Groundnut (Arachis hypogaea) originates from South America. Present annual world production of unshelled nuts is about 35.1 million tons from about 25.5 million ha.(FAOSTAT, 2001)
The crop is grown between 40°N and S latitudes. Its growing period is 90 to 115 days for the sequential, branched varieties and 120 to 140 days for the alternately branched varieties. The mean daily temperature for optimum growth is 22 to 28°C a reduction in yield occurs above 33°C and below 18°C. Germination is delayed at temperatures below 20°C. Groundnut is considered a day-neutral plant and daylength is not a critical factor influencing yield. For good yields, a rainfed crop requires about 500 to 700 mm of reliable rainfall over the total growing period.
The crop is best adapted to. well-drained, loose, friable medium textured soils. Heavy textures cause problems in lifting the crop at harvest. Also, the top soil should be loose to allow the pegs (on which the fruits are formed) to enter the soil easily.
Being a legume, groundnut can fix nitrogen from the air. However, a pre-planting nitrogen application of 10 to 20 kg/ha is often recommended to assure good crop establishment. Phosphorous requirements are 15 to 40 kg/ha; potassium requirements 25 to 40 kg/ha. A too high application of potassium can cause a decrease in yield. For proper kernel formation and pod-filling, 300 to 600 kg/ha of calcium i s required at the beginning of pod formation in the top soil where the fruits are formed. Limestone is used when soil acidity needs to be corrected and gypsum when only the Ca level needs to be increased. At pH lower than 6, liming may be necessary to avoid aluminium and manganese toxicity.
Groundut during ripening
The graph below depicts the crop stages of groundnut, and the table summarises the main crop coefficients used for water management.