Effect of Table Salt (Sodium Chloride) on Sedimentation of Sand, Silt or Clay in Soil Sample

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Our goal this year is to design a vegetable garden that is resilient to climate change by trapping water from the winter-spring rains into the soil. One step in that process is determination of soil texture by determining the percent sand, silt and clay in the soil present in the garden. Once the texture of soil is knwon, we can modify it accordingly to help trap water. We used simple method soil sedimentation test using glass jars to suspend soil sample in water, then allow it to sediment by gravity into layers of sand, silt and clay, which we can measure and use to calculate percentages of each layer. We conducted this experiment by suspending soil in water with (Jars 1 - 5) or without (Jars 6 - 10) dish soap. In addition, since clay particles are affected by sodium chloride to aggregate etc we determined the effect of adding sodium chloride at various time points during the sedimentation. Note: all Jars when shaken vigorously for 5 - 10 minutes, and set down on a flat table at time zero (T = 0). Thereafter, the jars were not disturbed during addition of salt ( teaspoon full sprinkled on top of suspension). Jars 1 and 6 = no salt Jars 2 and 7 = salt added before start of sedimentation test Jars 3 and 8 = salt added at 2 hours after the start of sedimentaion test (Jars were undisturbed during addition of salt) Jars 4 and 9 = Salt added at 4.5 hours after the start of sedimentation test(Jars were undisturbed during addition of salt) Jars 5 and 10 = Salt added at 8 hours after the start of sedimentation test (Jars were undisturbed during addition of salt) The results (as shown in the Figure below) read after 48 hrs show that, for this soil sample: In the absence of salt, there was about 81% sand, 15% silt and 4.2% clay when soil was suspended in water containing soap. The profile was essentially similar when the sample was suspended in water alone (without salt); except that the percent of clay was slightly lower (i.e. 2.5% when compared to the 4.2% in presence of soap). It is to be noted that clay particles remain in suspension for a long time (i.e not all clay precipitated by the time of reading at 48 hours from the start of sedimentation test-which limits the accuracy of determination of % clay in this soil sample- with little amount of clay).
In the prsence of salt, clay layer is unchanged when salt is added before the the start of sedimentation. However, the percent clay is increased in both suspensions with or without soap when salt is added at least 2 hours after the start of sedimentation; the increase is higher in the absence ( about 10%) than in the presence of soap (about 5 - 7%).
It appears that, for soil samples with little clay (as like the one we have in our garden sample), sedimentation test is more accurate in determining clay percent if no soap is used but sodium chloride is added at least 2 hours after start of sedimentation. The timing of addition of salt relative to the start of sedimentation is important: that is, for accuracy of measuring the clay layer, salt is added to the sedimentation at a time (at least 2 hours from start of sedimentation test) when the silt particles have completed precipitation. The salt appears to accelerate the precipitation of clay particles (perhaps through its effect on creating aggregates of clay particles). Therefore, it appears from the results of our experiment that, for soils with little clay, accuracy of determination of percent clay can be increased by adding table salt (sodium chloride) after silt particles have precipitated. Such information is essential when designing gardens to be resilient to climate change. By Peter Emau

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