**2. Soil electrochemical and physical properties in coffee crops**

Soil physical properties are essential to comprehension of the behavior of the soil when submitted to mechanization. Among the physical properties, the water content in the soil profile determines the reaction to tillage, and soil moisture is the most important for soil-machine interactions, since it controls the consistency of the soil [4] and governs the amount of soil deformation when subjected to external pressure [5].

In aerated soils, pH is considered to control available nutrients to plants. Also, pH is related to soil charges and is a key property expanding double layers of colloids. Points of zero charge (PZC) are pH values associated with specific conditions imposed on one or more surface charge densities of an electrified interfacial region between a soil solution and soil solid phase [6]. Point of zero charge indicates the pH at which the net surface charge on variable charge surface is zero [7].

The surface charge density is the most important physical characteristic of an electrified interface. It can be defined as the number of coulombs per square meter borne by surface functional groups, and it depends in sign and magnitude on the composition of the soil solution and structure of the solid phase to which the functional groups are bound [6].

PZC was estimated from pH-H2O and pH-KCl (1 mol L<sup>−</sup><sup>1</sup> ) through Eq. (1). According to these authors, ∆pH is the electrochemical soil properties that most affect the dispersion. When ∆pH tends to zero, charges are balanced with less dispersion.

$$\text{PZC} = 2 \text{ pH}\_{\text{KCl}} - \text{pH}\_{\text{H2O}} \tag{1}$$

*Coffee - Production and Research*

tent (Fe2O3 higher than 18 dag kg<sup>−</sup><sup>1</sup>

MAPA, 2019).

and harvest.

the total 1.47 million of hectare (≈80%) being cultivated with *Coffea arabica* L. species and 373.33 mil hectares (≈20%) with *Coffea canephora* Pierre (Secretary of Agricultural Policy in the Ministry of Agriculture, Livestock and Food Supply—

density system (>5000 plants per hectare) which guarantees the coffee farmers higher productivity by hectare [2] in soils derived from basalt with high iron con-

drainage is usually very good [3]. On the other hand, in the high coffee population density, spacing between coffee rows is 2–3.2 m and 0.5–0.75 m between the plants in the lines, which compromises the mechanization of operations to management

Soil slope is another important factor that compromises the mechanical operations and coffee management besides coffee planting spacing. In a previous study done in the climatic zoning of the State of Paraná for the cultivation of coffee (**Figure 1**), Höfig and Araujo-Junior [3] showed that 89% of the land area are not

Considering mesoregions of the State of Paraná, the northwest has the smallest area with a slope higher than 20% and therefore the greatest potential for mechanization of coffee plantations. On the other hand, in the Pioneer

*Map of the State of Paraná with slope classes and potential for mechanization in the area with climatic zoning* 

limited to mechanization by the criterion of soil slope classes.

In the state of Paraná, most coffee plants are cultivated in high coffee population

). In these soils, the workability is easier, and the

**46**

**Figure 1.**

*to cultivate coffee. Source: Höfig and Araujo-Junior [3].*

#### *Coffee - Production and Research*

where PZC is the point of zero charge, dimensionless; pHkCl is the hydrogenionic potential determined in potassium chloride solution (1 mol L<sup>−</sup><sup>1</sup> ); pHH2O is the hydrogenionic potential determined in water.

$$
\Delta \mathbf{pH} = \mathbf{p} \mathbf{H}\_{\text{KCl}} - \mathbf{p} \mathbf{H}\_{\text{H2O}} \tag{2}
$$

These electrochemical properties are affected by soil management and are related to soil physical properties like water-dispersible clay, aggregation indexes, and infiltration rate.

Water-dispersible colloid (WDC) is generally recognized as the fraction of clay that disperses in water. Dispersion is the ultimate state of breakdown that results in release of clay particles as a consequence of expanding double layers and dominating repulsive forces [8]. WDC is affected by the nature of soil including mineralogy [9–11], clay content and application of sewage sludges [7], soil management [8] in terms of crop sequence, application of organic manures [12, 13], soil tillage, and traffic [14] which have been shown to affect dispersion-flocculation of clay in soil structural elements.

Besides liming and gypsum on coffee crop, soil organic matter may have a dispersive or aggregating effect according to the quantity and quality of the fertilizer [12]. These authors observed that the addition of manure at the doses of 23 g kg<sup>−</sup><sup>1</sup> and 30 g kg<sup>−</sup><sup>1</sup> provided dispersion of the clay fraction in electropositive *red-yellow* Latosol caused by negative electric charge balance.
