Soil strength and macropore volume limit root elongation rates in many UK agricultural soils

Abstract

Background and aims: Simple indicators of crop and cultivar performance across a range of soil types and management are needed for designing and testing sustainable cropping practices. This paper determined the extent to which soil chemical and physical properties, particularly soil strength and pore-size distribution influences root elongation in a wide range of agricultural top soils, using a seedling-based indicator.

Methods: Intact soil cores were sampled from the topsoil of 59 agricultural fields in Scotland, representing a wide geographic spread, range of textures and management practices. Water release characteristics, dry bulk density and needle penetrometer resistance were measured on three cores from each field. Soil samples from the same locations were sieved, analysed for chemical characteristics, and packed to dry bulk density of 1.0 g cm(-3) to minimize physical constraints. Root elongation rates were determined for barley seedlings planted in both intact field and packed soil cores at a water content close to field capacity (-20 kPa matric potential).

Key results: Root elongation in field soil was typically less than half of that in packed soils. Penetrometer resistance was typically between 1 and 3 MPa for field soils, indicating the soils were relatively hard, despite their moderately wet condition (compared with <0.2 MPa for packed soil). Root elongation was strongly linked to differences in physical rather than chemical properties. In field soil root elongation was related most closely to the volume of soil pores between 60 µm and 300 µm equivalent diameter, as estimated from water-release characteristics, accounting for 65.7 % of the variation in the elongation rates.

Conclusions: Root elongation rate in the majority of field soils was slower than half of the unimpeded (packed) rate. Such major reductions in root elongation rates will decrease rooting volumes and limit crop growth in soils where nutrients and water are scarce.

Fig. 1.

Relationship between root elongation and soil properties in soil cores, showing distribution of root elongation over 48 h at 15 °C for Hordeum vulgare (‘Optic’) for FIELD and PACKED cores, as indicated.

Fig. 2.

Root elongation in repacked and field cores. Root growth over 48 h for PACKED and FIELD cores related to (A) air-filled porosity at –20 kPa and (B) penetrometer resistance. Grey dashed line, regression based on all cores; black continuous line, regression based only on FIELD cores (lines forward and backward extended).

Fig. 3.

Relationship between root elongation over 48 h for Hordeum vulgare (‘Optic’) and soil properties in repacked soil cores, showing (A) latent roots of principal component analysis of soil physical and chemical indicators; (B) regression analysis of root elongation against component 1 (scores) of the principal components analysis (P = 0·234); and (C) predictions of root elongation from the highest-ranked all-subsets regression model.

Fig. 4.

Linear regression of each soil parameter against root elongation in intact (FIELD) cores (mean ± s.e. of each indicator based on regional groupings). P indicates a significant positive correlation with root elongation, N indicates a significant negative correlation with root elongation; ns indicates no significant relationship; v.r. = variance ratio. Res > Res indicates residual variance exceeds variance of response variate. Also shown are mean soil physical indicators for Regions. Colour coding: green, parameter value indicates enhanced root elongation; red; parameter value indicates reduced root elongation. Shading indicates the strength of the response.

Fig. 5.

Regression analysis of root elongation against soil physical indicators: (A) highest ranking individual parameter models; (B, C) dual parameter models in which the size of the circles is proportional to root elongation [B, penetrometer resistance against AFV_{−20 kPa}; C, penetrometer resistance against volume of pores with diameter 60–300 µm (Pores_{60–300 µm})]; (D) relationship between root growth after soil strength has been accounted for, and volume of pores with diameter 60–300 µm (Pores_{60–300 µm}) and AFV_{−20 kPa} (circles: size equals residual of the regression of root growth against penetrometer resistance; closed, positive residuals; open, negative residuals).

Fig. 6.

Principal component analysis of soil physical and chemical indicators of intact field cores from 42 agricultural fields: (A–C) physical; (D–F) chemical; (A, D) loadings of the PCA; (B, E) scores of the PCA for individual fields coded by the field region groups; (C, F) regression analysis of root elongation of Hordeum vulgare (‘Optic’), in cores against component 1 from physical and component 2 from chemical PCA, respectively.