Molecular Soil Biology 2025, Vol.16, No.5, 272-286 http://bioscipublisher.com/index.php/msb 274 2.3 Overview of the adaptability of legumes on major soil types The growth performance of legume crops on different soil types is significantly different. In general, legumes are most suitable for loam environments with deep soil layers, good permeability and medium to high fertility. Under these conditions, the root system can fully penetrate and establish an effective symbiosis with rhizobia, thereby achieving high and stable yields (Li et al., 2024). On heavy clay soils, due to poor ventilation, the root system of legumes is susceptible to waterlogging, and improved tillage is needed to improve water permeability to adapt to this environment. However, the rich nutrient supply of clay is beneficial to legume crops. Practice has shown that soybeans and other crops can show high yield potential on improved black clay soil (Zhang et al., 2021). In poor sandy soil environments, legume crops often grow well during emergence and seedling stages, but are prone to drought and nutrient deficiency in the middle and late stages, resulting in reduced yields. At this time, water and fertilizer conservation measures can enhance their adaptability. Some barren-resistant legume varieties (such as mung beans and peas) have strong adaptability to sandy soils, short growth periods and relatively low nutrient requirements, but water and fertilizer management still needs to be strengthened to achieve high yields. Agronomic measures such as crop rotation and intercropping can also help legumes improve their adaptability in different soils. For example, intercropping legumes with grasses can improve soil nitrogen cycle efficiency and overall productivity. Therefore, selecting tolerant varieties and adjusting cultivation systems according to soil types are important means to unleash the potential of legume crops ( Zhang et al., 2019). 3 Soil Factors Limiting Legume Growth 3.1 Soil pH and rhizobium activity Soil pH directly affects the symbiotic nitrogen fixation efficiency of legume crops and rhizobia. Most rhizobia are most active in soils close to neutral, while their activity is significantly inhibited in strongly acidic soils with a pH below 5.5. Acidic soils often make it difficult for legume nodules to form and reduce nitrogenase activity, and the number and diversity of rhizobia are also significantly reduced (Han et al., 2020; Abulfaraj & Jalal, 2021). For example, studies have shown that when soil pH drops from 6.8 to 5.0, the number of soybean nodules and the amount of nitrogen fixed per plant drop significantly by more than 50% (Kollie and Semu, 2022; Lai et al., 2024). Acidic conditions not only produce aluminum and manganese toxicity that hinders root growth, but also inhibit the infection and colonization process of rhizobia, leading to symbiotic failure. On the other hand, too high pH (>8.0) in calcareous soil is not conducive to the growth of some legumes, and may cause a deficiency of trace elements such as iron and zinc, which in turn affects leaf photosynthesis and nitrogen fixation. However, compared with acid resistance, rhizobia are slightly more adaptable to weak alkaline environments, and some legumes such as alfalfa can still nodulate and fix nitrogen in soils above pH 7.5. Excessive acidity or alkalinity will limit the efficiency of the legume-rhizobium symbiotic system, and it is necessary to optimize its growth environment by adjusting the soil pH. Applying lime is a traditional measure to improve acidic soils, which can raise the pH to the appropriate range for legumes, thereby significantly increasing the number of nodules and nitrogen fixation. At the same time, inoculation of acid-tolerant rhizobia or combined use with growth-promoting bacteria has also been shown to alleviate the effects of acid stress on nitrogen fixation to a certain extent (Alemneh et al., 2020). Therefore, reasonable regulation of soil pH is one of the primary measures to ensure efficient nitrogen fixation and normal growth of legume crops. 3.2 Effects of drainage and aeration on root health The root system of legume crops is very sensitive to soil oxygen supply. An overly humid and oxygen-deficient environment will lead to restricted root growth and obstructed respiration. In severe cases, root rot and premature aging of plants will occur. When heavy clay soils and low-lying areas are overly waterlogged, the nitrogen fixation activity of legume nodules almost stops, and the plants may show symptoms of nitrogen starvation. Studies have shown that soybeans are extremely sensitive to field waterlogging. Waterlogging for more than 10 days during the growth period can cause yield losses of up to 60% (Figure 1) (Gangana Gowdra et al., 2025). Excessive water causes soil pores to be filled with water, reducing the oxygen diffusion rate, and the root system is metabolically disordered due to lack of oxygen, resulting in growth point necrosis and root hair shedding. At the same time, the anaerobic state will also lead to increased soil reductiveness and the production of reducing
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