It’s the final installment in our series on the Four R’s of Nitrogen Management and this week we are focusing on the Right Timing. Applying nitrogen (N) when crops can best access it is based on the N uptake timing for each crop.

This practice of targeting optimal N uptake timing is fairly common-practice in corn production in BC via side-dressing, but not utilized as much in other vegetable crops. Utilizing this one aspect of The Four R’s presents a big opportunity for growers to improve their nitrogen use efficiency!

What does N uptake timing look like in vegetable crops?

There are generally three phases of N uptake across the growing season:

Phase I: Slow early growth with slow N uptake, for approx. the first 20 to 40 days after seeding or transplanting.

Phase II: Rapid vegetative growth with rapid N uptake, for approx. the next 30 to 60 days.

Phase III: Maturity or reproductive phase with slow to no N uptake near the time of harvest. In some crops (e.g. sweet corn, squash, potato), N is redistributed from leaves to the seeds, fruit, or tubers after flowering. In other crops (e.g. broccoli, lettuce, celery), harvest actually occurs before Phase III is even reached.

“S-Curve” of N uptake:

These phases result in an “S” shape of N uptake over time (Figure 1), with Phase II representing the middle vertical part of the “S” – in other words, Phase II is shown as having the most N uptake over the shortest period of time.

Optimizing N uptake:

For optimal N uptake and yield, N must be adequate to support growth during every growth stage of the crop.

How does this translate into N application timing?

Ideally, we are applying the amounts of N needed according to each phase of N uptake, with a particular focus on the first two phases.

Phase I: Very small amounts of N (e.g. less than 30 lb/acre) are required during this phase.

Phase II: Crop N demand is greatest during this phase.

Phase III: Very small to zero amounts of N are required during this phase.

Plants as indicators of phases:

Fertilizing at these phases can be accomplished by tracking the plant stage. For example, the ideal time to fertilize for Phase II N uptake in potatoes is at tuber initiation. This is because tuber initiation is the growth stage before tuber bulking, and tuber bulking is the period of greatest N demand in potatoes. Tuber initiation can be determined by: digging up plants to track stolon development (enlargement to double the normal stolon diameter) or by tracking flower development (approximately 10-14 days before flowering).

Some methods that can be used to target different phases of N uptake over time include:
  • Split-applications
  • side-dressing
  • Fertigating
  • Gradual-release fertilizers
Common scenario in BC vegetables

All of the N that a crop will need through the season is typically applied at one time: pre-planting or at-planting. This means that most of the N must “wait” for the crop’s period of high N uptake (Phase II) – the longer the window for potential loss of N from the root zone, the greater risk of wasted N inputs.