Going STIR Crazy

Calculating the Tillage Intensity of Horsedrawn Implements

by Eric & Anne Nordell of Trout Run, PA

A big focus of tillage for our dryland market garden is improving rainfall infiltration and moisture retention. Beginning with skim plowing in the early 80’s, we have added a half dozen shallow tillage practices to maintain moisture conserving residues in the top of the soil.

Recently, we were introduced to an objective method for measuring and comparing these moisture preserving practices, thanks to joining the Soil Health Benchmark Study conducted by the Pennsylvania Association for Sustainable Agriculture. One of the benchmarks for this research project, which includes over 60 vegetable, grain and dairy farms, is tillage intensity. This numerical index is based on the Soil Tillage Intensity Ratings developed by the Natural Resources Conservation Service.

The NRCS has put together a large data base of STIR values for tractor equipment and hand tools, plus a few horsedrawn implements. Since our unusual applications of older equipment are not in the database, PASA Research Coordinator, Sarah Bay Nawa, arranged a conference call with Mike Kucera, a Conservation Agronomist and National Erosion Specialist / Database Manager in Lincoln, Nebraska. Mike walked us through the calculations for the tillage sequence depicted in this article.

The STIR equation uses the following parameters and their weighting factors:

SPEED: miles per hour times .5
TILLAGE TYPE: value times 3.25
TILLAGE DEPTH: inches times 1
SURFACE DISTURBANCE: percent area times 1

Multiplying the values of the four parameters provides the Soil Tillage Intensity Rating.

Although we were a little uncomfortable reducing our experience working the land to a mathematical equation, this rating system matched our tillage goals. The four parameters quantify the degree of soil disturbance and residue incorporation caused by each implement, indicating the potential for rainfall infiltration and moisture preservation, organic matter breakdown and carbon loss, and water and wind erosion. The STIR formula does not take into account compaction created by the power source — tractor, animal or human — or the rpm’s of the equipment, for example, a rototiller’s pulverizing effect on the soil.

The first photo provides a fairly good view of the soil disturbance and residue incorporation caused by the initial implement in the shallow tillage sequence. The 12” sweep and furrower mounted on the McCormick- Deering riding cultivator undercuts the rye stubble and throws the soil into low ridges, burying a good bit of the residue.

For the first parameter, we needed to measure the speed of the implement. Repeated time trials showed that we were holding the team to 2.7 mph, a sustainable working pace for primary tillage and steering this tool with the foot pedals.

For the second parameter, we needed to select a tillage type from the following list:

• 1.0 Inversion with some mixing. Typified by the moldboard plow, this form of soil disturbance buries most of the residue in the lower half of the tilled soil.
• .8 Mixing with some inversion. This tillage type leaves most of the residue in the upper half of the worked ground. Examples include the chisel plow, disk and field cultivator.
• .7 Mixing only. Exemplified by the rotovator, “mixing only” keeps residue in the upper third of the tilled soil.
• .4 Lifting and fracturing. Also keeping residue in the top three tenths of tillage depth, this tillage type has a very different effect on the soil, typified by the subsoiler.
• .14 Compression. Compression pushes residue into the earth without tilling the soil, such as the tires of a manure spreader or mower.

Although none of these tillage types seemed like a perfect match for the undercutter / ridger, we decided that “mixing with some inversion” best described it since some of the soil is inverted, but all of the residue remains in the upper half of the tilled soil.

The third parameter required measuring the deepest point of soil penetration by the implement. The forward tip of the angles sweep travels 4” deep in the soil.

Surprising to us, the area of surface disturbance includes the earth thrown on top of the undisturbed soil, called “soil splash” in NRCS jargon. By this definition, the combination sweep and furrowed disturbs 100% of the surface area.

To calculate the STIR value of the undercutter / ridger, we simply plug these measurements into the STIR equation.

Undercutter / Ridger
SPEED: 2.7 mph x .5 = 1.35
TILLAGE TYPE: .8 x 3.25 = 2.6
TILLAGE DEPTH: 4” x 1 = 4
SURFACE DISTURBANCE: 100% x 1 = 1
STIR: 1.35 x 2.6 x 4 x 1 = 14

The higher the STIR value, the greater the tillage intensity. For comparison, the NRCS database, found in the RUSLE Operation Table, gives the tractor moldboard plow a STIR value of 65, chisel plow 45.5, tandem disk for secondary tillage 32.5, light finishing disk 19.5, and manually tilling with a hoe 3.9.

The first pass with the spring tooth harrow, shown in the next photo, has a higher STIR value than the undercutter / ridger due to letting the team more-or-less set their own pace for secondary tillage. The second pass with the harrow has a much lower tillage intensity because we set the depth for smoothing the field. Both depth measurements were taken from the top of the loosened soil to the tip of the tines.

