Saline and Sodic Soils Chapter 10

This one.

Percent yield on y-axis and increasing level of salinity on x-axis.

OK, you can have natural salinity, especially where salts may concentrate, like in a seep area, but you can also salinize a soil with long-term irrigation. This has ruined many an acre, around the world. There are also sodium-affected soils and a combination.

Say you add a bit of salt with an irrigation. Next time you irrigate, add more water than just enough to wet the root zone and water percolates deeper, dissolving and leaching the salt from the previous irrigation. Of course, to make this design work, you need good internal drainage.

For salinity For sodicity

EC increase with concentration of electrolyte, right? Pure water is a poor conductor of electricity. The units of measurement are dS / m. Work at the USDA Salinity Lab in California showed that conductivity is (on a log-log basis) related to ionic strength, and the latter is something of a measure of electrolyte concentration.

This is an older measure of sodicity that has been replace in soil testing labs by an easier but indirect measure (sodium adsorption ration, next slide). Note the similarity of ESP with %BS.

The above threshold may seem surprising but the basis is that even a 15% saturation of the CEC with Na means that there is little Ca- or MgCO 3 in the soil. Since both Ca 2+ and Mg 2+ are strongly preferred to Na + adsorption, if there was appreciable Ca- or MgCO 3 present, the ESP would be lower. Further, in the absence of solid phase Ca- or MgCO 3, the pH is not controlled by carbonate equilibria (keeping it about 8.5) so that it increases to plant-bad values.

The basis of this is something called the Gapon model, which is empirical, Na Ex / (Ca Ex + Mg Ex ) = k [Na + ] / ([Ca 2+ ] + [Mg 2+ ]) 1/2 The left hand side is close to ESP, Na Ex / (Ca Ex + Mg Ex + Na Ex + …) = Na Ex / CEC

< 4.0< 13 < 4.0≥ 13 ≥ 4.0≥ 13 >

SAR = 13 ESP = 15 EC = 4 Sodic Saline and Sodic Not Affected Saline

Salt deposited on surface of soil when water evaporates = white.

≥ 8.5

Na is a dispersing cation. It is not tightly held to negatively charged colloids (figure left is supposed to show 2 parallel colloidal particles, one pair with tightly held Ca 2+ and one With loosely held Na +, off-setting the particle negative charge. Notice that for the Ca 2+, the repulsive negative charges are shielded by a concentrated Ca 2+ zone but not so for the diffuse Na + zone. So, Ca 2+ is flocculating, Na + dispersing. However, if the concentration of cation charge is sufficiently high, even with Na + (figure to right), the negative charge of adjacent particles is effectively shielded so that particles can closely approach, i.e., are flocculated. But decrease that high concentration and they disperse.

No, colloids dispersed and hydraulic conductivity too small. No, here too. See previous slide.

█2Na + + Ca 2+  █Ca Na +  Notice, also, that a solid phase of CaCO 3 forms, thereby dropping pH to ~ 8.5. So, there is a use for gypsum besides drywall.