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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, late, single-season, and double-season rice. In particular, in the southern regions where early rice is planted, and in the cold areas of the northeast, rice seedlings sometimes appear stiff after transplanting. This condition, known as "stiff seedlings" among farmers, is characterized by slow growth, short or delayed emergence of seedlings, and poor development. The causes of this phenomenon are complex, but one common cause is rice blast disease, which can be categorized into three main types.
One of these is **jaggedosis caused by zinc deficiency**. This condition typically appears 2 to 4 weeks after transplanting, peaking around day 20. Symptoms include yellowing at the base of new leaves, brown spots on old leaves, dry leaf tips, and narrow, short new leaves that emerge slowly. Whether the plant is stunted or not, farmers refer to it as "collapsed seedlings." If early rice fields show signs of stiffness, it can significantly impact yield.
The direct cause of this issue is a lack of zinc in the soil. The distribution pattern of affected fields shows that more cases occur in newly cultivated fields (Tiantang) compared to older ones, and in areas like Xinwei rather than traditional rice fields. Additionally, years with low temperatures and frequent rain tend to have more cases, while high-temperature, sunny years have fewer. These conditions suggest that low temperature and low available zinc levels in the soil are key factors. Soils such as calcareous or saline-alkali soils often have low zinc availability, making them prone to deficiency. Overuse of chemical phosphate fertilizers over time also reduces the availability of zinc in the soil, leading to induced zinc deficiency. Lack of organic fertilizer and trace elements like zinc further exacerbates the problem.
Another type of stiff seedling is caused by **potassium deficiency**, which usually appears after the rice turns green, peaking 20–30 days post-transplanting. Symptoms include stunted growth, dark green leaves, few tillers, and yellow-brown to reddish spots on the lower leaves, progressing from tip to base. In severe cases, leaf margins may rupture, leading to death. This occurs when the soil lacks available potassium, especially in sandy or poorly drained soils. Excessive nitrogen application can also disrupt the balance between nitrogen and potassium, worsening the deficiency. Root chilling due to low temperatures can also contribute to this condition.
A third type is **phosphorus deficiency**, which manifests as dark green leaves, purple discoloration on the lower leaves, small and erect leaves, long leaf sheaths, and shortened blades. Seedlings have fewer tillers, brown roots, and no white roots. This is primarily due to insufficient available phosphorus in the soil, especially in cold conditions where low soil temperatures hinder the release of organic phosphorus and limit the movement of inorganic phosphorus toward the root zone.
From analyzing these three types of stiff seedlings, it’s clear that multiple factors contribute to their occurrence. To address the issue effectively, a comprehensive approach is needed—combining symptom-based fertilization with proper management of temperature and moisture in paddy fields.
Symptomatic fertilization is crucial for resolving stiff seedlings. However, it must be based on accurate diagnosis. This involves observing plant symptoms and testing soil nutrient levels, especially for trace elements. For example, zinc deficiency should be confirmed by measuring available zinc in the soil. Once diagnosed, appropriate zinc fertilizers should be selected, applied at the right rate, and evenly distributed to ensure effective treatment. When applying phosphorus fertilizers, choosing the right type and placing them near the root zone is important. In southern rice fields, ordinary calcium phosphate or calcium magnesium phosphate is commonly used. For potassium deficiency, potassium fertilizers—especially those containing potassium oxide—are more suitable than potassium chloride.
By combining scientific diagnosis with targeted fertilization and environmental management, farmers can significantly reduce the occurrence of stiff seedlings and improve rice yields.