Springtooth Harrow
SPEED: 3.2 mph x .5 – 1.6
TILLAGE TYPE: .8 x 3.25 = 2.6
TILLAGE DEPTH: 1st pass 4” x 1 = 4 2nd pass 2” x 1 = 2
SURFACE DISTURBANCE: 100% x 1 = 1
STIR: 1st pass 1.6 x 2.6 x 4 x 1 = 16.64
2nd pass 1.6 x 2.6 x 2 x 1 = 8.32
1st and 2nd pass added together 24.96

To erase the erosion-prone furrows left by the harrow, we make several rounds with the rotary hoe, illustrated in the last photo of the shallow tillage sequence. The revolving spoonlike tips of the rotary hoe have a light mixing action, without inversion, which knocks soil off of cover crop clumps and weeds. They also flip a lot of the rye stubble back up on the surface and poke thousands of holes in the soil, improving both rainfall infiltration and moisture conservation. Another unique property of this tool is the spoons firm the soil underneath so that with each pass the hoe penetrates a half inch shallower and restores capillary action, bringing moisture closer to the surface.

Rotary Hoe
SPEED: 3.2 mph x .5 = 1.6
TILLAGE TYPE: .7 x 3.25 = 2.28
TILLAGE DEPTH: average 2.5” x 1 – 2.5
SURFACE DISTURBANCE: 100% x 1 = 1
STIR: 1.6 x 2.28 x 2.5 x 1 = 9.12
3 passes = 27.36

The STIR score for the whole shallow tillage sequence is the sum of the implements:

Total STIR: 14 + 22.28 + 27.36 = 64.24

To put this number in perspective, the NRCS definition of Conservation Tillage is an overall STIR value below 60. However, this STIR value is based on total annual fieldwork, averaged over the course of the rotation, not just an isolated tillage sequence.

To get a rough idea of our vegetable rotation’s overall tillage intensity, we went a little STIR crazy figuring out the value for all of our horse drawn tools. Then, using field records, we calculated the scores for a half dozen crops grown in 2019. For a complete picture of fieldwork, we included the preceding fallow year management for each crop.

The tillage intensity budgets at the end of the article show that all of our field practices have relatively low STIR values, but they add up to a relatively high score by the end of the growing season. The average for the six crops and six fallow fields was 92, well above the Conservation Tillage threshold.

Granted, it may not be realistic to stay under 60 growing certified organic vegetables unless we added a few years of sod to the rotation or converted the market garden to a compost mulch, no-till system. However, the numbers made us think we could do better. Initially, we are eliminating passes with the spring tooth harrow and rotary hoe, when possible. Down the road, we would like to develop a more efficient shallow tillage sequence.

Doing the math also helped us to see the role cover cropping plays in tillage intensity. Interseeded cash crops had a lower STIR value than mulched ones by making post-harvest tillage unnecessary for establishing a winter cover crop. Fallow fields with the lowest STIR scores transitioned quickly from one cover crop to the next, although this seed-and-till method is only possible when weed pressure is low. The best scenario for weed management, and worst for stirring the soil, was the fallow year before onions when two cover crops were planted, preceded by two periods of tillage.

To prevent the latter situation from occurring in the future we are experimenting with relay cover cropping interceded vegetables. Early this spring in Field 2, where 2019 fall vegetables had been interseeded with a single row of crimson clover in the pathways, we overseeded the rows of clover with a cover crop mix including Italian ryegrass. The relay cover crop filled in Field 2 nicely, extending living vegetation until midsummer.

One reason we were interested in using ryegrass in the cover crop mix is the fine roots of grasses improve aggregate stability. The soil testing component of the PASA benchmark study shows our fields are low in this important measure of soil health.

In addition to aggregate stability and tillage intensity, PASA is generating benchmarks for organic inputs, days of live cover, available water capacity, organic matter, active carbon, soil respiration, soil protein, and standard soil nutrient analysis. We hope to provide a complete report after a few years of testing have been completed. In the meantime, more information about the PASA Soil Health Benchmark Study can be found in the Resources section of the PASA website: www.pasafarming.org.

By May 21, 2019, the winter killed clover and vetch had decomposed sufficiently to form beds with seven teeth and a miniature cultipacker on the riding cultivator. This STIRling example of reducing tillage by eliminating primary and secondary tillage was partly offset by the need for bed renovation and extra cultivation to deal with chickweed that got established during the long period without tillage in the spring. To calculate the tillage intensity of a combination implement, it is necessary to figure out the STIR value for each function and add them together:

7 Teeth on Cultivator
SPEED: 2.7 mph x .5 = 1.35
TILLAGE TYPE: .8 x 3.25 = 2.6
TILLAGE DEPTH: 2.5” x 1 = 2.5
SURFACE DISTURBANCE: 100% x 1 = 1
STIR: 1.35 x 2.6 x 2.5 x 1 = 8.78

Mini-Packer
STIR value for cultipacker from database: 1.46
SURFACE DISTURBANCE: 55% x 1 = .55
STIR: 1.46 x .55 = .8
Total STIR: 8.78 + .8 = 9.58