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Reasons and Prevention of Rice Seedling "Frozen Seedling"
China has a wide variety of rice cultivation, with different types of rice grown across the country from south to north, including early, mid, and late-season rice, as well as single- and double-crop varieties. In some regions, such as the southern areas where early rice is cultivated and the northeastern parts where cold-tolerant rice is planted, rice seedlings may sometimes appear stiff after transplanting. This condition, known as "stiff seedling" among farmers, is characterized by slow growth, short or delayed emergence of leaves, and overall weak development. The causes behind this issue are complex and can vary depending on environmental and nutritional factors.
One common cause of stiff seedlings is zinc deficiency. This typically occurs 2–4 weeks after transplanting, peaking around day 20. Symptoms include yellowish or white discoloration at the base of new leaves, brown spots on older leaves, and shortened, narrow new leaves. Even if the plants are not stunted, they are still referred to as "collapsed seedlings." Zinc deficiency is often linked to low soil temperatures and poor availability of zinc, especially in calcareous or saline-alkali soils. Excessive use of phosphate fertilizers over time can also reduce zinc availability, making it harder for rice to absorb this essential nutrient.
Another cause is potassium deficiency, which usually appears after the rice starts to green up and peaks between 20–30 days post-transplanting. Affected plants show stunted growth, dark green leaves, and fewer tillers. The lower leaves develop yellow-brown or reddish-brown streaks, and severe cases may result in leaf margins breaking and dying. This is often due to low available potassium in sandy or poorly drained soils, or because of excessive nitrogen application that disrupts the balance between nitrogen and potassium.
Phosphorus deficiency is another factor contributing to stiff seedlings. Symptoms include dark green new leaves, purple discoloration on older leaves, and small, upright leaves. The roots become short and brown, with few white roots. Phosphorus deficiency is often linked to cold soil conditions, which hinder phosphorus release and movement toward the root zone. Since the seedling stage is critical for phosphorus uptake, any disruption during this period can lead to long-term issues.
To address these problems, farmers need to diagnose the specific nutrient deficiency accurately. This involves observing plant symptoms and analyzing soil nutrients. For example, zinc deficiency should be confirmed through soil testing before applying zinc fertilizer. Similarly, phosphorus and potassium deficiencies require careful selection of fertilizers and application methods. In general, using calcium or calcium-magnesium phosphate in southern regions and applying potassium chloride in paddy fields are effective strategies.
In addition to targeted fertilization, managing temperature and moisture levels in the field is crucial. Combining these approaches helps ensure healthy seedling development and prevents the occurrence of stiff seedlings. By understanding the underlying causes and taking timely action, farmers can significantly improve rice yields and quality.