Aggregatum onion (Allium cepa L var aggregatum Don) is widely grown and consumed in southern states of India viz Tamil Nadu, Andhra Pradesh, south Karnataka and some parts of Odisha and Kerala. Tamil Nadu accounts for five per cent of country’s area under onion and more than 70 per cent of the area is cultivated by small onion. Around 90 per cent of country’s small onion is produced in Tamil Nadu and 10 per cent in Karnataka. However the productivity is low in India mainly due to lack of inappropriate water management practices. The total water availability in Tamil Nadu is 5.5 million hectare meter (MHM) against the demand of 6.8 MHM resulting in a supply-demand gap of 24 per cent. Today lot of emphasis is being given on improving the irrigation practices to increase the water use efficiency and crop productivity and to sustain the production levels. Though India has the largest irrigation network in the world, its irrigation efficiency has not been more than 40 per cent. One of the reasons for the low irrigation efficiency is the predominant use of surface (conventional) irrigation where water use efficiency is very low which is only about 35 to 40 per cent because of huge conveyance and distribution losses (Dinar et al 1997).

By introducing micro-irrigation, it is possible to increase the yield potential of crops by three-fold with the same quantity of water. All these emphasize the need for water conservation and improvement in water use efficiency to achieve ‘more crop per drop of water’. India’s average productivity of onion is 16.41 tonnes/ ha which is lower than world’s average of 20.08 tonnes/ha (Anon 2009). One of the reasons for this low productivity is surface irrigation method predominantly adopted by farmers which results in low productivity. Onion being a shallow rooted (30 cm) crop requires frequent irrigation for achieving better productivity. The water use efficiency is low when surface irrigation methods are followed. By adopting drip irrigation system, high water use efficiency and better productivity can be achieved by always maintaining moisture near the root zone at field capacity. Dingre et al (2012) reported that in onion crop total irrigation water applied through surface and drip system was 840 and 520 mm respectively indicating 39 per cent water saving whereas field water use efficiency of drip irrigation was more by 2.5 times as that of control.

Drip irrigation is the system for accurate application of water to synchronize with the plant needs. Moreover three dimensional water flows with drip irrigation maintain soil moisture around field capacity or slightly above as compared to surface irrigation with moisture content between field capacity to available water capacity due to dominance of unidirectional vertical flow. Drip irrigation is often preferred over other irrigation methods because of the high water application efficiency (40-70%) on account of reduced losses.

The present investigations were done to evaluate and optimize the drip fertigation in aggregatum onion to study the effect of drip fertigation on water use efficiency, yield attributes and yield and economics of aggregatum onion.

The soil of the experimental field was sandy loam with a pH of 7.12 and 6.97 and electrical conductivity of 0.30 and 0.28 dS/m during 2015-16 and 2016-17 respectively. The infiltration rate of soil was 1.85 and 1.86 cm/h, field capacity 23.1 and 23.3 per cent, permanent wilting point 12.5 and 12.3 per cent, bulk density 1.37 and 1.39 mg/m³ and the organic carbon content was 0.48 and 0.50 per cent during both the years respectively. The nutritional status of the soil was low in available nitrogen (238 and 234 kg/ha), medium in available phosphorus (23 and 22.4 kg/ha) and medium in available potassium (387 and 396 kg/ ha) respectively. The experiment consisted of ten fertigation levels viz 100 per cent N through fertigation with 100 per cent P and K applied as basal (T₁), 100 per cent N and K through fertigation with 100 per cent P applied as basal (T₂), 100 per cent N, P and K through fertigation (T₃), 75 per cent N, P and K through fertigation and 25 per cent N, P and K applied as basal (T₄), 75% N through fertigation with 75 per cent P and K applied as basal (T₅), 75 per cent N and K through fertigation with 75 per cent P applied as basal (T₆), 75 per cent N, P and K through fertigation (T₇), 75 per cent N, P and K through fertigation and 25 of 75 per cent N, P and K applied as basal (T₈), 50 per cent N and 100 per cent P and K applied as basal and 50 per cent N applied on 30 DAT (T₉) and absolute control (T₁₀). N, P and K basal application was done through urea, single super phosphate (SSP) and muriate of potash (MoP) and NPK fertigation through 19:19:19 and/or 12:61:0 and/or 0:0:50 or other commonly available water soluble fertilizers.

