**1. Introduction**

Overall, about 70% of Brazilian soils are represented by Oxisols, Ultisols and Entisols, which are soil classes of predominantly low fertility. Thus, agricultural production might be restricted if there is no nutrient addition to the soil [1]. Mineral fertilizers are often the first choice used to improve the chemical properties of soil [2,3]. However, organic materials such as plant residues can also play an important role in the improvement of tropical agriculture systems. After decomposition, the organic materials provide nutrients and substrate for the synthesis of organic matter in the soil [4]. The chemical, physical, and biological properties of soils can be greatly improved using alley cropping, which represents an accessible option for the addition of organic matter to the soil [2].

Alley cropping involves the cultivation of annual crops among the hedgerows of multipurpose trees. Plant residues from the leguminous trees can be used as organic fertilizers, promoting improvements in soil fertility [5,6]. The benefits of this system of production include surface cover with plant residues, nutrient recycling, the biological fixation of atmospheric nitrogen (BNF), and the increase of the bearing capacity of the soil [7–9].

Alley cropping is a viable option to increase biomass production per unit area. Since the plant residues can be incorporated into the soil, the transference of nutrients from trees to annual crops can also occur [8]. Furthermore, since the leguminous crops used in alleys present a deep root system, the interception of percolated nutrients along the soil profile can occur, and nutrients accumulated in layers below the root zone of annual crops can be accessed. These nutrients absorbed by the root system of the trees become inputs when transferred to the soil surface in the form of litter and other plant residues [10].

Leguminous alleys disposed in annual crops represent relevant N inputs by biological fixation, reducing the need for N fertilization. For example, *Leucaena* (*Leucaena leucocephala* (Lam.) de Wit.) and acacia (*Acacia Mangium montanum* Rumph.) arranged in maize (*Zea mays* (L.) alleys produced large amounts of N due to the increase of biomass and soil fixation [11]. There is evidence that maize cultivated in *Gliricidia* (*Gliricidia sepium* (Jacq.) Kunth ex Walp.) alleys increase their foliar N content by up to 5 g kg−<sup>1</sup> when compared to single maize cultivation with mineral fertilization [12]. Legumes produce organic matter of greater bioavailability, which can also increase the cation exchange capacity (CEC) of sandy soils [13].

Among the trees and shrubs used in alley cropping systems, *Gliricidia* is widely used in the Brazilian northeast [5,8,14]. *Leucaena* is also common in alley cultivation. Though *Leucaena* has a higher competitive e ffect when compared to *Gliricidia*, it produces higher amounts of residues [11–15]. These species are widely used both in the incorporation of biomass into the soil and in animal feeding, and they are usually cut two to three times per year [5]. Furthermore, they are considered drought-resistant species that produce large amounts of biomass with high N levels and fast decomposition rates [16]. However, only a few scientific studies have thus far focused on the cultivation of forage sorghum (*Sorghum bicolor* (L.) Moench) in leguminous alleys, especially in areas of livestock activity [17,18].

Forage sorghum belongs to the Poaceae family, and it is among the most cultivated species in the world. Sorghum is widely used by farmers for forage production due to its high percentage of leaf and stem production when compared to other plant species. There are two categories of sorghum: Specific cultivars for grain production and specific cultivars for forage production [18]. Therefore, its high drought adaptability, high dry mass yield, and high nutrient recycling capacity make this crop attractive for forage production [13].

Agricultural crops, especially annual crops, require adequate fertility levels for their development. Therefore, the adoption of leguminous alleys in forage sorghum cultivation for sustainable soil fertility managemen<sup>t</sup> represents a grea<sup>t</sup> option for nutrient input, especially for resource-poor farmers [2].

The present study was based on the hypothesis that the presence of leguminous alleys in forage sorghum cultivation would promote greater growth and development, as well as higher foliar macronutrient contents and productivity, thus making alley cropping superior to the cultivation of single sorghum. Therefore, the objective of this study was to evaluate forage sorghum cultivation using a combination of leguminous alleys and mineral fertilization.

#### **2. Materials and Methods**

#### *2.1. Experimental Area and Treatments*

Two field experiments were conducted at the School of Veterinary and Animal Science of the Federal University of Tocantins (810751.01; 9213652.69 UTM, with an altitude of 243 m), Brazil. The first experiment was implemented in the agricultural year 2016/2017 and the second in 2017/2018. This region is classified as warm and humid (AW type according to the Köppen classification). The area presents two growing seasons: A dry period with a water deficit from May to September and a rainy period between October and April [19]. The rainfall and the average temperature throughout the experimental period are shown in Figure 1.

