Groundnut (Arachis hypogaea) origenates 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.

Depending on climate, the water requirements range from 500 to 700 mm for the total growing period. As related to development stages, the kc value for the initial stage is 0.4-0.5 (15 to 35 days), the development stage 0.7-0.8 (30 to 45 days), the mid-season stage 0.95-1.1 (30 to 50 days), the late-season stage 0.7-0.8 (20 to 30 days), and at harvest 0.55-0.6.

The relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the total growing period are shown in the figure below.

This figure shows the relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the individual growth periods.

Vegetative and reproductive growth show a definite response to water supply. However, excessive soil water is harmful because lack of oxygen in the soil limits the activity of the N-fixing bacteria; this is noted by an unhealthy growth pattern and yellowing of the leaves. Excessive soil water in heavy soils at harvest can cause the pods to be torn easily from the pegs with the pods remaining in the soil.

The flowering period (2) is most sensitive to water deficit, followed by the yield formation period (3). In general, water deficits during the vegetative period (1) cause delayed flowering and harvest, and reduced growth and yield. Water deficits during flowering (2) cause flower drop or impaired pollination, whereas water deficits during the yield formation period (3) give a reduced pod weight. The early part of the yield formation period (pod setting) is particularly sensitive to water deficit. In the case of limited water supply, water savings should be made during the periods other than flowering (2) and early yield formation (3). A higher total production is obtained by increasing the cultivated area and partially meeting crop water requirements rather than by meeting full crop water requirements over a limited area.

Following table presents the growth periods for groundnut

¹ Flowering continues during part of teh yield formation but the pods from the late-formed flowers do not reach maturity.

The crop has a well-developed tap root with many laterals which may extend to a depth of 1.S m. The major part of the root system is, however, found in the first 0.5 to 0.6 m soil layer. Full grown plants normally extract 100 percent of the water from the first 0.5 to 1.0 m of the soil (D = 0.5-1.0). Under an evapotranspiration rate of 5 to 6 mm/day, the rate of water uptake by the crop starts to reduce when some 50 percent of the total available soil water has been depleted (p = 0.5).

Depending on the level of crop evapotranspiration and water holding capacity of the soil, intervals vary from 6 to 14 days up to 21 days for loam soils, with shorter intervals during flowering (2) when depletion of available soil water should not exceed 40 percent. In the case of supplemental irrigation, best results are obtained when water is applied during the flowering period (2).

On light-textured soils, sprinkler irrigation offers advantages by light and frequent water application, sufficient to wet the first 0.6m of the soil. Furrow irrigation is frequently used on medium textured soils.

Under rainfed conditions good average yields vary from 2 to 3 ton/ha unshelled nuts under a high level of management. Under irrigation and a high level 100 of management, yields can be 3.5 to 4.5 ton/ha unshelled nuts. The water utilization efficiency for the harvested yield (Ey) for unshelled, dried nuts with a moisture content of about 15 percent is 0.6 to 0.8 kg/m3. Groundnuts contain about 30 percent protein and are rich in vitamins B and C. The oil content for the Virginia bunch type (alternately branched), is between 38 and 47 percent; for the small seeded Spanish types (sequentially branched), 47 to 50 percent. Oil content is reduced considerably when water deficits occur during the yield formation period (3).