How can we avoid the negative effects of tillage?

Sunday, February 6, 2011

Too much or the wrong kind of tillage can damage your soil. Some tips on retaining its benefits while avoiding its negatives

by KEITH REID

Most crop fields in Ontario are tilled at some point during the rotation. If you ask the average farmer why, you may get an answer about improving crop yields but few details beyond that. Tillage is simply a deeply ingrained habit.

Unfortunately, too much or the wrong kind of tillage leads to a number of undesirable outcomes:

Increased soil erosion by wind or water;

Scalping of topsoil from the knolls and moving it down slope;

Loss of soil organic matter;

High costs for equipment, maintenance and fuel. 

How can we avoid these negatives while retaining the benefits of tillage? Since the adverse effects increase with the frequency and intensity of soil disturbance, we need to understand what we are actually trying to accomplish so we can get the job done with the least amount of tillage. There will never be a single system that meets everyone's needs, but there are many benefits to putting some effort into designing the optimum system for your combination of soil type, topography and crop mix.

Seedbed preparation. A well-prepared seedbed will allow placement of the seed at a consistent depth, surrounded by soil that is loose but not too loose and with adequate pore space to allow unrestricted root growth. 

In the past, the issue of seed placement meant the elimination of all residue on the soil surface, since most drills and planters could not handle any trash without plugging. This became the standard for a "good seedbed" for our grandparents, but I question whether it is relevant to today's drills and planters, which have been designed to penetrate surface residue and place most seeds in a wide variety of conditions.

There is continual discussion about how much soil loosening is required to allow unrestricted root growth. We know that root growth is seldom the limitation in no-till systems for most crops, unless the soil has been abused, so tillage is often a remedy for poor soil management during the growing season and post-harvest. It may be worth considering changes to the rest of the cropping system rather than hitching up the plow.

A more recent challenge has developed from the trend to wider planters and air drills.  The concentration of the weight either at the centre of the planter or in a towed cart means there is not enough weight to ensure penetration into un-worked soil.

Modifying the physical environment. Crop residue on the soil surface reflects the sun's rays and blocks evaporation from the soil. This keeps the soil cooler and moister than it would be otherwise, an advantage in mid-summer but a significant drawback if you are waiting to get the crop planted.

Options to reduce the negative effects of heavy residue covers include tillage, but also the removal of some of the residue (e.g. by baling straw) or management to keep the residue standing rather than laying flat on the surface in a mat. The trend to corn headers that shred the residue has been directly opposed to successful management of these fields with limited tillage.

Incorporating fertilizer, lime and herbicides. If you have an acid soil, then tillage to mix the lime evenly throughout the topsoil is the best option, but most fertilizers do not require as much intimate contact with the soil to be effective. Banding below the surface – whether pre-plant, at planting or as a side-dress – can be equally or more effective in retaining nutrients and making them available to crops.

If you do wish to incorporate fertilizer or manure, the only requirement is that the materials have a shallow covering of soil over them, so the amount of tillage actually required to be effective is quite modest.

Pre-plant incorporated herbicides are a special case, since many of them will degrade if left on the soil surface, or vaporize and be lost. Part of the decision around choosing a herbicide that must be incorporated is whether the specific weed spectrum demands that herbicide, or if there are surface-applied alternatives that would be equally effective.

Weed control. Tillage can control young annual weeds effectively, but the disturbance of the soil can also bring a fresh crop of weed seeds into the germination zone. Weed populations adapt to the type of tillage system practised, so fields with regular soil disturbance will be dominated by rapidly growing annual weeds while fields without tillage will be dominated by perennial or winter annual weeds.

Judicious use of herbicides can eliminate most of the necessity for frequent tillage simply to control weeds. BF

Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs based in Stratford. Email: keith.reid@ontario.ca