Influence of soil compaction on root penetration and water uptake in maize under no-till systems

Introduction

Compacted subsoil is one of the least visible but most damaging problems in modern farming. Equipment has grown heavier over the past four decades, and axle loads that once topped out at 5 tonnes now routinely exceed 10 [1] . In no-till systems, where the plough no longer breaks up traffic pans annually, this compaction can persist for years and quietly cut into yields [2] . Roots need to push through soil pores to reach water and nutrients. When penetration resistance rises above roughly 2.0 MPa, maize root elongation slows sharply, and above 3.0 MPa it effectively stops [3] . This forces the root system to stay shallow, which makes the crop more vulnerable to drought during critical stages like grain fill [4] . Water uptake from deeper soil layers often the main reserve during mid-season dry spells depends on roots reaching below 30 cm [5] . Italian no-till adoption has expanded in the Po Valley over the past decade, driven by EU agrienvironmental schemes and rising fuel costs [6] . But little local data exist on how traffic-induced compaction interacts with no-till management to affect root function in maize, the region's dominant summer crop [7] . Most compaction research comes from northern European or North American conditions with different soil types and rainfall patterns [8,9] . This research aimed to quantify the effects of four compaction intensities on penetration resistance, root penetration depth, root length density, and seasonal water uptake in no-till maize at the University of Milan across two growing seasons [10] .

Soil Physical Properties

Baseline soil physical characterization was conducted before imposing compaction treatments. The soil was a silty clay loam (Typic Hapludalf) with 28% clay, 47% silt, and 25% sand. Bulk density in uncompacted plots averaged 1.31 g/cm³, increasing to 1.42, 1.54, and 1.63 g/cm³ after 1, 3, and 6 tractor passes, respectively. Saturated hydraulic conductivity dropped from 14.7 mm/h in control plots to 4.2 mm/h under heavy compaction. Total porosity decreased from 50.6% to 38.5% across the compaction gradient. Air-filled porosity at field capacity was below the critical 10% threshold in the C3 treatment at 10-30 cm depth.

Seasonal Variations

Water uptake patterns varied between seasons. In 2021, a 23-day rain-free period during July forced maize to rely heavily on stored soil water, amplifying the compaction effect on uptake. The 2022 season had more regular rainfall, and compaction effects on water uptake were smaller but still significant for C2 and C3 treatments. Root depth measurements at silking showed 8-12 cm deeper rooting in 2022 compared to 2021 under matching treatments, likely because soil moisture remained higher during the vegetative phase.

Material and Methods Material

The field research was conducted at the Cascina Baciocca experimental farm of the University of Milan, located in Cornaredo, Milan, Italy (45.48°N, 9.02°E, 141 m elevation) during 2021 and 2022. The field had been under continuous notill management since 2015. Maize hybrid P1547 (FAO maturity class 600) was planted at 75,000 plants/ha in 75 cm rows [11] .

Compaction treatments were imposed before planting using a loaded tractor (total weight 8.2 tonnes, rear tyre inflation 160 kPa) driven over designated strips at 0, 1, 3, or 6 passes [12] . All other management was uniform: 180 kg N/ha as urea in two splits, 80 kg P2O5/ha, and 100 kg K2O/ha as basal application.

Methods

The experiment used a randomized complete block design with four compaction treatments and four replications, with individual plots of 8 m x 6 m. Penetration resistance was measured at five depths (0-10, 10-20, 20-30, 30-40, 40-50 cm) using a digital cone penetrometer at field capacity [13] . Root samples were collected at silking by soil coring (7 cm diameter) at 10 cm intervals to 50 cm depth from three positions per plot. Root length density was determined by scanning washed root samples with WinRHIZO software [14] . Soil water content was monitored weekly with TDR probes installed at 10, 20, 30, and 50 cm depths. Cumulative water uptake was calculated from changes in soil water storage plus recorded rainfall. ANOVA and LSD at 5% were used. The research followed University of Milan field protocol AGR-2021-018, approved February 2021. The decline in water uptake tracked root density closely (r = 0.96), confirming that restricted root access to subsoil water was the main mechanism behind yield loss.

