How much tillage is enough?

Monday, March 5, 2012

When it comes to tillage, less is almost always more. For many situations, one pass in the fall and one or two more in the spring should be enough

by KEITH REID

As the days start to get longer, and bare soil appears from under a covering of snow, many farmers get itchy to head to the field with the tractor and cultivator. We almost seem to be hard-wired to enjoy the look, smell and feel of freshly turned soil, and I know that even some dedicated no-tillers need to fight the temptation each spring. Since most farmers still include tillage in their crop production package, the question for most is not whether to till, but how much?

The answer to this question will depend on what you are trying to accomplish with your tillage passes and there may be a number of different answers, depending on which unique outcome you are focusing on.

"I want to show the neighbours that I am a good farmer." We may chuckle at this one, but I wonder how much tillage is really motivated by vanity. There are really only two times that someone driving past on the road can admire your farming skill: immediately after planting and when the crop is near the peak of vegetative growth. Since the appearance of the growing crop can be affected by summertime weather extremes, the surest way to impress the neighbours is to have a perfectly smooth and even surface following planting, with arrow-straight rows.  

Unfortunately, it usually takes multiple passes to get the desired conditions, which also means increased fuel costs, wear and tear on equipment, and a soil surface that is left highly vulnerable to crusting and erosion. Perhaps a better approach is to educate the neighbours about what an ideal seedbed should look like.

"Harvest equipment works better and faster on smooth ground." This is true enough, particularly for crops that are harvested close to the ground like forages or beans, although I have never been able to figure out what advantage it gave for grain corn harvest, where the header seldom needs to run close to the ground.

Where primary tillage has been improperly done, it can take a lot of tillage passes to level off the humps and hollows in the field. This can leave the soil over-worked and vulnerable to crusting.  

"I want to dry the field out to speed up planting." There have been estimates that each tillage pass will result in the loss of one inch of available moisture from the soil. So, while it can speed soil drying, it can also remove moisture that the crop could use later in the season. In years with short windows of planting opportunity, tillage can delay planting unless there is enough equipment and labour to allow the planter to follow immediately behind the tillage. If it rains before the planter can get back to the field, the resulting delay will completely defeat the purpose of the tillage.  

"I want to conserve moisture." Harrows and packers can help to conserve moisture by creating a "dust mulch" at the surface which slows evaporation and creates fine pores in the soil that encourage the wicking of moisture up from the subsoil. This can be helpful if the weather stays dry. But, if it rains, the smooth surface will shed water rather than allowing it to infiltrate, so the net impact is a reduction in available water for the crop. This activity also seems to go along with multiple tillage passes that dry out the soil, so the same level of available soil moisture could have been achieved with less tillage rather than more.

"I want good seed-soil contact." It is necessary to have fine soil particles in contact with the seed to allow moisture to travel through the small pore spaces, encouraging rapid germination. How fine is a matter of some judgment, with a suggested range being one to six millimetres in diameter.  Larger clods will have large pore spaces that don't carry consistent moisture, while smaller soil aggregates will limit the rate that moisture can flow through the tiny pores and may block the growth of plant roots.

Smaller seeds, like canola or alfalfa, will need a seedbed with soil aggregates at the finer end of the range than larger seeded crops to ensure good seed-soil contact. The key point is that the proper-sized soil aggregates must be at the depth of the seed.

Since tillage will naturally "sort" soil aggregates of different sizes, with the finer ones settling to the bottom of the tillage zone, the surface should have coarser soil aggregates than the size range you desire for planting depth. This will allow rapid infiltration of spring rains and protect the soil from erosion by water and wind.

The other point to remember is that the planter or drill will also contribute some tillage action, and what counts is the soil aggregate size after planting rather than before. We need to condition ourselves to planting into a seedbed where the soil is a little bit coarser than we are used to.

When it comes to tillage, less is almost always more. Extra tillage passes are usually attempts to fix mistakes from previous tillage, so make sure your equipment is set up properly to avoid creating humps and hollows. Wait until the ground is fit before you touch it in the spring. Plan any nutrient applications so they fit in with the tillage program rather than needing extra passes for incorporation.

For many situations, the target should be one primary tillage pass in the fall (particularly on clay soils), and one or two secondary tillage passes in the spring, followed immediately by planting. BF

Keith Reid is Manager (Eastern Canada) – Soil Nutrient and GHG Management, Agriculture and Agri-Food Canada, Guelph.