has a great effect on the availability of residual nutrients in soil as well as on those added via fertigation. The balance between cation and anion uptake by the plant affects the pH in the rhizosphere. When a plant takes up more nutrient cations, such as when NH4+ is the main N source, protons are exuded by the roots and acidify the soil surrounding the roots. If the anion uptake is predominant, as when NO3− is the main source of N, the roots exude OH− or HCO3−, resulting in a pH rise in the rhizosphere. NO3− and NH4+ are the main forms of N available for plant uptake. NH4+ nutrition leads to excretion of 0.9 mol of H+ by roots with 1 mol of NH4+ assimilation; while, where there is NO3− reduction in the leaves, 0.1–0.3 mol of OH− is released from the root per mole of NO3− taken up. The
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