As we near the finish of the 2001 planting season, it is time to take a break and measure remaining crop residue--the stems and stalks remaining from last year's crop. Knowing how to measure existing residue as crops emerge helps to determine how well soil is protected from erosion. For most Iowa soils, high residue levels translate directly into soil conservation benefits. The consensus among experts is that the most effective conservation tillage practices leave at least 30 percent crop residue after planting.
How crop residue works
Individual raindrops strike the ground with a surprising force. For example, in normal rainfall, 6-millimeter-diameter raindrops may hit the soil's surface at 20 miles per hour. With no crop residue to cushion the impact, these raindrops dislodge soil particles, splashing them up to 3 feet away. Such soil splash also works to seal off the soil's surface, which reduces infiltration and allows rainwater to collect and move down slopes, carrying dislodged soil particles with it, and leading to gullies and severe rill erosion. On fields with bare soil, these combined effects may lead to severe soil erosion.
How to measure crop residue
There are several acceptable methods for estimating crop residue. For every method, repeat measurements at several sites within each field, and average them to ensure an estimate for the entire field.
The line-transect method (the preferred method) involves counting the number of times a marked line intersects a piece of residue. Use a 100-foot tape measure (or a rope with marks spaced at 1-foot intervals). Stretch the tape between two stakes placed diagonally at a 45 degree angle from the direction of the crop rows (exclude end rows). Looking from directly above the tape, count the number of times a mark intersects with crop residue. Make your judgement consistently on only the left or right sides of the mark to avoid over-counting residue. When you are done, the result converts into the percentage of crop residue remaining in that sample area. Record a minimum of five measurements, using areas that are typical of the field being measured. Then average the estimates to obtain the most accurate overall assessment.
Meter stick method
Place the meter stick on the soil (A yardstick with metric marks can be used.) Evaluate at each centimeter mark the crop residue occurring along one edge of the meter stick and total those measurements. For example, if the total residue occurring along the meter stick was present at 35-centimeter marks, the percentage of residue remaining is 35 percent. Again, sample several areas of the field. Places for measurement can be determined randomly by throwing the meter stick into the air.
Photos can provide an estimate by comparing fields to percentages in photos that show a known percentage of crop residue. Perspectives from angles are misleading, so look straight down when comparing photos.
Calculation is a good way to get a rough estimate of remaining residue without having to go to the field. Because there are many variables, however, such as weather and differences between individual operators of tillage equipment, it is less reliable (see top table, page 70).
A good cover of crop residue is one of your best allies in fighting soil erosion, and measuring after planting is the right time to find out how much residue remains.
Calculating residue losses from fall harvest to after planting (for corn).
|Percent residue remaining at harvest||0.05||95%|
|Reduction from winter decomposition
(multiply 95% residue remaining by 0.90)
|Reduction from spring chiseling (straight shank)
(multiply 85.5% residue remaining by 0.60)
|Reduction from spring disking (shallow)
(multiply 51.3% residue remaining by 0.75)
|Reduction from planting
(multiply 38.5% residue remaining by 0.90)
|Estimated residue remaining in this example after planting||35%|
Determine the percentage of existing residue after harvest then multiply that percentage by the percentage indicated for each of the field operations.
A planning tool: estimate your residue by field operation.
|Over winter decomposition||0.80-0.90||0.70-0.80|
|Chisel (twisted shank)||0.40-0.50||0.10-0.20|
|Disk (off-set, deep)||0.25-0.40||0.10-0.20|
|Chisel (straight shank)||0.50-0.60||0.30-0.40|
|Disk (tandem, shallow)||0.65-0.75||0.25-0.35|
Multiply each operation by the existing percentage of residue left to find how much ground cover will be left after each tillage.
This article originally appeared on pages 70-71 of the IC-486 (9) -- May 14, 2001 issue.