Seed Bed: Three ways to avoid soil compaction

Tuesday, February 3, 2009

The surest way to avoid yield losses from soil compaction is to prevent it from happening

by KEITH REID

Soil compaction is usually an invisible yield thief. The damage is below the soil surface, so unless it is severe the impact on root growth and water or nutrient availability is not obvious. In fact, the yield impact from compaction may be negligible in years with good growing conditions, only showing up when there is weather stress on the crop.

To understand compaction, it is helpful to think of the soil not as a solid, but as a matrix of holes of various sizes and shapes between the solid bits. Water forms a film over all the soil particles, and fills the small pores, while the larger pores will contain air. A soil in good condition will have a mix of pores of different sizes so that, when the soil has finished draining, they will contain about half air and half water. The continuity of the pores is also important, allowing water and air to move through the soil.

By the way, too much pore space in the soil is not a good thing either, since the soil cannot hold enough water for good plant growth. A very fluffy seedbed will benefit from firming, which is actually mild compaction.

Soil compaction occurs when the weight pushing on the soil is greater than the strength of the soil, pushing the soil particles closer together and reducing the size of the soil pores.

Since air is more easily displaced than water, most of the loss of pore space is in the larger, air-filled pores, leaving behind a higher proportion of small, water-filled pores. The soil can also be deformed, so that connections are broken between pores. In addition, squeezing the soil particles can make them stick together, creating a cloddy soil structure.  

The changes in a compacted soil can be profound. Fewer large, continuous pores mean slower drainage, so it takes longer before a field is fit to plant in the spring. These large pores are also pathways for plant roots, so root growth is constricted, which in turn limits the ability of the plant to take up water and nutrients.

Even where roots can get out into the soil, the movement of nutrients to the root surfaceis slowed by the long, crooked pathways the nutrients must follow to get through the fine pores and around the soil particles. Dry soil conditions make these conditions far worse. The surest way to avoid yield losses from soil compaction is to prevent it from happening. There are three approaches to this:
1. Reduce traffic.
2. Reduce tire pressure.
3. Reduce axle weight.

These three measures are not mutually exclusive, and an effective compaction management program will include all three.

Moist soils will become compacted more readily than dry soils, since the water acts as a lubricant to allow soil particles to slide past one another. Avoid travelling over the soil at all unless it is dry. But if you must be on a wet field, you can limit the damage through controlled traffic. By keeping all the traffic to a few laneways within the field, the impact on the following crop is limited to a small area.

Ag engineers have also measured the amount of compaction from field traffic, showing that about 70 per cent of the reduction in soil pore space happens on the first pass over the field. Subsequent passes will not compact the soil as much as the first pass, leading to the conclusion that there will be less impact on crop growth. I'm not convinced that this is always the case, since that second or third pass may compress the soil beyond some critical value for crop growth.

Reduced tire pressure works by increasing the footprint covered by the tire, since a tire with low pressure will "squat" and create a larger contact area. As the tire surface spreads out, the down pressure from the tire is only slightly greater than the air pressure within the tire. If this pressure is less than the strength of the soil, then traffic over the field should not cause compaction.

This will only work with radial tires, since bias-ply tires do not deform easily. The pressure in the tire must be great enough to support the weight of the equipment, so reduced pressure will also mean larger tires.

Finally, soft tires will wear more quickly when used for road travel, but inflating tires to full pressure before moving between fields can negate all the advantages for compaction reduction if they are not deflated when entering the next field.

Tracks can create the same conditions, but only if the soil deforms enough to spread the weight out over the entire surface of the track. When tracks travel over hard surfaces, it is clear that there is significant down-pressure from the drive wheels and each bogie because the track cannot have enough tension to spread the weight evenly. A tracked vehicle can only "float" if it is causing a little bit of compaction.

The final defense against compaction is to limit the weight of equipment travelling over the field. Greater weights mean that, if compaction occurs, it is transmitted deeper into the soil. Compaction at the surface can be broken up by tillage or by frost action, but deeper compaction can persist for years, or decades.

It is unrealistic to expect us to go back to small equipment, but you do have control over how full you fill the grain tank on the combine or the box on the manure spreader. Keeping to slightly smaller loads when conditions favour compaction can be enough to make a difference. 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