Physical and Economic Optimum of Response Model for NPK Application in Irrigated Groundnut ( Arachis hypogaea L.)

Field experiments were conducted to work out the physical and economic optimum of NPK fertilizers required for groundnut by using empirical model. The experiment was conducted in Randomized Block Design (RBD) with different NPK levels viz ., Control, 25 % Recommended Dose of Fertilizers (RDF) (4:9:14 kg NPK ha -1 ), 50 % RDF (9:17:27 kg NPK ha -1 ), 75 % RDF (13:26:41 kg NPK ha -1 ), 100 % RDF (17:34:54 kg NPK ha -1 ), 125 % RDF (21:43:68 kg NPK ha -1 ), 150 % RDF (26:51:81 kg NPK ha -1 ), 175 % RDF (30:60:95 kg NPK ha -1 ) and 200 % RDF (34:68:108 kg NPK ha -1 ). The experiments were conducted during rabi 2006-’07 and kharif 2007. The results revealed that among the different levels of NPK studied, application of 175 per cent recommended dose of fertilizers (30:60:95 kg NPK ha -1 ) registered significantly higher growth characters, yield parameters, yield and economic returns. The response model worked for different levels of NPK during rabi 2006-07 and kharif 2007 seasons indicated that the physical optimum levels for groundnut was 33.9 kg N, 67.5 kg P 2 O 5 and 107.3 kg K 2 O ha -1 , whereas the economic optimum was worked out to be 33.7 kg N, 65.8 kg P 2 O 5 and 105.5 kg K 2 O ha -1 .

Groundnut (Arachis hypogaea L.) is an important oilseed crop in India and commonly called as poor man's nut. India ranks first in acreage (6.41 m ha) which accounts for 23.87 per cent of the total world groundnut area and contributes 20.89 per cent (9.36 m t) to the world production (Damodaram and Hegde, 2000). The average productivity of groundnut in India is 1125 kg ha-1 which is below the world's average pod yield of 1449 kg ha-1. In Tamil Nadu, groundnut is cultivated in an area of 0.53 m ha with a production of 1.1 m t and the average productivity is 1724 kg ha-1, which is higher than global average (www. agricoop.nic.in).
The removal of major nutrients (NPK) at the present level of crop production has been estimated at 125 kg/ha whereas, the annual addition is hardly 75 kg/ha, resulting in depletion of nutrient reserves of soil (Tandon, 1994). Groundnut removes fairly large quantities of nutrients from the soil and, therefore, it depletes the soil nutrients rapidly unless the soil is adequately manured. Adequate manuring not only improves the yield but also maintains the soil health and sustains the productivity (Lourduraj, 1999). Ghosh et al. (2002) stressed that proper fertilizer management for groundnut crop with right kind of nutrients at right time adapting right method of application has significant effect on yield and quality.
Addition of N fertilizer generally increases rootshoot ratio and pod yield of groundnut. On the other hand, phosphorus is an important nutrient for all crops in general and legumes in particular. It is a key constituent of ATP and plays significant role in energy transformations in plants and also in various roles in seed formation (Sanker et al., 1984). Phosphorus application increases groundnut yield and yield contributing characters. In addition, K has a beneficial effect on N fixation and transformation of photosynthates from the leaves to the root nodules (Savani et al., 1995). Balanced fertilization of essential plant nutrients, particularly N, P and K in optimum quantity through appropriate method at suitable time in proper proportion always resulted in yield improvement.
Groundnut being a leguminous crop, basal application of little dose of fertilizer nitrogen is recommended by several workers. However, for the above said reasons, the recommended dose of NPK (17:34:54 kg ha-1) needs to be rescheduled. The application of entire nitrogen at the time of sowing was not preferable as compared to split application, since the pattern of maximum uptake was between 48 and 72 days after sowing (Panikar, 1981). At this juncture the present practice of basal application of 100 per cent of the recommended dose of NPK fertilizers in groundnut needs to be reviewed. Hence, the present investigation to work out the physical and economic optimum of NPK fertilizers for irrigated groundnut by employing empirical models was attempted.

Materials and Methods
Field experiments were conducted at the Regional Research Station, Tamil Nadu Agricultural University, Vridhachalam during rabi 2006-07 and kharif 2007 seasons. The soil of the experiment plot was sandy loam in texture, natural (pH 7.0), low in organic carbon (0.5%), low in available nitrogen (232 kg/ha), low in available phosphorus (13 kg/ha) and low in available potassium (243 kg/ha). The experiment was conducted in Randomised Block Design (RBD) with three replications and different NPK levels viz., Control, 25 % RDF (4:9:14 kg NPK ha-1), 50 % RDF (9:17:27 kg NPK ha-1), 75 % RDF (13:26:41 kg NPK ha-1), 100 % RDF (17:34:54 kg NPK ha-1), 125 % RDF (21:43:68 kg NPK ha-1), 150 % RDF (26:51:81 kg NPK ha-1), 175 % RDF (30:60:95 kg NPK ha-1) and 200 % RDF (34:68:108 kg NPK ha-1). The gross plot size was 5.0 m by 4.0 m, and one seed was planted per station at 10 cm within-row and 30 cm inter-row spacing. Irrigation was done as and when required. Seed treatment of Trichoderma viride was given @5 g/kg seed at the time of sowing. Need based plant protection measures were followed. The field was kept weed-free by hand weeding and moisture stress was avoided through irrigation. All recommended cultural practices and pest control operations were carried out as required to grow a good crop. The plants from each plot were labeled and kept separated. Various yield components such as number of pods/ plant, 100 kernel weight, shelling percentage, pod yield and haulm yield were taken at harvest. The statistical analyses of the experimental data were performed as per Gomez and Gomez (1984).

