Splendor of Gardening on Bainbridge Island Washington: Further Evidence that Adding Sodium Chloride (Table Salt) Improves the Accuracy of Determining Percent Clay in the Soil Sedimentation Test for Soil Texture

An essential aspect of sustainable agriculture is to develop resiliency to changes in climate. One aspect of this resiliency is to be able to capture and maintain soil moisture even during prolonged drought. Our goal this year (2021) is to capture the rain water from winter-Spring seasons into the soil of the garden for use in the Summer of 2022. We shall evaluate effectiveness in the summer by observing plant growth and determining whether or not they need watering from an external source. Since clay particles, due to their small size, have greater capacity to maintain/retain water/moisture, it is essential to accurately determine the percent clay, sand and silt in the soil; and thus, modify the soils in the garden accordingly. The profile soil in our area of Bainbridge Island, Washington, is considered to be gravelly sandy loam. We considered it neccessary to confirm the soil profile present in the specific site of the garden. We utilized soil sedimentation test by suspending soil samples (top soil and subsoils) in water (without dish soap), then removing gravel by filtration through a wire mesh sieve- to remove gravel and stones and obtain a suspension of sand, silt and clay in water. Filtration of soil suspension through a sieve to remove gravel and stones improves the accuracy of determination of the sand layer. Thereafter, the glass jars were shaken vigorously for at least 3 minutes and placed on a flat table to start sedimentation. A teaspoon of salt (sodium chloride) was added before start of shaking or at least 2 hours after shaking (i.e. after completion of precipitation of silt). One jar had no salt added. The layers of sand, silt and clay that sedimented in the jar are measured after 40 - 72 hours from the start of sedimentation when the jars are placed on the flat table (time zero) after vigorous shaking. Salt is added by carefully opening the jar (without disturbin/agitating the suspension), then sprinkeling the salt evenly over the surface of the suspension- salt particles will dissolve quickly as they descend through the liquid suspension. In one analysis, samples taken from topsoil and subsoil from a single site in the garden were analyzed. Layers of sand, silt and clay in the jar are measured at 44 hours after the start of sedimentation; and used to calculate the percent sand, silt and clay in the soil sample. Results shown below. Topsoil (Level 1)

Subsoil (Level 2)

Salt added at 3hours after the start of sedimentation increased measurement percent clay in topsoil from 8.6% (absent salt) to 13% (present salt). Similarly, in the subsoil of the same site, measurement of percent clay increased from 10.2% (absent salt) to 17% (present salt). The accuracy of clay determination is affected by the slow sedimentation rate of clay particles in suspension of water. Addition of sodium chloride accelerates this process to achieve a more complete precipitation of clay particles ( as shown by the clarity of the fluid layer in the glass jar)- resulting in higher percent clay being measured. Similar results are obtained in the sample below, in which both topsoil and subsoil from multiple sites in the garden were mixed and analyzed as described above, except salt is added at 2 hours after the onset of sedimentation and the layers of sand, silt and clay were measured at 69 hours after the start of sedimentation. Salt increased the measured percent of clay from 10% (absent salt) to 14% (present salt). The 4% increase is due the more complete precipitation of clay particles in the presence of salt relative to absence of salt.

In both experiments, the soil samples (after removal of gravel and stones) are mostly sandy loam (especially the subsoil); which is consistent with the records from the city which shows the soil profile for the site as gravely sandy loam, reflecting glacial till produced during the glacial period. This suggests the soil profile has resisted drastic change over the years. by Peter Emau .