3.1. Phosphorus effect
In the 2020/2021 crop season, there was an increase in soybean yields with increasing doses of phosphorus. With the use of the subsoiler, there was an increase in grain yield up to the highest level of phosphorus fertilization, close to 180 kg ha -1 of P2O5 where it reached a maximum grain yield of around 5,900 kg ha-1 of soybean, which represented the lowest maximum among the three breaking mechanisms (Fig. 3). With the use of the harrow, there was a higher response in terms of grain yield at around 100 kg ha-1 of P2O5 where it reached a grain yield of 6,200 kg ha-1 of soybean, which represented the highest grain yield achieved among the mechanisms.
Increasing fertilization in this case after 100 kg ha-1 of P2O5 reduced grain yield (Fig. 3). The highest yields in relation to soybean yield (~6,000 kg ha-1) were obtained with the scarifier breaker mechanism at around 110 kg ha-1 of P2O5; however, it was the breaker mechanism that had the least variation of the three (Fig. 3). The 2021/2022 crop season, opposite, had lower rainfall compared to the 2021/2022 crop season, as shown in figure 2. Increased levels of phosphorus fertilization also had an effect on soybean yields, but with lower total yields when compared to the 2020/2021 crop season (Fig. 3).
Phosphorus (P) plays an important role in plant metabolism, in the transportation of energy in the cell, in photosynthesis and respiration, forming part of nucleic acids, phospholipids, phosphoproteins and coenzymes (Melo and Mendonça. 2019). Due to the fact that it is considered a necessary nutrient to achieve high yields, low availability in the early phenological stages of the crop can irreversibly damage development, interfering with agricultural production (Pinto and Duarte. 2019). Therefore, plant development can be limited by the low availability of P in the soil (Hinsinger. 2001).
Therefore, the low availability of phosphorus in the soil can affect the main components of crop yield, leading to a reduction in the size of the plant and the height of insertion of the first pods, lower production and abortion of flowers and higher abortion of these structures, contributing to low crop yield (Guerra et al. 2006) and the physiological quality of the seeds (Carvalho et al. 2015).
The highest grain yield was observed with the scarifier compacted layer breaker, close to 90 kg ha-1 of P2O5, around 3,400 kg ha-1 of soybean. Both the harrow and the subsoiler had similar responses in the 2021/2022 crop season in relation to the increase in phosphate fertilization (Fig. 3). Both had an increase in grain yield as phosphorus fertilization increased, however, the subsoiler, when compared to the harrow, obtained grain yield between 1,000 and 1,400 kg ha-1 of soybean more per hectare when compared to the same doses (Fig. 3). The subsoiler without phosphorus fertilization yielded 2,600 kg ha-1 of soybean, while the harrow yielded 1,500 kg ha-1 of soybean, a 1,100 kg ha-1 difference in a crop season 2021/22 with water restrictions, while at a dose of 180 kg ha -1 of P2O5 the subsoiler yielded ~3,000 kg ha-1 and the harrow ~2,200 kg ha-1.
The disorder caused by tillage directly affects the chemical (Busari et al. 2015) and microbiological attributes of the soil (Balota et al. 2011). Conventional tillage often causes the disruption of the mycelial network due to the destruction of macro-aggregates as a result of mechanical intervention (Borie et al. 2006), decreasing soil organic carbon content, microbial biomass and fauna activity (Curaqueo et al. 2011; Mikha and Rice. 2004; Sainju et al. 2009).
Therefore, the low availability of phosphorus in the soil can affect the main components of crop yield, leading to a reduction in the size of the plant and the height of insertion of the first pods, lower production and abortion of flowers and higher abortion of these structures, contributing to low crop yield (Guerra et al. 2006) and the physiological quality of the seeds (Carvalho et al. 2015).
The highest grain yield was observed with the scarifier compacted layer breaker, close to 90 kg ha-1 of P2O5, around 3,400 kg ha-1 of soybean. Both the harrow and the subsoiler had similar responses in the 2021/2022 crop season in relation to the increase in phosphate fertilization (Fig. 3). Both had an increase in grain yield as phosphorus fertilization increased, however, the subsoiler, when compared to the harrow, obtained grain yield between 1,000 and 1,400 kg ha-1 of soybean more per hectare when compared to the same doses (Fig. 3). The subsoiler without phosphorus fertilization yielded 2,600 kg ha-1 of soybean, while the harrow yielded 1,500 kg ha-1 of soybean, a 1,100 kg ha-1 difference in a crop season 2021/22 with water restrictions, while at a dose of 180 kg ha -1 of P2O5 the subsoiler yielded ~3,000 kg ha-1 and the harrow ~2,200 kg ha-1.
