**1. Introduction**

It is well known that near the roots of plants, at the zone called rhizosphere, there are several types of microorganisms, whether pathogenic or organic matter decomposing organisms (saprophytes,) or even those organisms that live in symbiosis with the plant as is the case of mycorrhizae [1].

The mycorrhiza is the positive association between the arbuscular mycorrhizal fungi (AMF) and roots of plants, which is named as mutualistic association. Mutualism is an ecological relationship between individuals of different species, in which both benefit from the interaction. Since it occurs among individuals of different species, it is a relationship called interspecific, and due to it benefits everyone involved, it is called a harmonic relationship. As its name says "mycorrhiza" comes from fungi and the "rrizo" that are the roots. Recently, these associations between fungi and roots have drawn attention as a scientific research topic when was discovered the ability of this mycorrhiza in increasing the absorption of nutrients by plants. As an example, the researchers have found a great ability of these fungi to increase the uptake of phosphorus by plants, especially in high-leached and poor soils. It is important to understand that the mycorrhiza will not increase the phosphorus content in the soil, as others solubilizing microorganisms do, these fungi will help enhance the absorption by roots as it will increase the specific surface area of that plant [2]. It is important to have not only the phosphorous (P) but also all essential elements (nutrients) available in the soil so that they can perform their effect in nourishing the plant. For this, and other facts, it is important to combine the biological with the chemical factors in soil for a healthier and more balanced environment.

The AMF are classified into two major groups. The first one is the ectomycorrhizae (ECM), which are those fungi that will not penetrate the plant or will not penetrate the roots of the plants. They will live there around the plant and over the roots, forming a kind of "ball" (called mantle) and Hartig net. This mantle will not only have the function of absorbing nutrients it also protects against some pathogens. This type of ectomycorrhizae association will happen mainly in forest species [2]. Another classification of mycorrhizae is endomycorrhizae. They have the ability to penetrate the roots of plants through the hyphae, which form structures within the cortical cells and also grow intercellularly. Inside the cell, those hyphae swell forming a kind

of nutrient storage organ, which those elements will be used in the metabolism of that plant [1]. Therefore, the AMF receives carbohydrates provided by roots while the fungus transfers nutrients and water to the plant roots [3].

The endomycorrhizae occur mainly in crops of agricultural interest. These species are symbiotic obligatory since they need this lifestyle to survive. So, this kind of fungus will always have its host root being the connection between the plant's roots and soil. There are several species of fungi to establish this kind of connection and association, in which each species has its own ability to absorb a specific nutrient. Thus, depending on the species, they will absorb a particular nutrient or much more one kind of nutrient than others [1].

The soil system is interesting to have large varieties of these microorganisms, as each one will bring different benefits to the plants according to their specificities. It is important to emphasize that human being can directly or indirectly induce mycorrhizal survival and abundance in the crop (Hartmann et al., [4]). So, as we are working with microorganisms, which are living organisms, it is necessary to give specific conditions for their proper establishment in the soil. Inoculating various beneficial microorganisms such as mycorrhizal fungi in an attempt to bring benefits to the crop without appropriate and healthy soil management will be a waste of time and an expense. We cannot forget that soil health is the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals and humans, and connects agricultural and soil science to policy, thus soil health is not only focused on crop production, but also includes the role of soil in water quality, climate change, and human health [5]. As we can see, the soil is multifunctional and quite complex, so we must seek its most complete balance.

The soil management needs to be adequate for full-plant growth. For example, the excess of fertilizer and pesticides, and low addition of organic matter will affect the soil health, since the microorganisms are directly affected. Also, the monocrop causes negative impacts on the soil microorganisms, since the input of organic residue is limited from a single crop. It is important to keep in mind that the soil is alive and its biological compartment is as important as the chemical and physical factors [2]. Thus, based on what has been discussed, this chapter covers the main positive points under the symbiotic association between mycorrhizal fungi and agricultural crops, in addition, we discussed the main agronomic practices that negatively and positively impact the soil quality.
