Hill Labs Organic Amendment Planner
A planning tool for NZ organic growers, enter your Hill Labs soil or media test, pick your crop, and get a complete organic amendment plan: how much N to add from meals (OSU PAN model, your region's soil temperature and rainfall-leaching), plus the mineral side from the test (lime, phosphate, potassium, calcium, magnesium, sulphur). A built-in phosphate budget keeps the P that comes bundled in your organic N sources from pushing you over your crop's total P target.
What this is for: getting the best out of your crop, comparing sources, sizing a base dressing, and seeing roughly how much N your inputs will actually deliver this season.
What it is not: it is not a regulatory or council tool, a nutrient budget, or compliance evidence, use OverseerFM or a certified nutrient adviser for anything a council needs. Every figure is a planning estimate with real uncertainty; the soil test is the truth. Don't blindly top up to a target, and confirm with an in-season soil nitrate test before adding more.
The values loaded are a worked example, change every field to match your own situation.
Site & season
Soil temperatures are NIWA 1991–2020 normals, modelled to monthly values. Rainfall & drainage drive the leaching estimate.
For Existing soil N, the test type matters: use a Mineral N (nitrate + ammonium) result for the current snapshot, that one is safe to use alongside the organic-matter box below. If you instead enter an AMN / Available-N result, do not also tick organic-matter N (both estimate the same mineralisable pool, you'd double-count). Enter the kg/ha value; if your report is in ppm or mg/kg, multiply by ~2 for a 0–15 cm sample first. A Hill Labs Available-N result is already in kg/ha.
Your Hill Labs test optional · drives the mineral plan + phosphate budget
Enter your Hill Labs report. Soil = Basic Soil Profile (me/100g, Olsen P). Media = Media General (M1) test (mg/L, living-soil / potting mix). The pre-filled low/high ranges are typical land-use defaults, type your report's own printed Medium Range into the boxes (or hit Scan) for exact targets.
Nitrogen sources
N% & P% defaults are typical book / Seacliff values, overwrite with your product label or lab. The P% feeds the phosphate budget.
Bulk organics, compost, manure & effluent
These carry N differently from meals: part is already mineral (available now), the rest releases slowly on the C:N ratio. Add one; it feeds the totals below and obeys the same temperature, leaching and placement settings. Add up to two, e.g. a green crop and a compost, both come off your N total.
Available N this season
likely range · most-likely value shown below
| Source | Total N | PAN % | Final PAN |
|---|
Garden-bed amounts
Turns the per-hectare rates into how much product to weigh out for a bed, for market gardens, not just paddocks.
| Source | At your rate | To hit target |
|---|
Amounts per area don't change with working depth, you weigh out the same product whether you fork it into 10 or 15 cm; depth changes the soil concentration, not the dose. The soil-N pools in this tool assume a 15 cm (0–150 mm) working layer.
📋 Complete amendment recipe every product, every scale, on top of your compost & meals
💲 Mineral prices, $ per 25 kg sack, editable
Starting values are indicative NZ retail per 25 kg sack (mid-2026: AB Lime bagged rate ex-works; Egmont dolomite + GST; Daltons gypsum at Bunnings; Sprayshop Epsom incl GST; Farmlands SOP pro-rated from the 20 kg bag; NTS soft rock 25 kg). Edit these to your real quotes, bulk / bulk-bag is far cheaper per kg if you're buying tonnes. Blank = that product isn't costed. Freight & spreading not included.
Timing & splitting
Whether to apply it all at planting or split it across the season.
Plain-language glossary
- PAN, plant-available nitrogen
- The share of a material's total N a crop can actually take up this season. The rest is locked in organic matter and releases later (or not at all).
- C:N ratio
- Carbon to nitrogen, from a lab test. Low (under ~15) feeds N; around 20–25 is break-even; above ~25 the material temporarily "ties up" soil N (drawdown) as microbes use it to chew through the carbon.
- Mineral N (ammonium + nitrate)
- N that's already in the plant-available form, available straight away. On a soil test it's the current snapshot; on a compost test it's the small "available now" portion.
- AMN / Available-N test
- A soil test (e.g. Hill Labs) estimating the N your soil will mineralise from its own organic matter. Reported in kg/ha. If you've got this number, enter it as "existing soil N" and don't also tick organic-matter N, you'd count it twice.
- NDI, Nitrogen Drawdown Index
- An optional compost test. Above 0.5 means little drawdown; below 0.5 means the compost will tie up soil N for a while.
- Dry matter %
- How much of a compost or manure isn't water. A wet compost (low dry matter) carries far less actual material, and N, per tonne spread.
- Organic-matter N
- An estimate of extra N your soil releases from its own humus over the season, from your soil's OM%. A rough planning figure, not a measurement.
- ASC / drainage class
- How freely your soil drains and holds nitrate. Free-draining soils leach hardest; heavy soils hold on longer.
- VCSN
- NIWA's daily climate grid. If you have your own site's soil-temp and drainage figures from it, tick the VCSN box to use them instead of the regional averages.
- Volatilisation
- Ammonium lost to the air as ammonia gas when left on the surface. Incorporating or watering in keeps more of it.
Method & assumptions
1 · OSU first-season PAN. Plant-available N as a % of total N is predicted from the
material's N concentration: PAN% = −15 + (15 × N%) below 6% N, capped at 75%
at/above 6% N (OSU EM 9235, Sullivan & Andrews; Gale et al. 2006). Soluble amino-N is set to ~90%
(already plant-available, not bulk protein).
2 · Soil temperature by month. Each region's NIWA 1991–2020 annual mean 10 cm soil
temperature is expanded to a monthly curve (sinusoid peaking late January/February) using a
continentality amplitude. The tool averages the months in your chosen window and applies a
rate factor 2^((T−16)/10) (Q10≈2, capped at 1.0; no high floor, so genuinely cold soils
are discounted in line with the science). OSU's own model is not
temperature-corrected, this overlay is a conservative correction for cool/short NZ seasons.
