Crop nutrition and grain yield as affected by phosphorus fertilization and continued use of phosphogypsum in an Oxisol under no-till management

ABSTRACT The benefits of applying phosphogypsum to the soil are widely known. However, the effect of phosphogypsum on phosphate fertilization efficiency is still unclear. A long-term field experiment was performed on an Oxisol in Parana State, Brazil to evaluate the effects of triple superphosphate (TSP) and phosphogypsum application rates on nutrition and grain yield of soybean, wheat, and black oat under no-till. Applying TSP in the sowing furrow of soybean and wheat increased soybean yield by 14 to 24%, wheat yield by 57%, and black oat yield by 78%. Soybean yields varied in different cropping seasons in response to phosphogypsum application (with no response to increases of up to 15% in yield), while phosphogypsum increased wheat yield by 23% and black oat yield by 59%. The water balance during crop flowering possibly interfered on crop yield response to phosphogypsum. The use of phosphogypsum increased agronomic P-use efficiency by 20%, regardless of the TSP-P application rates. Changes in P-leaf concentration caused by TSP fertilization and phosphogypsum use had a positive impact on crop grain yield. Our results reveal that the continued use of phosphogypsum to alleviate subsoil acidity could increase the phosphate fertilization efficiency and improve crop yield performance under drought stress.


Introduction
Conservation agriculture systems are practiced globally on about 157 million hectares (Kassam et al. 2015). In Brazil, conservation agriculture based on no-till is a widely used practice for grain production. In Southern Brazil, the most important no-till grain-producing cash crops are soybean [Glycine max (L.) Merrill] during the spring-summer season and wheat (Triticum aestivum L.) during the autumn-winter season. Black oat (Avena strigosa Schreb.) is the most important cover crop grown during winter.
No-till systems are known to cause chemical stratification, including pH, where high pH levels are formed in the upper few inches of the soil profile (Caires et al. 2005;Godsey et al. 2007). Subsoil acidity is an important yield-limiting factor (Marsh and Grove 1992;Sumner 1995;Caires et al. 2016), especially in regions suffering from field drought effects (Tang et al. 2002;Bossolani et al. 2021).
Phosphogypsum, a by-product of the phosphoric acid industry that mainly contains calcium sulfate and small amounts of phosphorus (P) and fluorine (F), is largely available in many parts of the world. As an efficient way of reusing resources, phosphogypsum has been used as a calcium (Ca) and sulfur (S)-based fertilizer, and to alleviate subsoil acidity in order to enhance plant root growth (Zoca and Penn 2017). When applied to the soil surface, phosphogypsum moves down the profile during drainage, resulting in increases in the Ca and sulfate levels, and in a reduction in toxic levels of aluminum (Sumner 1995;Caires et al. 2011aCaires et al. , 2011b. As a result, better root growth and higher absorption of water and nutrients by plants roots have been observed (Carvalho and van Raij 1997;Caires et al. 2016;Bossolani et al. 2021).
Phosphorus is the nutrient most used in plant fertilization in tropical soils (van Raij 2011). Consumption of P fertilizer in Brazilian agriculture increased from 1.4 million tons in 2010 to 2.2 million tons in 2018 (Food and Agriculture Organization (FAO) 2021). This is mainly due to the low P reserve associated with the high P adsorption capacity by highly weathered soils in humid tropical and subtropical regions (Sanchez and Logan 1992;Campos et al. 2016;Antonangelo et al. 2020). Increases in P concentration in the topsoil were found when high rates of phosphogypsum were applied to the soil surface to alleviate subsoil acidity (Caires et al. 2003(Caires et al. , 2011cBouray et al. 2020). Increasing soil P availability from high phosphogypsum application rates improve plant P uptake (Caires et al. 2003(Caires et al. , 2011cBouray et al. 2020). However, information on the effect of phosphogypsum application on P use efficiency (PUE) is still scarce. In addition, a recent study indicated that phosphogypsum when used in elevated concentration, it is combined with high acidity may impair soil fertility and decrease microbial metabolic activity (Sengupta and Dhal 2021).
Since surface application of phosphogypsum can increase topsoil P concentration and P contact with plant roots due to increased root growth, we hypothesize that the continued use of phosphogypsum to alleviate subsoil acidity increases the efficiency of phosphate fertilization under no-till management. This study reports a field trial that examined the long-term effects of P and phosphogypsum application rates on the nutrition and grain yield of soybean and winter cereals (wheat and black oat) under a continuous no-till system.

