Spring time pore studies on Orthman Research Farm – Results!

Mike Petersen compares soil pores in Strip-Till after four years of continuous corn as well as 4 years of continuous corn in No-Till.

Coming back to the Benefits of Strip-Till and Pores in the Near Surface
Sept. 17, 2013    By: Mike Petersen, Lead Agronomist-Orthman Manufacturing

Early in the growing season we presented a blog regarding that pores were being looked at closely in the Strip-Tilled (ST) ground and No-Tilled (NT) ground at the Orthman Research Farm.  We have looked at this and 101 other details throughout the season and feel it is the right time to offer you some thoughts and conclusions.  My intern and I dug more soil pits than he cares to remember as he is deep into his college studies now, some of them when the temps were touching 100°F by 2:00 in the afternoon.

Developing the 10cm x 10cm block

During my stint of 34 years with USDA as a soil scientist we described pores, pore numbers, size, shape and continuity as we conducted soil surveys.  Pore research is intricate work and quite informative once the process and methodology is explained.  Let me be brief; pores are pretty much categorized by size – less than 1mm in diameter, 1 to 2 mm, 2 to 5mm and then greater than 5mm.  Get above that size and we are into void or holes.  Then we determine an actual count within a square decimeter (10cm x 10cm) and the pores shape.  The shape indicates whether they are old insect burrows, old root channels, solution channels, earthworm burrows and/or interstitial crevices.  All of these pores assist in the soil breathing and allowing water to move downward into the soil profile.  An observer may count several hundred pores of all sizes in native grassland soils or in soils that have suffered the fate of abundant tillage passes and heavy traffic to see less than one dozen in the square decimeter.  Essentially like concrete and impervious to water.

Our study at the Orthman Research Farm was fairly complete in that we looked at 27 in-field locations within the ST and 21 sites for the NT practice.  We made these observations after 4 years of continuous corn of NT and ST and moving the location of planting corn over 15 inches each year then digging out our blocks between the existing corn right after planting as corn was emerging.


Excavate a 30cm x 30cm block of soil from between the rows of emerging corn, shave off the upper two inches where all the residue was on top and the loose soil from winters freeze-thaw repeated and fluffed the soil surface.  Next we cut out with a sharp knife a 10cm x 10cm block that essentially allowed us to observe the 2 to 6 inch portion of the soil profile. Shaved the four surfaces flat when moist and with the point of the knife picked each face to be expressing a raw face.  Used the blades point to etch a quadrat on the face so we could count four smaller squares and begin counting the three sizes I mentioned above in the second paragraph (not the >5mm).

We would each count until our eyes crossed (well maybe not literally but we did run out of fingers and toes numerous times!) The observations are completed with a 10 magnification geologists hand lens and recorded.

Our results of the Pore Counts:
Table 1.  Data from the Strip-Tilled and No-Tilled sites at Orthman Farm, continuous corn from 2009-2014 and tillage practices remained in same rows all 4 years. Pores counted on 10cm x 10cm soil blocks from the 2 to 6 inch depth.

Location ST <1mm ST 1-2mm ST 2-5mm ST Total NT <1mm NT 1-2mm NT 2-5mm NT Total
Site 1 295 26 7 328 97 4 4 105
Site 2 218 20 9 328 82 8 3 93
Site 3 211 13 2 328 203 29 12 244
Site 4
Site 5 200 42 8 328 98 12 2 112
Site 6 188 59 14 328 74 6 11 91
Site 7 80 18 11 328 86 9 2 97
Site 8 96 25 5 328 142 8 7 157
Site 9 144 18 6 328 178 15 8 201
Site 10 113 12 4 328 147 11 3 161
Site 11 179 28 4 328 144 20 8 110
Site 12 149 22 7 328 81 21 8 90
Site 13 211 22 11 328 69 15 6 153
Site 14 242 32 20 328 135 15 3 131
Site 15 288 41 22 328 115 14 2 180
Site 16 326 39 30 328 155 17 8 204
Site 17 279 23 13 328 181 15 8 105
Site 18 176 18 13 328 99 4 4 107
Site 19 139 10 4 328 184 3 3 190
Site 20 135 21 14 328 89 9 2 100
Site 21 149 14 6 328 97 12 7 116
Site 22 106 12 9 328 100 12 5 117
Site 23 142 13 6 328
Site 24 137 14 8 328
Site 25 97 17 8 328
Site 26 113 12 7 328
Site 27 251 9 7 328
Mean 178.4 21.8 9.2 218 122 12.73 5.45 140.18
Median 154 18 8 98 12 6 124

