Variable rate fertiliser: what you need, what it costs, and whether it stacks up on an Irish farm

You already know fertiliser is your second-biggest input cost after feed. You probably also know you're spreading the same rate across fields that have completely different soil types. That's the problem variable rate technology tries to fix.

But does it actually work on an Irish farm? And can you afford the setup? Here's what we found.

What variable rate fertiliser actually means

Variable rate technology (VRT) adjusts how much fertiliser your spreader puts out — field by field, zone by zone — based on what the soil actually needs. Instead of blanket-spreading 30 units of nitrogen across every hectare, VRT might apply 35 units on a weak patch and 20 on a strong one.

It uses GPS guidance plus a prescription map — a digital plan that tells your spreader "more here, less there." The map comes from soil sampling, satellite imagery, or both.

The idea is simple: put fertiliser where it's needed. Stop wasting it where it's not.

How it works in practice

There are three pieces to a VRT setup:

1. Soil data You need field-by-field soil test results. Not one sample per field — zone sampling, ideally on a 1-hectare grid. This shows where pH, phosphorus, and potassium vary across your land. Most Irish farmers already soil test. VRT needs those results in digital format.

2. A prescription map This is where AI and software come in. Tools like Teagasc NMP Online, or third-party platforms like Gatekeeper or RHIZA, take your soil data and generate a map showing exactly what each zone needs. Some platforms use satellite NDVI data to refine the map further.

Teagasc NMP Online is free through your advisor. It generates nutrient plans based on your soil results, stocking rate, and derogation status. It won't create a VRT prescription map directly, but your advisor can export the data for one.

3. GPS-enabled spreader or controller Your fertiliser spreader needs a way to read the prescription map and adjust the rate on the move. This means either:

• A new spreader with VRT built in (Amazone, Kverneland, and Bogballe all offer this)
• A retrofit controller added to your existing spreader (cheaper, but not all spreaders are compatible)
• A GPS guidance system (Trimble, John Deere, or Topcon) that talks to the controller

This is where the money is.

What it costs

Here's the honest breakdown for an Irish setup:

| Item | Typical cost | Notes | |------|-------------|-------| | Zone soil sampling (1ha grid) | €8–15 per hectare | One-off, repeat every 3–4 years | | Prescription map software | €0–500/year | Teagasc NMP is free; third-party platforms cost more | | GPS guidance system | €2,000–8,000 | Depends on accuracy level (sub-metre vs RTK) | | VRT controller (retrofit) | €1,500–4,000 | Not all spreaders compatible | | New VRT-ready spreader | €15,000–30,000 | Versus €8,000–15,000 for a standard model |

Realistic total for a retrofit on an existing spreader: €4,000–12,000 depending on GPS accuracy.

Realistic total for a full new setup: €20,000–35,000.

Can TAMS help?

Yes. TAMS III includes a precision agriculture category that covers GPS guidance systems, variable rate controllers, and soil sampling equipment. The grant rate is 40% for most farmers and 60% for young farmers.

That means a €10,000 VRT retrofit could cost you €6,000 after TAMS — or €4,000 if you're under 40. Check the current TAMS reference costs list on agfood.ie, as the approved items and ceiling prices change between tranches.

Important: TAMS requires three quotes, and the equipment must be on the approved specification list. Not every GPS system or controller qualifies. Talk to your advisor before buying.

What are the actual savings?

This is where the claims get ahead of the evidence. Here's what the research says:

Precision lime application alone can return €50–100 per hectare over 3–4 years by correcting pH zones that blanket liming misses, according to Teagasc advisory guidance on soil fertility. That's real money on a 40-hectare farm.

Fertiliser savings from VRT are typically 10–15% on nitrogen and up to 20% on P and K, according to European trials. On a farm spending €150/ha on fertiliser, that's €15–30/ha saved per year.

The catch: Most Irish trials show the biggest gains on tillage farms with large, variable fields. On a 30-hectare beef farm with fairly uniform grassland, the savings may not justify a €10,000 setup.

Break-even guide:

| Farm size | Annual saving (est.) | Setup cost (after TAMS) | Break-even | |-----------|---------------------|------------------------|------------| | 30 ha grassland | €600–900/year | €6,000 | 7–10 years | | 60 ha mixed | €1,500–2,400/year | €6,000 | 3–4 years | | 100 ha tillage | €3,000–5,000/year | €6,000 | 1–2 years |

The maths work better the bigger and more variable your land is. For smaller grassland farms, the investment is harder to justify — unless you're already buying a new spreader and the VRT upgrade is marginal.

Where AI fits in

AI is improving VRT in two ways:

Satellite prescription maps. Platforms like EOS Crop Monitoring and Sentinel Hub use AI to analyse satellite imagery and generate variable rate maps without soil sampling every hectare. These are less precise than grid sampling but much cheaper — and they update through the season.

Nutrient modelling. AI tools can combine your soil data, weather forecasts, crop growth stage, and historical yields to generate more precise prescriptions than a static soil test alone. This is still emerging in Ireland but common in the UK and Northern Europe.

Teagasc NMP Online doesn't use AI yet, but it's the foundation. Your nutrient plan from NMP is where VRT starts. AI tools layer on top of it.

Where to get help

• Your Teagasc advisor can generate a nutrient management plan via NMP Online and advise on whether VRT makes sense for your farm size and soil variability
• TAMS III application through agfood.ie — check the precision agriculture category
• Your local machinery dealer can advise on retrofit compatibility with your existing spreader
• Soil sampling labs like Southern Scientific or Eurofins can do zone sampling on a grid

The bottom line

Variable rate fertiliser works — the science is solid and the savings are real on the right farm. But it's not for everyone. If you're farming 60+ hectares with variable soils, the sums stack up quickly, especially with TAMS covering 40–60% of the hardware. If you're on 30 hectares of fairly uniform grassland, you'll wait a long time to see a return. Start with proper zone soil sampling and a conversation with your Teagasc advisor — that's free, and it's where the data starts.

This guide is a starting point. For decisions about grants, animal health, or significant farm investments, always check with your Teagasc advisor or relevant authority.