by Amy Sangster
Compaction is a big issue in Nova Scotia. Wet weather, heavy soils, short growing seasons, and the pressure to get a crop in and off on time can all result in compressing and compacting soils. In some cases we’re also faced with naturally shallow soils and compacted subsoils that have been inherited from the ice age.
Machinery-induced compaction occurs when the soil cannot support the weight of farm equipment. The soil is forced into a smaller volume, and pore space between soil particles and aggregates containing air and water are eliminated. Soil compaction is greatest when the soil moisture content is high because water acts as a lubricant allowing soil particles to slide past each other as the soil is compressed.
I’ve had the opportunity to look at hundreds of soil pits across agricultural fields in Nova Scotia and I’m always amazed at the wide range of soils we’re dealing with in this province. I don’t think I’ve seen the same soil twice. Unfortunately, one thing that many of these soils seem to have in common is compaction. Ripping has gained ground as a means to try to address our compaction issues. But if it’s not done right, it can actually make your compaction issues worse. There is a lot you need to take into consideration before you decide to lower those shanks into the ground.
Ripping comes at a cost. The horsepower required to pull shanks through the ground equates to significant costs in fuel, time, and wear and tear on equipment. With a power requirement of 30 to 40 horsepower per shank, you need a decent-sized tractor to pull a relatively narrow implement. And because it’s narrow, you may be making a fair amount of passes across the field.
Many growers are going into the field not knowing the depth of their compaction layer. Shanks are often set at a relatively standard depth with intentions that the standard setting will help improve the compaction issue.
However, if you set the shank too shallow, the foot will travel on top of the compaction layer and can actually add to the problem by making that compaction layer thicker. If you are too far below the compaction layer, you can have the effect of lifting soil and pressing it underneath your compaction layer, making the problem worse.
The best placement is right along the bottom edge of the compaction layer. When the foot is placed at the bottom edge, you then get the result you are looking for – the breaking up or shattering of that layer. To maximize the benefits of subsoiling, it should be done when the soil is moist enough to allow penetration of the shank but not so moist that the shank causes smearing rather than fracturing and shattering of the soil. Under moist conditions, the opening in the soil formed by the shank may settle and reform to its compacted state. In addition, plant roots that will grow deep can help to keep that shattered compaction layer open.
There are some soils that I don’t think are good candidates for ripping. In my opinion, ripping glacial-induced compact subsoils is not a good use of your time or money. You’d be better off to find a crop that is more suitable to grow in those soils than to try to work against what you inherited from Mother Nature.
The only way to know where your compaction issues lie is to try to measure them. Penetrometers are available to sign out from the Nova Scotia agriculture department’s regional offices. Many dealers are also making penetrometers available and are helping growers to determine the depth of their compaction layer.
I prefer digging a soil pit to get the best handle on what you’re dealing with. Visual observation of the soil profile is a more accurate way to determine not only the depth of compaction but also the width of the layer and its severity. You may observe plate-like structure below the soil surface, which is often an indicator of compaction. You can also feel the changes in soil density if you use a jack knife to vertically probe the face of the soil pit. If you do go as far as excavating a soil pit, also take note of the rooting depth.
Digging a soil pit may seem like a lot of extra work to take a good look at what is going on in your field but the return on the time spent investigating will help ensure the investment in ripping is well spent.
I still maintain that the best subsoilers are the ones that take the least amount of horsepower – plant roots and earthworms. If you can create a soil environment that promotes good soil structure and you allow biology to do its job, you may find that subsoiling will no longer be a standard practice on your farm.
(Amy Sangster is a soil specialist with Perennia Food and Agriculture Inc. based in Bible Hill, N.S.)