The Physiology of Flowering Plants - KHAM PHA MOI

128 MINERAL NUTRITIONDeath and activity of microorganisms return the N of plants andother organisms to the soil as ammonia, which dissolving in soilwater yields ammonium ions. Some of the ammonia is convertedto nitrate by nitrifying bacteria, e.g. Nitrosomonas and Nitrobacter, inthe soil. But nitrate and ammonium ions are very soluble andeasily leached from the soil by rain into bodies of water; ammoniais volatile and escapes to the atmosphere. Moreover the soil containsalso denitrifying bacteria, e.g. Thiobacillus denitrificans, whichconvert nitrate to N 2 gas or to nitrous oxides. The N supply in thesoil, on which flowering plants depend, and hence on which mostother terrestrial organisms also depend, needs an input over andabove that which comes from the direct recycling of organicmaterial. There are no nitrogenous minerals in the soil. The supplycomes from the atmospheric N 2 via N 2 -fixing microorganisms, ofwhich the legume–Rhizobium symbioses account for 25–50%. Therest comes from the activities of symbioses involving non-leguminousplants, and from free-living N 2 -fixing bacteria, e.g. Azotobacter andClostridium. A non-biotic fixation is achieved by lightning: theelectric discharges convert N 2 to oxides of N which dissolve inrainwater to give nitrite and nitrate ions.4.5 Problems with mineral elements: deficiency andtoxicity4.5.1 Nutrient deficienciesWhen the supply of any of the essential elements falls below aminimal level, plants usually show visible deficiency symptomsand the yield of cultivated plants can be severely reduced; in extremecases the plants die. Many economically important and symptomcharacteristicplant diseases are recognized as resulting from mineralelement deficiencies. These have been given names descriptive of thevisible symptoms of the deficiency. Examples are ‘tea-yellows’– Sdeficiency of tea; ‘grey-speck’ of oats (Avena sativa) – Mn deficiency;‘sickle leaf’ of cocoa (Theobroma cacao) – Zn deficiency; ‘brown heart’of swede (Brassica napus) – B deficiency; ‘scald’ disease of beans – Modeficiency. Texts are available with such symptoms described andphotographed for a diagnosis to be made. Unfortunately, by the timethe symptoms are fully developed a crop may be beyond recovery.Earlier diagnosis can be achieved by leaf analysis, or by checkingfor curative effects of foliar sprays containing various elements,applied to crops suspected of incipient mineral deficiency. Plantspecies which rapidly develop characteristic deficiency symptomsfor particular elements are used as indicator species for testing suspectsoils.