Choosing CAN or urea under a fixed nitrogen budget
Mr. Otieno farms 1.5 hectares of maize on a sandy loam soil with low organic matter. A soil test recommends 60 kg of nitrogen per hectare. He has KSh 15,000 set aside for nitrogen fertilizer and wants to buy everything at once before prices rise. A nearby agrovet sells CAN (26% N) at KSh 2,400 per 50 kg bag and urea (46% N) at KSh 3,000 per 50 kg bag. Application labour costs KSh 900 per hectare per pass; incorporating fertilizer at planting counts as one pass, and top-dressing later counts as a second. The forecast is for a dry spell followed by heavy rain. Last season Mr. Otieno top-dressed urea at midday on wet soil, noticed a strong ammonia smell, and harvested poorly. The field is 300 m from his homestead, and he has no covered store. He asks whether to buy CAN or urea and how to apply it without exceeding his budget.
1. Calculate the total nitrogen required, the number and cost of bags of CAN and urea needed, and whether each option fits the KSh 15,000 budget before and after application labour.
Read the worked answer
Total N required = 60 kg/ha × 1.5 ha = 90 kg N.
CAN (26% N): product needed = 90 ÷ 0.26 = 346.2 kg. Bags = 346.2 ÷ 50 = 6.92, so he must buy 7 bags (350 kg). Cost = 7 × 2,400 = KSh 16,800. One incorporation pass costs 900 × 1.5 = KSh 1,350, making KSh 18,150 - over budget even before any top-dressing.
Urea (46% N): product needed = 90 ÷ 0.46 = 195.7 kg. Bags = 195.7 ÷ 50 = 3.91, so 4 bags (200 kg). Cost = 4 × 3,000 = KSh 12,000. One pass gives 12,000 + 1,350 = KSh 13,350, within budget. A two-pass split application costs 12,000 + (2 × 1,350) = KSh 14,700, also within budget with KSh 300 left.
Note that 4 bags of urea supply 200 × 0.46 = 92 kg N, slightly above the recommendation but agronomically acceptable; 7 bags of CAN supply 91 kg N. The key difference is concentration: urea delivers the same N in roughly half the product weight, reducing purchase and transport cost.
2. Recommend a fertilizer and application plan for Mr. Otieno, justifying your choice with the calculations and the field conditions.
Read the worked answer
Buy 4 bags of urea and apply it in two splits rather than one heavy dose. Apply 2 bags at planting and incorporate them into moist soil; apply the remaining 2 bags at 4–6 weeks, just before expected rain, and never at midday on a hot, wet surface. Total cost is KSh 14,700, leaving a small contingency.
The reasoning is financial and agronomic. CAN would cost KSh 18,150 for a single pass, which is KSh 3,150 above the budget before the second application. Urea’s 46% concentration makes the required nitrogen affordable. Splitting reduces the risk of leaching after heavy rain on a sandy soil and reduces volatilization losses. The previous season’s ammonia smell shows that surface-applied urea was being lost as gas; incorporation and timing address that directly.
If the dry spell delays the second split, he should wait rather than broadcast on dry soil. He should also store the unused urea in a closed bag under cover, because the homestead has no store and urea absorbs moisture. If he can obtain only one pass of labour, a single incorporated urea application is still better than CAN, but the split is preferred.
3. Explain why last season’s urea application may have lost nitrogen, and identify the practices in your plan that prevent a repeat.
Read the worked answer
Urea is not directly taken up by plants. In moist soil it hydrolyses to ammonium carbonate, and under warm, exposed conditions ammonium can be converted to ammonia gas and lost. Last season’s application at midday on wet soil, with no incorporation, created exactly those conditions; the ammonia smell was visible evidence of loss. A sandy loam with low organic matter has a low capacity to hold ammonium, so heavy rain after a dry spell could also have leached nitrate beyond the root zone.
The plan prevents this in three ways. First, incorporation places urea below the surface, reducing contact with air and sunlight. Second, splitting the dose means no more than 2 bags is exposed at once, and the second dose is timed before rain so it moves into the root zone rather than being washed away. Third, avoiding midday application reduces heat-driven volatilization. The residual risk is that if heavy rain falls immediately after the second application, some nitrogen may still leach; on this soil, a smaller third split would be safer but would exceed the labour budget.