Inoculation and co-inoculation practices associated with soybeans

For the cultivation of soybeans, nitrogen (N) is the nutrient required in the greatest quantity by the crop, due to the fact that its grains are very rich in proteins and have an average of 6,5% of this nutrient in their composition.

Therefore, to produce 1.000 kilos of soybeans, at least 65 kg of N are needed. In addition, at least another 15 kg of N are needed to maintain leaves, stems and roots, totaling an average need of 80 kg of N to produce 1.000 kg of soybeans.

In this sense, if we consider a production of 3 kg of grains, 000 kg of N are needed, of which 240 kg will be just for the composition of the grains.

The use of nitrogen fertilizers is the fastest way to supply N to plants, so that this nutrient is quickly absorbed and assimilated, however, this practice has a very high cost.

If this were the only way to supply the necessary nitrogen, soybean cultivation would be practically unfeasible, due to its high production cost. However, there are other options for supplying N to plants effectively and at a lower cost, one of which is inoculation with bacteria of the genus bradyrhizobium, which associate with the soybean root system, establishing an important symbiosis that results in the supply of all the nitrogen that the plant needs.

With this in mind, understanding the entire physiological process involving bacteria, plants and nutrient fixation is of paramount importance for both researchers and farmers, as it is through this knowledge that it will be possible to adapt management, aiming for greater efficiency in use of nitrogen and, consecutively, the increase in soybean productivity.

The physiological processes involving biological nitrogen fixation in soybean are quite complex, with several interactions between the plant and the fixing bacteria.

One of these processes is related to rhizobia infection, which involves different signaling agents between the plant and the bacteria. The model proposed by TIMMERS et al. (1999) shows that modulating bacteria migrate towards the roots due to a chemostatic response, which is due to the attraction caused by isoflavonoids and betaines secreted by the roots.

Nodulation itself occurs approximately two hours after contact between the bacteria and the root, which will then begin biological N fixation, which involves a series of processes, starting with the adaptation of the bacteria to the plant and resulting in the fixation of atmospheric N2. .

In addition to inoculating soybeans with bacteria of the genus bradyrhizobium, Another practice that has become increasingly evident is that of coinoculation, which consists of the use of bacteria of the genus azospirillum, which act as plant growth promoters, being able to increase the root system and the volume of soil explored and, thus, influence soybean nodulation and nutrient absorption efficiency due to their ability to stimulate the production of plant hormones in large quantities expressive. 

Also known as mixed inoculation, coinoculation consists of the use of different microorganisms, which when combined produce a synergistic effect, surpassing the productive results obtained with them when used in isolation (FERLINI, 2006; BÁRBARO et al., 2008).

In this way, products based on azospirillum brasilense has been recommended for soybean coinoculation, along with Bradyrhizobium (REIS, 2007), due to the occurrence of enhanced nodulation and greater root growth, in response to the positive interaction between symbiotic bacteria (Bradyrhizobium) and diazotrophic bacteria, especially those belonging to the genus azospirillum (FERLINI, 2006).

Thus, we can consider that inoculation and coinoculation are low-cost practices that increase productivity yields, however, they require favorable conditions for the good performance of microorganisms, some of them are:

·         Adequate population of microorganisms;

·         The temperature, with an optimal range for fixing between 25-32 degrees;

·         Need for soil moisture;

·         pH in the range of 5,5-6,5;

·         Correct nutrition.

In short, we can say that both practices mentioned, be it inoculation or coinoculation, have great potential for greater efficiency in N fixation, which when combined with good cultural and management practices can become an excellent tool for greater sustainability. in agriculture, through the various benefits that are generated for farmers, such as those mentioned previously.