Response model Quadratic response model
The quadratic model was proposed by Heady and Pesek (1954) to provide a continuous and differential response function that accommodated interactions with other nutrient forms. This model has found widespread use because of its simplicity.
Let 'y' be the yield in kg ha-1 'x' be the applied nutrient and a linear equation between and Z x could be estimated

Growth and Yield characters
Groundnut growth in terms of its dry matter production was more pronounced with application of 200 per cent of recommended dose of fertilizers (34:68:105 kg NPK ha-1). The least dry matter production was noticed with control. The excess dose of NPK application reported that increased dry matter production significantly with the enhanced rate of NPK supply at various stages of growth up to harvest.. This is in conformity with the finding of Barik et al. (1998).
On perusal of data on productivity of groundnut during rabi 2006-07 and kharif 2007 seasons, it is evident that the results were significantly favourable for application of 175 per cent of recommended dose of fertilizers (RDF) (30:60:95 kg NPK ha-1 ). The number of matured pods per plant, hundred kernel weight and sound matured kernel per cent were also higher under application of 175 per cent of recommended dose of fertilizers (RDF) (30:60:95 kg NPK ha-1) ( Table 1 and 2). The increase in the yield components by using the higher level of NPK might be due to the abundant nutrients in the soil solution which would have facilitated nutrients absorption through roots. These results are in full agreement with those obtained by El-Far and Ramadan (2000), Laxminarayana (2004) and Hossain et al. (2007), have also reported that increasing rate of NPK application, recorded higher yield and yield attributes of groundnut.
Among the different levels of NPK, application of 175 per cent of RDF registered significantly higher pod yield of 2634 and 2577 kg ha-1 during rabi 2006-'07 and kharif 2007 seasons respectively. However it was on par with application of 200 per cent of RDF (2618 and 2566 kg ha-1 ). Saxena et al. (2003) reported that pod yield of groundnut could be increased with increasing levels of N and K. Similar results were also reported by Kachot et al. (2001). The per cent increase in the pod yield was 13.4 and 12.4 per cent higher than 100 per cent RDF, during rabi 2006-07 and kharif 2007 seasons, respectively (Table 1 and 2). Moreover, the positive influence of these treatments through immediate supply of nutrients from inorganic sources especially at the early stage of the crop and slow and steady supply of nutrients from NPK as well as soil throughout the crop growth period might have improved adequate biomass production and improvement in yield parameters resulting in higher pod and haulm yield. Application of 175 per cent RDF (30:60:95 kg ha-1) significantly increased the pod yield, which was 66.3 and 62.0 per cent increase over control, during rabi 2006-07 and kharif 2007 respectively. In respect of groundnut, because of higher dry matter production, higher pod yield was reported by Nalawade and Patil (2000) and Sanjeev Kumar and Shivani (2001). Higher pod yield due to increase in yield parameters like number of matured pods per plant, pegging percentage and hundred kernel weight as reported by Samui and Ambhore (2000) and Sanjeev Kumar and Ngachan (2001). However higher rates of fertilizer addition beyond 175 per cent RDF showed a decline in yield and the higher reduction in yield was noticed with 200 per cent of RDF (34:64:108 kg NPK ha-1). Many workers including Obigbesan and Agboola (1978) and Kayode (1985) have reported in earlier studies that these application of higher amount of nutrients could result in the loss of nutrients easily from the soil through leaching (N), emission of gases (N and P) and possibly K fixation which is not lost from the soil but unavailable to the plants. These results were in accordance with the findings of Chitdeshwari et al. (2007).

Response model
In both the seasons, the calculated chi-square value was much lesser than chi square table value (15.2 for n-1 of 8) and this indicated that there was no significant difference between the observed value and predicted pod yield at harvest (Table 3). This  Chitdeshwari et al. (2007) also observed that application of higher dose of 34:64:108 kg NPK ha-1 was found to be an optimum dose for getting maximum pod yield in groundnut.

Economics
The application of 175 per cent of RDF (30:60:95 kg NPK ha-1) as compared to 100 per cent of RDF (17:34:54 kg ha-1) registered the highest returns above variable cost (Rs. 40,272 and 39,011 ha-1 and benefit cost ratio (3.28 and 3.21) during rabi 2006-'07 and kharif 2007 seasons, respectively). This treatment was closely followed by application of 200 per cent RDF which registered the returns above variable cost of (Rs. 39,560 and 38,416 ha-1 during rabi 2006-07 and kharif 2007 seasons respectively) and benefit cost ratio (3.19 and 3.13).

Conclusion
The response model worked for different levels of NPK during rabi 2006-07 and kharif 2007 seasons indicated that the physical optimum levels for groundnut was 33.9 kg N, 67.5 kg P2O5 and 107.3 kg K2O ha-1, whereas the economic optimum was 33.7 kg N, 65.8 kg P2O5 and 105.5 kg K2O ha -1 .