How do soil mechanical properties and mucilage affect the root penetration resistance to root growth?

Root elongation is affected by biological, physical, and chemical soil properties. Key soil physical properties determining soil strength are water content and bulk density. Highly compacted soils provide strong resistance to root growth. Therefore, it is vital to understand the effects of water content and density on the ability of roots to penetrate soil. Plant roots release a polymeric gel consisting of polysaccharides and lipids called mucilage. Mucilage also affects the physical, chemical, and biological properties of the soil, and thus is expected to have a significant effect on the penetration forces associated with root growth. In this study, penetration resistance is investigated for two soil types (sand and loam) treated with two types of mucilage obtained from flax and chia seeds. To determine penetration forces, a rheometer (MCR 102e, Anton Paar, Germany) equipped with a stainless steel needle with a shaft diameter 1 mm and an apex angle of 60° was used to mimic a root. In all measurements, the needle penetrated the soil with a velocity of 40 µm/s. Soil samples were prepared with various water content (6%, 9%, 12%, and 15%) while keeping the dry density of the soil constant following standard procedures of a mini-compaction test.  To investigate the effect of mucilage concentration, penetration tests were carried out for different concentrations (control, 0.1%, and 0.5%). Results suggest that an increase in water content significantly reduced the penetration forces. A clear effect of the type and the concentration of root exudate on the penetration resistance was also observed. It is concluded that root penetration forces are significantly affected by soil type, water content, and the type and concentration of mucilage in the rhizosphere.