The disorder caused by tillage directly affects the chemical (Busari et al. 2015) and microbiological attributes of the soil (Balota et al. 2011). Conventional tillage often causes the disruption of the mycelial network due to the destruction of macro-aggregates as a result of mechanical intervention (Borie et al. 2006), decreasing soil organic carbon content, microbial biomass and fauna activity (Curaqueo et al. 2011; Mikha and Rice. 2004; Sainju et al. 2009).
3.2. Potassium effect
The 2020/2021 crop season, the scarifier and subsoiler mechanisms had increasing responses as the doses of K2O increased. Yields of 4,600 kg ha-1 and 5,400 kg ha-1 were obtained with the subsoiler and scarifier, respectively, without the addition of K2O. Soybean yields responded to potassium fertilization up to the maximum dose of 180 kg ha-1 of K2O, where the subsoiler and scarifier produced yields of 5,300 kg ha-1 and 6,000 kg ha-1, respectively (Fig. 4). The harrow had an increase in grain yield with the increase in K2O concentration, around 6,300 kgha-1 at the dose of ~100 kg ha-1 of K2O. However, grain yield fell at the highest dose of 180 kg ha-1 of K2O, reaching 5,800 kg ha-1, which was in line with the grain yield found without the addition of K2O.
In the 2021/2022 crop season, where there was less rainfall, the scarifier and subsoiler behaved very similarly and yields were around 2,800 kg ha-1 of soybean without adding K2O, Tthe yields stabilized at around 3,200 kg ha-1 at the dose of 120 kg ha-1 of K2O and fell slightly with 180 kg ha-1 of K2O, where yields fell to 3,000 and 3,150 kg ha-1 of soybean with the use of a scarifier and subsoiler, respectively (Fig. 4).
One factor that could increase plant resistance to situations of water deficiency is the nutrient potassium, as it helps conserve water in the leaves and root development. It is worth pointing out that water shortages have negative consequences for crops. Since water determines the proper metabolic and physiological functioning of the plant, including thermal regulation, turgidity and acting as a solvent for minerals available in the soil, leading to nutritional absorption by the roots (Fioreze et al. 2011). In addition, water makes up approximately 90% of the plant's mass. The harmful consequences begin when the plant's transpiration is higher than the absorption of water available in the soil by the roots, characterizing water deficit (Taiz and Zeiger. 2013).
3.3. Soil physical attributes
Microporosity in the 0-10 cm layer was highest with the scarifier, followed by the harrow and then the subsoiler with values of 39, 37 and 35% respectively. In the 10-20 cm layer, the harrow was the mechanism with the lowest microporosity (33.5%) and the subsoiler and scarifier had a similar value of 36%, however, in the 30 cm deep layer, the subsoiler stood out, with a microporosity of 41%, where the scarifier had 40% and the harrow 35.5% (Fig. 5).
Macroporosity in the 0-10 layer was 3, 4.5 and 7.2% for the harrow, subsoiler and scarifier respectively, while in the 10-20 layer the scarifier and subsoiler had a similar value of 4% and the large one 3%. In the 30cm-deep layer, the scarifier and harrow showed 3.2% macroporosity and the subsoiler 4% (Fig. 5).
Evaluations of microporosity are very important in compaction studies, as it is responsible for the amount of water the soil is able to retain and, in turn, for root growth and crop yield (Lima et al. 2006).
With regard to total porosity, in the 0-10 cm depth layer the harrow and subsoiler had a similar value, 39.8 and 40.2% respectively. In the 10-20 cm layer, the scarifier and subsoiler had 40% total porosity and the harrow had 36.5%. In the 30 cm layer, the subsoiler had the highest total porosity value, 45%, 2.5% more than the scarifier, which had 42.5%. The harrow, however, obtained the lowest total porosity value, 38% (Fig. 5).
