In a paddy trial, 75kg/acre urea application resulted in short, weak and blackish roots while 25Kg/acre urea resulted in longer and healthy roots. what could be the possible reasons?
Root pathogens like Fusarium can be affected by soil pH and N application. First you need to discover the cause of this problem.
Endorse the comment by Lucas Braos. Another possible cause(s) for the observed symptoms may also be explored. Similar symptoms also develop in rice roots due to sulfide toxicity. However, the roots symptoms are usually accompanied by interveinal chlorosis of young leaves due to Fe deficiency, as the situation impairs Fe uptake by plants. Sulfide toxicity is most probable in coarse textured soils with low Fe, which match to most soils in Punjab. Besides, it may be induced through straw incorporation with high nitrogen fertilizer application. Few links provided below may give additional clues to solve the problem:
http://www.knowledgebank.irri.org/index.php?option=com_zoo&task=item&item_id=846&Itemid=782
http://ucce.ucdavis.edu/files/repositoryfiles/ca5702p55-69060.pdf
Previous discussions in this forum on similar topic may also be seen:
https://www.researchgate.net/post/What_is_the_effect_of_excess_sulphur_on_rice_plant/1
In general, rice roots are blackened due to the following reasons:
1. Attacked by fungal group mainly Pythium spp or Fusarium spp
2. Sulfide toxicity
3. Nematode attack
4. Depressed metabolism of Rice roots.
Now, you have to find out the exact reason in your case..
Madhu Dhingra, Could you send some photographs of the blackened roots and more detailed description. In addition do you have any information on the soil reaction and crop history? Alll of these will be helpful in diagnosing the condition and for suggesting some counter measures. Thanks Paul Reed Hepperly
Milan and Paul bring up blackening of rice roots related to anaerobic decomposition which mobilizes toxicity from metals such as Al Mn and Fe. The publication from California suggests this became an issue with the prohibition of burning. Decomposition of plant residue can cause toxicity but if it is allowed to decompose aerobically this does not last after 40 days. This type of metal toxicity is related to acidity as the heavy metals need acidity to become soluble. Using urea at 75 kg/ha would create some sizable amount of acidity. You may want to have a soil analysis to see what your soil pH is. If your soil pH is acidic to start the use of nitrate sources would provide N without generating acidity which would lead to metal toxicity which could blacken the roots. If your soil is acid nitrate would help the pH reaction rather than contribute to metal toxicity. I would suggest soil analysis would be recommendable to help devise better management to prevent issues.
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