Soils of Thotapalli Major Irrigation Project of North- Coastal Andhra Pradesh: Physical and Chemical Properties

Physical and chemical properties of the soils were assessed for the ayacut area of Thotapalli major irrigation project of North-Coastal Andhra Pradesh. Six representative soil pedons were studied in Devarapalli, Gujjangivalasa, Patikivalasa, Gangada, Aamiti and Maddivalasa villages falling in ayacut area of Thotapalli major irrigation project. The soil texture ranged from sandy loam to sandy clay loam in upland pedons while sandy clay loam to clay in pedons under cultivated plains. In general, relatively low bulk density was recorded at surface horizons compared to sub-surface layers. Maximum water holding capacity, pore space and volume expansion increased with soil depth. The soils were acidic to alkaline in reaction, non-saline, low to medium in organic carbon content. The CEC ranged from 6.4 to 32.5 cmol (p+)  kg-1. The soil exchange complex was dominated by calcium followed by magnesium, sodium and potassium. The soils were found to be low in nitrogen, low to medium in phosphorus and medium to high in available potassium status. Manganese and copper were sufficient but zinc and iron were deficient in soils. The total silica (SiO2) content of pedons varied from 48.10 to 71.40 per cent. Sesquioxide (R2O3) and the molar ratio of SiO2 / R2O3 ranged from 19.01 to 32.4 per cent and varied from 3.74 to 6.59, respectively in different pedons. The SiO2 / Al2O3 ratio and the SiO2 / Fe2O3 ratio varied from 4.43 to 7.65 and from 13.7 to 47.8, respectively. A significant positive correlation of clay content with cation exchange capacity (CEC), maximum water holding capacity (MWHC) was observed. Sand content was positively correlated with silica and negatively correlated with MWHC and volume expansion. Soil organic carbon had a significant positive correlation with soil pore space, available nitrogen, phosphorous, potassium, zinc, manganese, copper and iron, but a negative correlation with soil bulk density. Soil pH was positively correlated with CaCO3 but negatively correlated with available phosphorous, potassium, zinc, manganese, and copper and iron. Soil CaCO3 content was negatively correlated with available phosphorous, potassium, zinc, manganese, and copper and iron of the soil.