Availability in Alkaline Soils

There is no specific method that is best for assessing Mo availability in alkaline soils. In practically all of the reports on the status of the available Mo in alkaline soils, use has been made of one of the several methods essentially developed for determining available Mo in non-alkaline soils (Table 8.1). The principles and detailed procedures for such tests have been described in recent works on soil testing (Cox and Kamprath, 1972 Cox, 1987 Johnson and Fixen, 1990 Sims and Johnson, 1991) and have been described in relation to acid soils in Chapter 7 of this volume. 

The procedure of resin extraction of Mo proposed by Bhella and Dawson (1972) has not been well tested for evaluating Mo availability in alkaline soils. A comparison between that method and the AAO extraction (Karimian and Cox, 1979) did not show any positive correlation between the Mo extracted by either of the methods and plant Mo uptake. Lombin (1985) tried the resin extraction procedure for assessing the availability of Mo in semiarid savannah soils of Nigeria, but did not find it suitable. Compared with resin extraction, AAO extraction showed good correlation with Mo uptake by peanuts, provided that soil organic-matter content was used as an independent variable. 

The CEC is a major factor affecting Mo availability in alkaline soils. Prasad and Pagel (1976) reported a positive correlation between CEC and the amount of Mo extractable with ammonium acetate, at pH 4.0, for... 

Thus, B(0H)3 is a Lewis add rather than a Bronsted acid (see Chapter 1). Because boron adsorbs most effectively in the pH 8 to 9 range on A1 and Fe oxides and silicate minerals, its availability is generally low in coarse-textured, acid-leached soils and in calcareous soils. Deficiency in add soils is the result of boron depletion by leaching, while deficiency in calcareous soils is caused by strong adsorption and predpitation as relatively insoluble Ca borate salts. In contrast, B toxicity is most commonly found in alkaline soils of arid regions these soils often contain high levels of Na which forms quite soluble borate salts. A lack of rainfall allows soluble borate to accumulate to phytotoxic levels. 

The diagram also indicates that phosphate should precipitate in basic soils as one of several Ca phosphates, the least soluble of which are hydroxy- and fluoroapatite. Variscite and strengite are too soluble to exist under basic conditions, and they should not form in basic soils. Both variscite and strengite, in fact, would be good phosphate fertilizers in alkaline soils because of their solubility in basic soils, if they were applied as finely ground materials. Calcium phosphate ore ( rock phosphate, mostly hydroxy- and fluoroapatite) is similarly effective in acid soils. Rock phosphate is treated with sulfuric acid to make superphosphate, nominally CaHP04 treatment with phosphoric acid yields triple superphosphate, nominally Ca(H2P04)2. Both superphosphate and triple superphosphate are more soluble than rock phosphate and make phosphate more immediately available when added to soils at any pH. 

Adsorption of Mo to hydrous Fe and A1 oxides is pH-dependent (Reisenauer, Tabikh, and Stout, 1962), and the rate of adsorption is highest at acidic pH. It decreases with increases in pH from 4.45 to 7.5. Compared with acid soils, alkaline soils are high in soluble or available Mo (Davis, 1956). As in any other soil, in alkaline soil the availability of Mo is also influenced by several other factors. The relative Mo content of the parent rock, the process of soil evolution, and the physicochemical attributes of the soil (pH, calcareousness, organic-matter content, cation-exchange capacity, texture, moisture, relative concentrations of other mineral elements) all influence the availability of Mo. 

For assessing the availability of Mo in alkaline soils, various extractants have been used, including water-saturation extract, hot water (Lowe and Massey, 1965), acid ammonium oxalate (Grigg, 1953), ammonium carbonate (Vlek and Lindsay, 1977), and ammonium bicarbonate-DTPA (Soltanpour, 1985), as well as the bioassay procedure using A. niger (Mulder, 1948). 

Factors Affecting the Available Molybdenum in Alkaline Soils... 

Although pH is the predominant factor affecting the availability of Mo in alkaline soils, there are several other factors that influence its availability. Soil calcareousness, cation-exchange capacity (CEC), organic-matter content, and soil moisture are all known to affect the availability of Mo in alkaline soils. 

High moisture contents in alkaline soils have been shown to increase the availability of Mo (Nayyar, 1972). Poonamperuma (1972) observed an increase in soluble-Mo concentration under wetland rice conditions. Flooding of soils increases the availability of Mo. Soils in the tropics that remain flooded for considerable periods have relatively high contents of Mo (Lopes, 1980), because a high moisture content in the soil reduces ferric iron to the ferrous form, which favors fixation of Mo (Jones, 1956). 

Little information is available on the distribution of Mo in the soil profile. In alkaline soils. Mo is more mobile, and if it is not leached from the profile it can accumulate in plants. Pasricha and Randhawa (1971) reported that the available Mo in the surface horizons of recently reclaimed sodic soils in Punjab varied from 0.012 to 0.449 pggr with an average value of 0.112 pgg as compared with subsurface samples in which the values of available Mo ranged from 0.065 to 0.720 pgg with a mean of 0.207 pgg Ahmad, Khathak, and Perveen (1991) reported that the available Mo in the Dir district of Pakistan varied from 0.10 to 1.39 pgg in the topsoil and from 0.12 to 1.31pgg in the subsoil. Misra and Misra (1972) observed high amounts of available Mo in surface soils and a tendency for it to decrease with depth in an alkali soil profile, but not so in the black and red soils, in Uttar Pradesh. The available Mo content decreased with depth in salt-affected soils in Haryana and... 

Carbonic acid evolved from decomposing plant residues plays a role in iron availability. In acid soils this carbon dioxide forms bicarbonates which increase the solubility of iron compounds. In alkaline—calcareous soils, however, the carbon dioxide formed acts to keep the iron in insoluble forms and thus may increase chlorosis. Brown (1961) points out that green manure crops when incorporated into moist calcareous soil often cause severe iron chlorosis to develop in deciduous fruit trees. 

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