Are Reactive Rock Phosphate and Superphosphate Mixtures Suitable

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May 30, 2014 - Are Reactive Rock Phosphate and Superphosphate Mixtures. Suitable for No-Till Soybean? Ciro Antonio Rosolem* and Danilo Silva Almeida.
Published May 30, 2014 Soil Fertility & Crop Nutrition

Are Reactive Rock Phosphate and Superphosphate Mixtures Suitable for No-Till Soybean? Ciro Antonio Rosolem* and Danilo Silva Almeida Abstract

Several studies showed that rock phosphate efficiency may be increased when this fertilizer is mixed with soluble phosphates such as triple superphosphate (TSP), but most of these findings were obtained after 1 yr under conventional tillage. This study examined soybean [Glycine max (L.) Merr.] response to mixtures of Arad reactive rock phosphate (RRP) and TSP as affected by previous P fertilization. Plots received no fertilization and TSP or RRP broadcast on soil surface at a rate of 35 kg ha–1 of P in the first year. Mixtures containing 0, 20, 40, 60, 80, and 100% of RRP were then applied to soybean seed furrows at 35 kg ha–1 of P for 3 yr. Soybean responded to broadcast TSP from the first year on, while broadcast RRP increased available soil P, soybean leaf P content and yields from the second year on. Soybean responses to an increase of TSP in the mixture was linear, except for the third year, when the mixture of 40% RRP resulted in similar yields as TSP. The mixture of RRP with TSP applied to seed furrows did not produce similar soybean yields as those obtained with TSP applied alone in the first 2 yr. Combining rock phosphate and soluble phosphate to substitute for soluble phosphate is not a viable option for no-tilled soybean, because the efficiency of this fertilizer is relatively low and it does not increase when mixed with TSP.

Plant response to natural phosphates, mainly RRP,

may be increased when applied in mixture with soluble P sources by combining two fertilizers with different solubility (Chien et al., 2009). This can be done by applying phosphates separately to the same area or applying combinations of phosphates (Franzini et al., 2009a). The combination of rock phosphate and TSP has resulted in a yield at least similar to those obtained with the use of the water soluble source alone, and in some cases such results are obtained even after applying lower P rates (Franzini et al., 2009b). Increased efficiency may be a consequence of early development and increased root growth in the presence of a soluble form of P (Chien et al., 1996), resulting in more effective use of the less soluble P source (Zapata and Zaharah, 2002). In addition to P, most phosphate fertilizers contain Ca, which can improve root growth (Rosolem et al., 1998). The partial solubilization of reactive phosphate by acid generated from superphosphate hydrolysis in the soil (Mokwunye and Chien, 1980) may be another explanation for the increased efficiency of some P sources with the joint application. Chien et al. (1996) observed increased plant growth and P uptake from a moderately reactive phosphate rock. This effect is not always observed, as Xiong et al. (1996) reported

Dep. of Crop Science, Sao Paulo State Univ. (UNESP/FCA), Botucatu, Sao Paulo, Brazil. Received 28 Feb. 2014. *Corresponding author (rosolem@fca. unesp.br). Published in Agron. J. 106:1455–1460 (2014) doi:10.2134/agronj14.0115 Copyright © 2014 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

