As part of our recent Sustainable Agriculture Research and Education (SARE) Program Partnership Grant on zone-tillage, I purchased a penetrometer. This is a simple device composed of a pointed rod with handles and a pressure gauge which is used to measure soil compaction. The rod is scored every 3 inches so that you can quickly tell how deep you push the probe into the soil. To use this tool, you simply push the rod down through the soil profile slowly, to a depth of 18 inches, and record the maximum pressure in the top 6 inches and between 6 and 18 inches. This gives you a compaction measurement for both surface and subsurface hardness, so that you can compare the degree of compaction between different fields or farms. Of course, it is recommended that you sample at 10 or more different spots scattered across each field, using the average measurement to get an accurate picture of the true state of compaction in each field.
If the gauge reaches 300 p.s.i. while you’re pushing (which indicates a compacted layer or plow pan), you stop and record that depth and pressure, because plant roots cannot penetrate through soil that registers over 300 p.s.i. Multiple readings at 300 p.s.i. throughout a field can tell you the depth of your plow pan and thus, how much soil the plant roots can utilize. You can actually feel when you hit a plow pan because it temporarily stops the probe’s descent, but if you push hard and work through the 2-4 inches of compacted soil, the probe cuts through the soil below the plow pan like butter. You could accomplish the same thing with a homemade penetrometer made from a piece of rebar, with a shorter piece welded across the top horizontally for handles. All sampling with the instrument should be done when the soil is near field capacity (has adequate water for plant growth) to avoid times when the soil profile is unusually dry and hard.
A penetrometer is an important tool to use before converting to deep zone-tillage. It can tell you how deep you need to set the sub-soiling shank on the machine to successfully put a slit through the plow pan, which improves drainage under the plants and allows for deeper root growth (see article on ‘Measuring Soil Health before Transitioning to Deep Zone-Tillage’ this issue). In talking with growers using conventional tillage in their vegetable fields, we often found that people didn’t think they had a plow pan on their farm. So, I decided to conduct a compaction survey across CT to compare fields using reduced-till systems to those using conventional tillage.
By September, I had sampled 46 conventionally-tilled fields (on 44 farms) and 9 reduced-till fields with the penetrometer. Because it was difficult locating farms using reduced-till in vegetables, there were a variety of different techniques included in this group: no-till, shallow strip-till, preparing a field with a spader instead of a plow and harrow, and two fields tilled or about to be tilled for the first time in many years. I tried to choose just one sweet corn or pumpkin field per farm at random to sample, but there were 4 farms (2 conventional and 2 reduced-till) where the grower was very interested in sampling a second field, so I obliged them, and used that data in the survey. The growers seemed to know which of their fields were in better shape, because the second field sampled on each of these four farms had less compaction than the fields chosen at random. However, since there were an equal number of farms (we sampled two fields in each group), I decided it would still be a fair comparison to include the data from those fields the growers chose.
Since I was spending time in 55 fields measuring compaction, I decided I might as well take a standard soil sample while I was there, and have the UConn Soil Lab run a test for organic matter content as well as the normal pH, macro and micro nutrient tests. Tom Morris, UConn’s Soil Specialist, suggested that it might be important to compare soils of similar texture in the two groups, so that we weren’t comparing “apples to oranges.” In the end, we had 73% of the fields with sandy-loam soils, 25% with silt-loam or loamy-sand, and 2% with loam. Since the results of the study didn’t change if I used only sandy-loam fields or included the data from sandy-loam, silt-loam and loam fields, I used all 55 fields to generate the final results (below).
Who has a plow pan?
When we looked at farms where multiple penetrometer readings per field “maxed-out” at 300 p.s.i. (at least 2 of the 10 samples reached 300 p.s.i.), it included 98% of the conventionally-tilled farms and 89% of the reduced-till farms. This indicates that most fields were at least beginning to form a plow pan, regardless of their tillage practices. So, who has to worry about plow pans? Everyone! When we looked at farms where most penetrometer readings per field “maxed-out” at 300 p.s.i. (at least 6 of the 10 samples), it included 89% of the conventionally-tilled farms and only 33% of the reduced-till farms. This indicates that most conventionally-tilled fields have already formed an impervious plow pan, while most reduced-till fields have not.
