There is a common and expensive pattern: a farmer increases fertiliser, sees no yield response, increases it again the following season, and still sees nothing.
Often the fertiliser is fine. The soil chemistry is preventing the crop from reaching it.
What pH actually measures
pH is a measure of how acidic or alkaline the soil solution is, on a scale where the midpoint is neutral. Lower numbers are acidic, higher numbers are alkaline.
The scale is logarithmic — each whole step represents a tenfold change. That is why what looks like a small numerical difference on a report can mean a large practical difference in the field.
Why it decides whether fertiliser works
Nutrients have to be dissolved in soil water before a root can absorb them. Whether a nutrient stays dissolved or binds to soil particles depends heavily on pH.
Two examples that cost farmers real money:
Phosphorus in alkaline soil. Phosphorus reacts with calcium and forms compounds the plant cannot use. You apply it, it is physically present in the soil, and the crop still shows deficiency.
Phosphorus in strongly acidic soil. The same nutrient binds instead with iron and aluminium. Different chemistry, same outcome.
Phosphorus is most available in a band near neutral, which is one reason that range is so widely recommended.
The mirror problem also exists. In strongly acidic soils, aluminium and manganese become too available and can reach levels that damage roots directly.
How soils drift out of range
Toward acidic: high rainfall leaching basic nutrients downward, continuous use of ammonium-based nitrogen fertilisers, and removal of crop residue season after season without return.
Toward alkaline: irrigation with high-bicarbonate groundwater, arid conditions where salts accumulate rather than wash away, and naturally calcareous parent material.
Neither happens overnight. Both are gradual, which is exactly why they go unnoticed until yields have already slipped.
Correcting it
Acidic soils are typically treated with agricultural lime. The dose depends not just on current pH but on the soil's buffering capacity — a clay soil resists change far more than a sandy one and needs substantially more material. This is why a lab recommendation beats a rule of thumb.
Lime works slowly. Apply well ahead of sowing, incorporate it rather than leaving it on the surface, and expect the full effect over more than one season.
Alkaline and sodic soils are more complex. Gypsum is commonly used where excess sodium is the issue, but it needs adequate drainage to work — the displaced sodium has to physically leave the root zone. Applying gypsum to a poorly drained field achieves very little.
Organic matter helps in both directions, buffering the soil against swings and improving structure at the same time.
What to do before you spend anything
- Test properly. Composite samples from several points across the field, not one convenient corner.
- Compare against your crop, not a generic ideal. Preferred ranges differ — a soil that is a problem for one crop may be acceptable for another.
- Get a dose recommendation, not just a reading. The amount of amendment depends on buffering capacity.
- Check your irrigation water too. If the water is driving the problem, treating only the soil means repeating the work every year.
- Re-test after a season to confirm the amendment is moving the number.
When correction is not the answer
Sometimes the honest recommendation is to work with the soil rather than fight it. If a field is strongly alkaline and drainage is poor, the cost of full correction can exceed the benefit.
Switching to a crop or variety better suited to the existing conditions is a legitimate answer, not a compromise — and often the more profitable one.