Optimal cultivation measures for maize production in the drylands of the Loess Plateau

Experimental sites

The experiment was conducted from April 2022 to October 2024 at the National Soil Quality Observation Experimental Station (35°30′N, 107°29′E) of the Dryland Farming Institute, Gansu Academy of Agricultural Sciences. The area is located at an altitude of 1,279 m, with the soil being black loessial soil. The organic matter and total nitrogen content are 10.7 g kg⁻¹ and 0.91 g kg⁻¹, respectively, while the available nitrogen, available phosphorus, and available potassium contents are 91.3 mg kg⁻¹, 11.8 mg kg⁻¹, and 228.3 mg kg⁻¹, respectively, indicating medium fertility. The average annual precipitation is 533.4 mm, with 60% of the rainfall occurring between July and September, making it a typical rain-fed dryland farming area. The monthly rainfall distribution across different years is shown in Fig. 1.

Experimental materials

The experimental material is the maize variety Xianyu 1483, with a 100-kernel weight of 37.55 g, a fertility period of 128 days, and a planting density of 4,500 plants hm⁻². It was provided by the Institute of Dryland Agriculture, Gansu Academy of Agricultural Sciences.

Experimental design

The experimental treatments consisted of three factors at five levels (coded as −1.682, −1, 0, 1, 1.682). The corresponding levels for each factor were as follows: Organic fertilizer: M4: 2,220, M1: 3,570, M2: 6,000, M3: 8,250, M5: 9,780 kg hm⁻². Nitrogen fertilizer: N4: 23.85, N1: 75, N2: 150, N3: 225, N5: 276.15 kg hm⁻². Planting density: D4: 42,270, D1: 52,500, D2: 67,500, D3: 82,500, D5: 92,730 plants hm⁻². According to the rotatable orthogonal experimental design, which is an experimental design method used to study systems with multiple factors and multiple levels. It selects a subset of representative experimental points from the full factorial experiments based on the principle of orthogonality. These representative points are characterized by uniform distribution across the experimental domain and balanced comparability, ensuring both efficiency and statistical validity in analyzing factor effects. A total of 23 treatments were arranged. For operational convenience, treatments 15–23 were combined, resulting in 15 plots. Each plot measured 20 m × 4.8 m = 96 m², with a spacing of 0.5 m between plots. All treatments were conducted under wide plastic film mulching, with a film width of 1.2 m and four mulched planting strips per plot. The phosphorus application rate was 120 kg hm⁻² (P₂O₅). The coded levels of the experimental factors are presented in Table 1, while the specific experimental design and implementation plan are detailed in Table 2.

Experimental methods

Field preparation and plastic film mulching were carried out in early April during 2022–2024. Seeds were sown using a dibbler in mid-April, with harvesting conducted in late September. For yield assessment, a representative sampling area with uniform plant growth was selected from each plot. Within this area, 30 plants were systematically sampled row by row, with three replicates per plot. After threshing, grain moisture content was determined using a grain moisture meter (PM-8188-A), and the yield per unit area was calculated at a standardized moisture content of 14%.

Statistics analysis

The experimental data were organized, statistically analyzed, and subjected to variance analysis using Microsoft Excel 2010 (Microsoft, Redmond, WA, USA) and SPSS 27.0 (IBM Corp, Armonk, NY, USA) statistical analysis software. Graphs were created using Origin 2021 software.

Modelling and testing

Based on the experimental results (Table 3), a regression model of yield (Y) with organic fertilizer (X₁), nitrogen fertilizer (X₂) and density (X₃) was established.

Y₂₀₂₂ = 15,115.402+450.621X₁+507.927X₂+1,660.474X₃+127.222X₁²–287.767X₂²–1,013.066X₃²–147.271X₁X₂+109.674X₁X₃+167.111X₂X₃

Y₂₀₂₃ = 15,173.730+294.886X₁+275.302X₂+1,586.006X₃–34.898X₁²–368.183X₂²–933.130X₃²–10.690X₁X₂+62.085X₁X₃+71.170X₂X₃

Y₂₀₂₄ = 14,845.102+248.890X₁+478.774X₂+1,379.610X₃–251.469X₁²–586.648X₂²–1,028.403X₃²–179.584X₁X₂–56.099X₁X₃+194.096X₂X₃.

