Caption: Tracks show where the soil has been compacted.
By Harry Brook
There is a lot of talk about soil compaction, with plenty of machinery dealers willing to sell the newest and greatest implements to deal with it. However, do we really have a soil compaction problem? That is a question worth answering!
What is soil compaction? It occurs when soil particles are squeezed together, reducing the space available for air and water. Drier soils are resistant to compaction as soil particles will resist collapsing under pressure. Risk of soil compaction is highest when soils are moist, at or near field capacity. Soil aggregates become lubricated with water and will have little strength. Large air-filled pores collapse under pressure, resulting in compaction. A compacted soil will prevent plant root growth from penetrating as deep as it should, leading to a shallow rooting zone. And that leads to greater susceptibility to drought, poor nutrient uptake and plant stress.
Research on harvest ruts showed a 17% drop in yield from ruts in the field. Headlands, where traffic was heavier, caused even more yield loss.
There are a couple of types of compaction. Shallow compaction exists in the top six to eight inches of soil. It is directly related to the pressure applied to the topsoil. Ground pressure on the soil surface is one to two psi greater than the tire pressure. Part of this can be attributed to tire pressure. The proper inflation for a tractor tire for road speeds is two to three times that of the proper inflation rate for a tire in the field.
Deep compaction occurs below eight inches and is related to high axle loads. Compaction can be forced down to three or four feet with heavy axle loads. This is the same for both tracks and tires. To avoid this type of compaction, limit axle loads when soil is moist. Some of the heaviest axle loads come from grain carts. As they get bigger the force only increases.
How to manage compaction? Reduce total axle loads, maintain tire pressure in the appropriate range, minimize the number of trips over a field, use duals or large diameter tires or tracks, rotate to deep-rooted water-loving plants, and reduce tillage. About 80% of compaction happens on the first pass. Using controlled traffic or tram lines can help.
Can you fix a compacted soil? Deep ripping or subsoiling might not pay. Research in Iowa showed only a 50/50% chance that yield increases would pay for subsoiling costs. If you’re going to use ripping or subsoiling, try it on the headlands, where traffic is heaviest. When subsoiling, soil must be dry enough to fracture. Too wet and it won’t work and might actually contribute to more compaction.
Keeping your soil healthy is a way to prevent compaction. Good soil structure is the best defense and that can be helped with a good organic matter content in the soil. If you must work in the field under less than ideal conditions, decrease axle loads and try to minimize traffic on the soil surface.
We think winter, with its wetting and drying, freezing and thawing cycles will prevent compaction. However, in-season compaction may still negatively affect yield and crop growth. Also, don’t confuse some soil types in the Flagstaff County area as being the result of compaction. Gumbo soils (Solonetzic soils) have a hard pan due to soil chemistry. And that is a topic for another column.
If you do have compaction issues it can be addressed by being deliberate where and when you travel on your fields, especially during harvest. It is so true, “An ounce of prevention is worth a pound of cure.” A couple of videos on soil compaction are below.