Impact of crop rotation diversity on soil health indicators and economic sustainability

Introduction

Crop rotation has been part of agriculture since Roman times, when Virgil described alternating grain and legume crops [1] . Modern farming pushed many regions toward monocultures, especially in the US Cotton Belt where continuous cotton became standard [2] . The consequences declining soil organic matter, rising pest pressure, and increasing input costs are well documented [3,4] . Soil organic carbon serves as the best single integrator of soil health because it affects water holding capacity, nutrient cycling, aggregate stability, and microbial activity [5,6] . Diversified rotations rebuild organic carbon by varying root architectures and residue quality across seasons [7] . Yet Texas farmers hesitate to diversify because cotton remains the most profitable singleyear crop [8] . Long-term data comparing soil health and economic performance across rotation complexities are scarce for the semi-arid Southern Plains [9] . This research tracked soil organic carbon, microbial biomass, aggregate stability, and net returns over six years across three rotation systems.

Field experimental design

The trial followed a randomized complete block design with four replications at the Texas A & M Agri Life Research farm, College Station (30.6280°N, 96.3344°W). Individual plot size was 12 × 30 m. Three treatments were: T1 (continuous cotton), T2 (cotton-sorghum rotation), and T3 (cotton-sorghum-soybean-wheat rotation). Plots were maintained under dryland conditions with standard fertility recommendations. All crops were planted and harvested using commercialscale equipment. Buffer strips of 3 m separated adjacent plots to prevent root interaction [10] .

Material and Methods

Material: Cotton (variety DP 1646 B2XF), grain sorghum (DKS 37-07), soybean (AG 47X6), and hard red winter wheat (TAM 114) were planted following Texas A&M Extension guidelines. Soil sampling equipment included a Giddings hydraulic probe for cores, insulated coolers for microbial samples, and polyethylene bags. Laboratory reagents for organic carbon analysis (potassium dichromate, ferrous ammonium sulfate) and microbial biomass (chloroform fumigation) were analytical grade [10] .

Methods

Research was conducted from 2018 to 2023 under approval from the Agri Life Research Field Trials Committee (Protocol TAM-AG-2017-041). Soil samples were collected annually in March at 0-15 cm depth, five cores per plot composited. Organic carbon was measured by Walkley-Black wet oxidation [11] . Microbial biomass carbon used chloroform fumigationextraction with a K₂SO₄ extract [12]. Water-stable aggregates were assessed by wet sieving (0.25 mm sieve) using Yoder's apparatus [13] . Economic data included input costs (seed, fertilizer, herbicide, fuel, labor) and crop revenue at local market prices. Net returns were calculated as gross revenue minus total variable costs. Data were analyzed using SAS 9.4 with repeated measures ANOVA and Tukey's HSD (p<0.05).

Seasonal variations

Annual rainfall ranged from 684 mm (2022, drought year) to 1,103 mm (2019). Mean growing season temperatures were 26.1-28.7°C. The 2022 drought reduced cotton yields by 34% across all treatments, but the four-crop rotation showed the smallest decline (27%) compared to monoculture (41%), likely due to better soil water retention from higher organic matter. Monoculture earned more in year 1 but showed declining annual returns (Table 2). The four-crop rotation surpassed monoculture in cumulative net returns from year 3 onward, finishing USD 603/ha ahead after six years. Organic carbon trajectories diverged clearly from year 2 (Figure 1). The economic crossover point where diverse rotations outperformed monoculture appeared between years 2 and 3 (Figure 2), with the gap widening each subsequent year.

Results

Cost analysis

Input costs averaged USD 487/ha for continuous cotton, USD 423 for two-crop, and USD 396 for the four-crop system.

Discussion

The 50.8% increase in soil organic carbon under the four-crop rotation matches findings from long-term trials elsewhere.

McDaniel et al. [15] found in a meta-analysis that adding crops to a monoculture increased soil carbon by 3.6% on average and microbial biomass by 20.7% our results exceeded these, possibly because the semi-arid Texas baseline was lower, leaving more room for improvement [14] .

The economic crossover at year 3 is a key practical finding. Many farmers evaluate rotation on single-year returns, which favors monoculture [16] . But declining continuous cotton returns reflect mounting pest pressure and soil degradation [17] . The 2022 drought amplified these differences the four-crop systems superior water retention buffered yields more effectively [18] . The soybean phase contributed roughly 67 kg N/ha through biological fixation [19] , reducing fertilizer costs and supporting the following wheat crop.

Conclusions

Diversifying crop rotations from continuous cotton to a fourcrop system improved every measured soil health indicator over six years. Organic carbon increased by 50.8%, microbial biomass doubled, and aggregate stability rose by 65%. The diverse rotation earned less initially but overtook monoculture by year 3 and finished USD 603/ha ahead in cumulative returns. The soil health gains translate into both agronomic resilience and economic advantage over the medium term. For Texas cotton producers, a transition period of two to three years is needed before benefits materialize, but the payoff grows substantially afterward.

References

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