Actually, 81% of the penetrometer samples (8.1 of the 10) on the conventional farms and 40% of the samples (4 of 10) on the reduced-till farms had readings of 300 p.s.i. This means that fields on conventionally-tilled farms have twice the problem with plow pans that reduced-till farms have. So, who should be looking for ways to break up their plow pans? Almost all conventional farmers! The average depth of plow pans (depth to 300 p.s.i. reading) were similar (11-12 inches) under both tillage systems.
How hard was the ground?
The average penetrometer reading for the top 6 inches of soil was 142 p.s.i. for conventionally-tilled fields and 136 p.s.i. for reduced-till fields.
From these results, it seems like both tillage systems produced a similar level of hardness for surface soils. This is true, but is still interesting considering that almost all samples were taken while the crop was growing. This means that conventionally-tilled fields, which had just had their soils “fluffed-up,” were just as hard as reduced-till soils that were mostly undisturbed. This rapid “firming” of the topsoil may be evidence that the structure (clods & pore spaces) has broken down in conventionally-tilled soils. Soils with poor structure and little air for growing roots are not a great media for healthy plants.
The average penetrometer reading for 6 to 18 inches deep in the soil was 284 p.s.i. for conventionally-tilled fields and 253 p.s.i. for reduced-till fields. I’m not quite sure what conclusions to draw from this information except that the subsurface hardness is dangerously close to the point where all roots are excluded from subsurface penetration. So, our crops are probably much more shallow-rooted than they should be, which restricts their ability to explore the full soil profile for needed nutrients and water. Although there were no deep zone-till fields included in this survey, it should be noted that when I dug holes in fields using this system (which loosens soil under the plant row), roots extended to over two feet deep, while roots in conventionally-tilled fields with plow pans only extended down to the compacted layer (11”).
What about soil organic matter (OM)?
I think we all understand that organic matter is important in the soil for many different reasons. It absorbs excess moisture during wet periods, holds moisture through droughts, adsorbs and holds nutrients and pollutants (i.e. pesticides), reduces soil density, and feeds beneficial microorganisms (such as earthworms, fungi and bacteria) which help improve soil structure and drainage, and are important in the nitrification process.
For conventionally-tilled farms, 59% of the fields were low in OM (< 4% OM) and 41% were at moderate levels (4-8%). For reduced-till farms, 56% of the fields had moderate OM and 44% had high OM (>8%). The overall average for conventional fields was 3.9% OM, while reduced-till fields had 7.5% (almost twice as much). Perhaps all we can conclude from these results is that most conventionally-tilled farms could use more OM, and possibly, that OM levels in the reduced-till soils may be closer to the original (natural) levels before we started oxidizing it away through repeated tillage operations (plowing, harrowing, cultipacking, bedding, cultivating).
The question is, what can we do about low OM levels? This is an area where organic farmers seem to be miles ahead of conventional farmers. Organic farmers often make better use of composts, manures, summer cover crops, rotations, legume cover crops and sometimes reduced-till systems than do conventional growers. They basically have a head start in OM accumulation because they made long term soil care a priority from day one. Not that all organic growers do everything right.
Some tend to overuse composts or manures and build up excess levels of phosphorus and other nutrients in the soil.
So how does a conventional farmer improve OM levels, especially if he/she has much more land to care for than the average organic farmer? One simple solution is to reduce the amount of tillage used on your farm. Studies have shown that the level of OM tends to slowly increase in reduced-till systems, at least in the top foot of soil where the majority of important biological activity occurs. Consider switching to a system such as deep zone-tillage, which combines some of the best attributes of conventional and no-till systems, while helping to prevent soil erosion and compaction. Of course, resting a field for a year, using moderate levels of compost, or devoting part of the growing season to a summer cover crop can also help build OM levels.
One final cost-saving nugget from the survey
Twenty-four percent of the conventionally-tilled fields were low in pH (pH <6.0). Also, 70% of conventionally-tilled fields had higher levels of phosphorus than recommended. Since phosphorus is often the most expensive fertilizer element (much more expensive than lime), and is not available to the plant at low pH, about one of every four farmers can save money this coming season just by adequately liming his/her fields and by slightly reducing phosphorus use.
Conclusion: It’s cheaper to apply lime than phosphorus. We recommend the use of a soil test to determine proper lime and phosphorus.
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