Total porosity regulates the relationship between the solid, liquid and gaseous phases. The importance of quantifying total porosity is related to studies on the storage and movement of water and air in the soil and, consequently, the development of the root system. Soil density expresses the relationship between the amount of dry soil mass per unit volume of soil. The volume of soil includes the volume of soil solids and pores. Therefore, whenever there is a change in pore space, there will be a change in soil density. In this way, density provides information about the state of conservation of a soil, especially its influence on properties such as infiltration and retention of water in the soil, root development, gas exchange and the soil's susceptibility to erosive processes, and is widely used to assess soil compaction and/or densification (Guariz et al. 2009). Lowland soils, such as the one in this study, generally have higher density values and low macroporosity values. This is due not only to the excessive tillage mentioned above, but also to the low organic carbon content, which contributes to lower soil aggregation.
With regard to soil density, the values for the 0-10 layer were 1.2 and 1.25 g cm-3 for the scarifier and harrow respectively, and 1.4 g cm-3 for the subsoiler. For the 10-20 layer, the density was 1.41, 1.5 and 1.58 g cm-3 for the harrow, scarifier and subsoiler respectively. For the 30 cm deep layer, the values were similar, close to 1.5 g cm-3 (Fig.5). Soil density has an inverse relationship with porosity, so when considering the same soil, an increase in density reflects a decrease in the soil's pore space. Total soil porosity represents the total volume of pores, but does not provide information on the distribution of this porosity (Hillel. 1980).
It was observed that, in both crop seasons, the doses of maximum technical efficiency (DMTE) of phosphorus ranged from 94 to 181 kg ha-1 of P2O5 (Table 2). The maximum yield (Ymax) was close to 6,000 kg ha-1 in the 2020/21 crop season and around 3,000 kg ha-1 in the 2021/22 crop season, a reduction of around 50% compared to the first crop season. The phosphorus use efficiency (PUE) was around 10 kg grains-1 kg P2O5-1 in the 2020/21 crop season, being higher in the systems using the harrow and subsoiler. In the 2021/22 crop season, PUE decreased (-48%) on average to 5.2 kg grains-1 kg P2O5-1. In the second crop season, PUE was higher in the system with a scarifier and then a subsoiler. Finally, water use efficiency (WUE) was also quite different in the two crop seasons. In general, the highest efficiency was observed in the 2020/1 crop season, with the grid system standing out. In the second crop season 2021/22, there was a significant reduction in the WUE, with the highest values in the scarifier and subsoiler treatments and the lowest in the light harrow system.
With regard to potassium, the doses of maximum technical efficiency (DMTE) were 69 to 167 kg ha-1 of K2O (Table 3). The Ymax in the 2020/21 crop season was an average of 5,883 kg ha-1 and was higher in the harrow system. In the 2021/22 crop season, Ymax was 2,959 kg ha-1 and was higher when the scarifier and subsoiler systems were used. KUE was higher in the subsoiler system (6.3 kg grain-1 kg K2O-1) in the first crop season. In the second crop season, the highest KUE was observed in the scarifier treatment (3.8 kg grain-1 kg P2O5-1). The KUE in the potassium system was similar to that of phosphorus, with a high KUE in the 2020/21 crop season. In the 2021/22 crop season, the highest US was observed in the scarifier and subsoiling systems (Table 3).
In the second crop season, under conditions of water restriction, the average response to phosphorus and potassium showed a gain in grain yield of 1,038 kg ha-1 (+ 46%) with the scarifier and 650 kg ha-1 (+28%) with the subsoiler compared to the light harrow treatment. In the average of the scarifying and subsoiling breaker mechanisms and the response to phosphorus in the 2021/22 season, there was an increase of 2.3 kg grains kg P2O5 ha-1 (+ 123%) in PUE and 2.4 kg ha-1 mm-1 (+ 49%) in WUE compared to harrow management. As for the potassium response, the average of the scarification and subsoiling treatments showed gains of 0.6 kg grains kg K2O ha-1 (+ 20%) in KUE and 2.22 kg ha-1 mm-1 (+ 45%) in WUE compared to the harrow treatment in 2021/22 crop season.
3.4. Pearson's correlation and principal component analysis (PCA)
It was observed that grain yield had a high positive relationship with the amount of rainfall and the efficiency of phosphorus and potassium use (Fig. 6). The soil's physical attributes of macro and micro pores and total porosity had no influence on soybean grain yield (Fig. 6).
The principal component analysis showed that the two axes explained 84.7% of the variation in the attributes analyzed (Fig. 7). This PCA showed an overlap between the scarification and subsoiling management groupings. Grid management, opposite, had a different grouping compared to the other managements.