Nitrogen Acidity & Lime It Back to Neutral
Offsets urea
Nitrogen fertilizers acidify the soil as ammonium nitrifies — enter your annual N rate and fertilizer type to get the lime (CaCO₃ equivalent) needed to neutralise that acidity and hold your pH.
Enter your N program
Next: budget 216 kg/ha of lime to hold pH steady against this N program, or factor it into your maintenance liming schedule.
Lime-equivalent factors are typical CaCO₃ values per kg N; actual acidification also depends on nitrate leaching and crop removal. Switching to CAN or nitrate forms cuts the lime burden.
Fertilizer acidification — key facts
- Lime needed
- N rate × lime equivalent
- Urea
- ≈ 1.8 kg CaCO₃ per kg N
- Ammonium sulphate
- ≈ 5.35 kg CaCO₃ per kg N
- DAP
- ≈ 3.6 kg CaCO₃ per kg N
- CAN
- ≈ 1.0 kg CaCO₃ per kg N
- Cause
- ammonium nitrifies, releases H⁺
- Purpose
- annual offset to hold pH
- Privacy
- Runs in your browser; nothing uploaded
Every bag of nitrogen quietly pushes your pH down
When ammonium-based nitrogen nitrifies, soil bacteria release hydrogen ions and the pH drifts lower year after year. The effect is real and cumulative: a high-nitrogen field left unlimed slowly slides out of the range where phosphorus, molybdenum and the crop itself perform best. The fix is to budget a small annual lime application that neutralises exactly the acidity your fertilizer adds, so the pH you worked to build stays put.
This tool turns your nitrogen rate and fertilizer choice into the lime (CaCO₃ equivalent) that offset costs. Use it to plan a maintenance liming programme, to compare how much cheaper a CAN or urea programme is to neutralise than ammonium sulphate, and to add to any raise-the-pH dose your soil test calls for. Pair it with the Lime from Base Saturation and Lime Incorporation Depth tools for a complete liming plan.
Hold your pH
Neutralise exactly the acidity your N adds each year.
Compare fertilizers
See why ammonium sulphate costs far more lime than CAN.
Budget maintenance lime
Turn an N programme into an annual lime bill.
Stack with raise-pH lime
Add this offset on top of any pH-correction dose.
Frequently Asked Questions
How does this calculator work out the lime needed?+
It multiplies your annual nitrogen rate by the lime equivalent of the fertilizer you use. Each fertilizer has a published acidifying value in kilograms of calcium carbonate (CaCO₃) per kilogram of nitrogen — urea ≈ 1.8, ammonium sulphate ≈ 5.35, DAP ≈ 3.6, CAN ≈ 1.0. Lime needed (kg CaCO₃/ha) = N applied × that lime equivalent, so 100 kg N as urea calls for about 180 kg of lime a year.
Why do nitrogen fertilizers acidify the soil?+
Most nitrogen is applied as, or converts to, ammonium. When soil bacteria nitrify ammonium to nitrate they release hydrogen ions (H⁺), and those ions are what lower the pH. Crops also take up nitrate and leave acidity behind. The acidifying effect is real and cumulative, so without periodic liming a heavily fertilized field drifts steadily more acidic.
Which fertilizer acidifies the most?+
Ammonium sulphate is by far the strongest acidifier at about 5.35 kg of lime per kg of N, because the sulphate adds acidity on top of the nitrification. DAP (≈3.6) is next, then urea and ammonium nitrate (≈1.8), with CAN (≈1.0) the gentlest of the common straights because it carries its own calcium and magnesium. Choosing the fertilizer changes your lime bill markedly.
Is this lime in addition to lime for raising pH?+
It is a maintenance figure to hold the pH where it already is, by offsetting the acidity your nitrogen adds each year. If your soil is already too acidic you also need a separate lime requirement to raise the pH to target. Add the two together: the raise-pH dose first, then this annual offset to stop it dropping again.
What is lime equivalent or CaCO₃ equivalent?+
It is the amount of pure calcium carbonate that exactly neutralises the acidity produced. Quoting acidity in CaCO₃ equivalents lets you compare fertilizers and liming materials on one scale. If your lime is not pure (most field lime is 80–95% effective), divide the result by your material's neutralising value to get the actual product weight to spread.
How often should I apply this offset lime?+
Acidification accumulates every season, so the simplest approach is to apply the annual offset each year, or batch two to three years' worth into a single spread. Lime is slow to act, so a bigger less-frequent application is usually fine. Re-test soil pH every two to three years to confirm you are keeping pace and adjust the rate if your nitrogen programme changes.
Does the result depend on my soil type?+
This figure is the chemistry of neutralising the acid the fertilizer creates, so it is the same regardless of texture. Soil type matters for how fast the pH actually moves and how much buffering reserve acidity you have — sandy soils swing faster, clays resist change. Use this offset to budget lime, and a buffer-pH or base-saturation test to fine-tune on heavy soils.
Are these figures precise?+
They are solid planning figures based on published lime equivalents for each fertilizer. Real acidification varies a little with leaching, crop offtake and how much nitrate is lost versus taken up. Treat the result as a working annual estimate, confirm it against soil-pH trends over a few years, and lean toward the higher side if you run a high-nitrogen programme.