Introduction

Water is the most limiting resource for maximum yield potential in crop production, especially in semi-arid and arid climate zones. The primary water inputs to cropland include rain and snow, as well as irrigation. The primary pathways of water loss from cropland includes evaporation, transpiration, surface runoff, and deep percolation. Transpiration is the process by which water is absorbed from the soil by plant roots and used by the plants to ultimately be released as water vapor from the plant leaves to the atmosphere. On the other hand, evaporation, surface runoff, and deep percolation represent water losses with no use by the plant. Efforts to improve water conservation on farmland are aimed at minimizing these pathways of water loss and maximizing soil water storage. Practices that minimize soil disturbance and maximize soil cover will increase water infiltration and water holding capacity while decreasing runoff and evaporation. In this laboratory exercise, we will evaluate the effect of soil surface management on soil water storage.

Bucket Preparation, Soil Collection, and Wetting Soil to Field Capacity

In this experiment you will add soil to four 5 gallon buckets, determine initial and final soil moisture content, and determine the impact of four different soil management treatments on soil water loss. Note: Containers smaller than 5 gallons can be used if the quantity of available soil is limited, if you do not have 5 gallon buckets available, or if 5 gallon buckets filled with moist soil are too heavy to physically handle.

Bucket Preparation

The four 5-gallon buckets should be clean, and have holes drilled into the bottom of the buckets to allow gravitational water to drain out the bottom to prevent saturated conditions. Number the buckets 1-4 with a marker. Weigh each bucket and record the mass (kg) in the data sheet.

Adding Soil

The soil you will use will preferably come from your field site being used for other laboratory activities, but can come from other sources if needed. Large clods should be broken up into smaller aggregates, if applicable. Add soil to each bucket to within 5 cm of the top of the bucket. Weigh each bucket as you are filling them in order to add approximately the same mass of soil to each bucket. Tamping the soil lightly as you fill the bucket may help prevent voids.

Wetting Buckets to Field Capacity

Next, slowly add water to each bucket to bring it up to field capacity, which is achieved when water freely drains out of the drain holes in the bottom of the bucket. There may be preferential flow paths that allow water to drain freely out the bottom while dry pockets of soil remain. To fix avoid this, wait for approximately five hours after the initial wetting, then add water again until water freely drains out the bottom of the bucket. The buckets are at field capacity once the buckets stop dripping out the bottom. Collect a subsample of soil (approximately 500 grams) from each bucket and determine the the moisture content as described below. After collecting the subsample, weigh the buckets and record the mass at field capacity in the data sheet.

Establishing Treatments and Monitoring Moisture Loss

Treatments are designed to mimic typical soil management practices following crop harvest on farmland (Table 1). Bucket 1 will have wheat straw or some other crop residue added to completely cover the surface of the soil. For this bucket, weed seedlings should be removed periodically as seeds germinate and emerge from the moist soil. Bucket 2 will have bare soil. No crop residues will be added and weed seedlings should be removed periodically as needed. Bucket 3 will have no crop residues added, but weed seedlings that germinate will be allowed to grow through the duration of the experiment. The treatment used for bucket 4 is up to you. Think about what management practices might influence evapotranspiration, and develop a testable hypothesis that compares the four treatments. Record that hypothesis in the data sheet. Establish that treatment on the fourth bucket as appropriate. Once all treatments are prepared, store the buckets in a covered location to prevent rain or other elements from affecting the results of your experiment.

Calculate Soil Water Content and Water Loss

Calculate Gravimetric Soil Moisture Content at Field Capacity

Calculate the gravimetric soil moisture content (soil moisture content on a mass basis) of the buckets at field capacity using your subsample. Gravimetric soil water content is calculated as:

[latex]\text{Percent gravimetric water}=\frac{\text{mass of water}}{\text{mass of oven-dry soil}}×100\text{%}[/latex]

Where the mass of water is the difference in mass between the moist soil mass and oven-dry soil mass. To determine the difference between moist and oven-dry mass, weigh the 500 g subsamples immediately after collecting them from the buckets. Record this moist mass in the data sheet. Then dry the soil in an oven-safe pan using an oven under low heat (105°C, 221°F) for at least 24 hours, or until the soil mass reaches a steady-state condition where it no longer loses mass.

