Manganese (Mn) is essential for both plants and animals. McHargue (1923) showed that Mn was essential for the growth of higher plants and Kemmerer et al. (1931) established a role for Mn in animal health. Deficiencies of Mn can occur in cereal and root crops on soils of high pH and high organic matter. Excessive Ca and P in the diet interfere with the absorption of Mn (Welch et al., 1991). People with poor dietary habits can also be at risk where the optimal Mn is not being met (Kies, 1982).
Manganese in soils
Rocks, in general, contain relatively high levels of Mn when compared with values for most trace elements, except iron. Wedepohl (1979) estimated the Mn contents of various rocks as follows: ultra basic 1050, basalts 1300, gabbros 1100-1300, granites 350, rhyolites 620, limestones 550, gneisses and mica schists 600, shales 600, and sandstones 174 mg/kg, respectively, with an overall mean of 733 mg/kg Mn in the upper continental crust.
The concentration of Mn found in soils is a reflection of the levels that occur in their parent materials. The mean Mn content of 8354 world soils was reported as 761 mg/kg by Ure and Berrow (1982) with a range of <1-18,300 mg/kg. The Mn concentration of most Irish soils fall within the range 20-3000 mg/kg. Mn in soil can be divided into two main forms (1) Mn in primary minerals and (2) Mn in secondary minerals, the latter form being the more important because of its very high surface activity. The Mn oxides in soils have very high sorption ability and they can accumulate ions from the soil solution. The Mn oxides have a particularly strong affinity for Co ions which can render it unavailable to plants (q.v., under cobalt). Manganese content of soils tends to increase with increasing clay content.
An important soil factor which influences Mn mobility is soil pH, and the application of lime to acid soils will reduce Mn availability. The application of acid forming fertilisers will decrease pH and increase Mn availability (Miner et al., 1986; Conroy, 1961-1962). It has been shown that the amount of Mn solubilized in the rhizosphere is much greater than that in the bulk solution (Godo and Reisenaur, 1980), and the conclusion reached that Mn availability is neither controlled by soil nor by plant characteristics alone but by a combination of effects of soil and plant properties with the interaction of plant roots with the soil. The plant root can increase Mn availability by exuding organic compounds which can reduce Mn4+ oxides. Some forms of organic material such as humic acid can fix Mn whereas organic matter in general can provide an energy source for microbial reductions to occur in soils. The importance of impeded drainage in the mobilization of trace elements in soils is perhaps not fully appreciated and nowhere is the effect more clearly seen that in the case of manganese. Under the reducing conditions resulting from impeded drainage many manganese minerals become unstable with resultant release of Mn2+ ions. These are then readily assimilated by plant roots. The effect is illustrated in Table 1 (Mitchell et al., 1957). Manganese deficiency, however, can occur in poorly drained soils and arise from the solubilization during periods of low aeration followed by leaching of Mn2+ below the root zone (Robertson and Lucas, 1981). Drying of soil, combined with high temperatures, can affect the availability of Mn, and Mn deficiency in various crops can disappear after heavy rainfall following a dry period.
|Table 1 Manganese levels in herbage from well and poorly drained soils
The determination of the easily-reducible soil Mn (calcium nitrate/quinol) gives a good indication of the availability with values below 50 mg/kg, particularly if liming is contemplated.
Manganese in plants
Manganese is present in plants mainly as Mn2+ but it can be readily oxidized, and because of this it has a major role in redox processes such as electron transport in photosynthesis and the detoxification of oxygen-free radicals. In Mn deficiency situations because of restriction of photosynthesis, soluble carbohydrates are largely reduced (Romheld and Margehner, 1991).
The critical deficiency concentration of Mn for barley and wheat about 25 mg/kg Mn in mature leaves (Hannam and Ohki,1988). The critical deficiency concentrations for brassicas and sugar beet are of the order of 35 mg/kg Mn. Manganese deficiency symptoms in cereals appear as greenish grey spots, flecks and stripes (grey speck) on basal leaves whereas dicotyledons develop inter-veinal chlorosis of the less mature leaves. Manganese deficiency in the pea shows up as dark discolouration of the cotyledons (Marsh spot).
MacNaeidhe et al. (1986) obtained severe Mn deficiency in sugar beet growing on a soil with easily reducible (ER) Mn value of 30mg/kg. Mild symptoms were associated with soil ER Mn values of 60-75 mg/kg. The Mn deficiency symptoms developed when the foliar Mn levels fell below 30 mg/kg.
Manganese concentrations in herbage growing on typical agricultural soils range from 30 to 500 mg/kg, but, because of the influence of soil reaction, values are encountered outside this range. On very acid soils Mn concentrations in herbage can exceed 1000 mg/kg, whereas on alkaline soil herbage values can be below 20 mg/kg Mn. In a 130 km2 area sampled at a density of approximately two samples per km2. Mn concentrations in herbage ranged from 35 to 1184 mg/kg (Fleming and Parle, 1981). Soil pH values in this area varied between 4.0 and 7.7 and there was a significant relationship between the soil pH and herbage Mn content.
Correction of manganese deficiency in Ireland
Walsh and McDonnell (1956-57:personal communication) showed that fertiliser fortified with manganese sulphate drilled with the crop controlled Mn deficiency in cereals. O’Sullivan (1974) controlled deficiency in cereals by spraying with manganese sulphate and by drilling in manganese sulphate with the NPK compound fertiliser and showed that broadcasting was ineffective. Five sources of manganese were evaluated by MacNaeidhe et al. (1984) on spring barley and winter wheat and by O’Riordan and Codd (1984) on sugar beet. Responses varied in both trials between the sources of Mn and the different soil types.