What is the difference between N surplus, Purchased N surplus and N leaching? – Overseer Knowledge base

Putting the three N numbers in context

Metric	What it measures	Why it matters
N surplus	All nitrogen that comes onto the farm (fertiliser, imported feed, irrigation water plus clover/legume fixation) minus nitrogen that leaves in products (milk, meat, wool, crops, exported effluent, sold supplements). Reported as kg N/ha/yr. Modelled in OverseerFM to include the clover/legume fixation.	Straight-up efficiency score – how much of the N you invested in actually ends up in saleable product. The lower the surplus, the tighter the conversion. Includes biological N fixed by clover, so it captures the whole N story, not just what you paid for. A high surplus is a red flag that excess N is parked in soil or livestock excreta – a pool that can later be lost.
Purchased N surplus	Same calculation but it only counts the N you buy (fertiliser, imported feed). It ignores biologically fixed N.	Easy to understand because it follows the cheque book. Blind spot: on farms that utilise clover, purchased surplus can look great while total surplus – and potential losses – are still high. That’s why we prefer total N surplus for decision-making.
N leaching	Modelled estimate of the N that actually drains below the root zone and can enter groundwater and waterways. Factors in soil type, rainfall/irrigation, urine-patch dynamics, fertiliser timing, etc.	Shows the environmental loss, not just the in-system balance. Often the headline number councils and processors look at. Let’s you test “what-if” scenarios (e.g. lower N fertiliser rate, change winter crop) and see the predicted drop in losses.

Why focus on total N surplus and N leaching?

Double-lens view.

N surplus tells you how efficiently the system is using all sources of N; N leaching tells you how much of the leftover N is escaping from the root zone.
Put together, they show both the cause (inefficiency) and the effect (loss from the root zone).

Actionable insight.

If surplus is high and leaching is high, you know the priority is to lift efficiency and plug specific loss pathways (e.g. reduce autumn N, widen effluent area).
If surplus is high but leaching is modest, you’re still carrying unnecessary cost and risk – trimming inputs or lifting production will improve profit and buffer future regulation.
If surplus is low but leaching is high, the issue is likely soil, climate, or grazing management rather than input levels – focus on timing, paddock selection, wet-breaks, stand-off pads, etc.

Fair benchmarking.
Purchased N surplus ignores clover N, so legume-rich farms can appear to be performing better than in reality. Total N surplus levels the playing field and when utilised alongside N leaching can be used as the double-lens view detailed above.

Regulatory alignment.
Councils increasingly require both an efficiency indicator (surplus or N-conversion efficiency) and a loss metric (leaching). Tracking both now keeps you ahead of the curve.

Bottom line:
Monitoring total N surplus to run a lean, cost-effective system, and also watching N leaching to make sure any excess N isn’t slipping through the cracks is the most effective way of managing farm efficiency and understanding potential losses to the environment.

Purchased N surplus is handy for a quick check on bought-in inputs, but it shouldn’t drive the big decisions.

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