that rye growth and P uptake improved with the application of superphosphate and rock phosphate, but the increase was not correlated with the water soluble P in the less soluble fertilizer. Such effect also depends on the fertilizer application method (Franzini et al., 2009a). When the fertilizer mixture was applied to the seed furrow there was a linear response to the increase in watersoluble phosphate, but when the fertilizer was applied to the soil surface, a yield increase was observed up to 50% of water-soluble phosphate in the mixture. Thus, the combined application of P sources with different solubility may be a viable agronomic practice in the management of P fertilization for soybean (Oliveira Junior et al., 2011). In maize (Zea mays L.), the mixture of RRP with TSP improved the use of P from the reactive phosphate, and this effect increased with the rate of TSP in the mixture. Nevertheless, P recovery from the RRP (2.57%) is much lower than from TSP (10.52%), even when mixed in the proportion of 80% TSP:20% RRP. In this case there was no effect of increasing P doses in the mixture (1:1 ratio). Therefore, the availability of P from reactive phosphates is affected by the mixture ratio with water-soluble P, but not by P levels (Franzini et al., 2009b). The main factors affecting the efficiency of mixtures of reactive and soluble phosphates include the degree of reactivity and acidification, the amount of Fe and Al and impurities of raw materials, chemical soil properties such as pH and maximum P fixation capacity, the crop, and the immediate and residual effects of phosphates (Chien, 2003). The effect of mixtures applied under conventional tillage has been studied, but there is controversy about the effect of tillage on P reaction Abbreviations: RRP, reactive rock phosphate; TSP, triple superphosphate.

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in the soil (Pavinato et al., 2009), which can affect the mixture effectiveness. Most studies were conducted under greenhouse conditions or conventional tillage for only 1 yr, without taking into account the residual or cumulative effect of fertilizer, which can modify plant response. Moreover, there are no studies on crop response to mixtures of P sources with different solubility as affected by soil P availability. In the present experiment, the objective was to study soybean response to TSP mixtures with a reactive phosphate as affected by soil P availability for 3 yr, under no-till. Materials and Methods The study was conducted from 2001 to 2004 in Botucatu, State of São Paulo, Brazil (22°50¢00² S, 48°25¢31² W, and altitude of 806 m). The soil is a Rhodic Hapludox (Soil Survey Staff, 2010), sandy loam with 75 to 83% kaolinite. The climate is a humid subtropical with dry winters and a well-defined dry season between May and September. Mean rainfall averages approximately 1500 mm, while the highest monthly mean temperature is over 23°C and the lowest is below 18°C. The area selected for the trial had not been cropped for several years before imposing this study. The soil was limed to raise soil base saturation to approximately 60%, and for two seasons no-till soybean was grown in rotation with black oat (Avena strigosa Schreb.) or triticale (X Triticosecale Witt.) in the winter, and pearl millet [Pennisetum glaucum (L.) R. Br.] in the spring. When the P experiment was initiated, the soil had 18 mg dm–3 of P (resin) in the 0- to 5-cm layer, and 16, 5, and 3 mg dm–3 of P in the 5- to 10-, 10- to 20- and 20- to 40-cm layers, respectively. According to Raij et al. (2001) soil P (resin) ranging from 7 to 15 mg dm–3 in the surface 20 cm of soil is considered low for annuals (based on achieving relative yields ranging from 71 to 90 of the maximum attainable). In April 2001, after soybean harvest, three treatments with varying amounts of P were imposed. They were 35 kg ha–1 P as powder TSP, 35 kg ha–1 P as ground RRP, and a control (no phosphate fertilizer). The TSP had 46% of P2O5, 90% of which was soluble in the neutral ammonium citrate (NAC), while the RRP from Arad, in the region of Negev Desert in Israel, had 33% P2O5 of which only 7.1% was soluble in NAC (Zapata and Roy, 2004). A summary of P treatments and crop rotations is given in Table 1. The plots were 30 m long and 8 m wide, and subplots were 5 by 8 m. Fertilizers were broadcast on the soil surface over the previous crop residues. All plots received 42 kg ha–1 K as KCl and 100 kg ha–1 of phosphogypsum, also broadcast on the soil surface, Table 1. Summary of treatments and crop rotations in the experiment. Year 2001

2002

2003

Season fall/winter spring summer fall/winter spring summer fall/winter spring summer

Crop triticale pearl millet soybean black oat pear millet soybean triticale pearl millet soybean

P fertilization P broadcast† P mixtures in furrows‡

P broadcast P mixtures in furrows

† Main plots with 0.0 or 35.0 kg ha –1 of P as triple superphosphate and Reactive Rock phosphate. ‡ Subplots with 35 kg ha –1 of P as phosphate mixtures.