3 · Rainfall leaching. Each region's monthly rainfall and evapotranspiration run through a
simple soil-water bucket (regional rain in, regional ET out, capacity = your drainage class:
75/130/190 mm). Overflow = drainage. Leaching loss = 1 − e^(−drainage/capacity), capped
at 60%. Free-draining/low-ASC soils both drain more and hold nitrate less, so they leach hardest; a dry
summer window drains little and leaches little. The same leaching discount is applied to every
mineralised-N pool, inputs, compost and soil organic matter alike, while ammonium (held on the soil
until it nitrifies) is treated as not immediately leachable.
4 · Placement. The OSU coefficients assume the meal is tilled in. Surface-applied / no-till delivers less in the first season (poorer soil-microbe contact, surface drying, ammonia loss), field data show roughly a third of N released surface-applied versus about half incorporated, so no-till applies a ×0.6 factor.
Final = OSU PAN × temperature factor × (1 − leaching loss) × placement factor. Carryover is the N not released this season, total N minus first-season PAN, which mineralises slowly over following seasons (a later bonus, not available to this crop).
Crop targets. The crop list sets a typical first-season available-N goal (kg N/ha), a mid-range starting point that varies with yield, region, variety and soil supply. It's the N your inputs aim to provide; the soil itself mineralises extra N from organic matter that this tool doesn't count, so treat the target as an upper guide and confirm with in-season soil nitrate testing. Legumes (peas, beans, clover, lucerne) fix most of their own N, so their targets are low. On pasture, NZ caps synthetic N at 190 kg N/ha/yr; organic inputs aren't capped but the agronomy still applies. Figures drawn from NZ vegetable/arable nutrient guidance (Reid & Morton; Yara NZ) and OSU crop N tables.
Existing soil N & organic-matter N. Enter your current soil mineral N (from a nitrate / Available-N test, kg N/ha) and it's added to supply, so the meals only fill the gap to target. Ticking organic-matter N estimates the extra N your soil mineralises during the window from its own organic matter: OM% → organic carbon (÷1.72) → total organic N (C:N 12, top 15 cm) → a low-end annual release rate (1.5%/yr at a temperate 12 °C baseline, scaled up or down by the region's mean soil temperature with Q10≈2 and bounded to 1–2.5%/yr), then apportioned to your months by temperature. So warm regions mineralise more per year than cool ones, and a summer window captures more than a winter one. It's the conservative end of the 1–4%/yr range, and OSU doesn't predict this at all, so treat it as indicative. Don't tick OM if your "existing N" figure already came from an AMN / Available-N test, that measures the mineralisable pool and you'd double-count; use OM only when you have OM% but no N test.
Bulk organics (compost, manure, effluent). Unlike meals, these split into pools. In quick mode a single first-season fraction comes off the C:N ratio (≈16% of total N at C:N 8, ≈7% at 15, crossing to net tie-up past ~22), nudged by material type. In lab mode the report's ammonium + nitrate are counted as available now (ammonium discounted for volatilisation if surface-applied) and the organic remainder releases slowly on a C:N curve centred near 10%/yr; an NDI below 0.5 forces a net tie-up. Effluent counts its ammoniacal fraction as available (less volatilisation by application method) plus a small organic slow-release. All pools then pass through the same temperature and leaching adjustments as the meals. Anchored to NZ compost reporting (NZS 4454) and incubation data (Lazicki 2020; Geisseler 2021).
Honest limits. This is a planning estimate. Real leaching needs daily climate and soil hydraulics (Overseer-grade), treat the loss figure as a risk indicator, not a measured number. Evapotranspiration uses each region's annual total scaled by a shared seasonal curve. Assumes the meal is incorporated with adequate moisture (the placement setting adjusts for surface application). Sheep pellets, if well-composted, behave compost-like (far lower PAN). Always pair with in-season soil nitrate testing.
Known limitations (read this)
This is a planning tool, not a prediction. It multiplies several approximate sub-models, so uncertainty compounds, the true figure could be roughly half to double the central number. The displayed range reflects that. It's reliable for relative comparisons (source vs source, season vs season, region vs region) and for learning; it is not for regulatory nutrient budgets (use OverseerFM or a certified adviser).
Leaching is the weakest part. It uses a monthly water balance, which under-states loss because real leaching is driven by individual rain events on individual days. Rainfall and evapotranspiration are regional estimates, and the leaching fraction is uncalibrated. Pegging to your daily NIWA VCSN data is the best available fix.
The temperature overlay goes beyond OSU. OSU deliberately doesn't temperature-correct (high-N meals release fast even in cold soil); this tool adds a conservative Q10 discount, which can over-penalise cool regions over a long window. Toggle it off to see the unadjusted OSU figure.
The organic-matter estimate ignores soil moisture, so it may overstate release in a hot, dry summer. It's a stack of assumptions (the 1.72 factor, C:N 12, 15 cm depth, the annual rate) and is indicative only, an AMN / Available-N soil test is the real measure.
Other caveats. The OSU engine was validated in Oregon, not on NZ allophanic/pumice/peat soils; karanja/neem suppress nitrification so their N releases slower than shown; "sheep pellets" vary widely and may be compost-like; crop targets are typical mid-points, not recommendations; and the budget adds the N pools without modelling whether release timing matches crop demand (see Timing & splitting). Garbage in (wrong N%, region, OM%, or test type) gives a confident wrong answer. Because the tool runs conservative, don't blindly top up to target or you may over-apply. The soil nitrate test is the truth; this is the plan.