• Presence of certain minerals in soils like titamin compounds impart darker colour.
• Iron compounds like haematite, give red and limonite yellow colour.
• Preponderance of silica or lime results in whitish or grayish tinge to the soil.
• Accumulation of salts makes soils white or black, depending on the type of salts.
• If soil is well drained as in lateritic soil, ferric compounds of iron are commonly formed and they give red colour to the soil.
• If drainage is poor, soil colour is greenish or bluish in waterlogged soils.
• In subsoil layers, colour variegation or mottling of soil is due to alternate oxidizing and reducing conditions due to fluctuations in water table.
• Dark brown soils indicate high organic matter and fertility.
• Red colour shows good aeration.
• White colour of soil indicates accumulation of salts.
• Nutrient transformation and its availability in soils depends on pH, clay minerals, cation and anion exchange capacity.
• pH is defined as the negative logarithm of hydrogen ion activity.
• pH influences rate of nutrient release through its influence on decomposition, cation exchange capacity and solubility of materials.
• Important source of nitrogen and sulphur is organic matter.
• Decomposition of organic matter is slowest at pH below 6 and fastest between 6 to 8.
• Availability of phosphorus is high within a pH range of 6.5 to 7.5 due to higher solubility of phosphorus compounds.
• At low pH, phosphorus is precipitated as Fe and Al phosphates.
• At high pH, calcium phosphate is formed which is less soluble.
• Metallic cations such as Fe, Mn, Cu, Zn and also B precipitate at high pH, hence their availability is less in alkaline soils.
• Availability of K and B are influenced by pH through its effect on cation exchange capacity.
• At lower pH, H+ ions replace K and leaching of K occurs.
• At high pH, potassium compounds are converted into non exchangeable form and thus availability is reduced.
• Boron is leached out at low pH.
• Optimum pH range for availability of different nutrients
|Nutrient||Optimum pH range|
|S||6.0 and above|
|Ca and Mg||7.0-8.5|
|Fe||6.0 and below|
|B, Cu, Zn||5.0-7.0|
|Mo||7.0 and above|
• Soil with pH above 8.5 is alkaline with high sodium content which deflocculation soil colloids. It results in destruction of soil structure and movement of water and air are impaired in the soil.
• Rice and tea prefer acidic soil reaction and most of the other crops prefer neutral pH.
• pH preference of rice is 4.0 to 6.0.
• pH preference of tea is 4.0 to 6.0.
• Crops tolerant to acidity – paddy, potato, tea, millets
Semi tolerant – Bengal gram, maize, sorghum, peas, wheat, barley
Sensitive to acidity – Redgram, soybean, cotton, oats
• Within a pH range of 5 to 7, the acidity of soils is due to exchangeable hydrogen ions.
• At pH < 5.0, H+ ions replace Al3+ from the lattice of clay mineral.
• Aluminium released is hydrolysed with liberation of H+ ions. Iron behaves similarly and contributes to acidity.
• On soils with low CEC and in high rainfall regions, leaching of bases occurs resulting in reduction of pH.
• Addition of acid forming fertilizers increases acidity.
• KCL is an acid forming fertilizer.
• During the decomposition of organic matter, several acids are formed which increase the acidity.
• In acid sulphate soils, pH is low due to formation of sulphuric acid by oxidation of sulphur and sulphides.
• Acid soils can be converted into neutral soils by addition of lime.
• Liming materials are oxides, hydroxides and carbonates of calcium and magnesium.
• Application of lime at full lime requirement increased soil pH and reduces aluminium content.
• Continuous submergence reduces aluminium content below toxic levels compared to alternate submergence or irrigation regions at saturation.
• Availability of P and Fe is high under continuous submergence compared to alternate submergences or at saturation.
• Soils with pH higher than 7 are called alkaline soils.
• High base saturation other than H+ ions especially of sodium and presence of free carbonates of Ca and Na are the main reasons for alkalinity.
• Exchangeable sodium has higher effect on pH than of Ca and Mg since sodium hydroxide is a stronger base.
• Hydrolysis of carbonates of Ca and Mg release hydroxyl ions which increase pH.
• Presence of excess salts also increases pH.
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