Site description and soil
The experiment was carried out in Ponta Grossa, PR, Brazil (25°03ʹ23" S, 50°04ʹ55" W), on an Oxisol (clayey, kaolinitic, Rhodic Hapludox). The climate at the site is categorized as a Cfb type (mesothermal, humid, subtropical), with fresh summer and frequent frosts during the winter, without a defined dry season. The annual precipitation is about 1550 mm and average maximum and minimum temperatures are 22 and 13°C, respectively. At the beginning of the experiment, the field site had been used for grain cropping under the continuous no-till system for 10 years. Table 1 shows the results of soil chemical (Pavan et al. 1992;Cantarella and Prochnow 2001) and particle-size distribution (Empresa Brasileira de Pesquisa Agropecuária (Embrapa) 2011) analyses for different depths in July 2013 before the establishment of the experiment. Dolomitic lime was applied to the soil surface at 2.5 Mg ha −1 in July 2013, and reapplied at 2 Mg ha −1 in April 2020.

Experimental design and treatments
A randomized block design was used, with three replications in a split-plot scheme. The main-plot (36 by 5 m) treatments consisted of applications of triple superphosphate (TSP) to the crops sowing furrow at 0, 13, 26, and 39 kg P ha −1 , which were based on P fertilizer recommendations for Parana State (Pauletti and Motta 2017). The TSP used contained 200 g kg −1 total P, 166 g kg −1 P soluble in water, and 93 g kg −1 Ca. The subplot (9 by 5 m) treatments consisted of phosphogypsum applications on the soil surface at 0, 4, 8, and 12 Mg ha −1 , with half of the rates applied in October 2013 and the other half in October 2018. The phosphogypsum applications were guided by Ca 2+ saturation in the effective cation exchange capacity (ECEC) in the 0.20-0.40 m soil layer, according to the novel phosphogypsum application recommendation method proposed by Caires and Guimarães (2018). The phosphogypsum used in 2013 and 2018 contained 227 g kg −1 Ca, 181 g kg −1 S, 7.4 g kg −1 P, and 152 g kg −1 water. Figure 1 shows the timeline of cultivation in the field including the chronological sequences of phosphogypsum and TSP applications and crops grown over the years in the experiment.

Crop management and climatological water balance
In the period from 2013 through 2018, following the first phosphogypsum application rates, the experimental area was mainly cultivated with soybean or maize during the spring-summer season and with wheat or black oat during the autumn-winter season ( Figure 1). After phosphogypsum rates were completed in 2018, the crops rotated and considered for the evaluation of leaf diagnosis and grain yield in our study were: soybean [Glycine max (L.) Merrill], cv. NA 5909, sown in October 2018; wheat (Triticum aestivum L.), cv. TBIO Sinuelo, sown in June 2019; soybean, cv. BRS 511, sown in November 2019; black oat (Avena strigosa Schreb.), cv. Common, sown in May 2020; and soybean, cv. BRS 511, sown in October 2020. The applications of TSP to the crops sowing furrow were carried out with the aid of a no-till seeder. Soybean was sown each year at a seeding rate of 14 seeds m −l (inoculated with Bradyrhizobium japonicum), and row spacing of 0.45 m, without nitrogen fertilizer. Black oat was sown without fertilizers at a seeding rate of 80 kg ha −1 , and row spacing of 0.17 m. Black oat was used as a cover crop in 2018 and for grain production in 2020. Wheat was sown at a seeding rate of 160 kg ha −1 , and row spacing of 0.17 m. For the wheat crop, a rate of 70 kg N ha −1 was applied as urea in top dressing at the beginning of tillering. In all soybean and wheat crops, a rate of 50 kg K ha −1 , as potassium chloride, was applied soon after sowing.
A climatological water balance was calculated according to Thornthwaite and Mather (1955) for the soybean, wheat, and black oat crops ( Figure 2). There was practically no water deficit during soybean development in 2018-2019, although the water surplus during crop flowering this season was not very high. For winter wheat grown in 2019 there were several periods of water deficit, including shortly after flowering. Throughout the development period of soybean in 2019-2020 and black oat in winter 2020, a water deficit was recorded during the flowering phase. Soybean grown in 2020-2021 was not affected by water deficit and the highest water surplus during the flowering period was observed in this season.