Calculate Bulk Density

Soil bulk density is calculated as follows:

[latex]\text{Bulk density}=\frac{\text{mass of oven-dry soil}}{\text{volume of soil}}[/latex]

The entire volume of soil was not dried to determine the total mass of oven-dry soil. However, that total mass can be estimated using the soil moisture content at field capacity. The moisture content in the field capacity subsample is proportional to the moisture content of the entire bucket. That proportion can be expressed as follows:

[latex]\frac{\text{mass of water in subsample}}{\text{mass of oven-dry subsample}}=\frac{\text{mass of water in bucket}}{\text{mass of oven-dry in bucket}}[/latex]

The gravimetric water content can be considered the proportion of the total moist soil mass that comes from water. Thus, the mass of the dry soil can be estimated using the following calculation:

[latex]\text{Oven-dry soil mass}=(1-\frac{\text{mass of water}}{\text{mass of oven-dry soil}})\times\text{ Total mass of moist soil}[/latex]

Which simplifies to:

[latex]\text{Oven-dry soil mass}=(1-\text{ soil moisture})\times\text{ Total mass of moist soil}[/latex]

We will assume the soil volume is five gallons, which converts to 18.93 dm³. Based on the bulk density equation above, divide the mass of dry soil in each bucket (kg) by a volume of 18.93 dm³ to determine the bulk density (kg/dm³). This density unit is mathematically equivalent to the more common unit, g/cm³.

Convert Gravimetric Water Content to Volumetric Water Content

The gravimetric water content (percent or decimal) can be multiplied by the bulk density to determine the volumetric water content. Note: this is because one gram of water is equal to 1 cm³ of water. The mathematical explanation is as follows:

[latex]\frac{\text{Mass of water}}{\text{Mass of oven dry soil}}\times\frac{\text{Mass of oven dry soil}}{\text{Volume of soil}}=\frac{\text{Mass of water}}{\text{Volume of soil}}= \frac{\text{Volume of water}}{\text{Volume of soil}}[/latex]

Convert all gravimetric water contents to volumetric water contents by multiplying each gravimetric water content by the bulk density for each of the four buckets.

Calculate Depth Equivalent of Water

In many applications it is useful to express the water content as a depth equivalent of water. The depth equivalent of water can be calculated by multiplying the depth of soil by the volumetric water content as follows:

[latex]\text{Depth of water}=\text{Depth of soil}\times\text{ Volumetric water content}[/latex]

Measure the height of soil from the bottom of the bucket to the top of the soil in the bucket, then multiply that depth by the volumetric water content, as shown above, to determine the depth equivalent of water. Do this for each of the four buckets.

Quantify Water Loss

Allow the experiment to progress for approximately eight weeks. Remove any residue or above-ground plant biomass, then weigh each of the four buckets to determine the final total moist soil mass. Calculate the final gravimetric water content. Using the bulk density determined previously, calculate the final volumetric water content and the final depth equivalent of water. Finally, calculate the difference in the depth equivalent of water at the beginning of the study and the end to determine the depth of water that was lost to evaporation (or evapotranspiration in the case of the treatment with weeds present).

References

Ochsner, T. 2019a. Evaporation and Wind Erosion. In Rain or Shine: An Introduction to Soil Physical Properties and Processes. Stillwater, OK: Oklahoma State University. https://open.library.okstate.edu/rainorshine/part/soil-water-content-and-potential/.

Ochsner, T. 2019b. Soil and Water Content and Potential. In Rain or Shine: An Introduction to Soil Physical Properties and Processes. Stillwater, OK: Oklahoma State University Libraries.Libraries. https://open.library.okstate.edu/rainorshine/part/evaporation-and-wind-erosion/.

Presley, D., D. Shoup, J. Holman, and A. Schlegel, eds. 2012. Efficient Crop Water Use in Kansas. Manhattan, KS: Kansas State University Agricultural Experiment Station and Cooperative Extension Service. https://www.bookstore.ksre.ksu.edu/Item.aspx?catId=363&pubId=15559.

University of Nebraska-Lincoln. 2015. What is evapotranspiration? Lincoln, NE. https://www.youtube.com/watch?v=EuW9Sd3i_mY.

USDA-NRCS. 2022. Conservation Practices. https://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/national/technical/cp/ncps/?cid=nrcs143_026849.