble substance is used in making super-phosphate?