Leaf sampling and chemical analysis
Leaf samples of soybean (2018-2019, 2019-2020 and 2020-2021) and wheat (2019) were collected from 30 plants per subplot during the flowering period of the crops. In soybean, the third trefoil from the apex to the base was taken and in wheat, the flag leaf. The samples were washed in deionized water, dried in a forced-air oven at 60°C until a constant weight was achieved, and subsequently ground. The leaf tissue analysis was performed using sulfuric acid digestion for N and nitricperchloric acid digestion for P, K, Ca, Mg, and S. Nutrient concentrations were determined by the Kjeldahl method for N, metavanadate colorimetry for P, flame photometry for K, atomic absorption spectrophotometry for Ca and Mg, and turbidimetry as barium sulfate for S (Malavolta et al. 1997).

Crop grain yield
Soybean grain was harvested from a 9.00-m 2 plot (middle five rows by 4 m in length) and wheat and black oat grain was harvested from an 8.16-m 2 plot (middle 12 rows by 4 m in length). After harvesting by hand, the soybean pods and the ears of wheat and black oat were threshed in a stationary threshing machine. Grain yield was expressed at 130 g kg −1 moisture content.

Agronomic P-use efficiency (PUE)
Agronomic P-use efficiency (PUE) was calculated based on the results from cumulative yield of soybean, wheat, and black oat for each P and phosphogypsum application rate. The equation suggested by Roberts (2008) was used to determine PUE as follows: PUE = (CY F -CY C )/(P rate), where CY F is the cumulative grain yield with the P application rate at various phosphogypsum application rates, CY C is the cumulative grain yield in the control plot (with no TSP and phosphogypsum applications), and P rate is the amount of P applied via TSP.

Soil sampling and chemical analysis
Soil samples were taken after soybean harvest in April 2021, 7.5 years since the experiment was started, to assess soil-P and Ca 2+ status. To obtain a composite sample, 12 soil cores at 0-0.10 and  0.10-0.20 m depths, and 5 soil core samples at 0.20-0.40 and 0.40-0.60 m depths were taken in each subplot using a soil probe sampler. Then soils were air-dried and ground to pass a 2-mm sieve. Extractable P was extracted by a Mehlich-1 solution (0.05 mol L −1 H 2 SO 4 + 0.05 mol L −1 HCl) and exchangeable Ca 2+ with a neutral 1 mol L −1 KCl solution, in a 1:10 (v/v) soil/solution ratio, according to standard methods used by the Agronomic Institute of Parana State (Pavan et al. 1992). Phosphorus was determined by UV-visible spectrophotometry and Ca 2+ by atomic absorption spectrophotometry.

Statistical analysis
Data from the soil and leaf tissue chemical analysis along with crop grain yield were used in an analysis of variance (ANOVA) and regression using the SISVAR software. In the absence of a significant interaction effect of P application rates to the sowing furrow (main plots) and phosphogypsum surface-applied rates (subplots) on the variables studied, the treatment effects were compared by regression analysis using the means of the observations. When a significant interaction effect among the P and phosphogypsum application rates on the variables was observed, the regression analysis was conducted separately for each P or phosphogypsum application rate. The criterion adopted for choosing the model was the magnitude of the determination coefficients  Thornthwaite and Mather (1955). Data obtained from Instituto Nacional de Meteorologia (INMET) (2021) significant at P < 0.05. A discriminant analysis of principal components was used to determine the components account for variation. For this, a correlation matrix of leaf nutrient concentrations and grain yield of crops was analyzed using the CANOCO for Windows software, version 4.56.

