Productivity and Nodulation Cowpea Inoculated in Function of Phosphorus and Potassium

The cowpea represents one of the most important agricultural species of the brazilian Northeast, and proper management of mineral fertilizer that culture becomes essential to their production. The present work aimed to study the effect of the interaction between potassium and phosphorus in the presence of the inoculant INPA 03-11B in the culture of cowpea for Production Pole of Bom Jesus, Piaui, Brazil. The experiment was conducted in the experimental area of the Federal University of Piaui, Campus of Bom Jesus, PI, about a Quartzarenic Neosol. The experimental design was a randomized block with four replications. The treatments consisted of four levels of potassium (0, 35, 70 and 105 kg K2O ha) and four levels of phosphorus (0, 40, 80 and 120 kg P2O5 ha). Nodule number, nodule dry and fresh biomass, shoot dry biomass and grain yields were evaluated. The potassium and phosphorus levels and their interaction affected nodulation and parameters indicators of production of cowpea. The interaction between the phosphorus and potassium levels favored the nodulation, shoot dry biomass and grain yield. The highest grain yield of cowpea was obtained with the fertilization of 67.73 kg of P2O5 ha and 35 kg of K2O ha.


Introduction
In the North and Northeast region of Brazil the cultivation of cowpea [Vigna unguiculata (L.) Walp.] gives major importance for food production and contributes effectively to generate employment and income in countryside areas.However, despite the socioeconomic relevance of culture, the average yield levels recorded are significantly low, which might be related, among other factors, the natural nutrient deficiencies that occur in soils of the producing regions (Leite et al., 2009), as well as low levels of technology employed by farmers in the cultivation of the crop.
The cowpea is produced in all regions of the state of Piaui in system upland or irrigated form (Amaral et al., 2011) and consists one of the most cultivated crop species, occupying an area of approximately 210,000 hectares (IBGE, 2010).The Production Pole of Bom Jesus, located in the Southwest region of the state, is remarkable both in acreage as production, producing approximately 4,100 tons (IBGE, 2010).However, the soils of this region have low natural fertility and correction and fertilization practices are nonexistent or performed with low frequency.
The application of fertilizers in the soil is a essential practice to increase the availability of nutrients in the soil and promote the growth of agricultural crops.In Vigna unguiculata and Phaseolus vulgaris, the use of phosphate fertilizers (Miranda et al., 2002;Valderrama et al., 2009;Silva et al., 2010;Oliveira et al., 2011) and potassium (Melo et al., 2005;Gualter et al., 2008;Oliveira et al., 2009) promote significant responses in growth and yield of these crops.In sandy soils, the addition of phosphorus at levels ranging between 60 and 80 kg P 2 O 5 ha -1 favored the growth and levels between 60 and 90 kg P 2 O 5 resulted in major yield grain (Silva et al., 2010).The K is the nutrient extracted and exported in larger quantities by cowpea (Oliveira et al., 2009) and the best response to cowpea yield can be obtained with the application of K 2 O levels ranging between 20 and 40 kg ha -1 (Melo et al., 2005).
The availability of nutrients consists one of the main factors influencing crop production and biological nitrogen fixation (BNF), with emphasis mainly for P and K (Gualter et al., 2008).The P is considered the nutrient that most limits the yield of cowpea (Freire Filho et al., 2005), due to its importance to the establishment of nodulation and BNF (Silva et al., 2010).The K has direct participation in activation of various enzymatic systems, which have acting in the processes of photosynthesis, respiration (Oliveira et al., 2009) and BNF (Duke & Collins, 1985).
The present work aimed to study the effect of the interaction between potassium and phosphorus levels in the presence of the inoculant INPA 03-11B in the culture of cowpea for Production Pole of Bom Jesus, Piaui, Brazil.

Material and Methods
The present work was conducted between March and May of 2010, the experimental area of the Federal University of Piauí, Campus of Bom Jesus, city of Bom Jesus, Piaui (09º04'28'' S and 44º21'31"'W and altitude of 277 meters), in a Quartzarenic Neosol.
According to the Köppen (1931) climate is Aw, which is characterized by being hot and humid with summer rains.In this region the average annual rainfall varies between 900 and 1200 mm and the average temperature of 26.5ºC (Viana et al., 2002).
The precipitation and temperature averages between January and June 2010 in Bom Jesus, Piaui are shown in Figure 1.Before installation of the experiment samples were collected of the soil layer from 0 to 0.20 m for the physical and chemical characterization (Table 1).
Before planting, the soil was prepared with light disk harrow.Nitrogen fertilization was via inoculation with the strain INPA 03-11B (Bradyhrizobium sp.), recommended by the Ministry of Agriculture, Livestock and Supply (Brasil, 2011) for the cultivation of cowpea.The inoculant was prepared with autoclaved peat and log phase culture (after six days of growth with a concentration of 10 8 cells g -1 inoculant) in semi-solid 79 medium (Fred & Waksman, 1928) in the proportion 3:1.The dose of 500 g of inoculant per 50 kg of seed was used.
The sowing was carried immediately after inoculation, distributing three cowpea (BR 17 Gurguéia variety) seeds per pit at a spacing of 0.80 x 0.20 m, to obtain, after cutting out 15 days after sowing, a density of 10 plants per linear meter and a density of 125,000 plants ha -1 .The useful area of each plot corresponded the two central lines, ignoring 0.5 m from each end.During the driving test was conducted weed control, to keep the culture free of weeds during their cycle.
In the first evaluation performed during flowering stage at 45 days after planting, ten plants of experimental useful area were collected randomly.Nodule number (NN), nodule dry (DBNod) and fresh (FBNod) biomass, dry biomass of shoot (DBS).
In the second evaluation performed at harvest, 10 plants of each experimental useful area were collected.Number of pods per plant (NPPL), the biomass of pods per plant (BPPL), pod length (PL) and grain yield per hectare (GY) were evaluated.For the calculation of the GY the moisture was adjusted to 13%.
To test the normality was applied Shapiro-Wilks, verifying that all P values obtained from this test were higher than 0.05.After verification of normality, the results were submitted to analysis of variance and regression, using the statistical program SISVAR 4.2 version (Ferreira, 2011).

