Nitrogen Rate on Irrigated Potatoes with a Leaching Risk
A grower near Virginia irrigates 40 hectares of potatoes under centre pivot on sandy soil with low water-holding capacity. The crop is at 30 days after planting and the agronomist has recommended 220 kg N/ha in total. So far 90 kg N/ha has been applied. Soil tests show 25 mg/kg nitrate-N in the top 30 cm, and a leaching rain of 45 mm is forecast within four days. Deep soil sampling to 90 cm shows nitrate has already moved below the root zone in wheel tracks. The grower's nitrogen budget assumed 60% recovery of applied N. The processor requires a minimum tuber size, and late nitrogen tends to delay maturity and increase the risk of tuber rots if the crop is still green at harvest.
1. Decide how to adjust nitrogen application before and after the forecast rain, and explain the reasoning.
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The key issue is that nitrogen applied just before a 45 mm rain on sandy, low-water-holding soil will largely leach below the root zone, so the timing matters more than the total rate.
The grower should:
- Not apply the remaining nitrogen in the four days before the rain. Any urea or nitrate applied now would move with the wetting front, and the deep sampling already shows nitrate below the root zone, so the soil has a proven leaching pathway.
- Wait until after the rain and re-test. The 45 mm event will dilute and displace nitrate, so a post-rain soil test to 60–90 cm is needed before deciding the next rate.
- Recalculate the budget using realistic recovery. If 60% recovery is assumed, 90 kg N/ha applied has delivered about 54 kg N/ha to the crop, so the crop is behind the 220 kg/ha target even before leaching losses. The remaining requirement is therefore larger than the simple 130 kg/ha difference, but the leached fraction should be replaced cautiously because over-application also leaches.
- Split the remaining nitrogen into two or three small applications with irrigation scheduled to avoid heavy rain, and stop nitrogen by the date that protects maturity. Late nitrogen that keeps the crop green risks tuber rots and delayed maturity, which is a quality and harvest problem, not just a cost problem.
The best answer weighs leaching risk against crop demand and the processor's maturity constraint, and uses post-rain soil testing to reset the budget rather than applying a fixed remaining rate.
2. The grower proposes switching to a slow-release nitrogen product to solve the leaching problem. Evaluate this proposal.
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Slow-release products can reduce leaching by matching release to crop uptake, but they are not a complete solution here.
Strengths: on sandy soil with low water-holding capacity, a controlled-release product reduces the peak of nitrate in the soil at any one time, so a 45 mm rain event has less mobile nitrate to displace. That directly addresses the observed problem of nitrate moving below the root zone. It also reduces the number of passes and the risk of a single badly timed application.
Limitations: release rate depends on soil temperature and moisture, so in a cold or very dry period release may be too slow to meet early demand, and in a hot wet period it may release faster than expected. The cost per kilogram of nitrogen is higher, which matters on a 40-hectare pivot where the margin per hectare is thin. Slow release does not fix over-application: if the total rate exceeds crop demand, the excess still leaches eventually. It also does not remove the need for post-rain soil testing or for stopping nitrogen in time to protect maturity.
A defensible judgement: trial slow-release nitrogen on a strip of the pivot and compare soil nitrate at depth, tuber size and yield against the standard program before converting the whole 40 hectares. Combine it with split application and irrigation scheduling rather than treating it as a substitute for good timing.