The regression coefficients, total regression coefficients and lack of fit of the equations were tested separately by F-test to obtain the ANOVA table (Table 4). The ANOVA results showed that the total regression coefficients of equations Y₂₀₂₂, Y₂₀₂₃ and Y₂₀₂₄ reached the highly significant level (P < 0.001), and the lack of fit did not reach the significant level (P > 0.05), which indicated that the models of equations Y2022, Y2023 and Y2024 were valid, and the regression equations were reasonably and reliably fitted. Finally, the optimised regression equations were obtained after the ANOVA test to eliminate insignificant terms (P > 0.05).

Y’₂₀₂₂ = 15,115.402+450.621X₁+507.927X₂+1,660.474X₃–287.767X₂²– 1,013.066X₃² +109.674X₁X₃+167.111X₂X₃

Y’₂₀₂₃ = 15,173.730+294.886X₁+275.302X₂+1,586.006X₃–368.183X₂²–933.130X₃²

Y’₂₀₂₄ = 14,845.102+248.890X₁+478.774X₂+1,379.610X₃–251.469X₁²–586.648X₂²–1,028.403X₃²–179.584X₁X₂+194.096X₂X₃.

Factor main effects analysis

The contribution rate of each factor to yield was calculated based on the F value of each treatment, and the effect of each factor on yield in each year could be obtained by calculating the contribution rate of each factor to maize yield in different years (Table 5). The effects of organic fertilizer, nitrogen fertilizer, and density on maize yield exhibited significant interannual variations. Quantitative analysis of the 2022–2024 growing seasons revealed a consistent hierarchy of contributing factors: density > nitrogen fertilizer > organic fertilizer. Meanwhile, the contribution of density to maize yield was much greater than that of fertilizer.

One-way effects analysis

Dimensionality reduction method was used to study the effect of organic fertilizer, nitrogen fertilizer and density on maize yield, fixing the level of other factors at zero, the effect of a single factor on maize yield could be obtained. Fixing the nitrogen fertilizer and density factors at zero level gives an equation for the effect of organic fertilizer on maize yield:

Y₂₀₂₂ = 15,115.402+450.621X₁

Y₂₀₂₃ = 15,173.730+294.886X₁

Y₂₀₂₄ = 14,845.102+248.890X₁–251.469X₁².

Similarly, the equation for the effect of nitrogen fertilizer on maize yield:

Y₂₀₂₂ = 15,115.402+507.927X₂–287.767X₂²

Y₂₀₂₃ = 15,173.730+275.302X₂–368.183X₂²

Y₂₀₂₄ = 14,845.102+478.774X₂–586.648X₂².

Similarly, the equation for the effect of density on maize yield:

Y₂₀₂₂ = 15,115.402+1,660.474X₃–1,013.066X₃²

Y₂₀₂₃ = 15,173.730+1,586.006X₃–933.130X₃²

Y₂₀₂₄ = 14,845.102+1,379.610X₃–1,028.403X₃².

From the equation, a one-factor effect plot was obtained (Fig. 2). Different factors and levels had different effects on maize yield and, also, varied among years. In 2022 and 2023, maize yield increased with increasing organic fertilizer content. In 2024, maize yield showed an increasing and then decreasing trend with increasing organic fertilizer content, and maize yield in 2024 was significantly lower than the other two years. This may be related to the rainfall in that year, where sufficient moisture effectively dissolved the nutrients of organic fertilizer in the soil, which in turn contributed to the increase in yield. The maize yield showed a trend of decreasing with increasing nitrogen fertilizer content and planting density. The highest maize yield was recorded at 225 kg hm⁻² of N fertilizer in 2022, while the highest maize yield was recorded at 150 kg hm⁻² of N fertilizer in 2023 and 2024. All planting densities were highest at 82,500 plants hm⁻². It indicates that the appropriate nitrogen fertilizer and density can help to increase maize yield, while too high nitrogen fertilizer application and planting density will rather inhibit the formation of yield.

Analysis of intercropping effects

The results of ANOVA showed that significant intercropping existed among the crosses. Organic fertilizer-nitrogen fertilizer, organic fertilizer-density and nitrogen fertilizer-density intercropping all affected maize yield (Table 4). The equations for the intercropping effects of organic fertilizer, nitrogen fertilizer, density and yield were obtained using the downscaling method.

Equation for the effect of organic fertilizer-nitrogen fertilizer interactions on yield:

Y₂₀₂₂ = 15,115.402+450.621X₁+507.927X₂+127.222X₁²–287.767X₂²–147.271X₁X₂

Y₂₀₂₃ = 15,173.730+294.886X₁+275.302X₂–34.898X₁²–368.183X₂²–10.690X₁X₂

Y₂₀₂₄ = 14,845.102+248.890X₁+478.774X₂–251.469X₁²-586.648X₂²–179.584X₁X₂.