Results and Discussion
In the first and second evaluation there was significant effect of P 2 O 5 and K 2 O levels for all variables evaluated (Table 2).Significant interactions between the levels of P 2 O 5 and K 2 O were checked to FBNod, DBNod, DBS, NPPL, PL and GY, and individual effects of P 2 O 5 levels for variables NN and BPPL.NN (Figure 2) adjusted to a quadratic equation, with the estimated maximum value of 23.84 nodules plant -1 obtained in the estimated dose of P 2 O 5 (84.08 kg ha -1 ).The greater availability of P, as reported by Okeleye and Okelana (1997), stimulates nodulation due to the transfer of energy in form of ATP, contributing to increase root hair formation and providing more sites of infection to N 2 -fixing bacteria.
FBNod and DBNod (Figures 2 B and C, respectively), the best fit adjusted was quadratic regression model, with the maximum values of 399.68 and 262.33 mg plant -1 in the estimated levels of 93,19 and 62,94 P 2 O 5 , respectively, in the presence of 105 kg ha -1 K 2 O. Higher levels of K 2 O influence on the biomass of nodules, as in conditions of greater availability of this nutrient, there is increased photosynthetic efficiency of cowpea (Oliveira et al., 2009) and higher encouragement of BNF (Duke & Collins, 1985).Mean NN, the FBNod and DBNod show that the symbiosis between plant and microorganism was satisfactory, since the values found are similar to those reported in several studies on nodulation in cowpea inoculated in the presence of phosphorus and potassium fertilization planting (Gualter et al., 2008;Xavier et al., 2008;Costa et al., 2011).DBS adjusted to the quadratic model, with the maximum estimated average of 16.06 g plant -1 , obtained from the estimated dose of 50.84 kg P 2 O 5 ha -1 in the presence of 70 kg K 2 O ha -1 (Figure 3A).It indicates that the soil fertility through the application of P 2 O 5 and K 2 O favored the increase of DBS cowpea, including averages of DBS above those obtained by Almeida et al. (2010) and Costa et al. (2011) in studies using inoculation with rhizobia (strain INPA 03-11B) in the presence of P 2 O 5 and K 2 O do levels reported for culture (Freire Filho et al., 2005) and similar to those obtained by Oliveira et al. (2011) in levels of P 2 O 5 similar study of the response of cowpea to water levels and phosphorus levels in the Cerrado of Roraima, Brazil.This result also indicates that when the cultivation of the crop is done in order to produce biomass to be used as green manure or cover crops, higher content of K 2 O associated with P 2 O 5 , contribute positively to production DBS cowpea.
Among yield components of cowpea, variables NPPL and PL (Figure 3 B, C), were best fitted to quadratic regression model showed maximum values of 7.45 units plant -1 and 18.16 cm plant -1 , in the estimated levels of 55.88 and 59.35 kg ha -1 of P 2 O 5 in the presence of 35 and 70 kg ha -1 K 2 O, respectively.For the variable BPPL significant responses were observed only for levels of P 2 O 5 , expressing average estimated maximum of 3.01 g plant -1 at a dose estimated 64.33 kg ha -1 P 2 O 5 (Figure 3D).
For GY (Figure 4) was observed quadratic regression for the levels of P 2 O 5 .The estimated maximum yield was obtained with the application of 67.73 kg ha -1 of P 2 O 5 in the presence of 35 kg ha -1 of K 2 O.This result is similar to the maximum grain yield of cowpea obtained by Cardoso & Ribeiro, (2006) who used 45 kg of P 2 O 5 and 30 K 2 O in medium textured soil in the city of Teresina, Piaui, Brazil.In this sense, it can be inferred that the estimates of increase in crop yield with application of the respective levels of P and K were approximately 40%, compared to control treatments.It indicates the efficiency of this management in terms of increased grain yield of cowpea.

Figure 2 .
Figure 2. Number of nodules (A), fresh biomass of nodules (B) and dry biomass of nodules (C) of cowpea due to the application of potassium and phosphorus in the presence of the inoculant INPA 03-11B in the southern state of Piaui, Brazil.*, ** -Significant at 5%, significant at 1% by Tukey's test, respectively

Figure 3 .
Figure 3. Dry biomass of shoot (A), number of pods per plant (B), pod length (C) and pod dry biomass of pods (D) of cowpea due to the application of potassium and phosphorus in the presence of the inoculant INPA 03-11B in Southern the state of Piaui, Brazil.ns, ** -not significant, significant at 1% by Tukey's test, respectivel

Table 2 .
Summary of analysis of variance of the number of nodes (NN), fresh biomass of nodules (FBNod), dry biomass of nodules (DBNod), dry biomass of shoot (DBS), number of pods (NPPL), pod biomass (BPPL), length of pods (PL) and grain yield (GY) ns , *, ** -Sources of variation, not significant at the 5% probability, not significant at the 1% probability by Tukey test, respectively.