On Monday the week of April 9, 2019, we were trying to create compaction using a 36-ton manure truck for a demonstration in preparation for Penn State’s Diagnostic Field Clinic (July 16-17, 2019). Unfortunately, there was too much of a bad thing — surface soil moisture exceeded 30% in some areas and was above 21% everywhere — and the soil was in the plastic or even liquid state.
The result — the truck got stuck within 60 feet from the field edge and we needed to get a tractor to pull it out of the muck. Let’s review a couple basics of compaction and how they apply to conditions this spring.
The first thing to consider is how to avoid causing compaction — remember that it is much more cost effective to avoid compaction than to remediate it after you caused it. To avoid soil compaction:
- Monitor soil moisture — when you can make a ball out of the soil it is in the ‘plastic’ state and most compactible (this is called the ‘ball test’). If soil acts as a fluid (‘mud’) it is in the liquid state and you will cause deep ruts, so that’s no good either. Soil that is in the plastic or liquid state is too wet to be on it. Many times, the surface is dry, but a few inches down the soil is still too wet to be in the field.
- Limit traffic across the whole field as much as possible — it is amazing how much of the field can be trafficked during manure spreading, fertilizing, liming etc. Because most compaction is caused in the first trip it is important to limit those second and third passes. Organize some traffic lanes that are ‘sacrifice areas’ — these can then be loosened later with a deep tillage tool without having to address the whole field.
- Use flotation tires on trucks and farm equipment. Road tires are typically inflated to 95-120 psi — causing severe surface compaction. When soil is moist (typical in a Pennsylvania spring) it is very important to use flotation tires.
- Adjust inflation pressure to lowest allowable pressure. Research has shown that the same tire inflated to high pressure will cause a lot more compaction compared to when it is inflated to low pressure. Therefore, use the lowest allowable pressure (but watch out for side-wall failure — which happens when a tire is underinflated). Check with your tire dealer to determine the right inflation pressure. Tire pressure in the field should be less than 35 psi, and preferably less than 20 psi.
- Reduce axle load — the primary cause of deep compaction. More axles help reduce deep compaction. Typically 10 ton axle load should be the limit to avoid compaction below 12 inches.
To alleviate the effects of compaction and make soil resist compaction:
- Avoid tillage except when absolutely necessary (such as in high traffic lanes). Evaluate penetration resistance with a penetrometer, and your soil structure using a shovel and the Pennsylvania Soil Quality Assessment Worksheet to check if tillage is really required. If the soil falls apart in natural units called peds, it is doubtful that tillage is required. On the other hand, if you see significant platiness or massive soil structure, it may be beneficial to break that up. But avoid inverting the soil and try to keep crop residue at the soil surface for erosion protection, to reduce water evaporation in summer, and to create a favorable habitat for soil organisms such as nightcrawlers. Before you put a tillage tool in the ground, think about the effects — your soil is becoming soft and this will make the soil more susceptible to re-compaction, rutting, and getting stuck. You will mix surface organic matter to tillage depth, destroying the high surface organic matter content in continuous no-tillage soil that make soil resist compaction and bounce back from its effects quickly. You will break up soil peds and roots structure that maintain soil structure. Tillage also causes continuous macropores to be destroyed. Although tillage looks like a great fix, it causes short and long-term harm by exposing the soil to erosion and by making the soil highly susceptible to re-compaction.
- Build organic matter. The proven practices to build organic matter are to use crop diversity and cover crops, have continuously living root systems in the soil, return crop residues to the soil, add manure with high-solid content, use compost, and use no-tillage.
In our compaction demonstration, we tried to cause maximum damage: The field had been tilled last year, making it very soft; there was no cover crop, so no roots to keep the equipment up; soil was in the plastic or liquid state, making it highly susceptible to compaction; we loaded our truck up to 36 tons of total weight with only 3 axles — this caused the axle load to be on average 12 tons, elevating the threat of subsoil compaction; and we used road tires inflated to 120 psi, maximizing the threat of surface compaction.
Think about your management to determine if you are in danger of causing compaction, and what you can do to avoid it.