ABSTRACT
Soil is a major component of all agroecosystems encountered across different continents of the earth. Soil forms the belowground portion and provides an-chorage to crop. Soil acts as repository of moisture, mineral nutrients, organic matter, gaseous material and most importantly the soil microbes that mediate large number of nutrient transformations. Soil phase of any agroecosystem is a variegated system. Soils vary markedly for physicochemical properties, fertility and moisture held within the profile. Several types of soil that differ with regard to parent material, texture, structure, bulk density, pH, mineral nutrient distri-bution, organic matter and moisture-holding capacity are congenial to support cropping expanses. Soil fertility per se and ability to support cultivation of crops and productivity levels attained also vary in an agrarian region. Farmers may en-counter different kinds of constraints to crop establishment, growth and yield formation. The number and types of soil maladies felt may also vary. Over all, soil phase almost decides the crop species, cropping pattern and productivity in a given agrarian belt. 2.1 SOILS OF AMERICAS
Agroecosystems in Americas thrive mostly on fertile soils classified as Udolls and Udalfs. Soil types, such as Mollisols, Aqualls, Ustolls, Fluvents, Aqualfs and Haplaquepts also support large expanses of crops (Table 1; Plate 1). Soils found in Great Plains are endowed with optimum level of organic fraction. Despite it, large amounts of crop residues are recycled. We should note that intensive cropping necessitates removal of massive quantities of nutrients into aboveground portion of ecosystem, mainly into grains/forage. Incessant cropping depletes soil fertility and diminishes SOM, therefore, irrespective of soil type; much of the cropping region is provided with inorganic and organic fertilizer replenishments. In New England zone, crops thrive on Udults. Soil productivity is held high through the use of fertilizers and farmyard manure. Crop yields are held high. For example, maize grown exclusively for forage may produce 25-30 t fresh forage/ha. In Southern Plains, crops thrive on Uderts and Ustolls. In the South-east of USA, (Georgia, Florida) crops are grown on Psamments, Quartzipsammets, Ultisols and Aquults. Piedmonts of Georgia, Carolinas and Virginia also support large-scale farming.
