Fig. 6.3 shows how the pH of soil affects the availability of nutrients to plants. Acid rain is caused by the release of sulfur dioxide and nitrogen oxides ...

(a) From Fig. 6.3, nitrogen is most available to plants at approximately pH 6 to 8. [1]

The bar for nitrogen in the diagram is widest (indicating highest availability) across this range, centred around neutral pH.

(b) How soil pH affects nutrient availability: [3]

• Most major plant nutrients (nitrogen, phosphorus, potassium) are most available in slightly acidic to neutral conditions, around pH 6 to 7.5. In this range, these elements exist in soluble ionic forms that plant roots can absorb through their root hair cells. [1]
• In very acidic soils (below pH 5), the availability of nitrogen, phosphorus and potassium decreases. At low pH, these nutrients become chemically bound to soil particles or converted into insoluble compounds (e.g. aluminium phosphates) that plant roots cannot take up. Aluminium and manganese may also become toxic at very low pH. [1]
• Iron is an exception to the general pattern. It is more available in acidic conditions (pH 4-6) and becomes less available in alkaline soils. This is because iron forms insoluble hydroxides at high pH. Plants such as rhododendrons and blueberries that thrive in acidic soils have adapted to exploit this iron availability. [1]

(c) How a farmer could change the pH of acidic soil: [3]

• The farmer can add lime (calcium carbonate, CaCO₃) to the soil. Lime is an alkaline substance that reacts with the excess hydrogen ions (H⁺) in the soil, neutralising the acidity and raising the pH towards neutral. [1]
• Chemically, the calcium ions (Ca²⁺) from the lime replace hydrogen ions adsorbed on the surface of soil particles. This exchange reduces the concentration of free H⁺ ions in the soil solution, directly raising the pH. [1]
• The amount of lime required depends on the current soil pH, the target pH and the soil type (clay soils need more lime than sandy soils because they have a higher buffering capacity). Soil testing before liming allows the farmer to calculate the correct application rate and avoid over-liming, which could make the soil too alkaline. [1]

(d) Why testing soil pH is important before planting: [3]

• Different crops have different optimal pH ranges for growth. For example, blueberries prefer acidic soil (pH 4.5-5.5), while most vegetables and cereal crops grow best at pH 6-7. Planting a crop in soil with the wrong pH will limit its growth and yield. [1]
• Knowing the soil pH allows the farmer to choose appropriate crops for the existing conditions, or to amend the soil (adding lime to raise pH, or sulfur to lower it) to suit the intended crop before planting. [1]
• Applying fertiliser to soil with an unsuitable pH may be wasteful. If the pH is outside the optimal range, the added nutrients (especially phosphorus) may become chemically locked in insoluble forms that plants cannot access, meaning the money spent on fertiliser produces little benefit. [1]