With the cold, wet months approaching, it’s time to think about how you are going to minimise the risk of pugging and compaction damage this season. These two problems are similar, in that they both involve physical damage to the soil. Pugging is caused by the weight of stock on the soil surface – it occurs when the soil is too soft to support the animal’s weight. Compaction is similar, but is caused by machinery such as tractors, and it can affect the topsoil and the subsoil.
So what exactly is happening in a soil that is pugged or compacted? A normal, well-structured soil contains solid material – sand particles, clays, and silts – plus organic material, water and air, not to mention biological life and nutrients. The solid components are all different shapes and sizes, and they collect together in aggregates, also of different sizes.
Because the soil aggregates are not all the same, there are spaces between the particles. These can be large (macropores) or small (micropores), and they play a very important role in soil health. Macropores allow water to flow freely through soil; they provide pathways for worms and other soil life (worms also create macropores as they move through the soil); they allow plant roots to easily penetrate soil; and they provide an area for gasses to be exchanged. In a well-structured soil the pores create a interconnected network through the solid material.
When soil is pugged or compacted, the pores in the soil are crushed; they become smaller or disappear altogether, and the connections between the pores are lost, so there’s a large reduction in soil macroporosity.
As a result of the breakdown of the pore structure of the soil, moisture can’t flow through the soil effectively, so drainage is compromised. If a pan develops, then water will not be able to penetrate it, and the soil will be at risk of becoming waterlogged. As well as creating an anaerobic environment, which can cause nitrogen loss through denitrification, waterlogged soils can contribute to increased loss of soil (and nutrients) through surface run-off and erosion.
In compacted soil, plant roots can’t work their way deep into the ground, which restricts the area they can access for nutrients, which in turn restricts their growth.
A compacted soil also has less area for air to circulate. This can result in an environment that does not favour beneficial biological activity, so activities such as organic matter breakdown (and nutrient release) decrease.
Plants growing in a pugged or compacted soil might become nutrient deficient, even though soil tests show that soil fertility levels are adequate. Sometimes this can be revealed by comparing the results between herbage and soil tests. Adding more nutrients in the form of fertiliser is only a short-term solution to this problem. What’s more, increasing Olsen P levels in soil above agronomic optimums is known to increase the risk of phosphorus loss through run-off, so is not desirable from either an economic or environmental perspective.
It can be difficult to completely prevent pugging and compaction, especially in farming situations with heavy soils and wetter climates. However, it is possible to minimise the damage by careful stock and soil management.
Minimise pugging by building pasture covers going in to winter. Longer grass will protect the underlying soil. If possible, adjust the round length so that paddocks are grazed the minimum number of times when they are at risk. If you have paddocks that you know are particularly prone to pugging, aim to graze them before the wettest conditions develop. Block grazing stock when pastures are wet will also help to minimise damage, especially if a back fence is used to prevent back grazing. If you can, use on-off grazing on your most vulnerable soil; reduce the time animals spend on at-risk paddocks to as little as 6 hours a day by shifting stock to stand-off pads or even into farm lanes. Use supplementary feed if required, but don’t feed this out in paddocks.
Minimise compaction by reducing soil tillage – consider the use of no-tillage or minimal tillage techniques. If you do use cultivation techniques, vary the depth each time, which will reduce the chance of a pan developing. Where possible, avoid working the soil when it is wet, and reduce tyre pressures on machinery to the lowest, practical, safe level. The less pressure in the tyre, the less pressure on the soil.
