Rates of Agricultural Gypsum in Soil Under No-tillage System With Surface Lime in the Southern of Brazil

At the no-tillage system, the surface liming is a good environmental practice, which aims to maintain the physical structure and stocks of carbon in the soil. However, the acidity amelioration is restricted to the surface layer and the use of the gypsum can be an alternative to improve the chemical conditions in subsurface without the soil revolving. The purpose of this experiment was to evaluate the effect of different rates of gypsum, estimated by different methods, in acid soils with application of superficial limestone since the beginning of the implementation of the no-tillage system. The experiment was conduct at a commercial cropping field located at the municipally of Muitos Capões, RS, Southern Brazil in a Red Oxisol. The experiment was conducted in randomized completely blocks design (RCBD), with six treatments and four replications. The treatments consisted of the superficial application of gypsum in the rates: 0, 3100, 6014, 7875, 9750 and 12400 kg ha. At this area, soybean and corn were cultivated and have their productivity evaluated. After 4 and 16 months of the experiment, Ca, Mg and K levels were evaluated at different depths (0-5, 5-10, 10-20, 20-30 and 30-40 cm). The results showed an increase in Ca contents to the depth of 10 cm at 4 months after application and at all depths evaluated at 16 months after their application. The application of gypsum decreases the Mg contents to the depth of 20 cm and of potassium at all the depths after 16 months of their application. The effects on soil chemical properties with increasing rate of applied gypsum resulted in higher maize grain yield. So, agricultural gypsum applied in the soil with surface liming is efficient in improving soil chemical conditions in deeper layers in a no-tillage system.


Introduction
No-tillage system has emerged as an efficient alternative to ensure sustainability in tropical and subtropical soils, especially in Brazil.In most of these soils the acidity has been limiting for the development of the grain crops, being used the practice of surface liming and/or incorporated.Yagi, Fidalski and Tormena (2014) found that the incorporation of limestone in soil reduced the carbon stocks of the macroaggregates in the 0-10cm layer and concluded that surface liming is fundamental for the maintenance and increase of these stocks.However, the action of lime without incorporation can be restricted to the surface layer of acid soils (Miyazawa, Pavan & Franchini, 2002) and limiting the deepening of roots.
Agricultural gypsum, a byproduct of phosphate fertilizer manufacture, in surface applications is an alternative for improvements in soil chemical characteristics in depth, avoiding soil mobilization in no-tillage system areas and increasing plant tolerance to periods of water deficit.Its effect is known to carry basic cations in subsurface, reduce aluminum saturation (Ritchey, Sousa, Lobato, & Correa, 1980) and aluminum toxicity (Sumner, 1995).However, these potential changes do not necessarily translate into increases in crop yields (Zoca & Penn, 2017).Improvement in crop yield may be the result of an additive or synergistic effect of each of these potential changes, and they vary with crop, soil type and rainfall regime (Zoca & Penn, 2017).
The available knowledge about the effect of gypsum in subtropical soils of Southern Brazil, about the crop yield is incipient mainly for soils under no-tillage system (Tiecher, Pias, Bayer, Martins, Denardin, & Anghinoni, 2018).Therefore, there are many doubts about the most appropriate methods to estimate their need (Guimaraes, Caires, Silva, & Rocha, 2015) and there are no official recommendations on gypsum for the states of Rio Grande do Sul and Santa Catarina (CQFS-RS/SC, 2016), although it is a common practice among farmers.Mistaken rates can lead to large leaching of nutrients and, consequently, loss to subsurface layers such as Magnesium (Caires, Kusman, Barth, Garbuio, & Adilha, 2004).
For Souza, Lobato and Rein (2005) if saturation by aluminium (Al) is higher than 20% or calcium (Ca) content is lower than 0.5 cmol c dm -3 , there is a probability of a response to gypsum, and under these conditions the amount of gypsum to be applied according to these authors is based on the clay content of the soil.Malavolta (1992) recommends the use of agricultural gypsum whenever there is less than 40% of Ca in the effective cation exchange capacity or when the saturation in Al is greater than 20%; Under these conditions, the rate of gypsum is based on the assumption that 2.5 t ha -1 of gypsum is required to raise or lower 1 cmol c dm -3 of Ca and/or Al.According to Guimaraes, Caires, Silva and Rocha (2015) the saturation by Ca in the effective cations exchange capacity was a more important attribute than the saturation by Al to estimate the need for gypsum in soils under no-tillage.Recently, Tiecher, Pias, Bayer, Martins, Denardin & Anghinoni (2018) analyzing the results of 73 experiments, reported in 20 scientific publications, concluded that for grasses cultivated in subtropical latosols under no-tillage system, the use of 10% Al and/or 3.0 cmol c dm -3 of exchangeable Ca in the subsurface soil layer (0.20-0.40 m) is more suitable than the current recommendation (Al saturation of 20% and/or 0.5 cmol c dm -3 Ca) for tropical soils.Soils with superficial application of limestone, since the implantation of the crops under no-tillage system, can present these limitations even in the layer of 10-20cm of depth.In these cases, the incorporation of limestone is considered (CQFS-RS/SC, 2016).
Therefore, it is necessary to evaluate efficient rates for surface application in soils with high potential acidity and high levels of organic matter, searching for alternative strategies of soil correction in depth without its mobilization and that can reach the deeper layers of soil.The objective of this study was to evaluate the effect of different rates of gypsum, estimated by different methods in acid soils with surface liming application from the beginning of the no-tillage system.