Table 2. The range in pore counts for all sites at Orthman Research Farm

Pore Sizes ST-hi NT-hi ST-mean NT-mean ST-low NT-low
1mm 326 203 178.4 140.2 80 69
1-2mm 41 29 21.8 12.7 9 2
2-5mm 29 11 9.2 5.5 2 2

As you contemplate all the numbers there is definitely a trend that strip till has some higher counts in all three pore sizes, even as we look at the lowest counts on the right side of Table 2 for ST-low and NT-low. I will provide some clues and observations as to what I believe has occurred here over the four years of the side-by-side tillage comparisons.

Discussion and Conclusions:
As we made these observations we also excavated 30cm x 30cm blocks and pulled them apart and counted earthworms at the same time of the pore counts. Think about it, earthworms burrow and leave nice round tunnels – PORES. Who is helping out both tillage systems, certainly the “Tunnel Kings of the Earth”. Our worm counts ranged from 8 to 36 earthworms per square foot in mid-May.
Because we at the Orthman Farm alternate each year where we run the 1tRIPr tool between the previous year’s corn row we allow the old corn crown to remain and disintegrate slowly where worms are very active and those root channels/holes are routes and places for earthworms to live, breed and eat. Our tillage from what we have seen encourages worms to move in and out of the till zone (soil density is generally less than where it is easier to burrow and leave tunnels and burrows. I have been observing this now for 32 years. During this four year side-by-side study and digging during the growing season I have seen and pointed out to whoever is with me that in the NT surface compaction becomes problematic with row crop systems with tractor, combine and grain cart traffic. This has broken down soil pores in this 2 to 6 inch zone where we carry out the soil pore counts. With strip-till we can alleviate this in the spring and see more pores in the spring months.

Figure 1: Pore counts in comparing high counts and mean values between Strip-Till (ST) and No-Till(NT), 2013

In this graphic we compare the ST high counts of pores to the NT high numbers along with what the mean values of the counts. In the green text box you can read that the Strip-Till numbers are slightly fewer than the highest counts made in the No-Till

Figure 2: Total pore counts, mean values compared as Strip-Till and No-Till and the lowest number of pores for each tillage type

In the above chart (Fig 2) I offer a different look at the mean pore count numbers and the lowest numbers of pores counted to demonstrate the differences.
As I consider all what is here and the 1300+ root pits I have engaged into and then thousands of holes when I was involved in the National Soil Survey Program in 4 different states as a soil scientist, it is my opinion we see consistently more of all three major pore sizes in a Strip-Tillage System. Will it always be 25 to 40% more in numbers? No. If any conservation tillage farmer really makes a serious program to control all season traffic in and out and across of their fields the pore counts will be pretty close to the same. At the Orthman Farm we carry out harvest operations with grain carts moving all about to load and unload the combine to keep the harvest smooth and quick, similar to large corn growers in the Great Plains. We are confident that our tillage efforts will take care of 98% of all issues.
So as I conclude that as we advocate Strip-tillage, we will see more pores (three sizes We have observed that earthworms like to burrow deep in the tilled zones deep into the non-till zone of the strip-till and feed on the surface and then return. The diminished effort they have to make in the strip-till allows them quicker access to the previous crops residue. As they return to their burrows they stabilize the burrows due to their skin secretions making for larger conduits for water to enter the soil subsoil’s and substratum or underlying layers. Roots follow these tunnels with ease and I have observed 2 to 5 roots going on down one burrow frequently in the thousands of pits I have dug. Yes it sounds weird but folks, this is what makes the soils so dynamic and helpful to support healthier crops, improve soil sustainability and ultimately better yields.
All of the above offers to the grower a better chance to improve output of each seed he/she plants. I ask then, sharing our evidence and facts that soil physical characteristics do respond positively in the strip-till system to make a difference, it begs the question: are you helping your soils?

Download a copy of the report here: Benefits of StripTill_pores2013.

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