Yellowing leaves on corn seedlings and wilting after fertilizing: what should you do?
Corn tastes sweet and can be used in all kinds of dishes, such as corn pancakes and corn juice. It is also the main raw material for industrial alcohol and shochu, so corn seedlings are planted in large quantities across both northern and southern China. So why do the leaves of corn seedlings turn yellow? And what should you do if they wilt after fertilizing?
1. Yellowing leaves on corn seedlings can be caused by the following:
1. Sowing too deep. Sowing too shallow makes it hard for seedlings to emerge, while sowing too deep produces weak, yellow seedlings. Sowing depth should be kept at 3-5 cm.
2. Not thinning and spacing the seedlings in time. After corn emerges, thin the seedlings when the corn has 3-4 leaves and set the final spacing when it has 6-7 leaves, to avoid crowding of the young plants and competition for fertilizer, water and light, which forms weak, diseased and yellow seedlings.
3. Insufficient watering. If watering is insufficient before or after sowing corn, the seeds cannot get enough moisture, which affects normal germination and emergence; the longer time to emerge results in weak seedlings and yellowing at the seedling stage.
4. Waterlogged seedlings. The seedling stage of waterlogged corn often coincides with the rainy season; poor drainage in low-lying plots or water accumulating in places compacted during wheat harvest causes yellow seedlings.
5. Disease. If the seedling stage meets long periods of low temperature and rainy weather, it can cause the occurrence and spread of corn seedling blight. Seedling blight mostly starts at the 2-3 leaf stage; diseased seedlings have yellow, dry leaves with scorched edges, and the leaves gradually turn yellow and dry from bottom to top; there are fewer root hairs, the root system turns brown and develops poorly, resulting in yellow, weak seedlings.
6. Insect pests. The main insect pests at the corn seedling stage include cotton bollworm, wireworm, aphids, armyworm, thrips, Swedish frit fly, cutworm, and the mealybug Phenacoccus solani. The mealybug Phenacoccus solani gathers as nymphs and female adults at the base of the stem near the soil surface, on the roots and inside the leaf sheaths of corn seedlings, sucking sap, causing the leaf sheaths of the affected corn to turn yellow and dry first.
7. Herbicide damage. Improper use of herbicides, arbitrarily increasing the herbicide dosage; blindly mixing with other pesticides; using too high a concentration; sharing sprayers; and the effect of counterfeit or substandard herbicides on the following crop can all cause yellow corn seedlings.
8. Nutrient deficiency. Corn is a zinc-sensitive crop; zinc deficiency can produce white-flowered seedlings that look like yellow corn seedlings.
2. Wilting of corn seedlings is caused by ammonia gas released from the decomposition of urea.
Because the corn seedlings are still small, with tender leaves close to the ground, and because a large amount of urea is applied on the soil surface, in the hot season watering only a small amount of surface water cannot leach the granular urea deep into the soil; it can only dissolve a large amount of urea at the surface. Calcareous soil is alkaline, and the dissolved urea molecules decompose relatively quickly at high temperatures, producing large amounts of ammonia and carbon dioxide, which causes seedling fumigation.
In this situation, plots with the necessary conditions should be watered first, then cultivated and earthed up. This can both dilute the nitrogen fertilizer and aerate the soil to promote root growth. For plots without the conditions for irrigation, immediately carry out deep cultivation followed by earthing up; this can both retain nitrogen and reduce ammonia volatilization, and also remove soil moisture to encourage roots to grow downward. Case note: urea is a high-concentration nitrogen fertilizer; when used as a topdressing, the amount applied at one time should be appropriate. Calcareous soil is alkaline, so both urea and ammonium nitrogen fertilizers used as topdressing should be covered with soil.