How to read a soil test: pH, phosphorus, potassium, sulphur and mineral N

pH: check the method

A pH of 7 is neutral; lower is acidic and higher is alkaline. Most Australian labs report either pH (water) or pH (CaCl2). The CaCl2 value is usually 0.5 to 1.1 units lower than the water value and is less affected by seasonal swings, so it suits long-term monitoring. For pastures Agriculture Victoria suggests keeping pH above 5.3 (water) or 4.5 (CaCl2). Low pH can raise exchangeable aluminium, which can be toxic: aluminium generally falls to harmless levels once pH (water) passes about 5.6 to 5.8, and an exchangeable aluminium level of 10% or more calls for lime.

Phosphorus and the PBI

Soil minerals can bind applied phosphorus, so only part is available to plants. In Victorian pasture soils plant-available P is usually tested as Olsen P.

The phosphorus buffering index (PBI) measures how strongly soil locks up applied P. A soil with a high PBI needs more phosphorus fertiliser than one with a low PBI. The classes run from extremely low (below 15) through very very low (15–35), very low (36–70), low (71–140), moderate (141–280) and high (281–840) to very high (above 840).

Potassium and sulphur

Available potassium (Colwell K or exchangeable K) targets depend on soil texture. For example, “ideal” is 101–150 mg/kg on sands, 121–200 on sandy loams, 161–250 on clay loams and 181–300 on clays. When potassium is high, inputs can be reduced until levels fall. Sulphur is usually measured by the KCl 40 test, and above 8 mg/kg is considered adequate.

Organic carbon, CEC and salinity

• Organic carbon shows organic matter, which helps hold nutrients, water and structure. Pasture ranges differ for low and high rainfall areas.
• CEC (cation exchange capacity) is the soil’s capacity to hold cations such as calcium, magnesium, sodium and potassium. It depends mostly on clay and organic matter, and it matters for nitrogen too: higher-CEC soils lost less urea nitrogen in GRDC trials (see urea volatilisation).
• Salinity is measured as electrical conductivity (EC). Ideal is below 0.2 dS/m. To judge plant tolerance, labs convert EC to ECe using a soil-texture factor.

Mineral nitrogen is different

Nitrogen moves and changes quickly, so it needs its own test. GRDC’s guidance: sample in the month or so before sowing and before fertiliser N goes on; take 6–8 cores per production zone and bulk them (about an 80% chance of being within 20 kg/ha N of the true mean); sample to 0.6–1.0 m in layers; keep samples cool and get them to the lab fast. The lab reports nitrate and ammonium in mg/kg, which you convert to kg N/ha using bulk density and depth. Our nitrogen budget walk-through shows the arithmetic.

What to do with the report

• Write down the method next to each number (water or CaCl2 pH; Olsen or Colwell P; KCl 40 for sulphur).
• Compare against targets for your crop and region, not a pasture table from another state.
• Take it to your agronomist before choosing products or rates.