This post was created in cooperation with Paul-Tech
Let’s be honest – the vast majority of farmers care about yield. In good times, yield mattered most and costs less so. Today, yield still matters, but so does achieving it as cheaply as possible.
Yield potential can be significantly influenced by timely nutrient application, especially nitrogen, because it increases leaf area and photosynthesis. Until now, I mainly relied on intuition, fertilization guideline tables, and the variety’s potential when applying nitrogen.
But one question remained: how long does it take for nitrogen fertilizer to become available to plants, how do plants use it, and is there a risk of leaching?
The 24-hour myth and the real timeline
After the first spring nitrogen applications in winter wheat, I realized that the claim “you can see plants turning greener after 24 hours” is about as believable as a psychic prediction.
The 24/25 season – as mentioned in my previous post – had very good growing conditions (at least initially): not too warm and plenty of rain. Logically, the crop should have turned greener the moment the last spreader load of nitrogen hit the field.
To be fair, the plants did become greener fairly quickly, but that was more likely a combination of temperature, sunshine, plant “awakening,” and the farmer’s belief.
In reality, the nitrogen I applied became available to plants only after 5–6 days. Even in ideal conditions, fertilizer takes time to become “plant-ready”.
For me, the important part was that I could actually see the nutrient graph rising – meaning the “money” I spread on the field really reached the soil.
What the graph showed: rise, decline, and uptake
The graph moved upward, but then quickly started to decline as the crop began consuming nitrogen. Around that same time, it was also visible that the crop canopy became denser and greener.
Because the wheat plants were growing vigorously, nitrogen disappeared from the soil quickly. For example, in the Murumäe field, 74 kg N was “consumed” within two weeks. The next 75 kg no longer increased the graph, but it didn’t decrease either.
Why did this happen?
- During the first fertilization, plants were smaller and still “sleepy”, so initial uptake was slow.
- By the second round, growth was already in full swing – everything applied was essentially consumed immediately.
Does spring nitrogen leach?
A lot of water + plenty of nitrogen = leaching? It would be especially painful if fertilizer washed into deeper soil layers (where plants can no longer access it) and eventually into groundwater.
The good news: with spring fertilization I did not observe leaching, at least in the Murumäe field.
In the Veski field, sensors at 8 cm and 20 cm depths reacted similarly to fertilization – the graphs moved in sync. My theory is that there was so much water in the soil that fertilizer moved deeper more quickly instead of remaining in the root zone.
A clear leaching example: Oonurme (25/26 winter wheat)
However, I did clearly observe nutrients being washed into deeper layers in the Oonurme field during the 25/26 season.
As a test, I applied an NPK fertilizer (16 kg N) at the three-leaf stage. The nutrient graph rose rapidly and reached a value of 958 at 8 cm depth.
Then more than 26 mm of rain fell in one day, and the graph dropped by 400 points.
Likely reasons:
- A large rainfall event on soil already close to waterlogging.
- The soil could not retain the nutrients.
- Low temperatures and slow crop growth.
- Insufficient plant biomass to rapidly absorb the nitrogen.
Where I expected to save money
Times are challenging in agriculture – grain prices are too low and costs are too high. In crop production, fertilizers make up the largest share of direct costs (fertilizer, crop protection, seed), so any savings there are economically significant.
I hoped to use soil sensors to reduce nitrogen fertilizer use, but during the 24/25 season it didn’t quite work out as expected. I could see nutrients entering the soil and being consumed, but essentially no matter how much I applied, it was eaten up.
The real benefit came with the following crop. Previously I could roughly estimate how much nitrogen might remain in the soil after harvest, but as the Oonurme example shows, nutrients can also disappear quickly.
Protein fertilization and the “extra round”
During the 24/25 season I added about 20 kg N at the flag-leaf stage because all indicators suggested a strong yield and the premium for food-grade wheat compared to feed wheat looked promising.
The nutrient graph did not move up or down for 40 days. Based on all assumptions, the applied nitrogen seemed to have been consumed. But before grain maturity the graph began to rise again, and I realized that the last nitrogen application may have been unnecessary.
Possible explanations:
- Reduced nitrogen demand by the plants.
- Late nitrification.
- Mineralization of organic nitrogen.
For protein fertilization I used YaraBela Axan 27+4S, where ammonium and nitrate nitrogen are present in a 50/50 ratio. My conclusion: part of the slower ammonium nitrogen was converted to nitrate at a time when the crop was no longer consuming it sufficiently. Root decomposition may also have contributed to the small increase seen in the graph.
A practical decision: no nitrogen needed for straw decomposition
Fortunately, this nitrogen did not disappear after harvest, and considering the following crop I decided that adding nitrogen fertilizer before sowing was unnecessary. Looking at the graph, nutrient levels returned to the pre-fertilization level on 24 December 2025, which confirmed that the decision was correct.
I want to emphasize that not fertilizing (especially with major nutrients) is certainly not sustainable in the long term, and seasons differ greatly. As seen in the Oonurme example, surface-applied nitrogen can disappear after heavy rainfall and it does not come back.
Soil analyses show that phosphorus and potassium levels are good. To survive difficult times, I’m “borrowing from the soil” and will replenish it in better years. Since triticale followed winter wheat in the Murumäe field, the rule of thumb suggests adding about 10 kg N per ton of yield/straw – about 50 kg N in my case. However, I skipped it with confidence because enough nitrogen was already present in the soil for straw decomposition and the triticale’s start.
What new features have been added, what the future looks like, what the biggest weakness of this soil station is, and what I consider the most important aspect – I’ll cover that in the next and final post.
FAQ: nitrogen in soil and leaching risk
How quickly does nitrogen from granular fertilizer become available to plants?
In practice, visible changes based on sensor readings may appear only after several days. In my experience, this was around 5–6 days after application even in good conditions.
Why doesn’t the crop turn green “within 24 hours”?
The visual greening may result from temperature, sunshine, and the start of plant growth rather than rapid fertilizer uptake.
When is the risk of leaching highest?
If soil is already waterlogged, a large rainfall event occurs, and plant growth is slow (cold conditions, low biomass), the risk increases that nitrogen will move below the root zone. In other words: most likely in autumn.
