Why Your Soil pH Affects Whether Fertilisers Actually Work

You can apply the perfect fertiliser at the perfect rate and still see deficiency symptoms if your soil pH is wrong. pH determines the solubility of every nutrient in the soil — whether it dissolves into the soil water where roots can reach it, or locks up into insoluble compounds that roots cannot access at all. Understanding this relationship is one of the most important insights in gardening, and it explains many puzzling crop failures.

What Soil pH Actually Measures

Soil pH is a measure of acidity or alkalinity on a scale from 0 to 14. A pH of 7.0 is neutral. Below 7 is acidic; above 7 is alkaline. Most garden plants grow best between 6.0 and 7.0, with a sweet spot around 6.5 for the widest range of vegetable crops. At this range, the greatest number of nutrients are in their most available, soluble forms.

How pH Affects Individual Nutrients

Each nutrient has its own pH sensitivity:

• Nitrogen, phosphorus, potassium, sulfur, calcium, magnesium: best available at pH 6.0–7.5; marked reduction in availability below 5.5 or above 7.5
• Iron, manganese, zinc, copper, boron: more available at lower (acidic) pH; can become toxic at pH below 5.0 but unavailable above 7.5 — this is why acid-loving plants that end up in alkaline soil show iron chlorosis
• Molybdenum: the opposite — most available at higher pH, deficient in very acidic soils

Testing Your Soil pH

Basic soil test kits are inexpensive, easy to use and take minutes to give a result. More accurate professional soil tests (posted to a lab) give pH plus macronutrient levels for a modest cost and are very worthwhile for new garden plots or problem areas. Test from several spots across the garden — pH can vary significantly from one bed to another, especially if some areas have been limed in the past.

Correcting Acidic Soil

To raise pH (make soil less acidic), apply garden lime (calcium carbonate) or dolomitic lime (which also supplies magnesium). The amount needed depends on soil type — sandy soils need much less than heavy clay. Apply in autumn, water in well and retest after six to eight weeks. Changes are gradual; large pH adjustments take more than one application over more than one season. Never over-lime — going too far alkaline creates a different set of problems.

Correcting Alkaline Soil

Reducing soil pH is slower and harder than raising it. Flowers of sulfur (elemental sulfur) acidifies gradually as soil bacteria convert it to sulfuric acid. Acidifying fertilisers such as ammonium sulphate or diammonium phosphate also reduce pH slightly with repeated use. Incorporating acidic organic materials like composted pine bark, leaf mould or ericaceous compost helps, particularly for raised beds and containers. Very strongly alkaline soils are difficult to change permanently — building raised beds and using appropriate compost can be more practical than continually fighting the underlying soil.