Soil management for high production levels

The impact of soil profile formation on agricultural productivity has been increasingly evident, especially in crops where rainfall is insufficient or irregular. Losses in productivity are more significant when insufficient precipitation levels coincide with periods of flowering, maturation and grain filling, stages of crops that require a high water supply.

Root growth in depth with the aim of greater tolerance to water stress has been one of the main objectives that drove the search for soil profile formation (Figure 1). However, in addition to improving the efficiency of using the water available in the profile, the greater development of plant roots brings several other benefits, such as the efficient recovery of nutrients that can be leached, better interaction with the microbiota and greater capacity for tolerate attack by pests and pathogens. Therefore, even for areas where water supply is not a limitation, the formation of the profile will enable more adequate development of plants, increasing their ability to tolerate biotic and abiotic stresses.

The formation of the soil profile is often associated only with the greater availability of nutrients distributed to soil layers below 20cm deep, giving greater relevance to chemical attributes. However, the construction of the profile goes beyond increasing nutrient levels, as parameters related to soil physics and microbiology have a great influence on the development of roots and, consequently, plants. Therefore, these factors must be worked together to enable greater nutrient balance, microbial diversity and soil structuring.

Regarding the supply of nutrients for the formation of the soil profile, although all macronutrients and micronutrients are essential to plants, some assume greater importance depending on the characteristics of Brazilian soils and the nutrients themselves. Calcium, for example, participates in several essential functions for the proper growth of plants, mainly related to root development, as it is a component of the cell wall. This effect is clearly demonstrated in Figure 2, where increased calcium availability in the soil promoted a linear growth in sugarcane root mass.

Despite the important role of calcium in adequate root growth, the supply of this nutrient in Brazilian soils is often low throughout the entire soil profile. Additionally, due to their limited mobility in plants via the phloem, root growth in depth is dependent on their availability throughout the soil profile.

Another element that has a great influence on root growth is aluminum, which in the Al3+ form is toxic to root growth, reducing its length and consequently the absorption of nutrients by plants. In their natural state, most Brazilian agricultural regions have high levels of aluminum throughout the profile and, for areas already corrected, it is common to have inadequate levels of aluminum saturation in deeper layers (below 40cm). This condition limits root growth to the superficial layers of the soil, where correction was normally carried out using limestone.

The reduction in aluminum toxicity and the increase in calcium levels at depth favor the growth of the plants' root system. The use of limestone for this purpose promotes an increase in calcium levels and a reduction in aluminum toxicity, mainly in the layer where it is incorporated, but it has little effect on deeper layers. The combined application of limestone with sources that have greater solubility, such as gypsum, can be an alternative for increasing calcium levels in the profile and reducing aluminum toxicity without the need to disturb the soil, which is known to have a negative effect on other parameters.

This effect is demonstrated in Figure 3, where the application of limestone and gypsum promoted an increase in calcium levels up to 60cm deep after one year of application when compared to the isolated application of limestone.

In addition to factors related to nutrient supply, the construction of the profile is influenced by the structure of the soil. This structure is composed of the aggregation of soil particles with mineral and organic components, which results in the formation of macro and microaggregates. This property influences root development, microbial activity, water infiltration and retention, organic matter and a multitude of other properties.

The management used in agricultural areas has a great influence on the structure of the soil, both positively and negatively. Practices that provide an increase in the input of plant biomass, formation and maintenance of straw and minimal soil disturbance favor good soil structure. On the other hand, operations with inadequate humidity and soil disturbance are good examples of management that often cause deformation of soil aggregates and result in the formation of zones that impede root growth, reducing the plant's ability to use available water and nutrients. (Figure 4).

Among the management practices that contribute to good soil structure is the use of cover crops, which provide numerous benefits to the system. This effect occurs mainly due to the greater contribution of plant biomass, both aerial and roots, in addition to the greater diversification of plant materials with different root growth and decomposition times. Some cover crops have been widely used in Brazil, such as millet (Pennisetum sp.), brachiaria (brachiaria sp.), oats (Avena sativa/Avena strigosa), turnip (Raphanus sativus) and sunn hemp (crotalaria sp.)

The way in which cover crops are implemented within the production system varies greatly between regions and properties, however the most common has been their sowing after the harvest of the summer crop, which in most areas is soybeans. This normally occurs at the end of the rainy season in most regions, imposing different scenarios of water availability for the growth of cover crops. Therefore, the choice of the cover plant needs to consider, in addition to the main objective of its implementation, the environmental conditions to which it will be subjected and which condition its development.

Cover crops play an important role in shaping the soil profile, as they condition its chemical, physical and biological properties. From the point of view of soil chemistry, cover crops promote the cycling and incorporation of nutrients, enabling greater efficiency in their use and an increase in organic matter content in the soil. In the case of physical and biological attributes, cover plants help to structure and protect the surface layers of the soil, reducing thermal oscillation, promoting greater water infiltration and reducing the evaporation rate. The sum of these factors, associated with the input of plant residues, contributes to an increase in the biodiversity of microorganisms in the soil, creating a more balanced environment between beneficial and phytopathogenic microorganisms.

The achievement of higher production levels and the stability of these potentials is directly related to the ability of agricultural crops to tolerate stress situations, whether biotic or abiotic. The construction of the soil profile plays a fundamental role in the development of plants with deep and voluminous root systems, making them more resilient to the numerous stresses to which they will be subjected. Given this scenario, to construct the profile, the factors of soil chemistry, physics and biology must be considered and managed together, in order to create soil environments that support high levels of production. 

Larissa Gomes Araújo Tormen, Phytus Institute