(a) Acid toxicity – The H+ ion concentration is toxic to plants under strong acid conditions of soil.
(b) Toxicity of different nutrient elements – The concentration of Fe2+ and Mn2+ in soil solution depends upon soil pH, organic matter and soil redox condition. Due to increase in organic matter content in the soil, the population of soil microbes increases and very rapidly use the soil oxygen and results in reduced soil condition. As a result of reduction, the nutrient elements like Mn4+ and Fe3+ reduce to Mn2+ and Fe2+ respectively and toxicity of these elements develops. Due to such toxic effects, a physiological disease of rice is found in submerged soils, which is popularly known as “browning disease”.
(c) Toxicity of Aluminium (Al) – Al toxicity in soils affects pant growth in various ways:
• It restricts the root growth.
• It affects various physiological processes like division of cells, formation of DNA and respiration etc.
• It restricts the absorption and translocation of nutrients like P, Ca, Fe, Mn etc., from soil to plant.
• It causes wilting of plants.
• It also inhibits the microbial activity in soil.
(d) Nutrient imbalance – It is evident that soluble iron, aluminium and manganese are usually present in higher concentration under strong acidic conditions. Phosphorus react with these ions and produces insoluble phosphatic compounds rendering phosphorus unavailable to plants. Besides these, fixation of P by hydrous oxides of iron and aluminium also reduces the P availability. In acid soils Fe, Mn, Zn, Cu are abundant but molybdenum and boron availability are decreased. N, P and S become less available in acid soil having pH less than 5.5.
(e) Microbial activity – Bacteria and actinomycetes function better in soils having moderate to high pH values. They can not sustain their activity when the soil pH drops below 5.5. Free living N fixing bacteria (Azotobacter sp.), and symbiotic N fixing bacteria (Rhizobium sp.) activities are inhibited under acidic condition. Fungi can grow well under very acid soils and causes disease like root rot of tobacco, blights of potato etc.
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