Soluble Calcium Amendments: Effects on Soil Structure.

Soil structure is a surrogate for soil physical fertility.  Soil structure, the arrangment of aggregates and pores, is responsible for creating an environment for plant root system to uptake nutrients and water, while allowing flow of water and air in and through the soil.

Soil structural stability depends on the mineralogy of the layered silicates; texture, ratio, and concentration of sorbed cations; and the concentrations of various organic substances, silica, and oxyhydroxides.  Dispersivity of soil aggregates is affected by the nature of soil organic materials and the clay type.  

Clay minerals influence properties that affect aggregation, such as surface area, cation exchange capacity, charge density, dispersivity and expandability. The addition of gypsum to soil decreases clay dispersivity through changes in the electrolyte concentration and composition.  The cation exchange capacity and surface area of clay minerals, soil organic matter and other soil properties interact to form intraparticle interactions that influence dispersivity.

Surface-applied phosphogypsum materials improve infiltration rates and reduce erosion by increasing the electrolyte concentration in the soil solution, enhancing clay flocculation, and maintaining aggregate stability of a great variety of soils.

In this paper we use, as study case, a phosphogypsum material called fluidized bed combustion (FBC) bottom-ash, which is produced from the capture of SO₂from coal-fired electric power generating plants.  In the FBC desulfurization process, crushed calcite and coal are injected together in the furnace and two byproducts may be collected: granular bottom-ash material and flue gas dust removed by cyclones.

The composition of the material is dependent on the desulfurization process and the coal and sorbent used. In general, the material is a mixture of coal ash, unspent sorbent, and the sorbent plus SO₂ reaction product, which is primarily CaSO₄ (anhydrite), CaO, Ca(OH)₂, and CaCO₃, in the case of FBC bottom-ash. Both CaO and Ca(OH)₂ are more soluble than CaSO₄.2H₂O and result in greater OH^-concentrations upon dissolution.  

Therefore, FBC bottom-ash is a stronger source of electrolytes than gypsum but it will also increase the pH of soil systems, whereas gypsum dissolution will not affect pH.  

Increasing pH enhances dispersion of variable-charge soils, and for some soils, if the amount of electrolytes necessary for flocculation increases dramatically with increased pH, the electrolytes released by the FBC bottom-ash will not keep the soil flocculated and aggregated, and erosion may well be increased.

On less weathered soils (Vertisols and Alfisols) with 2:1 type clay minerals, surface sealing and erosion is significantly reduced by the surface-applied FBC bottom ash on all studied soils, but with a lesser effectiveness on the illitic and highly smectitic soils.  For water infiltration, FBC bottom-ash is most effective on the smectitic soil, except when swelling is high.  The increased electrolyte concentration reduced swelling and enhanced flocculation and, possibly, sedimentation of soil clays, thus decreasing sediment concentration in the runoff and water and sediment yield, and increasing the size of the sediment in runoff.

On highy-weathered soils, such as Oxisols and Ultisols, surface application of FBC bottom-ash to improve infiltration and reduce erosion may or may not be effective, and can even be detrimental.  Since the material contains CaO, Ca(OH)₂, and CaSO₄(anhydrite), the electrolyte concentration and pH of the soil solution were both increased. Increased pH causes an increase in negative charges and dispersion.  

Thus, the amount of electrolytes added to the soil with FBC bottom-ash dissolution may be, depending on the soil's final pH and its buffer capacity, insufficient to counteract dispersion caused by raindrop impact, low electrolyte levels of rain, and increased negative charges with phosphogypsum material dissolution. The FBC bottom-ash increased infiltration and reduced erosion and runoff only for soils that had low pH (ΔpH < 0.6), negative charge (<2.0 cmol_c kg^-1), and final pH below 7. Unfavorable responses were observed when ΔpH was >2.1, negative charge increase was at least 6.8 cmol_c kg^-1, and final pH was >9. The soils demonstrated greater resistance to favorable or unfavorable changes in erosion than in infiltration or runoff with the surface application of the phosphogypsum material.

With increasing amounts of smectite or illite in soil, a longer lasting effect of FBC bottom-ash was observed; thus suggesting that this material has great potential for reducing sealing and erosion on permanent charge soils.  Kaolinitic soils were more stable, generating lower runoff and soil loss rates, and response of these soils to FBC bottom-ash was dependent on flocculation and surface charge characteristics.

Image analysis is used to quantify crust morphology, porosity and characteristics of the seal and the unsealed soil below it.  A conspicuous feature is a structural crust at the surface with a continuous dense layer of lesser porosity and smaller pores than untrusted soil. The seal shows no evidence of a “washed-in” zone of illuvial clay in the control treatment, although dispersed clay was observed in the percolating water for some soils. Where FBC bottom-ash was effective in increasing water infiltration, an increase in total porosity attributed to planar pores is observed.  A layer of eluvial silt and fine sand is observed at the surface of soils where considerable dispersion occurred on non-treated soils.

For studies on surface sealing, erosion and water infiltration, aggregate stability testing by a settling device generates direct information on aggregate slaking, sediment transportability, and speed of deposition, which are directly related to soil erosion and sedimentation.

We demonstrated in this abstract the influence of clay mineralogy and soluble calcium amendments on dispersion/flocculation, surface sealing/crusting, water erosion, sedimentation, and water infiltration.