Field Crop 2025, Vol.8, No.6, 258-264 http://cropscipublisher.com/index.php/fc 261 fully supplied, there will be more pods, larger seeds, and naturally higher yields (Si et al., 2022). Like other oil crops, soybeans respond very well to fertilization. The more pods, the higher the yield. In addition, management measures such as crop rotation and timely topdressing can also help increase yields (Assefa et al., 2019). 5.2 Improvements in protein and oil content The quality of soybean seeds mainly depends on protein and oil content. Studies have shown that if part of organic fertilizer is used, combined with appropriate nitrogen fertilizer management, the protein content of seeds can be increased. Sometimes, whether it is organic nitrogen or chemical nitrogen, as long as there is more nitrogen fertilizer, the protein content will increase, but this may reduce the oil content, because the two sometimes affect each other (Digrado et al., 2024). However, in general, if the fertilizer is properly matched, not only can the protein content be increased, but the oil content will not decrease too much, so that the nutritional value and selling price of the seeds can be maintained. 5.3 Effects on seed safety (nitrate, heavy metals) Soybean safety is also very important, especially harmful substances such as nitrates and heavy metals. Although many studies mainly look at yield and quality, there are also data showing that if too much chemical fertilizer is used, nitrates and heavy metals may accumulate in the soil or seeds (Maity et al., 2023). Relatively speaking, good use of organic fertilizers can reduce such problems. As long as the source of organic fertilizers is safe, coupled with regular inspections of soil and seed quality, soybeans can be guaranteed to be both safe and high-yielding. 6 Regional Trials and Practical Applications 6.1 Yield increase under partial substitution in Northeast China In Northeast China, a study found that soybean yields have been rising in recent years, increasing by about 1.68% each year. This increase is mainly due to better management, such as the use of conservation tillage, more reasonable planting methods, and often the use of organic + chemical fertilizers. Studies in the Northeast Agricultural Region also pointed out that without expanding the planting area, as long as the planting methods are improved, such as replacing chemical fertilizers with part of organic fertilizers, soybean yields can be increased by up to 60%. Among them, the Songliao Plain is considered to have the greatest potential for yield increase (Zhao et al., 2023). Variety improvement and improved field management (such as more scientific fertilization) also make each plant have more pods, and the yield naturally increases (Figure 2) (Zhang et al., 2023). Figure 2 Spatial distribution of the national unified soybean variety testing (NUSVT) sites between the period of 2006 and 2020 across China (a). Effects of the interactions of cultivars, sowing dates and nitrogen inputs on soybean performance in the field experiment (b, photograph by the authors). NEC, HHH, and SMR represent Northeast China, Huang-Huai-Hai Plain, and Southern Multi-cropping Region, respectively. The background image represents the harvested soybean area in 2010, which was obtained from the Spatial Productional Allocation Model provided by International Food Policy Research Institute (https://www.mapspam.info/). The field experiment represented the interactions of 2 cultivars, 3 sowing dates, and 4 nitrogen managements (Adopted from Zhang et al., 2023)
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