Fundamentals of Soil Science (1+1)

Nutrient Availability

Nutrient Availability:
The availibility of plant nutrients are more at a pH range of 6-7 except Mo ( Fig 2.)
Nitrogen

• One of the key soil nutrients is nitrogen (N). Plants can take up N in the ammonium (NH₄⁺) or nitrate (N0₃^-) form.
• At pH near neutral (pH 7), the microbial conversion of NH₄⁺ to nitrate (nitrification) is rapid, and crops generally take up nitrate. In acid soils (pH < 6), nitrification is slow, and plants with the ability to take up NH₄⁺ may have an advantage

Phosphorus

• The form and availability of soil phosphorus is highly pH dependent.
• When the soil is neutral to slightly alkaline , the HPO₄^-- ion is the most common form. As the pH is lowered both the HPO4^-- and H₂PO4 ^- ion prevail. At higher acidities H₂PO4 ^- ions tends to dominate. The most plants absorb phosphorus in HPO₄^-- .
• Between pH 6-7, phosphorus fixation is at minimum and availability to higher plants is maximum.

Potassium:

• The fixation of potassium (K) and entrapment at specific sites between clay layers tends to be lower under acid conditions. This situation is thought to be due to the presence of soluble aluminum that occupies the binding sites.

Calcium, Magnesium and Sulphur:

• The availibilty of Ca and Mg is more above pH 7.0.
• Sulphate (S0₄^2-) sulphur, the plant available form of S, is little affected by soil pH.

Micronutrients

• The availability of the micronutrients manganese (Mn), iron (Fe), copper (Cu), zinc (Zn), and boron (B) tend to decrease as soil pH increases.
• The exact mechanisms responsible for reducing availability differ for each nutrient, but can include formation of low solubility compounds, greater retention by soil colloids (clays and organic matter) and conversion of soluble forms to ions that plants cannot absorb.
• Molybdenum (Mo) behaves counter to the trend described above. Plant availability is lower under acid conditions.

Fig.2. Effect of Soil pH on Nutrient Availability

Soil pH and soil organisms:

• Growth of many bacteria and actinomycetes is inhibited as soil pH drops below 6
• Fungi grow well across a wide range of soil pH
• Therefore fungi are dominant under acid conditions
• Less competition from bacteria and actinomycetes
• Earthworms do best when soil pH >6.5
• Nitrification greatly inhibited at pH <5.5
• N fixation greatly restricted a pH <6
• Decomposition of plant residues and OM may be slow in acid conditions (pH <5.5)

Soil Buffering Capacity

• The ability to resist a change in pH refers to buffering capacity of the soil
• The buffering capacity increases as the cation exchange capacity increases. Thus, heavier the texture and the greater the organic matter content of a soil, the greater is the amount of acid or alkaline material required to change its pH.
• The colloidal complex acts as a powerful buffer in the soil and does not allow rapid and sudden changes in soil reaction.
• Buffering depends upon the amount of colloidal material present in soil. Clay soils rich in organic matter are more highly buffered than sandy soils.
• Importance of buffering in agriculture
• The stabilization of soil pH through buffering act as a effective guard against deficiency of certain plant nutrients and excess availability of others in toxic amounts which would seriously upset the nutritional balance in the soil.