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How to read a Hill Laboratories soil report

by Seacliff Organics in Information

A Hill Laboratories Basic Soil report lists pH, Olsen P, four exchangeable cations, cation exchange capacity (CEC), base saturation and volume weight, each beside a bar showing where your result sits against a medium range for the crop named on the form. Read the bars for excesses first, then shortages, and judge every number against Hill's own New Zealand ranges. American targets were built on a different extraction, different units and a different CEC method, so they cannot be laid over this report.

How the page is laid out

Each line gives the test, its unit, the level found, a medium range and a bar marked low, medium and high. Hill describes the medium range as a generalised level suited to the land use or crop: a guide, not a precise interpretation (Hill Laboratories n.d.c). Crop specific ranges are printed when you name the crop on the request form (Hill Laboratories n.d.i), so two reports can show different ranges for the same test. On the 2026 reports we have handled, the cations are repeated below the table as base saturation percentages and in MAF units.

Olsen P is measured on a scoop of known volume, not a weighed sample, which is why it is reported in mg/L (Drewry et al. 2013; Cavanagh et al. 2025).

The Basic Soil lines

Unless another source is given, the ranges and notes in this section are Hill's (Hill Laboratories n.d.a). The range printed on your own report takes priority.

pH

Acidity, measured in water (Hill Laboratories n.d.f). General medium range: 5.6 to 6.5 for a loam, 5.9 to 6.8 for a sandy soil, 5.1 to 5.5 for peat. Low: lime. On an average silt loam about 1 t/ha of good lime lifts pH by 0.1. High: stop liming. Bringing pH down is slow: fine elemental sulphur may need 1 to 1.5 t/ha per pH unit (Reid and Morton 2019).

Olsen P

The standard New Zealand test for plant available phosphorus: a bicarbonate extraction at pH 8.5 that gives an index, not a weight of phosphorus (Hill Laboratories n.d.d). General medium range: 20 to 30 mg/L for agricultural soils, 30 to 80 for horticultural, 70 to 150 for glasshouse.

Low: lifting Olsen P one unit in the top 15 cm takes about 10 kg P/ha on sedimentary soils and about 25 on volcanic ash. High: withhold phosphorus and let it fall, because high values raise the risk of phosphorus reaching waterways (Reid and Morton 2019). If your bar is high you do not need BioPhos, fish bone meal or rock phosphate.

Olsen P under-reads after recent liming and where reactive phosphate rock has been used. Where slow release phosphates are in the history, including the soft rock phosphates we sell, Hill's advice is to add the Resin P test (Hill Laboratories n.d.a).

Potassium, calcium, magnesium and sodium

The exchangeable cations, reported in me/100g: milliequivalents of charge per 100 g of soil, the same number as cmol/kg (Hill Laboratories n.d.b). Hill prefers these actual amounts to percentages when judging cations. General medium ranges for horticultural soils: potassium 0.5 to 1.0, calcium 6 to 12, magnesium 1 to 3, sodium 0 to 0.5.

  • Potassium. Low: add potassium, such as potassium sulphate. One quick test unit takes roughly 33 kg K/ha on Brown sedimentary soils and 50 on volcanic ash (Reid and Morton 2019). High: withhold it; excess potassium suppresses magnesium uptake.
  • Calcium. Most New Zealand horticultural soils already hold an excess (Reid and Morton 2019). If pH is low, lime supplies it. If pH is in range, gypsum supplies calcium (about 23%) and sulphur (about 18%) without changing pH (Nicholls et al. 2012; Espinoza and Ismanov 2022).
  • Magnesium. Hill suggests about twice as much magnesium as potassium, and warns that uptake can be suppressed when magnesium is the lower of the two. Low with low pH: dolomite, about 11% magnesium with liming value. Low with pH in range: a magnesium sulphate, which has no liming value (Nicholls et al. 2012). A slightly low bar is not an emergency: Reid and Morton (2019) found no New Zealand evidence that vegetable brassica yields respond to magnesium fertiliser.
  • Sodium. Generally ignorable for crops unless high, as on low coastal ground; then add a soluble salts test.

The MAF units line is the same four results converted to a volume basis with the volume weight. These are the MAF Quick Test units on which New Zealand field calibration was built (Analytical Research Laboratories n.d.), and the vegetable manual still gives its potassium advice in them.

CEC

The soil's capacity to hold cations, measured at pH 7. It describes the soil; it is not a nutrient to top up. Hill rates 5 to 12 me/100g as low, 12 to 25 medium, 25 to 40 high and over 40 very high (Hill Laboratories n.d.b).

Base saturation

Total base saturation is the four cations added together as a percentage of CEC. It tracks pH, and Hill's lime requirement uses a default target of 70%. The individual percentages Hill calls desirable for horticultural soils are potassium 3 to 6, calcium 50 to 75, magnesium 7 to 15 and sodium 1 to 2.

