Soil Moisture Relationship in a Sandy and Clayey Soil | University Research | Raleigh Lawn Care

By Ryan Walsh | Student, North Carolina State University

Introduction

Many factors are important for plant growth but one of the most critical factors is the availability of soil moisture.  Unavailable water is water held at wilting point, it is mostly hygroscopic water held too tightly within micropores.  Available water is water held at field capacity minus water held at wilting point; it is the water in the mesopores.  Gravitational moisture content is the difference in moisture content at saturation and field capacity.  Capillary water is moisture held in the tiny spaces between soil particles. It is the principal source of moisture for a plant’s roots.  Hygroscopic water is water that forms a thin film around individual soil particles, it is considered to be unavailable.  Saturation or water saturation is the ratio of the volume of water to the volume of voids.  Field capacity is the amount of water content held in soil after excess water has drained away and the rate of downward movement has materially decreased, which usually takes place within 2–3 days after a rain or irrigation in previous soils of uniform structure and texture.  Wilting point is the condition in which a plant begins to use water from its own tissues for transpiration because soil water has been exhausted.  All of these are influenced by texture and structure because these two soil properties determine the pore size distribution.  The purpose of this study was to examine the influence of texture and density on water holding capacity and pore size distribution.

Materials and Methods

Each table was responsible for a soil sample. Four 120-g samples of a sandy and a clayey soil were placed in cups. The cups each designated for different treatments: Saturated, field capacity, wilting point, oven dry.  All of the treatments but oven dry; the cups were perforated at the bottom and placed into a pan of water to saturate.  Periodically the cups were removed, allowed to dry and then weighed.  The wilting point was calculated by removing after 1 day allowed to dry for 6 days and then weighed.  Field capacity cup was removed after 6 days, allowed to dry for 1 day and then weighed.  The saturation cup was removed only just prior to weighing.  The volume of the soil in the cups can be determined by filling an empty cup with water to the same level as the soil and then pouring the water into a graduated cylinder.

By dividing the soil weight by the volume of the water in the cylinder we found bulk density.

Percent soil water by weight was found by (mass of moist soil minus mass of oven dry soil) divided by mass of oven dry soil multiplied by one hundred.

1. Percent water by volume is calculated by bulk density time’s percent water by weight.
2. Gravitational water is calculated by volumetric moisture at saturation minus field capacity
3. Available water is found by volumetric moisture at field capacity minus moisture at wilting point
4. Porosity is the volumetric moisture at saturation
5. Macro pore volume is volumetric moisture at saturation minus field capacity
6. Mesopore volume is volumetric moisture at field capacity minus moisture at wilting point

Results and Discussion

1. Summary of density, moisture, and porosity in sandy and clayey soil.  According to the data that was gathered from the experiment, the clayey soils tend to have more total, available (except for percent by volume) and unavailable water content. The saturation percent by weight for sandy soils is 28.91, while clayey soils are 49.16.  Gravitational water percent seems to be higher in the sandy soils than the clayey soils for percent of both weight and volume.

From the data collected from the experiment, the clayey soils seem to have a higher percent total porosity then the sandy soils.

From the data collected from the experiment, the sandy soils seem to have a higher percent macro-porosity then the clayey soils, but the clayey soils have a higher percent micro-porosity than the sandy soils.

Conclusion

Several inferences can be made from this set of data. One is that the moisture content at gravitational water was considerably higher in sandy soils than clayey soils.  The moisture content at saturation, field capacity and wilting point all were considerably lower in the sandy soils than the clayey soils. This implies that clayey soils are much better suited for plant growth. The availability of this water means that the capillary force required to pull the water into the roots will not be too high causing plants to die from lack of soil water. Total porosity and macro-porosity percent was greater in sandy soils, but micro and meso-porosity were significantly lower in sandy soils than clayey soils.

Appendix

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Hi and thank you so much for visiting Capital City Groundskeeping. We’re a full service lawn care and groundskeeping company based out of Raleigh, North Carolina. I started performing lawn maintenance to my clients with one philosophy in mind: provide consistent, superior service while building lasting relationships within our community. As a recent graduate of North Carolina State University, B.S. Turfgrass Science and Biological Sciences, I’ve grown passionate about lawn care and enjoy learning about the constantly growing green industry. After founding CCG over ten years ago, I’m proud in the growth we’ve achieved and excited about our future projects. My name is Ryan Walsh and I look forward to speaking with you about your lawn care questions and groundskeeping goals.