Bean nutritional management

The direct planting system (SPD) has been one of the best alternatives for conserving natural resources in the agricultural use of Brazilian soils. One of the advantages of this cultivation system is the protection it provides to the soil, reducing the impact of raindrops, surface runoff and erosion. However, one of the limiting factors of SPD is the difficulty of permanently maintaining straw on the soil surface, especially in regions with dry winters, where straw production is low.

To try to overcome this difficulty in straw production in the SPD, several alternatives have been sought, one of which is the intercropping of grain-producing plants with tropical forages, which has increased significantly in these regions with low water supply in winter. This technique has been used with the aim of producing forage in the period of lowest supply and at the same time increasing straw production for the SPD in the following harvest. In this system, intercropping corn with brachiaria has been an option widely used by producers.

Among the most used brachiaria species are Urochloa ruziziensis (Syn. Brachiaria bryzantha) and the Urochloa brizantha (Syn. Brachiaria bryzantha). Several studies carried out at Universities and Research Institutions have demonstrated that the intercropping of corn with U. brizantha promotes high biomass production and adequate soil coverage, does not reduce corn productivity and in some cases can even increase it.

In this scenario of agricultural production, mainly in the Center-South region of Brazil, one of the main annual crops cultivated in the SPD is beans (Phaseolus L.). In this system, bean cultivation has benefited from the inclusion of forage grasses in the production system, which has allowed bean yields to be obtained well above the national average. However, as the bean plant is a demanding crop in nitrogen (N), even in well-managed SPD, it is necessary that the supply of N to the bean plant is made adequately, in order to obtain high productivity ceilings, since the supply Deficiency of N in beans causes a reduction in grain productivity.

Nitrogen fertilization of beans has normally been carried out with the application of part of the N in the sowing furrow, together with phosphorus and potassium and the remainder as top dressing. However, in some situations the use of high amounts of N in the sowing furrow, mainly in association with potassium fertilization, can cause a reduction in the plant population and compromise productivity. On the other hand, the application of N in top dressing increases the cost of production and can cause damage to the bean plant, caused by the traffic of agricultural machinery within the crop. Furthermore, supplying N to the bean plant at a later stage of the cycle is less efficient than applying N at the initial stage of crop development.

Some research has shown that the application of N in SPD, broadcast or incorporated even before sowing the beans, results in yields equal to or even greater than in those situations in which N is applied as cover. However, despite research indicating the possibility of anticipating N for the bean plant in SPD, there are still doubts as to whether this technique can be routinely used in conditions in which the bean plant is grown in succession to corn intercropped with perennial forages, as in this condition normally the production of plant mass and the amount of straw on the soil surface are greater.

In this sense, research was developed at the Faculty of Agricultural Sciences of Botucatu, under the coordination of professor Rogério Peres Soratto, with the aim of better understanding the bean plant's response to times of application of nitrogen fertilizer, in a direct planting system after single corn or combined with brachiaria (U. bryzantha). In this case, the forage was sown by mixing 10kg/ha of seeds (VC=24%) with the corn base fertilizer, summer harvests 2004/2005 and 2005/2006. Thus, the brachiaria seeds were distributed at the same depth as the fertilizer and simultaneously with the corn sowing. After harvesting corn from the 2005/2006 harvest, the area was only managed with a straw crusher every six months until beans were sown approximately a year and a half later. At the time of sowing the beans, the areas (single corn and corn + brachiaria) had very different amounts of straw.

In this study, three times of application of a dose of 100kg/ha of N were investigated in bean plants grown after single corn or corn intercropped with brachiaria. The N application times were: 33 DAS - 33 days before sowing the beans (19 days before the plants present in the area desiccated); SEM - on the same day as sowing the beans; 23 DAE - 23 days after bean emergence; and a control without application of N. At all times, N was applied to the soil surface, using ammonium sulfate as a source.

The results obtained in the study demonstrated that in the corn + brachiaria consortium, straw production immediately before sowing beans was 20.415kg/ha of dry matter with an amount of N present in this straw of 376kg/ha. In the single corn system, straw production and the amount of N accumulated in this vegetation cover were much lower and did not exceed 12.311kg/ha of dry matter and 251kg/ha of N. This was one of the factors that directly interfered with nitrogen nutrition. of the bean plant grown in succession (Table 1).

In the presence of nitrogen fertilization, N levels in bean leaves were not influenced by the previous cultivation systems (Table 1). However, when nitrogen fertilization was not applied, foliar N levels were higher in bean plants grown in succession to the corn + brachiaria consortium, indicating that the inclusion of forage in the system provided a greater amount of N for beans grown in succession, due to the greater amount of straw and N accumulated in this straw. Furthermore, it was observed that nitrogen fertilization increased the foliar N content of the bean plant, however, significantly only in the beans grown after single corn. It is worth highlighting that in this study, the corn + brachiaria consortium was cultivated in the area for two consecutive years (summer harvests 2004/2005 and 2005/2006) and that the forage remained in the area for approximately 18 months, between the harvest of the last harvest of corn and desiccation for sowing beans, providing high biomass accumulation.