Leaf nutrient concentrations and crop yields
Leaf nutrient concentrations of soybean (Table 2) and wheat (Table 3) were not significantly influenced by the interaction of P application rates to the sowing furrow × phosphogypsum application rates to the soil surface, and remained at adequate or close to normal levels for these crops.
Increasing the P rate to the sowing furrow increased the P-leaf concentration in all soybean cropping seasons (2018-2019, 2019-2020, and 2020-2021), the N-leaf concentration of soybean in 2019-2020 and 2020-2021, and leaf concentrations of K, Ca, and S of soybean in 2019-2020 (Table 2). Applying the phosphogypsum rates on the soil surface increased the N-leaf concentration of soybean in 2018-2019, the P-leaf concentration of soybean in 2018-2019 and 2019-2020, the Caleaf concentration of soybean in 2019-2020 and 2020-2021, and the S-leaf concentration of soybean in 2018-2019 and 2019-2020. There was also a reduction in Mg-leaf concentration of soybean in 2019-2020 and 2020-2021 with increasing phosphogypsum application rate.
The P-leaf and Ca-leaf concentrations of wheat in 2019 increased with increasing P rate to the sowing furrow (Table 3). The Ca-leaf and S-leaf concentrations of wheat increased with increasing phosphogypsum application rate.
Fertilization with TSP and application of phosphogypsum increased soybean, wheat, and black oat grain yields (Figure 3). Grain yields of soybean and wheat were not significantly influenced by the interaction of P application rates to the sowing furrow × phosphogypsum application rates to the soil surface (Table S1). A significant interaction effect of P × phosphogypsum application rates was found only on black oat grain yield in 2020. The largest increases in soybean yield due to TSP fertilization were obtained with P application at 39 kg ha −1 in 2018-19 (14%), 26 kg ha −1 in 2019-2020 (24%), and 39 kg ha −1 in 2020-2021 (16%) (Figure 3a). In wheat (2019), the largest increase in yield was obtained with TSP application at a rate of 39 kg P ha −1 (57%). Since black oat was sown without fertilizers, a largest increase in black oat grain yield occurred due to residual effect of TSP application at a rate of 24 kg P ha −1 (78%) only in the plots without application of phosphogypsum. The largest increases in soybean yield due to phosphogypsum application were obtained at a total rate of 5.8 Mg ha −1 for soybean grown in the 2018-19 season (7%) and 8.5 Mg ha −1 for soybean grown in the 2019-2020 season (15%) (Figure 3b). Soybean yield in 2020-2021 was not significantly influenced by the application of phosphogypsum. In wheat (2019), the largest increase in yield was obtained with application of phosphogypsum at a total rate of 12 Mg ha −1 (23%). A largest increase in black oat yield (2020) was also obtained with application of phosphogypsum at a total rate of 12 Mg ha −1 (59%), but only in the plots without any P application.
At the discriminant analysis of principal components for the soybean in 2018-2019, wheat in 2019, and soybean in 2019-2020 and 2020-2021, the two principal components accounted for 67.4%, 76.3%, 83.5%, and 68% of the total variance, respectively (Figure 4). Soybean grain yield in 2018-2019 was positively correlated with the P-leaf concentration, which was accompanied by a positive correlation with N-leaf and K-leaf concentrations. Wheat yield in 2019 was positively correlated with the P-leaf and Ca-leaf concentrations, and there was a weaker correlation with the N-leaf concentration. Soybean yield in 2019-2020 was strongly correlated with the Ca-leaf and P-leaf concentrations, which were accompanied by a positive correlation with K-leaf and S-leaf concentrations. Soybean yield in 2020-2021 was strongly correlated with the P-leaf and N-leaf concentrations. In this soybean growing season, Ca-leaf and S-leaf concentrations were influenced by the highest  phosphogypsum application rates (8 and 12 Mg ha −1 ), but there was no influence of these nutrients on grain yield. The cumulative grain yield of the crops, in the period from 2018 to 2021, increased with increasing TSP-P and phosphogypsum application rates ( Figure 5). There was no significant interaction effect of P × phosphogypsum application rates on cumulative grain yield of the crops. The largest increases in cumulative grain yield were achieved with TSP application at 32 kg P ha −1 for each soybean or wheat crop (22%) and with phosphogypsum application at a total rate of 8.3 Mg ha −1 (7%).

