Production of Caupi Beans and Chemical Attributes of a Latosol Under Soil Correction Levels

The cowpea [Vigna unguiculata (L.) Walp] represents an important source of income and employment in the northeastern region of Para, also being a valuable source of protein in the diet, especially for the rural population. However, the productivity is low, mainly due to the low fertility of the soils used in the region for the cultivation of this plant. In this context, the correction of the factors responsible for the soil limitations that lead to nutrient deficiency, by means of lime, is of great practical importance for obtaining greater yields of crops grown in acidic soils. The objective was to evaluate the effects of five doses of lime in the production of cowpea beans, BR3-Tracuateua and BR2-Bragança cultivars and chemical attributes of a dystrophic oxisoil, at Tracuateua city. The study was a randomized block designed experimental in a factorial (5 × 2) with four replications. The terms of the factorial refer to five doses of limestone and two cowpea cultivars. Liming increased pH and concentrations of phosphorus, calcium and magnesium, reducing the Al concentration and saturation. Lime dosages did not provide significant increases in grain production. The cultivar BR3-Tracuateua was more efficient in all liming levels, producing more than 30% of grain than the BR-2 Bragança.


Introduction
The cowpea [Vigna unguiculata (L.) Walp.], also known as black-eyed peas and/or macassar beans, is a plant that belongs to the Fabaceae family (subfamily Faboideae).Considered a culture of great economic, social, nutritional and functional, it is a good alternative to increase family incomes in many regions (Bennett et al. 2013).
The state of Para stands out as the fourth national producer of cowpea (Freire Filho, 2011).Its annual average production in the period of 2005-2009 was 513.619 t.In the same period, the Northeast annually produced 426.367 t, confirming its superiority over other producing regions.However, it had the lowest average yield (330 kg ha -1 ) compared to other regions (Freire Filho et al., 2012).
The greater extent croplands used in the North are Oxisols and Argisoils, prevalent in 75% of Amazon having high acidity, low cation exchange capacity, high phosphorus fixation and low concentrations of exchangeable bases (Junior Vale et al., 2011).Thus, the establishment of rational cropping systems with the adoption of more appropriate technologies that enable improvement of soil conditions, combined with the use of more adapted cultivars in each region of cultivation can leverage the low average productivity values (330 kg ha -1 ) currently achieved (Furtado et al., 2012).
In the case of soil acidity correction liming promotes neutralization of Al 3+ , pH elevation and supply Ca 2+ and Mg 2+ , allowing the proliferation of roots with positive effects on shoot growth of plants (Natale et al., 2007).
The objective of this experiment was to study the effects of liming on the chemical properties of a Yellow Oxisol and the productivity of two cultivars of V. unguiculata.

Research Methodology
The study was conducted from June to September of 2003, in the Agropecuária Milênio farm, Tracuateua city, mesoregion of the northeastern of Para state, between the geographical coordinates of 00°46′18″ South latitude and 47°10′35″ West longitude of Greenwich.The climate in the region is the Awi type and, according to the National Institute of Meteorology, it is divided into two seasons: rainy from December to May and less rainy from June to November, with an average of 2,500 mm rainfall, 27.7 °C of temperature and 84% of relative humidity per year.
The experiment was established in a randomized block design in a factorial arrangement (5 × 2), referring to the five levels of liming (0; 1; 2; 3 and 4 t lime ha -1 ) and two regional cultivars of cowpea (BR2 Bragança and BR3 Tracuateua), with four replications, consisting of 40 experimental units.

Soil Sampling
The samples collected at soil layer 0-0.2 m from the experimental area, classified as Dystrophic Yellow Latosol sandy texture (Embrapa, 2006), were air-dried and processed in sieve with 2 mm mesh diameter for chemical and granulometric analysis (Table 1).

Soil Characterization
Table 1.Chemical characteristics and grain size of the Yellow Dystrophic Oxisol before the experiment Twenty-five days before planting cultivars BR2 and BR3 Tracuateua Bragança, the application of lime was held and incorporated into the depth of 0 to 0.2 m by means of two disk harrowings and one disking.The applied correction doses (0; 1; 2; 3 and 4 t ha -1 ) corresponded to 0; 0.5; 1; 1.5 and 2 times the need for liming to neutralize exchangeable aluminum of the soil, calculated in accordance with the recommendations of Cravo et al. (2007).
After liming, seeding cultivars was performed with a 0.50 m space between rows and eight plants per linear meter.The experimental area of 1.452 m 2 was divided into plots measuring 48 m 2 .In all plots a basic fertilizer was made up of 150 kg ha -1 P 2 O 5 in a triple superphosphate form, 120 kg ha -1 K 2 O in a potassium chloride form, complemented with a micronutrients fertilizer on the basis of 30 kg ha -1 FTE BR-12.Plants were harvested 72 days after planting.The beans were adjusted to 13% of moisture and determined the productivity (kg ha -1 ).The variables were pH, aluminum saturation, concentration of Al, P, K, Ca, Mg, sum of bases and grain yield.

Statistical Analysis
The results were submitted to analysis of variance, compared by 5% SNK test and when relevant, carried regression studies by adjusting the equations according to the data obtained in the treatments.

Results and Discussion
The pH, aluminum saturation (m%), exchangeable aluminum (Al) and the concentration of phosphorus (P) adjusted to the quadratic regression model.The pH values were better adjusted to the increasing linear regression model growing significantly with the increase of applied limestone rates (Figure 1a).
Figure 1 The  The highest yield obtained for the BR3 Tracuateua was 2188.36 kg ha -1 at a dose of 2.66 t ha -1 of lime and 1677.90 kg ha -1 to 1.80 t ha -1 of lime to the BR2 Bragança.It is noted that the cultivar BR3 Tracuateua required a higher amount of limestone for maximum productivity, leading one to believe that this cultivar is little tolerant to high acidity.In both cases, the achieved productivity was higher than the regional average of 1,000 kg ha -1 of grains (Cravo et al., 2012).
In a study conducted by Cravo et al. (2012) by testing increasing doses of lime in four cowpea varieties, including the BR2 Bragança and BR3 Tracuateua, it was observed that the BR3 Tracuateua showed to be more sensitive to soil acidity, similar to the results found in this experiment.Cowpea production increases, after liming, have been demonstrated in several research papers (Novais et al., 2007;Cravo et al., 2012).

Conclusions
The application of lime in the studied Yellow Oxisoil provides an increase in the levels of available phosphorus, Ca +2 , Mg +2 , pH and the sum of bases with reduction of Al +3 levels and the percentage of saturation of this element; The culture of cowpea does not respond to liming in yellow oxisoil, with more than 2.4 cmolc dm -3 ; Productivity found among cowpea cultivars was higher in BR3 Tracuateua.

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Figure 2. L