Malcolm E. Sumner

Use of gypsum on soils: a review

Gypsum is a relatively common mineral that is widely available in agricultural areas and has a number of specialized agronomic uses, principally as a Ca source on legumes and as a soil conditioner on sodic soils. Recent research has suggest that the utility of gypsum may extend to a greater range of soils and crops than previously acknowledged. The purpose of this review is to document both current practices and potential new uses of gypsum in agriculture, in order to establish the state of the art in current gypsum use, and to point out needed research in this important area.

Composition and element solubility of magnetic and non-magnetic fly ash fractions.

Magnetic and non-magnetic fractions of coal fly ashes from SE US electric power plants were characterized with special emphasis on the potential environmental consequences of their terrestrial disposal. Quartz and mullite were the crystalline minerals dominating the non-magnetic fractions. Magnetic fractions contained magnetite, hematite, and, to a lesser extent, quartz and mullite. Chemical analyses revealed that magnetic fractions had about 10 times higher concentrations of Fe, and 2-4 times higher concentrations of Co, Ni, and Mn. Non-magnetic fractions were enriched in K, Al and Ca. Iron content within fly ash particles was negatively correlated with elements associated with aluminosilicate matrix (Si, Al, K, Na). Solubility of most elements was higher in the non-magnetic than in the magnetic fractions of alkaline fly ashes at comparable pH. Calcium was associated with the non-magnetic fraction of the alkaline fly ashes which resulted in a higher pH buffering capacity of this fraction.

Sorption of heavy metals in strongly weathered soils: an overview

Current knowledge of sorption processes in tropical soils is reviewed. Landscapes throughout the tropics are dominated by oxisols which occupy extensive areas of potentially highly productive soils. These soils are dominated by low-activity sesquioxide minerals and clays that have variable charge surfaces. The limited information on tropical soils available suggests that the composition of the ambient soil solution can influence sorption through changes in particle surface-charge density. Thus the observed decrease in sorption in the presence of divalent index cations may be related to the effect of ionic charge on the double-layer thickness which is manifested through a change in surface-charge characteristics. However, much work needs to be done to differentiate the effect of cation charge on surface-charge density from the competitive effect between the index cation and heavy-metal ions for the sorption sites. The effects of inorganic and organic ligands on adsorption of Cd by variable charge surfaces are also reviewed.

Measurement of soil pH: Problems and solutions

Abstract pH measurements in soil systems present unique challenges in terms of the interpretation of the values obtained. The principles behind the glass/calomel electrode system are discussed as a backdrop to pH measurements in both pure solutions and soils. The influence of the liquid junction potential and salt concentration on the pH values of soil water suspensions are discussed in detail from which it emerges that the current practice of measuring soil pH in stirred soil suspensions is likely to result in the greatest errors being incurred. This is due to the large liquid junction potential of uncertain magnitude developed in such systems. Measurements in salt solutions, such as M KCl and 0.1 M CaCl2, reduce the magnitude of the liquid junction potential substantially and make the pH values obtained more reproducible and consistent. The position of the calomel electrode salt bridge is crucial in measuring soil pH and should always be positioned in the clear supernatant solution.

Relationship Between Ionic Strength And Electrical Conductivity For Soil Solutions

Activity of an element is a better predictor of plant growth response than its concentration. Ionic strength of the soil solution is an important property that is required to calculate ionic activity. Since its calculation requires measurement of all major cations and anions in solution, an alternative approach was sought to estimate it on the basis of a single measured property of solution. Griffin and Jurinak reported a linear relationship between ionic strength and electrical conductivity which was employed to predict ionic strength using soil solution samples from a wide range of soils subjected to varying treatments with respect to source of amendment, concentration of supporting solution, soil moisture content, and duration of incubation. This study has shown a very close agreement between ionic strength predicted using the Griffin-Jurinak relationship and the ionic strength calculated using concentration and valence values for all cations and anions in solutions. Furthermore, for the conditions of soil solutions from variable charge soils, the Griffin-Jurinak relationship was modified.

Beneficial use of effluents, wastes, and biosolids

Abstract Anthropogenic wastes are accumulating at ever increasing rates. As an alternative to stockpiling and landfilling, land application of wastes is considered in terms of benefits to agriculture while protecting the environment. Beneficial reuse of wastes such as municipal wastewater, sewage sludge, animal manures, composts, byproduct gypsum, food processing and paper and pulp wastes are discussed both in terms of their benefits to agriculture and requirements from the standpoint of analyses required for monitoring. Clearly, many of these wastes are highly beneficial to crop production as fertilizer substitutes and soil ameliorants.