**Figure 1.** Average monthly rainfall and temperature for the experimental site during the 2016–2018 growing season.

Table 1 shows the physical and chemical attributes of the soil prior to the cultivation of the first crop cycle. The soil is classified as Entisol (quartzipsamment) [1].

**Table 1.** Data from chemical and physical analysis of the soil in preplant in the experimental site (0.00–0.20 m layer).


pH (H2O) at a ratio of 1:2.5 *m*/*v*; organic matter determined by the Walkley–Black method; available P e K: Mehlich-1 extraction; exchangeable Ca, Mg and Al: Extraction with KCl; H + Al: Extraction with calcium acetate; clay content: The pipette method.

The experiment followed a randomized block design with a factorial arrangemen<sup>t</sup> of 3 × 2 and five replications. The first factor refers to the cultivation system (single sorghum, sorghum cultivated in *Gliricidia* alleys, and sorghum cultivated in *Leucaena* alleys), and the second factor refers to mineral fertilization (the presence and absence of fertilization) (Figure 2). The total area of the experiment was 900 m2, and each experimental unit had a total area of 30 m<sup>2</sup> (6 × 5 m).

**Figure 2.** Scheme of the arrangemen<sup>t</sup> of cultivation systems and mineral fertilization in the experiment.

#### *2.2. Establishment of Gliricidia and Leucaena Alleys and Forage Sorghum*

*Gliricidia* and *Leucaena* were sown in 2013, with a spacing of 6 m between double rows and 0.75 m between single rows and plants. The legumes were only pruned prior to the first sorghum cultivation in 2017, and the biomass that was deposited on the soil surface was composed of leaves and stems. All plots of the same treatment received the same amount of biomass, and all available dry plant residues were added to the soil, which was added according to the dry mass content shown in Table 2. The single sorghum treatment received no plant residue.

**Table 2.** Macronutrient content and dry mass of plant residues deposited between lines of sorghum cultivation (double crop) in March 2017.


The legume biomass that was deposited in the crops was quantified using a metal frame of 0.25 m<sup>2</sup> and dried in a forced circulation oven at 55 ◦C until constant weight for chemical analysis (Table 2). One month after the cutting of the alleys, the planting furrows were manually opened. Sorghum was sown on 31 March 2017, as it was characterized as double crop. Thirteen seeds per linear meter were sown, with a spacing of 0.5 m between rows.

After collecting data on growth, yield, leaf macronutrient levels and forage sorghum productivity of the first experiment, the area remained fallow. In 2018, the alley treatments containing *Gliricidia* and *Leucaena* were again pruned, and the residues were deposited between the rows of the subsequent single sorghum cultivation, which was characterized as a crop. The experimental procedures, the cultivar, and the amount of biomass deposited were the same as the previous year, and planting was carried out on 13 January 2018.

Planting and fertilization were only carried out in plots containing mineral fertilization with nitrogen-phosphorus-potassium (NPK), according to the requirements of the crop: 20 kg ha−<sup>1</sup> of N, 90 kg ha−<sup>1</sup> of P2O5, 75 kg ha−<sup>1</sup> of K2O, and 30 kg ha−<sup>1</sup> of micronutrients based on fritted trace elements (FTE) [20]. Fertilization was divided into two applications: When sorghum plants had four and seven fully expanded leaves by adding 100 kg ha−<sup>1</sup> of N and 75 kg ha−<sup>1</sup> of K2O, respectively.

#### *2.3. Analysis of Plant Tissue and Sorghum Production*

When sorghum plants reached up to 50% of flowering, leaves were sampled—the fourth leaf was collected from the apex of the plants from the central rows. Eight leaves were sampled per experimental plot, and these were oven dried at 55 ◦C and milled in a Willey-type stationary mill for the determination of the foliar contents of N, P, K, Ca, and Mg [21].

At 85 days after sowing (DAS), eight plants of the two central rows of each plot were evaluated. Plant height and panicle length were measured from the lap of the plant up to the last expanded leaf. The stem and panicle diameters were measured using a digital caliper.

Sorghum plants were cut near the soil surface, and the plant parts were separated into stem, leaf, panicle, root, and dead material. The roots were removed with the aid of a hoe in depth of 20 cm and then washed under running water through a 2 mm sieve. Thus, the green mass of each part of the plant, as well as the leaf/stem ratio and the productivity, were obtained. The dry mass of each component was determined after drying in a forced-air oven at 55 ◦C until constant weight.
