By Sjoerd Willem Duiker
After a dry start in the fall, the rain, drizzle, and cloudy conditions have saturated our soils and prevent them from drying out. This increases the threat of causing soil compaction. Always remember that it is much more difficult to fix compaction than to prevent it. If you still have grain to harvest or want to graze your animals in pastures or on cover crops, remember to take every effort to avoid soil compaction.
Soil compaction is defined as ‘increasing the soil bulk density, and concomitantly decreasing soil porosity, by the application of mechanical forces to the soil’ (Soil Science Society of America Glossary of Soil Science Terms). Soil compaction also increases the penetration resistance of the soil. Soil compaction has many detrimental effects – although it is a physical process, it also affects chemical and biological properties of soil.
Soil compaction causes a loss of large pores in the soil – and this has a negative effect on soil aeration and water infiltration and percolation. The result is poor root growth of crops, water runoff and erosion, and increased soil wetness. Soil compaction has been shown to decrease the activity of large soil organisms – such as earthworms, mites, springtails and fly larvae. It has also been shown to increase the activity of facultative anaerobic bacteria such as Pseudomonas, Bacillus, Micrococcus, and Achromobacter. If the oxygen content of the soil is less than 10%, these bacteria use nitrate instead of oxygen in their metabolism. Nitrate is subsequently converted into N2 (nitrogen gas) or N2O (nitrous oxide) or NO (nitric oxide). As much as 50% of nitrogen fertilizer applied to soil can be lost by denitrication on soils that suffer for long periods of wetness.
Besides, the small root systems of crops growing in compacted soils have decreased ability to take up potassium. So soil compaction can actually decrease nutrient use efficiency and cause nutrient deficiencies such as nitrogen, phosphorus or potassium deficiencies! Soil compaction can cause huge yield decreases in the short term. In one trial in Kentucky, corn yield in an extremely compacted no-till soil was decreased to almost nil the following year. Soil compaction effects on yields can have long-lasting effects. In a series of long-term compaction trials in North America and Northern Europe, scientists concluded that after 10 years there was still a 3% yield loss due to subsoil compaction caused 10 years earlier.
Soil compaction can be caused by farm vehicles, tillage equipment, hoof traffic and intense rainfall in crop or pasture land. Soil moisture content is key. At a moisture content called the ‘plastic limit’ soil is most sensitive to compaction. The water acts as a lubricant causing soil particles to move under stress and since there are some empty pores, the particles pack tighter together. You can determine the plastic limit using the ‘ball test’ – take a handful of soil, knead it in your hand – if you can make a ball out of it, the soil is in the ‘plastic state’ and wetter than the plastic limit and highly sensitive to compaction. Soil texture (sand/silt/clay composition of the soil) affects the plastic limit: You will notice that soil with high clay content tends to be in the plastic state more often than a sandy soil.
Soil structure (the organization of soil particles in aggregates of different sizes with a network of pores of different dimensions) is also important – a soil with a stable soil structure is not in the plastic state as often as a soil with a weak structure. By increasing organic matter content you promote more stable soil structure. Some practices that promote more stable structure are those that increase organic matter, increase fungal hyphae, earthworm activity and living roots. Practices such as dairy manure application, packed manure application, permanent no-tillage, and permanent living roots in the soil by consistently using cover crops therefore help make soil resist compaction better.
At this moment, however, you may be wondering what to do if you still have crops to harvest or need to graze your animals. Here are a few tips:
- Wait a little so the soil can dry out below the plastic limit (defined above).
- Wait until soil is frozen (yes it might happen!)
- Increase vehicle footprint by using flotation tires or tracks - Tire configuration determines to a large extent how much surface compaction is caused – by using tires with low inflation pressures or tracks that have a large foot print, you reduce surface stress and therefore surface compaction. Remember that low tire inflation pressure is key. I recommend that tires should be inflated to a pressure of 35 psi or less.
- Reduce axle load below 10 tons. Axle load largely drives subsoil compaction – and if it is higher than 10 tons you are likely to compact soil below 12 inches which is very difficult to get rid of.
- Don’t take trucks with road tires in the field.
- Avoid making many passes over the field.