Many farmers are torn between using Urea and CAN for topdressing. Some believe urea will trigger their crops to grow extraordinarily tall. Other farmers believe that combining the two will give a maximum yield. But to know whether you should use CAN or urea for topdressing, you need to understand two things. First, the overall goal of topdressing, and second, how differently CAN and Urea can help meet that goal.
Topdressing involves adding a thin layer of nutrient source to provide the nutrients your plants require at that stage. On many occasions, farmers top dress with a nitrogenous fertilizer. Many crops require nitrogen during their vegetative growth in quantities that may not be available in the soil.
If you are topdressing to provide nitrogen to your crops, the excellent sources available are UREA and CAN. So, is CAN better than Urea for top dressing or vice versa? You won’t be wrong to use any of them. However, each choice has its pros and cons. Several factors and considerations should guide your decision to use one over the other.
Nitrogen Composition
Urea contains 46% N while CAN has 26% N. Meaning if you were to apply 100kg of Urea, the soil would have a resource of 46kg of N. Applying a similar amount of CAN would mean the soil would have an N resource of 26kg.
But when you apply any of the two fertilizers, volatilization losses occur i.e., change of chemicals from solid into vapor. Although Urea has a higher amount of Nitrogen, it is highly volatile, especially in certain conditions. CAN is also a culprit of volatilization but the risk is lower compared to UREA. Note that half of the nitrogen in CAN is in the nitrate form while the remaining amount is in the ammonium N form. On the other hand, the 46% nitrogen in Urea is entirely in the ammonium form.
When rain is not guaranteed, you may want to avoid topdressing using UREA. It hydrolyzes and gets lost in the atmosphere if there are no rains to help in the dissolution and absorption of the granules in the soil within 24-48 hours after application. That doesn’t mean it is purely safe to apply CAN when there is no rain. Only that CAN has a lower nitrogen volatilization risk.
Impact on Soil pH
If you are concerned with soil acidity, you may want to use CAN over Urea. CAN has a near-neutral effect on soil pH.
Ammonium (NH4+) has a positive charge while nitrate (NO3–) has a negative charge. Plant roots release an ion with a similar charge to the charged ion they take up in the soil. This helps maintain a balanced pH in plant cells.
When plants take up ammonium ions, they release a hydrogen ion (H+). On the other hand, an uptake of the nitrate ion triggers a release of a hydroxide ion (OH–). The hydroxide ion combines with a hydrogen ion in the soil to form water, thus doesn´t contribute to soil acidity. Remember, soil acidification occurs when hydrogen cations build up in the soil.
Urea VS CAN on Maize
One acre fund conducted a trial to compare maize yields when Urea and CAN are applied at the rate of 123.5kg per hectare and 247kg per hectare. The trial used SC Duma 43 maize variety planted at a spacing of 25cm by 75cm and weeded twice.
A standard one-acre fund maize program was used as the control experiment. The program involved planting the SC Duma 43 maize variety at a spacing of 25cm by 75cm, weeding twice, and topdressing with 123.5kg per hectare of CAN. The yields were 3.35 tonnes per hectare in the standard program.
One acre fund discovered an 11.6% increase (to reach 3.79 tonnes/ha) in yield by doubling the amount of CAN used to top dress. In the block where the company used 123.5kg/ha of urea, the yields were 3.72 tonnes/ha (9.9% higher than the standard program). A double amount of the urea resulted in a yield of 3.79 tonnes/ha, 11.6% higher than the control experiment. Therefore, using urea instead of CAN would be effective at 123.5 kg/ha level. The only downside would be the soil acidification that would result from urea application.
Choosing Between CAN and Urea
Topdressing using either CAN or Urea has its merits and drawbacks. A farmer will need to consider the local weather patterns, soil conditions, and crop-specific requirements to make informed decisions.