Equation for the effect of organic fertilizer-density interactions on yield:

Y₂₀₂₂ = 15,115.402+450.621X₁+1,660.474X₃+127.222X₁²–1,013.066X₃²+109.674X₁X₃

Y₂₀₂₃ = 15,173.730+294.886X₁+1,586.006X₃–34.898X₁²–933.130X₃²+62.085X₁X₃

Y₂₀₂₄ = 14,845.102+248.890X₁+1,379.610X₃–251.469X₁²–1,028.403X₃²–56.099X₁X₃.

Equations for the effect of nitrogen fertilizer-density interactions on yield:

Y₂₀₂₂ = 15,115.402+507.927X₂+1,660.474X₃–287.767X₂²–1,013.066X₃²+167.111X₂X₃

Y₂₀₂₃ = 15,173.730+275.302X₂+1,586.006X₃–368.183X₂²–933.130X₃²+71.170X₂X₃

Y₂₀₂₄ = 14,845.102+478.774X₂+1,379.610X₃–586.648X₂²–1,028.403X₃²+194.096X₂X₃.

The organic fertilizer-N fertilizer interaction equation yielded a plot of the interaction effect (Fig. 3). The highest yield of maize was obtained when organic and nitrogen fertilizers were at higher levels. In all three years, when the organic fertilizer content was at a high level, the yield showed a trend of increasing and then decreasing with increasing nitrogen fertilizer content. In 2022 and 2023, yield increased with increase in organic fertilizer when nitrogen fertilizer content was higher, whereas in 2024, yield showed an increasing and then decreasing trend with increase in organic fertilizer. It showed that the interaction of organic and nitrogen fertilizer had a synergistic effect on yield at both organic and nitrogen fertilizer levels.

The organic fertilizer-density interaction equation yielded a plot of the interaction effect (Fig. 4). The best yield was obtained only when both organic fertilizer and density were at high levels. At low planting densities, the effect of increasing organic fertilizer content on yield improvement was not significant, indicating that planting density had a greater effect on maize yield. Meanwhile, in all the three years, when the organic fertilizer content was constant, the yield showed an increasing and then decreasing trend with increasing planting density. In 2022 and 2023, when planting density was constant, yield increased with increase in organic fertilizer, while in 2024, yield showed an increasing and then decreasing trend with increase in organic fertilizer.

Nitrogen fertilizer-density reciprocal equations yielded a plot of reciprocal effects (Fig. 5). The highest maize yield occurred at higher levels of nitrogen fertilizer and density, indicating a synergistic effect of nitrogen fertilizer and density on yield. In the three years, when the planting density was constant, the yield showed a trend of increasing and then decreasing with the increase of nitrogen fertilizer content. Meanwhile, when nitrogen fertilizer application was constant, yield also showed a trend of first increase and then decrease with increasing planting density. This indicates that both excessive nitrogen fertilizer application and planting density are not conducive to the formation of maize yield.

Analysis of the optimisation of cultivation measures

With maize yield as a function of the target, the integrated agronomic measures can be optimised by the statistical frequency method to derive the target. Considering the production conditions, climatic conditions, and soil conditions in the Loess Plateau, a maize yield of 15,000 kg hm⁻² can be positioned as a high-yield target. A total of 24 treatments were calculated to have theoretical yield higher than 15,000 kg hm⁻² in the experiment, and the frequency of occurrence of the 24 treatments at the level of each factor was counted. It can be seen that the highest frequency of yield higher than 15,000 kg hm⁻² occurred when the coded levels of organic fertilizer, nitrogen fertilizer and density were zero. By calculating the distribution of 95% confidence intervals, stable organic and nitrogen fertilizer application rates and planting densities could be optimised, i.e., organic fertilizer application rates of 6,429.30–7,895.10 kg hm⁻², nitrogen fertilizer application rates of 159.30–203.55 kg hm⁻², and planting densities of 72,465–80,940 plants hm⁻² resulted in high maize yield (Table 6).