Soil chemical properties
Extractable P (Mehlich-1) and exchangeable Ca 2+ concentrations throughout the soil profile were not significantly influenced by the interaction of P application rates to the sowing furrow × phosphogypsum application rates to the soil surface (Table S2). Increasing both TSP-P and phosphogypsum application rates increased extractable P (Mehlich-1) concentration in the topsoil (0-0.10 m) at 7.5 years from the beginning of the experiment (Figure 6). Although there was also a slight increase in extractable P (Mehlich-1) concentration at 0.10-0.20 m soil depth with increasing TSP-P application rate, the largest increase in soil P concentration due to the application of TSP occurred in the topsoil (0-0.10 m). Fertilization with TSP to the soybean and wheat sowing furrow did not change the exchangeable Ca 2+ concentration throughout the soil profile. Exchangeable Ca 2+ levels in all soil profiles (0-0.60 m) were significantly increased with increasing phosphogypsum application rates.

Agronomic PUE
PUE was not significantly influenced by the interaction of P × phosphogypsum application rates (Figure 7). Cumulative grain yield gains decreased with increasing TSP-P application rates in the sowing furrow of soybean and wheat, decreasing the PUE (Figure 7a). PUE was significantly lower with TSP application at 26 or 39 kg P ha −1 than at 13 kg P ha −1 in each soybean or wheat crop. Phosphogypsum application at a total rate of 7.6 Mg ha −1 led to an increase by 20% in PUE (from 50 to 60 kg of grain for each kg of P applied to the sowing furrow) (Figure 7b).   Crop grain yield of soybean, wheat, and black oat as affected by application rates of P as triple superphosphate (TSP) (a) and phosphogypsum (b) under a no-till cropping system. TSP was applied to the crops sowing furrow and black oat was grown without fertilizers. Phosphogypsum was surface-applied with half of the rate in 2013 and the other half in 2018. P × PG application rates interaction significant only for black oat grain yield in 2020. ns: not significant, *P < 0.05, **P < 0.01, and ***P < 0.001.

