Effects of bio-subsoiling species on the recovery of compacted subsoils: a literature review

Soil compaction is seen as a major challenge in modern agriculture. It could result in a decrease in soil qualities such as permeability of water and air, infiltration capacity, water storage, oxygen supply and rootability. Especially subsoil compaction is problematic because of its more permanent nature. Several techniques have been developed to recover compacted subsoils, with bio-subsoiling as one that is promising due to its alleviation potential and limited drawbacks. This study addresses the knowledge gap about the efficacy of different crops as bio-subsoilers and how these effects can be quantified. A literature-review has been performed on 57 different experiments performed in 20 studies. 19 different bio-subsoilers were investigated with the main focus on radish, alfalfa and chicory. Experiments ranged from a few months till several years and were mainly performed on sandy and silty loam either in controlled pot (soil column) experiments or in the field.

In general, compaction has several effects on roots such as decrease in root length and number, increase in root diameter, shallower root systems which are concentrated above compacted zones (increase in root growth outside of compaction) and decrease in root hair length. Plants can, however, adapt in several ways to increase the probability of penetrating dense zones.

In almost none of the studies the bio-subsoiler was able to significantly improve bulk density or total porosity in the subsoil even after many years of crop growth. There can be several reasons why these indicators do not significantly change with bio-subsoiling. The root systems of the crops that are grown and their created channels can be too small to change the bulk density of the soil. Or, the variability of these indicators can be so large that the effects of the small roots seem negligible. It can also be that after a short period of time the roots that would have improved the subsoil characteristics, are currently not decomposed and therefore the results do not show the improvements that are made.

Although differences in large scale indicators are not seen across the studies, several studies have shown that some of the potential bio-subsoilers are able to significantly affect other soil structural or functional indicators in the subsoil like macro- and microporosity. These indicators seem to be more sensitive to changes compared to bulk density and total porosity and therefore might be more useful to assess the effects of different bio-subsoilers.