Reasons for Yield Reduction in Continuous and Alternate Soybean Cropping and Yield-Preserving Techniques

I. Reasons for Yield Reduction in Continuous and Alternate Soybean Cropping

Continuous and alternate soybean cropping causes plants to grow slowly and become short, with yellowing leaves, fewer pods, and fewer seeds, ultimately leading to a significant reduction in yield. In the short term, continuous and alternate cropping intensifies the damage from cyst nematodes, root maggots, root rot, pod borers, gray leaf spot, and malignant weeds, causes unbalanced depletion of soil nutrients, leads to deficiencies in soil nutrient elements, increases pathogenic fungi and decreases beneficial bacteria in the soil, and produces certain toxins, resulting in a severe decline in soybean yield and quality. In the long term, after long-term accumulation and reproduction, once the density of diseases, insects, and weeds reaches a certain level, it will cause total crop failure of soybeans and harm the land for many years. The longer the continuous cropping period, the more severe the harm to soybean yield and quality.

II. Yield-Preserving Techniques for Continuous and Alternate Soybean Cropping

1. To choose suitable plots for planting continuous and alternate soybean crops well, plot and rotation selection is key. The principles of "plant alternate crops rather than continuous crops, choose fertile land rather than lean land, choose low-lying land rather than upland, and avoid disease and pest areas" should be followed in selecting suitable plots. First, under the same conditions, "plant alternate crops rather than continuous crops" should be achieved, and the traditional concept of "rather plant continuous crops than alternate crops" must first be abandoned. Continuous and alternate cropping should be treated differently: continuous cropping is monocropping, while alternate cropping is rotation in alternate years. The yield reduction caused by continuous and alternate cropping differs; alternate cropping reduces yield far less than continuous cropping. Alternate soybean cropping reduces yield by about 10%, while continuous soybean cropping reduces yield by about 30%. Therefore, "prefer alternate cropping over continuous cropping" should be followed, and continuous cropping for more than two years is strictly prohibited. Second, under conditions of different terrain, different soil types, and different fertility levels, low-lying, fertile black soil plots should be selected. Saline-alkali soil, albic soil, sandy loam soil, and plots with low organic matter content should not be planted with continuous or alternate crops. Third, in plots with severe disease and pest problems, especially plots with obvious damage from cyst nematodes, sclerotinia, and dodder, continuous and alternate cropping should be avoided.

2. When selecting disease-resistant varieties, integrated control techniques combining agricultural measures with biological and chemical measures should be considered. The key is to ensure proper seed selection. Based on local accumulated temperature, soil, and pest and disease occurrence characteristics, varieties with strong growth vigor, relatively luxuriant growth, preference for fertilizer and water, resistance to pests and diseases, and strong stress resistance should be selected, and seed chemical treatment should be done well to reduce the severity of pest and disease damage.

3. Scientifically increase the application of two types of fertilizer Continuous and alternate cropping causes single-nutrient depletion in soybean fields, resulting in an imbalance of nutrient elements in the soil and an improper ratio of nitrogen, phosphorus, and potassium, thus causing yield reduction. Therefore, supplementing and regulating soil nutrients is an important technical measure to reduce the harm of continuous and alternate cropping. Farmyard manure and chemical fertilizer should be combined, increasing the amount of farmyard manure; nitrogen, phosphorus, potassium, and micronutrient fertilizers should be combined, increasing the amounts of trace elements such as potassium, zinc, copper, and molybdenum, and soybean multi-element compound fertilizer should be used. The specific amounts should be determined according to soil fertility, cultivation level, and other conditions. Generally, for soil of average fertility, 10 kg of diammonium phosphate and 5 kg of potassium fertilizer per mu should be used as seed fertilizer applied in layers and deeply, and 5 kg of urea should be used as topdressing. In poor and thin land and plots with poor growth, 0.75-1 kg of urea per mu can be sprayed diluted with water at the flowering stage. For plots using multi-element compound fertilizer, while applying 20 kg of compound fertilizer per mu, 2.5 kg of potassium fertilizer should be applied. In plots with severe potassium deficiency, large amounts of ash and plant ash fertilizer can be applied, and at the same time, 0.15 kg of monopotassium phosphate per mu can be sprayed diluted with water at the flowering stage, and sprayed again after 7-10 days. The application methods for trace elements can use foliar spraying or seed treatment. The common amounts for seed dressing with micronutrient fertilizers are: borax, 0.4 g per 1 kg of soybean seed; ammonium molybdate, 0.5 g per 1 kg of soybean seed; zinc sulfate, 5 g per 1 kg of soybean seed.

4. Strengthen pest and disease control To control underground pests such as root maggots, 2 kg of phorate per mu or 4-5 kg of 3% carbofuran granules can be mixed into fertilizer and applied; to control aboveground pests such as soybean pod borers, aphids, red spiders, and meadow moths, corresponding agents such as dichlorvos, deltamethrin, and abamectin can be selected respectively; to control diseases, corresponding agents such as carbendazim, thiophanate-methyl, and agricultural streptomycin can be selected respectively; to control dodder, 50 ml of 48% dinitroaniline emulsifiable concentrate per mu can be diluted with water to a 150-200 times dilution and sprayed at the flowering stage.