Amendment effects on plant nutrition
Fertilization with TSP to the sowing furrow was efficient in supplying P to the plants, since increasing P rate increased P-leaf concentrations in all cropping seasons, i.e. soybean (Table 2) and wheat ( Table 3). The importance of superphosphate application in increasing P uptake by wheat plants was well demonstrated in the study by Takahashi and Anwar (2007). Soybean has required an even greater input of P due to the high consumption of adenosine triphosphate (ATP) for biological N 2 fixation (Cooper and Scherer 2012). In a study with the oat-soybean succession under no-till, the application of TSP in the sowing furrow provided greater N, P, and K uptake by oat and soybean crops due to the increased soil P availability (Caires et al. 2017). The increase in Ca-leaf concentration of soybean (2019-2020) with increasing P application rate in the seed furrow could have been due to the presence of Ca in the TSP composition (93 g kg −1 Ca). In this same growing season, there was also an increase in S-leaf concentration of soybean with increasing P rates applied in the seed furrow. This   (2018-2019, 2019-2020, and 2020-2021) and wheat (2019) as influenced by application rates of P as triple superphosphate (TSP) and phosphogypsum (PG, Mg ha −1 ) under a no-till cropping system. TSP was applied to the crops sowing furrow and phosphogypsum was surface-applied with half of the rate in 2013 and the other half in 2018. effect could be due to the action of phosphate ions displacing sulphate adsorbed in the soil because of its higher adsorption energy (Chao 1964;Metson 1979), thus increasing the concentration of sulphate in the soil solution.
The increase in N-leaf concentration of soybean due to the use of phosphogypsum (Table 2) may have been a consequence of improved biological N 2 fixation provided by increased Ca (Weaver et al. 1991) and S (Zhao et al. 2008), and also by an increase in the gene abundances of microorganisms responsible for biological N 2 fixation (Bossolani et al. 2020). However, this effect was observed in only one (2018-2019) out of three soybean cropping seasons. The use of phosphogypsum showed efficiency in supply P to the plants, since the P-leaf concentration increased with its application in two (2018-2019 and 2019-2020) out of three soybean cropping seasons (Table 2). Increases in P-leaf concentrations of soybean by applying phosphogypsum were also observed in other studies conducted on no-till soils (Caires et al. 2003(Caires et al. , 2011a(Caires et al. , 2011c and may be related to both increased root development enlarging the explored soil volume for P acquisition and supply of P contained in phosphogypsum as an impurity. The increases in the Ca-leaf and S-leaf concentrations of soybean (Table 2) and wheat (Table 3) with the phosphogypsum application were certainly caused by an increase in the availability of these nutrients in the soil, since phosphogypsum is an excellent source of Ca and S (Caires et al. 2011a(Caires et al. , 2011b(Caires et al. , 2011cVicensi et al. 2016). Decreases in Mg-leaf concentrations of soybean as a result of phosphogypsum application, such as those observed in our study in the 2019-2020 and 2020-2021 growing seasons (Table 2), were also found in other studies (Zambrosi et al. 2007;Caires et al. 2011aCaires et al. , 2011bCaires et al. , 2011cVicensi et al. 2016) and are due to the movement of Mg downward the soil profile caused by the formation of ion pair with sulfate (MgSO 4°) .

