Effects of soil acidity

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Soil microbial activity

Like plants, the functioning of soil microbes is impaired by acidic conditions.

Pasture legumes may fail to persist in acidic soils.

Most microbial processes, including the breakdown of organic matter and cycling of nutrients, are reduced in acidic soil because growth and reproduction of the soil microbes, primarily bacteria and fungi, are reduced. Micro-organisms break down organic matter and use the carbon and nutrients for their own growth.

Nutrients excess to requirements are released into the soil where they are available to be taken up by plants. The rate of mineralisation of nutrients by soil microbes into plant-available forms is slower in acidic soil, potentially limiting plant uptake.

Importantly, legume nodulation can fail in acidic soil. Legumes are a group of plants that fix their own nitrogen from the air through a symbiotic relationship with specialised bacteria. Under favourable conditions, nitrogen-fixing rhizobia bacteria form a symbiosis (in essence a partnership) with crop and pasture legumes in root nodules (Figure 6).

Well nodulated subclover plants produce more shoot and root biomass than poorly nodulated plants
Figure 6 Well nodulated (left) compared to poorly nodulated subclover plants (right). Photo: Centre for Rhizobium Studies, Murdoch University

Acidic soils limit both root growth and rhizobia survival, reducing the chances of roots contacting enough bacteria to form a nodule and inhibiting the performance of nodules that do form. In acidic soils, the failure of a functioning symbiosis results in plant nitrogen deficiency.

Species of rhizobia bacteria vary in their tolerance to soil acidity, for example, medic rhizobia are very sensitive and may fail to persist. Grass-dominated pastures can result from the failure of pasture legumes to thrive in acidic soil. Under more appropriate pH levels, the activity of detrimental soil microbes can also be increased and may need to be managed.

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