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How Acidity Harms Root Growth: Toxicity, Nutrient Deficiency, and Reduced Microbial Activity
Largely because acidity affects element availability. Aluminium and manganese become available (easier for crop roots to take up), while major plant nutrients such as phosphorus become less available.
Aluminium is normally bound to organic matter and clay in the soil—an innocuous bystander to crop root growth. However, as soil pHCa drops below 4.8, aluminium becomes more soluble, damaging crop roots. Roots affected by aluminium are brittle, deformed, and have fewer branches.
Generally, 2–5 ppm of aluminium in the soil is toxic to the roots of sensitive plants, and above 5 ppm is toxic to most species. Aluminium toxicity is more of a problem below about 10 cm depth; it tends to remain bound to organic matter in the topsoil.
Manganese toxicity is less problematic than aluminium toxicity but can still stunt root growth. It’s more likely to show up in lucerne. Manganese toxicity is not normally an issue in South Australia but is a problem in NSW.
Nutrient deficiency
As soil pH decreases, so does macronutrient availability. Trace elements fare a little better (except for iron), but as pH drops below 5, availability decreases. To top it off, poor root growth in acidic soil means crops can’t explore the soil to find more nutrients to make up for lower nutrient availability.
Poor nodulation
In legumes, if the soil is too acidic Rhizobia won’t survive, limiting nodulation and nitrogen fixation. New rhizobium mixes containing more acidic strains are being released in 2024 for lentils and beans.
Microbes
Most microbes that break down organic matter and cycle nutrients don’t like very acidic soil, slowing the rate at which nutrients are made available to crops.