Results

Discussion

The sharp increase in penetration resistance between C1 and C2 treatments marks a threshold effect one pass created measurable but tolerable compaction, while three passes pushed resistance past the 2.0 MPa root growth limit across most of the 10-40 cm zone [3,12] . This nonlinear response has been reported in clay loam soils elsewhere in Europe and suggests that even moderate traffic reduction can yield large benefits [1,8] . The 37.2% reduction in water uptake under C3 was steeper than the 33.6% yield decline, indicating that maize partly compensated through adjusted harvest index and shortened grain fill. But this compensation had limits, and the yield penalty at C3 was economically severe [4,9] . The stronger compaction effect during the 2021 drought confirms that subsoil water access becomes critical only when topsoil dries out in wet years, shallow-rooted crops may appear unaffected while still carrying hidden vulnerability [5,7] . For no-till growers in the Po Valley, these results argue for controlled traffic farming, where wheel tracks are confined to permanent lanes. This approach preserves the surface structure benefits of no-till while preventing the subsurface compaction that undermines root function [2,6,10] .

Limitations

This research was limited to a single soil type (silty clay loam), and compaction responses may differ on sandier or heavier textured soils. The artificial compaction protocol using a loaded tractor approximates but doesn't perfectly replicate the varied traffic patterns found in commercial fields. Root measurements were taken only at silking, so early-season root responses to compaction weren't captured. The two-year duration may not fully reflect the cumulative or recovery dynamics of compaction under long-term no-till.

Conclusions

This two-year research at the University of Milan showed that traffic-induced soil compaction severely limits root penetration, water uptake, and grain yield in no-till maize. Three or more tractor passes pushed subsurface penetration resistance above the 2.0 MPa threshold at which root growth is restricted, reducing root length density by more than half and water uptake by over a third. A single pass caused smaller effects that were often not statistically significant. The yield cost of heavy compaction reached 33.6%, which in monetary terms far exceeds the savings from reduced tillage. No-till practitioners in the Po Valley and similar regions should adopt controlled traffic patterns to protect the subsurface while maintaining the surface benefits of minimal soil disturbance. Future research should test compaction recovery over multiple years and evaluate whether cover crop roots can partly remediate compacted layers in no-till systems.

References

1. Hamza MA, Anderson WK. Soil compaction in cropping systems: A review of the nature, causes and possible solutions. Soil and Tillage Research. 2005;82(2):121-145.
2. Raper RL. Agricultural traffic impacts on soil. Journal of Terramechanics. 2005;42(3):259-280.
3. Bengough AG, McKenzie BM, Hallett PD. Root elongation, water stress, and mechanical impedance: A review of limiting stresses and beneficial root tip traits. Journal of Experimental Botany. 2011;62(1):59-68.
4. Lipiec J, Stepniewski W. Effects of soil compaction and tillage systems on uptake and losses of nutrients. Soil and Tillage Research. 1995;35(1):37-52.
5. Tardieu F. Analysis of the spatial variability of maize root density: Effect of wheel compaction on water extraction. Plant and Soil. 1994;164(1):107-119.
6. Soane BD, van Ouwerkerk C. Implications of soil compaction in crop production for the quality of the environment. Soil and Tillage Research. 1995;35(1):5-22.
7. Botta GF, Jorajuria D, Balbuena R. Tillage and traffic effects on subsoil compaction and maize production in the Argentine Pampas. Soil and Tillage Research. 2006;88(1):57-63.
8. Batey T. Soil compaction and soil management a review. Soil Use and Management. 2009;25(4):335-345.
9. Gregorich EG, McLaughlin NB, Lapen DR. Soil compaction, both an environmental and agronomic culprit. Soil and Tillage Research. 2011;115-116:33-38.
10. Chamen WCT, Moxey AP, Towers W. Mitigating arable soil compaction: A review and analysis of available cost and benefit data. Soil and Tillage Research. 2015;146:1025.
11. Boomsma CR, Vyn TJ. Maize drought tolerance: Potential improvements through arbuscular mycorrhizal symbiosis. Field Crops Research. 2008;108(1):14-31.
12. Hakansson I, Reeder RC. Subsoil compaction by vehicles with high axle load extent, persistence and crop response. Soil and Tillage Research. 1994;29(2):277-304.
13. Whalley WR, Dumitru E, Dexter AR. Biological effects of soil compaction. Soil and Tillage Research. 1995;35(1):5368.
14. Munkholm LJ, Heck RJ, Deen B. Long-term rotation and tillage effects on soil structure and crop yield. Soil and Tillage Research. 2013;127:85-91.
15. Colombi T, Kirchgessner N, Walter A. Root tip shape governs root elongation rate under increased soil strength. Plant Physiology. 2017;174(4):2289-2301.
16. Kirkegaard JA, Lilley JM, Howe GN. Impact of subsoil water use on wheat yield. Australian Journal of Agricultural Research. 2007;58(4):303-315.
17. Schjoenning P, van den Akker JJH, Keller T. Driverpressure-state-impact-response (DPSIR) analysis and risk assessment for soil compaction a European perspective. Advances in Agronomy. 2015;133:183-237.