Gypsum in Soil (M.Sc. Thesis)
How do we measure soil salinity?
The soil saturated paste method, created by James D. Rhoades at the USDA-ARS Soil Salinity Laboratory (Rhoades, 1989), was developed to mimic salts experienced by crop roots when the soil is saturated. We add deonized water to the soil, let it sit overnight, then extract water out of this paste and measure the electrical conductivity of the solution to determine salt content in the form of something called ECe (measured in dS/m or mmho/cm). ECe can be used to develop soil remediation strategies, as well as estimate potential yield losses in various crops.
Gypsum in Soils
Gypsum salt is chemically known as CaSO4, or calcium sulfate. Ca is a cation with a 2+ charge, as opposed to the Na in NaCl (table salt, another common soil salt), which only has a 1+ charge. As such, Ca ions usually bind soils together (like in the figure) and encourage aggregation because soil particles have a negative charge and look for ions to "hold on to".
Illustration "A" shows a calcium ion (2+ charge) binding negatively charged soil particles together, as is the case with gypsum. Illustration "B" shows how soils are dispersed by sodium ions (1+ charge).
The problem with gypsum and ECe
Because of gypsum's ability to bind soils together, microaggregates form l that may “protect” the gypsum from dissolving into water solution in-situ (i.e., in the field), but when we grind it to make saturated paste, we alter gypsum’s solubility. This can make ECe values derived in a lab markedly higher than what the plants experience in-situ (approx. 0 – 2 dS/m greater). This ECe inflation results in over estimations of potential yield losses and water required to leach the soil, resulting in negative environmental and economic impacts for land managers.
My Location of Study:
This is the location of my research. It's located in Southeast Colorado, where the climate, hydrology, and irrigation practices are all very similar to agricultural areas in Pakistan. The region is often referred to as the Fairmont Drainage District. This refers a series of ceramic drainage tiles installed by the Catlin Canal Company in 1918 in an attempt to lower the water table and remove salts from the region.
Our Team's Scalable Solutions
1. Equilibrium Geochemical Modeling (EGM)
Callaghan et al. (2016) used ExtractChem , a equilibrium geochemical model (EGM) to simulate ions of saturated paste solutions along with field pore water within the model but omitting gypsum from portions of the model simulations, thereby deriving a soluble salt equivalent ECe, called ECeg
2. Selective Dilution
We can predict ECe in gypsiferous and non-gypsiferous samples using methodology adapted from Bernstein, (1962). Taking the difference between a prediction in both scenarios results in a shift that represents a correction factor (ΔEC) to adjust gypsifersous samples to a soluble-salt equivalent, ECeg.
3. Linear Meta-Modeling
Strong linear correlation between original ECe and corrected ECeg values prompted the use of the linear model for spatial interpolation using the same EMI methods as before but correcting the calibration samples with the linear model prior to mapping.
Final Results and Takeaway
Using the linear meta-model approach, we estimated the impact that correcting for gypsum in ECe maps could do to resulting yield estimations.
Yield loss and salinity predictions were significantly reduced when accounting for gypsum bias in ECe values. This could save land managers money, time, and valuable water when it comes to remediation and agronomic decisions!
For more details, feel free to contact me for my technical report, or view my oral defense here on my research blog!