Let This Sink In

Let this soak in from Central Valley Ag on Vimeo.

by Mike Zwingman

by Mike Zwingman

It’s no secret that I have some pet topics; Nitrogen, the planter, and compaction. This spring Keith and I both said that the issues of compaction plague us all year long. So we’ve talked about it when it came to tillage, during planting, when evaluating corn roots, but now are we going to talk about how it impacts water infiltration and irrigation strategies. Settle in while I get my soapbox out.

Keith could tell you that when we look at the AquaSystems probes that we have around Central Valley Ag country, we see some weirdness when it comes to water holding capacities, infiltration rates, and root activity. This weirdness comes from soil structure issues, and those issues are due to one, if not more, forms of compaction.

Just a quick review of soil structure and compaction; your soil is comprised of either soil or pore space, and the voids of the pore space is a variable mixture of air and water. Now the amount of those voids varies by soil type, and that’s how we determine the water holding capacity and how we schedule your irrigations. When we have compaction at any depth, we reduce or sometimes remove the voids in the soil increasing the density of the soil, when there is a reduction or removal of those voids there is nowhere for the water to go when rains or we irrigate it. That is what we see in a fair number of fields with our capacitance probes.

A perfect example is at our Bellwood RD Innovation site. We strip tilled the site in April which changed (increased) the bulk density (the amount of mass per volume of soil). Today when we probe the strips with a penetrometer we see a soil tension of about 100-150 psi at all depths of the soil. Then when we go in between the rows we push the penetrometer into the soil we start to see 250-320 psi at as little as the 2-3-inch depth. This becomes a problem.

Imagine this visual, you go to the sink to do the dishes, and there is a sponge sitting there on the sink where you left it from the last time you did dishes. The longer that sponge sits there the less moisture is in it but when you look at it has shrunk and is hard. When a sponge looks like that, the pore space (voids) is where the volume was lost.  When you first try to wet that sponge, it acts like it wants to repel water rather than absorb it. The same thing happens to your soil, the harder and dryer the soil is, the more hydrophobic it becomes. If you don’t believe it, pour a bucket of water on a dirt road.

The shallower that compaction is, the more hydrophobic the soil becomes, especially when it comes to rainfall where we can’t infiltrate the water at the same rate of which it is falling making rainfall less effective. If infiltration rate problem wasn’t bad enough the water holding capacity is effectively cut in half so we can’t even hold the water, we are getting anyway. Going back to our example of Bellwood, the water runs from between the rows to the stripped zone and re-infiltrates to the rows and the root zone. This difference in compaction is evident after a rainfall or irrigation event you can walk on the compacted soil hours later while the stripped zone is wet.

Take this issue across a whole field due to varying soil types, slope, aspect, residue cover we get wildly variable water related issues. There isn’t much we can do about it today, but it is something that we can go back to at the end of the year when we look at yield and start to ask questions about remediating soil compaction, variable rate irrigation, or some combination of the two.

This is something that we don’t normally think about, but with some of the new technology we have available we can evaluate and make decisions around compaction. We can improve our water use efficiency in both irrigated and dryland situations for you in the future.