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Journal of Arid Land  2020, Vol. 12 Issue (4): 666-675    DOI: 10.1007/s40333-020-0017-6
Research article     
Untangling the influence of soil moisture on root pullout property of alfafa plant
Chaobo ZHANG1,*(), Yating LIU1, Pengchong LIU1, Jing JIANG1, Qihong YANG2
1College of Water Resources Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China
2Changjiang River Scientific Research Institute, Changjiang Water Resources Commission, Wuhan 430010, China
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Root pullout property of plants was of key importance to the soil reinforcement and the improvement of slope stability. To investigate the influence of soil moisture on root pullout resistance and failure modes in soil reinforcement process, we conducted pullout tests on alfalfa (Medicago sativa L.) roots at five levels (40, 30, 20, 10 and 6 kPa) of soil matric suction, corresponding to respectively 7.84%, 9.66%, 13.02%, 19.35% and 27.06% gravimetric soil moisture contents. Results showed that the maximal root pullout force of M. sativa decreased in a power function with increasing soil moisture content from 7.84% to 27.06%. Root slippage rate increased and breakage rate decreased with increasing soil moisture content. At 9.66% soil moisture content, root slippage rate and breakage rate was 56.41% and 43.58%, respectively. The threshold value of soil moisture content was about 9.00% for alfalfa roots in the loess soil. The maximal pullout force of M. sativa increased with root diameter in a power function. The threshold value of root diameter was 1.15 mm, because root slipping force was greater than root breaking force when diameter >1.15 mm, while diameter ≤1.15 mm, root slipping force tended to be less than root breaking force. No significant difference in pullout forces was observed between slipping roots and breaking roots when they had similar diameters. More easily obtained root tensile force (strength) is suggested to be used in root reinforcement models under the condition that the effect of root diameter is excluded as the pullout force of breaking roots measured in pullout tests is similar to the root tensile force obtained by tensile tests.

Key wordsshallow landslides      root reinforcement      soil moisture content      threshold of root diameter      root pullout force      soil conservation     
Received: 16 December 2019      Published: 10 July 2020
Corresponding Authors: Chaobo ZHANG     E-mail:
About author: *Corresponding author: ZHANG Chaobo (E-mail:
Cite this article:

ZHANG Chaobo, LIU Yating, LIU Pengchong, JIANG Jing, YANG Qihong. Untangling the influence of soil moisture on root pullout property of alfafa plant. Journal of Arid Land, 2020, 12(4): 666-675.

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Fig. 1 Alfalfa (Medicago sativa L.) samples planted in containers (a) and root samples (b)
Fig. 2 Diagram of pullout test of alfalfa roots from the cultivation container
Fig. 3 Relationship between soil moisture content and soil matric suction
Fig. 4 Relationships of root slippage and breakage rates with soil moisture content
Fig. 5 Maximal root pullout forces under different soil moisture contents (a) and soil matric suctions (b). Different lowercase letters indicate significant differences among different soil moisture contents and soil matric suctions at P<0.05 level.
Root diameter
Root number of slippage Root number of breakage Root diameter of slippage
Root diameter of breakage
FP of slippage
FP of breakage
D≤1.15 16 20 0.85±0.03a 0.76±0.04a 8.88±0.58a 9.79±0.53a
D>1.15 14 7 1.30±0.05a 1.36±0.05a 17.47±0.70a 18.23±0.92a
Total D 30 27 1.06±0.05a 0.93±0.06a 12.89±0.91a 11.98±0.85a
Table 1 Difference of root diameter (D) and the maximal pullout force (FP) between slippage and breakage of roots under 9.00% soil moisture content
Fig. 6 Relationships between the maximal root pullout forces of slipping and breaking and root diameter under 9.00% soil moisture content
Source Degree of freedom Mean square F P
SMC 4 35.319 32.703 0.008
D 92 14.896 13.793 0.025
SMC×D 15 2.670 2.472 0.248
Error 3 1.080
Total 114
Table 2 Summary of analysis of covariance for root diameter (D) and soil moisture content (SMC)
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