Drip irrigation was given on alternate days at 100 per cent crop evapo-transpiration (ETc). Irrigation water was pumped through 7.5 HP motor and conveyed to the mainline of 63 mm OD PVC pipes after filtering through sand and disc filters. Ventury was installed in the main line for fertigation. Sub-mains of 40 mm OD PVC pipes were drawn from the main pipes. Laterals of 12 mm LLDPE pipes were drawn at an interval of 1.2 m within line emitters spaced at 50 cm and at a discharge rate of 4 lph from the sub-main. Individual control valves were provided for each treatment for imposing different fertigation treatments. Single lateral was used for each bed containing four rows of onion crop. The wetting diameter of a single dripper was 0.5 m. Sub-mains and laterals were closed at the end with end cap. After installation, trial run was conducted to assess mean dripper discharge and irrigation efficiency. This was taken into account for fixing the irrigation water application time. During the irrigation period an average irrigation efficiency of 90 per cent was observed.

Seeds of Co 5 onion variety were sown in raised beds of 1.2 m width and 5 m length at the rate of 8 kg/ha in a separate field which was surface irrigated and the seedlings of 15 to 20 cm height were transplanted after 50 DAS. Fifty days old healthy seedlings were used for transplanting and one seedling per hill was planted adopting a spacing of 20 cm x 15 cm. Before transplanting, the field was completely made wet by drip irrigation.

The crop was raised with all recommended package of practices except the irrigation and fertilizer application. Treatment-wise onion yield was recorded and total water used, water use efficiency, water productivity, yield attributes, yield and economics were driven out.

The outcome of the investigations indicated the advantage of drip fertigation over conventional method. In aggregatum onion, the treatment comprising 75 per cent N, P and K through fertigation using water soluble fertilizers at weekly interval was found to be the best fertigation treatment for achieving higher water use efficiency, yield and economic returns.

T₁- 100% N through fertigation with 100% P and K applied as basal, T₂- 100% N and K through fertigation with 100% P applied as basal, T₃- 100% N, P and K through fertigation, T₄- 75% N, P and K through fertigation and 25% N, P and K applied as basal, T₅- 75% N through fertigation with 75% P and K applied as basal, T₆- 75% N and K through fertigation with 75% P applied as basal, T₇- 75% N, P and K through fertigation, T₈- 75% of 75% N, P and K through fertigation and 25% of 75% N, P and K applied as basal, T₉- 50% N and 100% P, K applied as basal and 50% N applied on 30 DAT, T₁₀- Absolute control; Total water use included effective rainfall also; Data not statistically analysed

T₁- 100% N through fertigation with 100% P and K applied as basal, T₂- 100% N and K through fertigation with 100% P applied as basal, T₃- 100% N, P and K through fertigation, T₄- 75% N, P and K through fertigation and 25% N, P and K applied as basal, T₅- 75% N through fertigation with 75% P and K applied as basal, T₆- 75% N and K through fertigation with 75% P applied as basal, T₇- 75% N, P and K through fertigation, T₈- 75% of 75% N, P and K through fertigation and 25% of 75% N, P and K applied as basal, T₉- 50% N and 100% P, K applied as basal and 50% N applied on 30 DAT, T₁₀- Absolute control

T₁- 100% N through fertigation with 100% P and K applied as basal, T₂- 100% N and K through fertigation with 100% P applied as basal, T₃- 100% N, P and K through fertigation, T₄- 75% N, P and K through fertigation and 25% N, P and K applied as basal, T₅- 75% N through fertigation with 75% P and K applied as basal, T₆- 75% N and K through fertigation with 75% P applied as basal, T₇- 75% N, P and K through fertigation, T₈- 75% of 75% N, P and K through fertigation and 25% of 75% N, P and K applied as basal, T₉- 50% N and 100% P, K applied as basal and 50% N applied on 30 DAT, T₁₀- Absolute control; Cost of cultivation includes seasonalised drip cost; Data not statistically analysed

References