Arsenate Displacement from Fly Ash in Amended Soils

Arsenic (As) is the biggest environment contaminant in most of the soils where fly ash is applied. Usually, it is not mobile and strongly adsorbed on to soil particles. However, in gypsum and phosphorus amended soils As may be much more mobile. A study in repacked columns was conducted to determine whether or not As becomes mobile when Ca(H2PO4)2and CaSO4are used as leaching solutions, and to compare the competitive interactions between PO4-AsO4and SO4-AsO4. Arsenic concentration in leachate was found to be approximately ten times greater when Ca(H2PO4)2was used to leach the columns as compared to CaSO4. A maximum concentration of 800 μg As L-1was found in the leachate in this case, which is much higher than the groundwater limit of 50 μg L-1for drinking water established by the United States Environmental Protection Agency. In fly ash, the portion of arsenate non-specifically adsorbed is believed to be much lower than that of specifically adsorbed. Sulfate anions were able to displace only non-specifically adsorbed arsenate. In this case the concentration of As in leachate was found to be within acceptable limits. On the other hand, phosphate can compete with arsenate for all available adsorption sites, non-specific and specific. Phosphate displacement of both forms of arsenates increases As mobility in both control and fly ash treatments.

RETENTION AND TRANSPORT OF CALCIUM NITRATE IN VARIABLE CHARGE SUBSOILS

Salt retention has been observed in many variable charge soils. However, studies on the extent of the nitrate salt retention in tropical and subtropical soils are not documented in the literature. No attempts have been made to relate the extent of nitrate salt retention with soil mineralogy and the parameters of the soil chemistry, such as cation and anion exchange capacities (CEC and AEC) and soil solution pH and ionic strength (IS). Our objective was to provide evidence of the retention of Ca(NO3)2 in variable charge subsoils with extremely diverse mineralogical and chemical characteristics and to estimate the change in the extent of salt retention as a function of pH and IS. Laboratory leaching experiments were conducted with disturbed subsoil materials collected from the southeastern United States and other areas. Four lime treatments and four Ca(NO3)2 solutions of different concentrations were used to test the effect of pH and IS on nitrate salt retention. The results show that the transport of Ca(NO3)2 is affected by soil mineralogy and chemistry. The magnitude of salt retention was higher in subsoils with appreciable AEC and equivalent amounts of CEC, where both kaolinite and Al/Fe oxides dominate the clay fraction. In the subsoils with the highest magnitude, the electrical conductivity breakthrough curves (EC BTC) have an extended plateau of low EC values, and the first pore volumes of leachate contain virtually no salt. The cation and anion of the leaching electrolyte (Ca++ and NO3−) are adsorbed with no net release of other ions from exchange sites. Both pH and IS of the subsoil solution affect ion transport and alter the extent of Ca(NO3)2 retention. This study leads to the conclusion that mineralogy, surface charge, pH, and IS affect the magnitude of salt retention in variable charge subsoils, and their effects should be considered when predicting ion mobility and transport in these subsoils.

Boron availability to plants from coal combustion by-products

Agronomic use of coal combustion by-products is often associated with boron (B) excess in amended soils and subsequently in plants. A greenhouse study with corn (Zea mays L.) as test plant was conducted to determine safe application rates of five fly ashes and one flue gas desulfurization gypsum (FDG). All by-products increased soil and corn tissue B concentration, in some cases above toxicity levels which are 5 mg hot water soluble B (hwsB) kg−1 soil and 100 mg B kg−1 in corn tissue. Acceptable application rates varied from 4 to 100 Mg ha− for different by-products. Leaching and weathering of a high B fly ash under ponding conditions decreased its B content and that of corn grown in fly ash amended soil, while leaching of the same fly ash under laboratory conditions increased fly ash B availability to corn in comparison to the fresh fly ash. Hot water soluble B in fly ash or FDG amended soil correlated very well with corn tissue B. Hot water soluble B in fly ash amended soil could be predicted based on soil pH and B solubility in ash at different pH values but not so in the case of FDG. Another greenhouse study was conducted to compare the influence of FDG and Ca(OH)2 on B concentration in spinach (Spinacia oleracea L.) leaves grown in soil amended with the high B fly ash. The Ca(OH)2 significantly decreased tissue B content, while FDG did not affect B uptake from fly ash amended soil.

FARMING WITH ACIDITY

In many parts of the world notably the US Midwest and Europe, soils have been limed to a particular target pH (pHH2O 6.5–7.0) according to the crop. Because plants do not directly respond to the activity of H+, it is pertinent to enquire why this approach to liming has enjoyed such widespread popularity. Among the reasons, the original near-neutral pH of many of the soils was no doubt a consideration and the use of acid-sensitive legumes to supply N in rotations in the early stages of agricultural development also played a part. The introduction of the pH meter at about the same time as N fertilizers found widespread popularity replacing legumes in rotations facilitated the measurement of soil acidity and removed the focus from the real problems of soil acidity, namely, toxic levels of Al3+ and Mn2+ and deficiencies of nutrients such as Ca, Mg, N, S, P and Mo. Even after legumes disappeared from the rotations, the high target pH values were retained. Liming experiments throughout the world reveal that, with very few exceptions, all crops including legumes cease to respond to lime above pHH2O=5.5–5.8, provided that the factors (Ca, Mg, Mo, B, P, etc) negatively impacted by soil acidity are optimized. In fact on highly weathered soils, liming to near neutrality can have disastrous effects on yields of many crops. Many examples are presented as illustrations of the few benefits accruing to liming to neutrality and the many benefits of farming with levels of acidity somewhat more intense than has normally been the case. Among the latter benefits are increased profitability from higher nutrient efficiencies, reduced diseases and pests, slower nitrification with less water pollution, improved soil tilth, improved availability of many metals and P.