Compacted subsoils in organic farming
Mechanical loosening and the risk of recompaction
By Lars J. Munkholm & Per Schjønning, Department of Agroecology, Danish Institute of Agricultural Sciences
Subsoil compaction caused by heavy traffic has been highlighted as the biggest threat to soil quality in modern agriculture. This concern is even more relevant in organic farming, as poor crop growth conditions can not be compensated for by the use of mineral fertilizers or pesticides. Subsoil compaction has been shown to severely hamper root development, limit rooting depth, reduce utilization of nutrients and water and reduce crop yield. Subsoil compaction may also cause increased impact on the environment due to increased risk of erosion, nitrogen leaching and denitrification. Even though subsoil compaction is a widespread and serious concern, subsoiling of compacted subsoils often gives discouraging results. In many cases this has been related to a traffic-induced recompaction of the loosened soil, which is highly prone to recompaction. It is thus a key issue in organic farming, whether mechanical loosening should take place and how recompaction should be avoided.
Blocks of soil sampled from different cultivations are shown here.
For an organically farmed sandy loam we evaluated the degree of recompaction after subsoiling and studied effects of loosening and recompaction on root and yield response for a winter wheat crop. Plots were mechanically non-inversion loosened (NINV) to a depth of 35 cm in 1997 and again in 1998. Non-loosened plots (CONV) referenced these plots. Perennial grass/clover was grown with limited traffic intensity in 1999-2000. The perennial grass/clover was ploughed under in spring 2001 and oats established. Winter wheat was established in the autumn about a month after the harvest of oats. A recompaction experiment was conducted in 2001 and 2002, using the NINV plots from the subsoiling experiment. On-land ploughing was compared with traditional mouldboard ploughing. In addition, the loosened plots were either heavy trafficked (10-18 Mg axle load and ~200 kPa inflation pressure tires) or light trafficked (<6 Mg axle load and <100 kPa inflation pressure).
On-land ploughing mitigated recompaction
Our results showed that on-land ploughing mitigated recompaction of the upper part of the subsoil. Traditional ploughing with the tractor wheels in the furrow caused a relative loss of 22% macroporosity and resulted in a more than 50% reduction in air permeability in the upper subsoil than in on-land ploughed soil. This clearly indicates reduced conditions for root growth, water infiltration and microbial activity in the upper subsoil for the traditionally ploughed soil. Despite that, we found no clear difference in crop response between the differently ploughed soils. This could be related to favourable weather conditions for wheat growth in 2002 (i.e. no periods of drought or water logging).
Surprisingly, we found only minor differences in upper subsoil properties between the heavy and light trafficked soils. A clear negative effect of heavy traffic on crop yield was interpreted as a residual effect of soil compaction on the plough layer soil.
Non-loosened soil performed similar or better than loosened soil
In general the CONV soil performed similar or better than the least recompacted NINV soil. In 2002 - 4 years after loosening the NINV-treatments - the CONV soil showed similar pore characteristics as the least recompacted NINV soils. The results indicated that the structural conditions in the upper subsoil improved for the CONV soil within the 4-year period. Supposedly, this was related to the growing of grass/clover ley and a change to a more cautious tillage and traffic system when growing cereals in 2001 and 2002.
The CONV soil also facilitated higher root intensity than the NINV soils and produced similar yield. The deep rooting in the CONV soil occurred despite increased root diameters indicating a hampered root growth in the CONV plough pan layer. The deep rooting in the CONV soil may be due to preferential growth in biopores (earthworm and root channels). Subsoiling undoubtedly destroyed the inherent system of continuous biopores in the upper subsoil although at the same time produced new cracks and pores. These new pores were probably not as efficient pathways for roots as the inherent biopores.
On-land ploughing is an efficient means of mitigating recompaction of mechanically loosened subsoils. However, our results show that mechanical subsoiling may create even more constraints than benefits to crop development. We thus recommend that mechanical subsoiling should only take place in situations with severe compaction of subsoil layers. Our results indicate that biological amelioration induced by appropriate changes in cropping system as well as tillage and traffic intensity comprises a favourable alternative to mechanical subsoiling.
Our study highlights the need for focussing on prevention of further subsoil compaction especially in organic farmed systems. Presently there are some solutions available on the market in form of low-pressure tyres on twin wheels and/or dual or triple axles. On-land ploughing is certainly also a means to reduce the stress on the subsoil. Controlled traffic farming (CTF) systems would be very effective in preventing subsoil compaction except in the permanent wheel tracks. However, intensive research and development is needed to make CTF systems practically available in Europe and adapted to organic farming.
Many soils already exhibit harmfully compacted subsoils. For these soils, improvement of the present poor subsoil structure is needed. Our study showed that biological amelioration comprises a favourable alternative to mechanical subsoiling. Root and earthworm channels constitute effective pathways through compacted layers and significantly improve soil functions like aeration, infiltration and deep rooting. Already, organic farming promote earthworm activity due to the use of diverse crop rotations, catch crops and organic amendments. However, also in organic farmed systems, substantial changes in soil management may be needed to significantly enhance the number of channels in the subsoil. The changes in soil management may include simultaneous changes in tillage and traffic, crop rotation, and rate, type and timing of organic amendments. Exploring management strategies that secures a fast and persistent improvement of subsoil structure represents a great challenge for future research in organic farming.