Soil Compaction: Quantifying Penetrative Ability

Written by Patrick Skilleter:  In my blog post last year, I gave an overview on how soil compaction inhibits potato root growth and tuber quality by increasing the force required to overcome the soil and reducing availability of nutrients, water and oxygen in the soil. In this blog post, I will explain about the difficulty of identifying plants able to grow through impeding material, known as penetrative ability.

Compacted soils are typically formed by pressure being applied to soils that are wet. Dry soils are more resistant to compaction (Al-Kaisi, 2001), but the majority of crop plants are grown in moist or wet soil, and thus are susceptible to compaction. In agricultural fields, most compaction is caused by the use of heavy machinery. The risk of compaction has increased greatly over time thanks to increased size and mass of machinery used, with a typical tractor weighing twice as much in the 1990’s as in the 1950’s (Duiker, 2004). This results in a soil pore density, mean pore size and soil water holding capacity decreasing, with soil resistance increasing. A soil resistance of more than 2 MPa is considered restrictive, with a resistance greater than 4 MPa being inhibitive (Stalham et al., 2007). As such, soil compaction growth of underground plant architecture, which in turn results in reduced shoot growth and crop yields.

When determining whether a plant is better adapted to grow in compacted soils, most studies use penetrative ability as a measure of success. However, penetrative ability is not a term with a set definition, and can be quantified in many different ways. One of the simplest is to measure changes to root diameter. When plant roots encounter mechanical impedance caused by soil compaction, root diameter increases (Potocka and Szymanowska-Putka, 2018). This increase in root diameter increases the force the root can exert without buckling, making it more likely that the roots can force their way through the impedance. However, there is evidence that diameter by itself does not determine penetrative ability, but instead relative increases in diameter, as the presence of other root features such as a fibrous sheath can improve buckling resistance irrespective of root diameter (Materchera et al., 1992). Some studies use a waxy layer (usually a mixture of soft and hard paraffins) with a high resistance to mimic soil compaction, and determine penetrative ability as the proportion of roots that reach this layer that are then able to penetrate it. These studies usually find huge differences in penetrative ability (Clark et al., 2002), (Botwright and Wade, 2013), but there is no evidence that the ability to penetrate a wax layer translated into better root growth in compacted soil. In fact, whilst numerous studies exist comparing different plant species and varieties for differences in penetrative ability, there is very little evidence of any inter-specific or inter-varietal studies using soil compaction to determine penetrative ability.

A large part of my PhD involves investigating the role of penetrative ability in potato plants. Overcoming the difficulties in quantifying penetrative ability and to what extent inter-varietal penetrative ability can be used to overcome constraints imposed by soil compaction in the field is a major part, and one that I hope will succeed in answering these questions posed by previous studies into penetrative ability. As long as heavy machinery is still used on fields, soil compaction will continue to be a constant obstacle that must be overcome to ensure sustainable food production in the future.

References

AL KAISI, M., (2001). Wet soils vulnerable to compaction. ICM News Archive. IC-486 (13), 106-107.

BOTWRIGHT, T., WADE, L., (2013). Use of genotype 3 environment interactions to understand rooting depth and the ability of wheat to penetrate hard soils. Annals of Botany. 112, 359-368.

CLARK, L., WHALLEY, W., BARRACLOUGH, P., (2003). How do roots penetrate strong soil? Plant and Soil. 255, 93-104.

DUIKER, S., (2004). Effects of Soil Compaction. Publications Distribution Center, The Pennsylvania State University.

MATERCHERA, S., ALSTON, A., KIRBY, J., DEXTER, A., (1992). Influence of root diameter on the penetration of seminal roots into a compacted subsoil. Plant and Soil. 144, 297-303.

POTOCKA, I., SZYMANOWSKA-PUTKA, J., (2018). Morphological responses of plant roots to mechanical stress. Annals of Botany. 122 (5), 711-723.

STALHAM, M., ALLEN, E., ROSENFELD, A., HERRY, F., (2007). Effects of soil compaction in potato (Solanum tuberosum) crops. Journal of Agricultural Science. 145, 295-312