Fertiliser, such as manure, contains ammonium compounds. Explain how the presence of soil bacteria and the use of manure improve crop yield. (6 marks)

Ammonium compounds in the manure are converted into nitrite, then into nitrate by nitrifying bacteria. Nitrate is then taken up by plants via active transport. Nitrogen fixing bacteria living in the nodules on the roots of plants also fix nitrogen from atmosphere into plants by converting gaseous nitrogen into nitrogen-containing compounds. Nitrogen is an essential component of amino acids, ATP and nucleotides in DNA. Therefore nitrogen-containing fertilisers improves crop yield by increasing rate of photosynthesis and plant growth.