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and triticale was planted over the standing straw. In September 2001, millet was planted in the experimental area after triticale harvest. In November, after chemical desiccation of millet, soybean (cultivar IAC 17) was planted, and fertilized with phosphate mixtures applied to subplots 5.0 by 12.0 m at 35 kg ha–1 of total P. Fertilizers were mixed to reach the following proportions of soluble and reactive P: (i) 0% TSP and 100% RRP; (ii) 20% TSP and 80% RRP; (iii) 40% TSP and 60% RRP; (iv) 60% TSP and 40% RRP; (v) 80% TSP and 20% RRP; and (vi) 100% TSP and 0% RRP. In addition to P, soybean received 42 kg ha–1 K, as KCl. Phosphorus and K fertilizers were placed in the seed furrows, next to and below the seeds. In the next cropping season the crop rotation was repeated on residual P. Black oat was planted without fertilizers in May, after soybean harvest, and chemically desiccated when it was blooming in August. Pearl millet was planted late September and desiccated in November. Soybean (cultivar BR 48) was planted early December with 42 kg ha–1 K, as KCl, and harvested in the following May. In May 2003 the treatments were repeated as in the first year (i.e., no P, 35 kg ha–1 total P as TSP, or 35 kg ha–1 total P as RRP broadcast on the soil surface). Triticale was then planted, with 33 kg ha–1 of K (KCl) in the planting furrow. In September 2003, after triticale was harvested, pearl millet was planted without fertilizer in all plots. In December, after chemical desiccation of pearl millet, soybean (BR 48) was planted using phosphate mixtures (as described for the first year), and 42 kg ha–1 K, as KCl. Phosphorous mixtures and K fertilizer were placed in the seed furrows next to and below the seeds. Four soil samples were randomly collected from each subplot before soybean planting in each year at the depths of 0 to 5, 5 to 10, 10 to 20 and 20 to 40 cm, and combined into one composite sample per soil layer and per subplot. The samples were air dried and ground to pass through a 2-mm mesh sieve. Available P (resin) was determined according to Raij et al. (2001). Soybean grain samples taken each year, and triticale grain samples from the first year were dried to constant mass in an air-forced oven (65°C), ground and sieved through a 1 mm mesh sieve, and P was analyzed as in Malavolta et al. (1997). Grain yields were adjusted to 130 g kg–1 humidity. The experimental design was a complete randomized blocks design (RCBD) with subplots, with four replications. Analysis of variance was used to analyze P treatments and fertilizer mixture effects and to test their interaction using SAS/STAT (SAS Institute, 2001), where replicates were regarded as random effects and P and fertilizer mix as fixed effects. Data were subjected to ANOVA and means were compared using protected LSD (P < 0.05). Results and Discussion Broadcasting soluble or RRP increased soil P in the layer from 0 to 0.05 m compared with the absence of phosphate fertilizer (Table 2). In deeper layers, an increase in P concentration was observed from the first to the second year, and broadcast P interacted with furrow P in the soil depth from 0.10 to 0.20 m, since generally there was less soil P with soluble phosphate irrespective of the way it was applied. It is worth mentioning that there is a large concentration of roots at this soil depth, where a higher P uptake is usually observed (Rosolem and Calonego, 2013). Agronomy Journal  •  Volume 106, Issue 4  •  2014

Table 2. Available soil P(resin) in the 3 yr of the experiment as affected by phosphates with different solubility and soil depth. Soil samples were taken just before soybean planting each year. Treatments 0– 5 cm No P† RRP§ TSP¶ LSD 5– 10 cm No P RRP TSP LSD 10– 20 cm No P RRP TSP LSD 20– 40 cm No P RRP TSP LSD

Year 1 Year 2 Year 3 ——————— mg dm–3 ——————— 30 37 48 13

24c‡ 50a 42b 8

25b 41a 40a 6

9 12 17 6

18 21 19 4

16b 20a 17a 4

7b 7b 10a 3

17a 15b 14b 3

11b 13b 14a 1

2 2 3 2

9 10 9 1

9 10 11 1

† No P broadcast on the soil surface. ‡ Means followed by the same letter within a column are not significantly different at P < 0.05. § 35 kg ha –1 of P broadcast on the soil surface in the first and third year, as Arad reactive rock phosphate (RRP). ¶ 35 kg ha –1 of P broadcast on the soil surface in the first and third year, as triple superphosphate (TSP).

