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Symptomatic fertilization to prevent rice seedlings
China has a vast area dedicated to rice cultivation, with a wide variety of rice types grown from south to north, including early, mid, and late-season rice, as well as single- and double-crop varieties. In particular, in the southern regions, early rice often faces challenges after transplanting, while in the cold climate of the northeast, rice seedlings may develop a condition known as "stiff seedlings." This phenomenon is characterized by slow growth, short or stunted seedlings, and poor establishment, which can significantly impact yield if not addressed promptly.
Stiff seedlings are caused by multiple factors, but one of the most common is nutrient deficiency, particularly zinc, potassium, and phosphorus. Among these, zinc deficiency is especially prevalent and can lead to a condition called "jaggedosis." Symptoms typically appear 2–4 weeks after transplanting, with the most severe cases occurring around day 20. Affected plants show yellowish-white discoloration at the base of new leaves, brown spots on older leaves, and dry tips on lower leaves. The newly emerged leaves are short, narrow, and grow slowly. Whether the plant is dwarfed or not, farmers refer to this condition as "collapsed seedlings," which can severely reduce productivity, especially in early rice fields.
The primary cause of zinc deficiency is insufficient availability of this essential micronutrient in the soil. Factors that contribute to this include low temperatures, high rainfall, and poor soil conditions such as calcareous or saline-alkali soils, which naturally have low levels of available zinc. Long-term overuse of chemical phosphate fertilizers can also deplete soil zinc, leading to induced deficiencies. Additionally, the lack of organic matter and trace elements like zinc in paddy fields exacerbates the problem.
Potassium deficiency, another major cause of stiff seedlings, usually appears after the rice plants start to green up, peaking 20–30 days post-transplanting. Symptoms include stunted growth, dark green leaves, and fewer tillers. The lower leaves gradually turn yellow-brown or reddish from the tip to the base, with streaks forming across the leaf surface. In severe cases, the leaf margins may rupture, leading to plant death. This condition often occurs in sandy or poorly drained soils where potassium is not readily available. Excessive nitrogen application or root chilling can also contribute to potassium imbalance.
Phosphorus deficiency, on the other hand, manifests through dark green leaves, purple discoloration on the lower leaves, and small, erect leaves with long sheaths. Seedlings show reduced tillering, brown roots, and no white roots. Phosphorus is critical during the seedling stage, and low soil temperatures can hinder its availability. Cold environments slow down the release of organic phosphorus and limit the movement of inorganic phosphorus to the root zone, making it difficult for plants to absorb enough.
To address these issues, a comprehensive approach is necessary. Farmers must first identify the specific nutrient deficiency based on visual symptoms and soil analysis. Symptomatic fertilization—applying the right nutrients in the correct amounts—is key. For zinc deficiency, measuring available zinc in the soil is crucial before applying appropriate zinc fertilizers. For phosphorus, using the right type of fertilizer and placing it near the root zone can improve uptake. For potassium, choosing suitable fertilizers like potassium chloride and applying them effectively can help restore balance.
In addition to fertilization, managing temperature and moisture in the field plays a vital role in preventing stiff seedlings. A combination of proper nutrition and environmental control can significantly improve rice growth and yield, ensuring healthier and more productive crops.