Volume weight

The weight of a known volume of the dried, ground sample, in g/mL: 0.4 to 0.5 for raw peat, 0.6 for pumice, 0.8 for clay and 1.0 to 1.3 for sandy soils. It is not field bulk density. A low figure warns that by-weight results such as me/100g will look bigger than the same soil measured by volume.

Common add-ons

Line What it measures General medium range Reading it
Sulphate sulphur Readily available sulphur 10 to 20 mg/kg Leaches from low phosphate retention soils. Low: add a sulphate such as gypsum. High: leave sulphur out.
Organic matter Total carbon × 1.72 7 to 17% Linked to CEC, nitrogen supply and structure. Track the trend.
Available nitrogen Nitrogen released in a laboratory incubation (15 cm sample) 150 to 250 kg/ha A potential, not a promise: field release depends on temperature and moisture.

The table's ranges are Hill's (Hill Laboratories n.d.a). On trace elements, Hill treats soil tests as indicators only and encourages a plant tissue test before any are applied (Hill Laboratories n.d.g). We sell trace element salts and chelates, and would still rather you confirmed the need in the leaf first.

Worked example: turning the units over

An illustrative report (not a real client's): potassium 0.60, calcium 9.0, magnesium 1.50 and sodium 0.20 me/100g; CEC 18; volume weight 0.90 g/mL; sampled to 15 cm. The factors are Hill's (Hill Laboratories n.d.a).

  1. Total base saturation: 0.60 + 9.0 + 1.50 + 0.20 = 11.30, and 11.30 ÷ 18 × 100 = 62.8%.
  2. Potassium in MAF units: 0.60 × 20.8 × 0.90 = 11.2.
  3. Potassium by volume: 0.60 × 391 × 0.90 = 211 mg/L. The top 15 cm of a hectare is 10,000 m² × 0.15 m = 1,500 m³, or 1.5 million litres, so 211 × 1.5 = about 317 kg/ha of exchangeable potassium.
  4. Lime to reach 70% base saturation: (70 - 62.8) × 18 × 0.90 × 0.0083 = 0.97 t/ha, or 97 g per square metre.

Whether to spread that lime is debated. Hill regards base saturation as the more reliable liming guide (Hill Laboratories n.d.b); a review of New Zealand pastoral soils concluded that pH alone should decide (Edmeades and Perrott 2004).

Why American targets do not fit this report

  • Different extraction. Many overseas laboratories run Mehlich 3, an acid extract at about pH 2.6, as their whole basic profile (Hill Laboratories n.d.e). Olsen is alkaline. Hill's typical ratio is Olsen P = Mehlich 3 P × 0.45, so a Mehlich 3 figure of 100 sits near Olsen 45, but Hill calls the relationship highly variable and says results from different laboratories' methods cannot in general be converted (Hill Laboratories n.d.d).
  • Different units. American guides such as Oregon State's work in pounds per acre with an assumed depth and bulk density (Horneck et al. 2019). One pound per acre is 0.4536 kg ÷ 0.4047 ha = 1.12 kg/ha.
  • Different CEC. Some overseas laboratories estimate CEC from pH, which Hill says tends to underestimate it on New Zealand soils, so base saturation percentages only compare within one method (Hill Laboratories n.d.b).

The Oregon guide agrees: its sufficiency ranges hold only for the methods listed. Hill offers Mehlich 3 as an add-on with its own bars, and we can send samples to Logan Labs. Read each report against its own laboratory's ranges.

Calcium to magnesium: Dan's position

If a test shows calcium and magnesium are out of balance, fix it: gypsum when pH is in range, ag lime or crushed oyster shell when pH is low. But the workable range is wide and there is no single magic number. Hill's desirable bands of 50 to 75% calcium and 5 to 15% magnesium span ratios from 50 ÷ 15 = 3.3 to 1 up to 75 ÷ 5 = 15 to 1 (Hill Laboratories n.d.b). The "ideal soil" of 65% calcium, 10% magnesium and 5% potassium dates from 1940s American work, and a review found that, within the ranges commonly found in soils, fertility is generally not influenced by these ratios (Kopittke and Menzies 2007).

Peat and coco mixes need a different test

A bagged living soil, whether it grows tomatoes or licensed medical cannabis, is a potting medium. Hill's Potting Media test is a water extract in mg/L that shows only what is immediately available, and it is read against its own ranges (Hill Laboratories n.d.h). The Basic Soil ranges above do not apply to that report.

Putting it into the planner

The Market garden soil planner takes the lines above. Paste the results table from your report and it fills the boxes, including your printed medium ranges, which replace its defaults. It suggests lime only when pH is below your range. It is a planning aid, not an agronomist. Which tests to order and where to post the sample are on Where to test your soil.

What to do

  • Check the sample type and depth on the report: 15 cm for gardens and crops (Hill Laboratories n.d.c).
  • Read the high bars first and stop adding those nutrients.
  • Confirm trace elements with a leaf test before buying any.
  • Retest at the same depth, in the same month, with the same laboratory, and not within three months of fertiliser or lime (Morton et al. 2024).

References

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