Crop yields and PUE
Compared with no P treatment, TSP fertilization to the sowing furrow increased soybean yield by 14 to 24%, wheat yield by 57%, and black oat yield by 78% (Figure 3). Water balance during crop development ( Figure 2) did not play an important role in the yield response of crops to TSP fertilization. Soybean grain yield varied in different cropping seasons in response to phosphogypsum application (with no response to increases of up to 15% in yield), while phosphogypsum increased wheat yield by 23% and black oat yield by 59%. The water balance during crop flowering possibly interfered on crop yield response to phosphogypsum application. Phosphogypsum provided the greatest increase in soybean yield (15%) in the growing season marked by a water deficit during the Figure 6. Changes in extractable P (Mehlich-1) and exchangeable Ca 2+ concentrations for different soil depths as affected by application rates of P as triple superphosphate (TSP) (a) and phosphogypsum (b) under a no-till cropping system. TSP was applied to the crops sowing furrow and phosphogypsum was surface-applied with half of the rate in 2013 and the other half in 2018. Soils were sampled after the soybean harvest in April 2021. P × PG application rates interaction not significant for extractable P (Mehlich-1) and exchangeable Ca 2+ concentrations at 0-0.10, 0.10-0.20, 0.20-0.40, and 0.40-0.60 m depths. ns: not significant, *P < 0.05, **P < 0.01, and ***P < 0.001.
flowering period (2019-2020) and there was no soybean yield response to phosphogypsum in the growing season with the highest water surplus during the flowering period (2020-2021) (Figure 2). In other studies, reported by Pias et al. (2020), no-till soybean yield response to acid-subsoil amelioration using phosphogypsum was also higher when water was more limited during the crop cycle. Cereal crops (wheat and oat) grown in seasons marked by a more pronounced water deficit than that observed in soybean during the flowering period ( Figure 2) were more responsive than soybean to phosphogypsum. Discriminant analysis of the principal components clearly showed the positive influence of TSP-P application rates on crop grain yields (Figure 4), regardless of the water balance occurring in each crop ( Figure 2). Overall, the P-leaf concentration had a positive impact on crop grain yield in all cropping seasons. Crop grain yields were influenced by the highest application rates of phosphogypsum (8 and 12 Mg ha −1 ) only on crops grown in the driest seasons during the flowering period ( Figure 2). For soybean in 2020-2021, since there was a water surplus throughout the crop development cycle, especially during flowering (Figure 2), there was no influence of the application of phosphogypsum on grain yield. Thus, the discriminant analysis of the principal components also revealed that the crop yield response to phosphogypsum application was dependent on the water balance during the flowering period.
Crops responded more to TSP fertilization than to phosphogypsum application ( Figure 5). Thus, in our study, the availability of P was an important yield-limiting factor. The application of TSP at an average rate of 32 kg P ha −1 for each soybean or wheat crop was efficient in maintaining topsoil P (Mehlich-1) concentration (Figure 6a) above the critical level of 12 mg dm −3 (Pauletti and Motta 2017) and highest crop yields (Figure 5a). Increases in soybean and cereal yields with P application in weathered soils were also observed in other studies (Vieira et al. 2015;Caires et al. 2017). However, the use of phosphogypsum also increased cumulative grain yield (Figure 5b), especially due to increases in cereal yields (Figure 3), as reported in several studies conducted under no-till systems (Caires et al. 2011a(Caires et al. , 2011b(Caires et al. , 2011cMichalovicz et al. 2014;Vicensi et al. 2016;Dalla Nora et al. 2017). A decrease in cumulative grain yield gains with increasing TSP-P application rates to the sowing furrow ( Figure 7a) is explained by the law of diminishing returns (Tilman et al. 2002). In our study, it was expected that the continued use of phosphogypsum would increase the efficiency of phosphate fertilization, especially at lower P application rates. Our results showed that the continued use of phosphogypsum increased PUE by 20%, regardless of the P application rates. The benefits of applying phosphogypsum to the soil are widely known (Zoca and Penn 2017). The increase in PUE as a result of the application of phosphogypsum should have been due to an increase in topsoil P levels with increasing phosphogypsum rates (Figure 6b). However, since increasing phosphogypsum application rates also increased exchangeable Ca 2+ levels throughout the soil profile (Figure 6b), a possible involvement of phosphogypsum use increasing PUE as a result of increased P contact with plant roots caused by increased root growth (Carvalho and van Raij 1997;Caires et al. 2016;Bossolani et al. 2021) is difficult to exclude entirely in our study.

Conclusions
In our study conducted under a continuous no-till management, fertilization with TSP in the sowing furrow of crops and the continued use of phosphogypsum on the soil surface to alleviate subsoil acidity increased soybean, wheat, and black oat yields. Applying TSP to the sowing furrow increased soybean yield by 14 to 24%, wheat yield by 57%, and black oat yield by 78%. The application of TSP at an average rate of 32 kg P ha −1 for each crop was efficient in maintaining topsoil P concentration above the critical level and highest crop yields. Soybean yields varied in different cropping seasons in response to phosphogypsum application (with no response to increases of up to 15% in yield), while phosphogypsum increased wheat yield by 23% and black oat yield by 59%. The water balance during crop flowering possibly interfered on crop yield response to phosphogypsum. Phosphogypsum provided the greatest increases in crop yield in the growing seasons marked by a water deficit during the flowering period. We hypothesize that the continued use of phosphogypsum to alleviate subsoil acidity increases the efficiency of phosphate fertilization under no-till management. The continued use of phosphogypsum increased PUE by 20%, regardless of the P application rates. Changes in P-leaf concentration caused by TSP fertilization and phosphogypsum use had a positive impact on crop grain yield. The continued use of phosphogypsum to alleviate subsoil acidity could increase the phosphate fertilization efficiency and improve crop yield performance under drought stress.