The ability of maize roots to grow through compacted soil is not dependent on the amount of roots formed

Mechanical impedance is a primary constraint to root growth and hence the capture of soil resources. To investigate whether rooting depth and root length under mechanical impedance caused by compaction are correlated we evaluated 12 maize lines at two field sites. To distinguish between lateral and nodal roots, roots were sorted into different diameter classes. Coarse roots had diameters >1 mm and represent nodal root axes. Greater proportions of coarse roots on compacted plots were found at both field sites however results were driven by genotypic variation. Soil compaction reduced total rooting depth (in all diameter classes) and coarse rooting depth at both sites compared to non-compacted plots. Root distribution was influenced by compaction with greater root length densities closer to the soil surface. Root length and root depth were not related to each other under impeded conditions. Coarse roots of some genotypes became obstructed on the compacted plots, while other genotypes were capable of growing through the impeding soil and reached deeper soil strata resulting in differential distribution of roots through the soil profile. On compacted plots we observed genotypes with similar root depths but with contrasting coarse root lengths. The ability of roots to grow through compacted soils is therefore not dependent solely on the coarse root length formed by the root system.