Introduction

Plant residue, mulch, and other soil covers are important because cover helps protect the soil from the impact of raindrops. The three components of soil erosion are detachment, transport, and deposition. Soil erosion rates can be greatly reduced when detachment of soil particles from soil aggregates is prevented. When assessing the existing erosion risk of a site it is important to accurately quantify the amount of soil cover present as well as predict the fate of residue during subsequent decomposition or field operations. This laboratory exercise is the first of a four-part activity to conduct a cropland in-field soil health assessment based on the USDA-NRCS Technical Note No. 450-06 (USDA-NRCS 2021).

The risk of soil erosion is also influenced by the slope. As slope increases the velocity of water flowing in sheets, rills, and gullies increases, making the water more erosive. In addition to percent cover, you will be determining percent slope, which will be used in estimations of erosion rates in later laboratories.

Determining Percent Slope and Length of Slope

Determining Percent Slope with a Clinometer

You will now measure the slope and length of slope for hill slopes at the site, or for individual terraces if the field has already been terraced. This can be done with a clinometer in two ways. If you have a partner of approximately the same height you can aim the clinometer at their eyes, or a facial feature about the same height as your eyes as you use the clinometer. If you do not have a partner, then drive two stakes or shovels into the ground at the top and bottom of the slope to be measured. The tops of each shovel or stake should be at an equal distance above the soil surface. Using the clinometer, measure the slope from the top of one stake/shovel to the other. Most clinometers display two numbers – the slope angle and the sloper percent.  The percent slope should be the higher number, and is typically the number on the right of the scale inside the clinometer. You can check this by looking at a 45º angle. The slope percent should be 100% while the slope angle should be 45º. Alternatively, a unit should be visible when looking directly upward or downward through the clinometer. Record the percent slope of each major hill at your site.

Note About Terraces

When determining the slope of a terrace: the slope of a broad-based terrace is measured from the top of a terrace to the channel behind the next terrace downslope. The slope of step-back terraces are measured from the bottom of one terrace to the edge of the next terrace downslope.

Measuring Length of Slope

You will now measure the length of slope using a long measuring tape, or if you know the length of your gate you can measure the length of slope using the number of steps it takes to walk along the slope. Alternatively you can measure the length of slope using the Google Earth app. Navigate to your location in the Google Earth app, click on the ruler icon near the top-right of the screen. Click on the “Add Point” button to add a point at the bottom or top of the slope, then repeat for the opposite end of the slope. Once the second point is added the length of that slope will be reported on screen in the units selected at the bottom-left of the screen.

Report the length of slope in the data sheet, and save this data for future erosion modeling labs.

Determining Percent Cover

There are two primary methods that can be used to determine percent cover. Choose a method based on the materials available to you, and report the method used in your data sheet.

Meter stick Method

This method utilizes a meter stick (or yard stick) and its graduations as an easy, quick method of determining ground cover, and is completed entirely in the field. Place a meter stick perpendicular to the rows of the field. Add up the number of centimeter marks (or inch marks) along the length of the stick that are underlain by ground cover. Also, add up the number of total marks on the stick. Calculate the ground cover according to the following equation:

[latex]\text{Ground cover(%)}=\frac{\text{marks underlain by ground cover}}{\text{total number of marks}}\times100\text{ %}[/latex]

Repeat this measurement at least five times for each section of the field. If this is for a terraced slope, repeat this measurement five times for each terrace. Report the measured values and the average percent cover for each section of the field in the data sheet.

Line-Transect Method

The line-transect method is essentially the same as the meter stick method, but uses either a roll-up tape measure, or a rope/string specifically designed for this method.

With a partner, stretch out the length of a tape measure and align it so that the tape is perpendicular to the rows. Walk the length of the tape, and record the number of graduation marks (meter, yard, or foot marks) that are underlain by ground cover. Also record the total number of graduation marks. Calculate the ground cover using the same equation as above. This method captures a larger sampling area, so repeated measurements are not needed. However, you are welcome to repeat it if field conditions are variable.

A variation of this method is sometimes used in which a rope or string is used instead of a tape measure. In those cases, a knot, or similar marker, is used in place of the graduation marks on a tape measure.

Assessing Residue Breakdown and Predicting Residue Loss

Crop residue and ground cover is broken down by soil organisms through shredding, fragmentation, cycling, and decomposition. The rate at which this occurs can be an indicator of biological activity. This process is also influenced by soil moisture, soil and air temperature, C:N ratio of residues, and more. Assess residue breakdown by examining residue for signs of decomposition, shredding, and incorporation by soil organisms. In addition, note the condition, color, and depth of the residue most recently deposited. If possible, discuss management of the field with the land manager to determine residue age and types of plants. Note that this indicator can be assessed at any time, but may be influenced by full-width tillage.

Based on your observations at the site, is the natural decomposition of crop residues or ground cover as expected with crop and conditions? Report your response as yes or no in the data sheet.