One of the symptoms that a deficient N supply causes in beans is reduced growth and branching of the crop, which results in plants with fewer flowers and pods. In this study, it was observed that the number of pods per plant increased with nitrogen fertilization, only when the bean plant was grown in succession to single corn (Table 1). In treatments fertilized with N, the previous cultivation of the forage in intercropping did not affect the number of bean pods, because there was no N restriction for the plants. However, when N was not applied, the bean plant grown in succession to the intercrop produced more pods. This demonstrates that the presence of forage in consortium with corn in the previous cultivation improved the nitrogen nutrition of the bean plant in succession, resulting in larger plants, with greater branching, number of flowers and consequently a greater quantity of pods, even without application of mineral nitrogen fertilizer. .

Regarding bean grain productivity, it was observed that regardless of the time of fertilizer application, there was an average of 33% increase in grain productivity with the supply of N, when beans were cultivated in succession to single corn (Table 1) . This indicates that it is possible to anticipate nitrogen fertilization in beans grown in SPD, as high doses of N in the sowing furrow can compromise the plant population. And application as cover, in addition to increasing production costs, can cause damage to the crop due to the traffic of agricultural machinery. Furthermore, the anticipation of N in bean plants can be beneficial, as high concentrations of nutrients in the initial stages of development can promote good initial growth of beans in SPD, as there is a lack of N in the initial phase of development for the main annual bean crops. grains. The bean plant must absorb most of its nutritional needs in the vegetative phase, transforming into a vigorous, tall and strong plant to, in the future, form the grain. This implies that the plant must be well formed before reaching the reproductive phase to achieve good grain productivity.

When cultivated in succession to intercropped corn, the beans did not respond to nitrogen fertilization (Table 1), and even without the application of N (control), productivity was almost 3.000kg/ha. This result demonstrates that the inclusion of forage (Brachiaria brizanta) in the previous cultivation reduces the need for N application in the bean plant grown in succession, given the high nutrient cycling capacity of Brachiaria.

However, it is worth highlighting that in this study there was a good distribution of rainfall during the development of the crop, including immediately after N applications, which favored the supply of N and the productive performance of the bean plant. Furthermore, the soil at the site was clayey and had a good content of organic matter in the surface layer. This, associated with the high accumulation of decomposing residues on the soil surface, may have contributed to reducing the losses of N applied in advance. However, in sandy soils, with a low organic matter content, with limited fertility and with a reduced amount of plant residues on the surface, the anticipation of N may not be an efficient practice.

Another aspect that must be taken into account is that when urea is used as a source of N, its application to the straw can cause large losses due to volatilization.

Table 1 - Effect of nitrogen fertilizer application times on N content in leaves, number of pods per plant and grain yield of beans grown after single corn or corn intercropped with brachiaria(1)

N(2) application time	Predecessor culture
	Single corn	Corn + brachiaria
	N content in leaves (g kg-1)
Witness	29,4bB	37,9bA
33 THE	41,1aA	40,0abA
SEM	42,6aA	45,6abA
23 DAE	42,9aA	48,3aA
	Number of pods per plant
Witness	8,4bB	12,3aA
33 THE	14,0aA	14,3aA
SEM	12,4abA	14,4aA
23 DAE	15,9aA	14,1aA
	Grain productivity (kg ha-1)
Witness	2.549bA	2.939aA
33 THE	3.366aA	3.072aA
SEM	3.538aA	2.850aB
23 DAE	3.258abA	3.557aA

N(2) application time

Predecessor culture

Single corn

Corn + brachiaria

N content in leaves (g kg-1)

Witness

29,4bB

37,9bA

33 THE

41,1aA

40,0abA

SEM

42,6aA

45,6abA

23 DAE

42,9aA

48,3aA

Number of pods per plant

Witness

8,4bB

12,3aA

33 THE

14,0aA

14,3aA

SEM

12,4abA

14,4aA

23 DAE

15,9aA

14,1aA

Grain productivity (kg ha-1)

Witness

2.549bA

2.939aA

33 THE

3.366aA

3.072aA

SEM

3.538aA

2.850aB

23 DAE

3.258abA

3.557aA

(1) Averages followed by the same letters in the column, within each factor, do not differ from each other using Tukey's test at 5% probability. (2) Dose of N (100kg/ha-1) applied by broadcast at times: 33 DAS – 33 days before sowing the beans (19 days before the plants present in the area dry out); WITHOUT – on the same day as sowing the beans and; 23 DAE – in coverage on the bean plant 23 days after emergence.

Click here to read the article in issue 178 of Cultivar Grandes Culturas.