Importance of density on yield

The Loess Plateau, as one of the major areas of dryland food production in China, provides an important guarantee for national food security. Maize is the main food crop in the region, and its yield is the result of a synergy of planting density, nutrient management and environmental conditions (Jia et al., 2018). Planting density directly affects population structure and resource use efficiency and is the most important factor affecting yield (Hou et al., 2020). This was also confirmed in this study, with the effect on maize yield: density > nitrogen fertilizer > organic fertilizer. Increasing planting density is one of the most direct and effective cultivation measures to improve crop yield. Moderate densification can improve the growth rate and dry matter accumulation of maize in the pre-reproductive period by improving the canopy light interception rate and productivity, but too high a density is not conducive to the accumulation of dry matter in the post-reproductive period, which is prone to cause a decline in the growth rate of the population. At the same time, over-densification will increase the competition among plants for light, water and nutrients, leading to individual stunting and increased risk of failure, which will ultimately affect the formation of yield (Han et al., 2015; Liu et al., 2017; Tokatlidis & Koutroubas, 2004). In this study, maize yield increased with increasing density when planted below 82,500 plants hm⁻². However, when the density exceeded 82,500 plants hm⁻², the yield decreased instead with the increase of planting density. Therefore, while increasing planting density, soil fertility and hydrothermal conditions, etc., need to be considered.

Effect of water-fertilizer synergy on yield

Water and nutrients are the main influences on crop development and yield formation in arid regions. Insufficient precipitation inhibits nutrient uptake and insufficient nutrients reduce water use efficiency, both of which are detrimental to crop yield formation. (Mingde, 2019; Pinheiro & Chaves, 2011). In this study, maize yields were significantly higher in 2022 and 2023, when rainfall was more abundant, than in 2024. Under the organic fertilizer treatment, maize yield tended to increase as the amount of organic fertilizer applied increased. This is due to the fact that organic fertilizer addition improves soil structure, enhances water and fertilizer retention capacity and slow release of nutrients, which contributes to kernel filling and resilience in the later stages of maize fertility (Gathorne-Hardy, 2016; Haque et al., 2015). And with the increase of nitrogen fertilizer application, the maize yield showed a trend of first increase and then decrease. This may be due to the fact that nitrogen addition can quickly replenish key nutrients for crop growth and promote the accumulation of dry matter in maize. Whereas, excessive nitrogen application can cause problems such as vigorous nutrient growth, soil acidification and organic matter loss, which ultimately lead to a reduction in maize yield (Marques et al., 2017; Okalebo et al., 1999).

Effect of organic fertilizer, nitrogen fertilizer and density interactions on yield

Reasonable fertilizer application and planting density are key to improving crop yields. Density and fertilizer application interactively influence root spatial distribution. Under high nitrogen conditions, roots tend to concentrate in shallow soil layers, whereas organic fertilizer promotes deep root development, effectively mitigating water competition (Ma & Zheng, 2018). Fertilizer supply at high planting densities can effectively alleviate problems such as unbalanced nutrient supply to maize and fertilizer removal at the later stages of fertility (Lai et al., 2022). At the same time, organic fertilizers increase the abundance of ammonia-oxidising bacteria by providing a carbon source to stimulate microbial activity, promote organic nitrogen mineralisation, and form a slow-release complementary effect with chemical nitrogen fertilizers to improve nitrogen deficiency under high density conditions. The combination of organic fertilizer and nitrogen fertilizer can increase bioproductivity and reduce leaching losses, playing an important role in achieving high-yield and green agriculture (Li et al., 2024). This study showed that organic fertilizer-nitrogen fertilizer intercropping, organic fertilizer-density intercropping and nitrogen fertilizer-density intercropping all had significant yield enhancement in maize. It is noteworthy that the interaction effect of the three yield increases was significant. For example, at medium densities, organic- nitrogen fertilizers are most effective in increasing yields because of a well-structured population and a balanced supply of nutrients. In contrast, high-density planting may result in lower yields if it is not supported by adequate amounts of fertilizer (Workayehu, 2000). Combined with the actual environmental conditions and production conditions in the Loess Plateau, the optimal range of each cultivation measure was obtained when the yield was higher than 15,000 kg hm⁻²: the application of organic fertilizer was 6,429.30–7,895.10 kg hm⁻², nitrogen fertilizer was 159.30–203.55 kg hm⁻², and the planting density was 72,465–80,940 plants hm⁻². Considering the arid climate characteristics of the Loess Plateau, the combination of moderate planting density + slow-release nitrogen fertilizer + organic fertilizer can be adopted, along with appropriate moisture-conserving and rainfall-harvesting cultivation measures (such as double ridge-furrow mulching, straw mulching, etc.), to achieve high maize yields. The results are reliable and can provide theoretical basis and practical reference for maize production in the Loess Plateau.