The Biology of Soil Compactionadmin | February 15, 2012
Soil compaction is clearly a physical process, but did you know that it also has biological aspects?
Soybean seedlings struggling to emerge through a crusted soil. Photo from USDA.
The usual explanation for soil compaction is excessive traffic, such as tractors and livestock. It stands to reason that if you press down on a soil, it will become compacted. This, after all, is the basis for road building.
But the causes go further. In the agricultural setting, the chief offender is ploughing. The reason goes to the basis of soil structure.
Healthy soils have a complex structure. Not only do they have layers (most, anyway). Within any layer, you can see small lumps. These are called peds. Peds are composed of smaller particles, called macroaggregates (considered to be >250 µm across). The macroaggregates are composed of smaller particles, called microaggregates (considered to be <250 µm across).
The microaggregates are composed of clay particles, organic matter and fungal hyphae (threads). They are bound by fine roots and microbes into macroaggregates. The macroaggregates are linked more loosely by fungal hyphae, roots and polysaccharides (sugar polymers exuded by roots).
The net result of this hierarchy of soil particles is a complex network of channels for air and water, through which roots can grow, soil invertebrates such as worms can dig, and microorganisms can decompose plant litter. This hierarchy depends for its existence on living organisms in the soil.
Clearly, then, tilling the soil will break apart the loose bonds. The first casualties are plant roots and fungal hyphae. Breaking them cuts off their food supply. (This is, of course, what we want to happen to weeds.) But because plant roots continuously exude polysaccharides, and fungal hyphae similarly exude a glycoprotein (sugar protein) called glomalin, which act as soil glue, killing the organisms that produce them cuts off the supply of soil glue. This glue is called “active carbon”, in that it is actively used by soil microbes on account of its ease of breakdown. Bacteria, being much smaller than fungi and so being unfazed by tilling, then build up in numbers, consuming soil carbon and nutrients and converting soil nitrogen into gaseous forms that are then lost. The introduction of extra oxygen into the soil by tilling further benefits the bacteria.
The physical action of tilling breaks down the soil aggregates in smaller pieces, which can fit into a smaller space, and the loss of the glue producers prevents the aggregates from reforming. So the soil compacts, reducing the movement of water and oxygen into the soil and thus reducing yields and profits.
Deep ripping is a short-term solution that can restore yields, but only until the next time you drive over the field or till the soil again. The only effective long-term solution is to mimic the natural conditions that created the soil in the first place:
- Leave plant residues on the soil surface. This provides a cushion against equipment and stock movement, and leaves a supply of organic matter for soil organisms to digest instead of the soil glue. In addition, it reduces soil erosion due to rain and wind.
- Minimise your tilling so as to leave plant roots intact. A legume cover crop will compete with weeds and supply nitrogen. The roots will allow water and air to penetrate the soil, and will supply a constant stream of organic material to support soil fungi.
Hoorman JJ, de Maraes Sá JC, Reeder R. 2009. The biology of soil compaction. Fact Sheet SAG-10-09, AEX-543-09. Ohio State University, Columbus, OH, USA.
Primary Industries and Fisheries, Queensland. Soil compaction.