Materials and Methods
The experiment was carried out in the municipality of Muitos Capões-RS-Southern Brazil (28°20′ S 51°7′ W) in Red Oxisol, for two consecutive harvests, in no-tillage areas established under native land without any soil disturbance.
Before the installation of the experiment the superficial application of acidic corrector, Macro-calcium ES ® , relative power of total neutralization (PRNT) 75.6% (CaO = 51% to 53% MgO = 0.2%) in the medium rate to reach saturation of bases of 80% The experiment was conducted in randomized completely blocks design (RCBD), with six treatments and four replications.The treatments consisted of different rates of gypsum applied manually at the soil surface (Table 1).The rates of gypsum used were adapted from Souza, Lobato, and Rein (2005), and Malavolta (1992).The recommendations of the agricultural gypsum rates were calculated based on the soil layer of 10-20 cm depth.The gypsum applied rate treatment followed the recommendation of treatment 2: Souza, Lobato, and Rein ( 2005), which recommends annual crops to 50 Kg ha -1 of gypsum by the clay content.In treatments 3 and 6 (Table 1) an adaptation was carried out, using 97 and 200 Kg ha -1 respectively per clay content, both treatments mentioned above did not consider the subsurface layer of 20 to 40 cm as recommended by the author, but the layer of 10 a 20cm deep.Since chemical conditions at depths of 10 to 20 cm are not ideal for root development and should be ameliorated.
In the experimental area, the soybean cultivar BMX Titan 5.3i RR ® , was implanted.The spacing between lines was 0.45 m.The fertilization was carried out in the sowing line, and 80 Kg ha -1 of diammonium phosphate (DAP) was applied.Potassium chloride was applied in the furrow to achieve base saturation of 0.4%.The following year the corn crop, AS1555g hybrid, was implanted.The fertilization used was 300 Kg ha -1 of Urea Plus ® .
After the cultivation, the cultures were harvested manually.In order to evaluate the yield components of each crop (soybean and corn), it was considered a useful area per plot consisting of five central rows, 5.0 m long, totalizing 11.25 m 2 , always discarding the ends of the plots in the lengthwise direction (3 m on one side and 2 m on the other side) and discarding the external rows (6 rows) of the plot.Productivity calculation was corrected to 13% moisture.
Soil chemical evaluations were performed at 4 and 16 months after the application of gypsum to the depths of 0-5, 5-10, 10-20, 20-30 and 30-40 cm.The sub-samples were collected using the two center rows of each useful plot, there was obtained two subsamples per plot to form a sample.They were collected with two cutting blades with a blade of 45cm depth, and a soil profile was drawn in a line perpendicular to the direction of the rows, with a width of 14cm and a thickness of approximately 4cm.In these samples the Ca, Mg and K contents were determined (Tedesco, Gianello, Bissani, Bohnen, & Wolkweiss, 1995).
During the crop cycle, rainfall monitoring was carried out through the use of an analogue rain gauge installed in the crop.
The results of the evaluations were submitted to analysis of variance (p ≤ 0.05); and, in case of significance, rate effects were compared by analysis of regression.The magnitude of the significant coefficients of determination at 5% of probability was adopted as the criterion for choosing the model.

Soil Chemical Parameters
The application of agricultural gypsum significantly influenced the Ca contents in the soil in the two evaluations, and only 4 months after the application there was interaction with the soil depth (Figure 1).These results demonstrate that the effect of the application occurs gradually over time, and at 16 months the effect of the application was linear and reached all depths evaluated (0-40 cm).
jas.ccsenet.Ca content in cmol c dm -3 ) × 6.4.Using this equation, but with the decision criterion the 0-10 cm layer, the required rate of gypsum in this soil would be 16 ton ha -1 , higher than the rate tested in this work.So, suggesting that higher rates to this work are tested under these conditions and especially, the losses of Mg and K in the system were evaluated.

Conclusion
The gypsum in soil with surface liming is efficient in improving soil chemical conditions in deeper layers without the need for soil mobilization in no-tillage cropping systems.