The higher soil P content in plots without phosphate application compared with the initial content (Table 2) was probably due to mineralization of crop residue and old roots. Crop and cover crops residues contain significant amounts of P, and act as an important factor in nutrient cycling (Buchanan and King, 1993; Wisniewski and Holtz, 1997). Moreover, the disturbance is minimized under no-till, the surface contact with soil particles is low, and the decomposition of plant residues after harvest is slower, allowing for better nutrient cycling, and eventually an accumulation of labile P fractions in the soil (Pavinato and Rosolem, 2008). Soil P decreased from 0.05 to 0.20 m in the soil profile as the proportion of RRP in the mixture increased after the first soybean crop (Fig. 1 A). This soil layer is the most explored by plant roots in regard to P uptake (Rosolem and Calonego, 2013), hence it is possible to infer that there was a higher use of soluble phosphate compared with rock phosphate, and a high dependence of plants on this source of P to achieve maximum growth. The linear soybean response to the proportion of soluble P in the mixture (Fig. 2) supports this hypothesis. This effect was observed because RRP react more slowly in the soil than TSP, although its efficiency can equal the soluble phosphates when assessed for longer periods (Novais and Smyth, 1999). By the third soybean planting, soil available P levels of P were increased linearly with the proportion of soluble phosphate in the mixture (Fig. 1B) down to 0.40 m. Correa et al. (2005) have shown that P fertilization is more efficient with the use of cover crops under no-till because this practice reduces the contact surface of soil with phosphate,

Fig. 1. Available soil P(resin) as affected by the proportion of soluble phosphate applied in the soybean seed furrows and soil depth. Samples taken before the (A) second and (B) the third soybean crop. ns, nonsignificant; *, ** Significant (P < 0.05 and 0.01, respectively); vertical bars show the standard error (n = 12), averaged over treatments with and without phosphate broadcast.

resulting in less P fixation. Moreover, the mineralization of plant residues on the soil surface releases organic acids, which may increase P availability to plants and the distribution of this nutrient in the soil profile (Galvani et al., 2008; Pavinato and Rosolem, 2008). Although it has been observed that reactive phosphates could be as effective as water-soluble phosphates, particularly when considering the residual effect (Oliveira et al., 1984), this effect was not observed in the present experiment, showing that under no-till, with lower contact of soil particles with the fertilizer, the reaction of the less soluble phosphates in soil was slowed down. Chemical soil properties such as pH and maximum P fixation capacity, as well as the crop species, were reported to affect phosphate efficiency use (Chien, 2003). Therefore, an interaction between broadcast and furrow fertilizations was expected, but it was not observed. Pavinato et al. (2009) observed no accumulation of soil available P under no-till after 10 yr, and a large amount of the freshly applied P as fertilizer

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Table 3. Triticale yield, P concentration and exported in grain as affected by broadcast phosphates. Treatments No P† RRP§ TSP¶ LSD (P < 0.05)

P Concentration Year 1 Year 3 ———— g kg–1 ———— 2.9b‡ 3.0a 3.1ab 2.6b 3.4 a 2.8ab 0.4 0.4

P in grains Yield Year 1 Year 3 Year 1 Year 3 —————————————— kg ha–1 —————————————— 6.1b 3.1b 2044b 1062b 6.3b 2.8b 2060b 1078b 8.2a 3.8a 2436a 1342a 1.1 0.5 351 191

† No P broadcast on the soil surface. ‡ Means followed by the same letter within a column are not significantly different at P < 0.05. § 35 kg ha –1 of P broadcast on the soil surface in the first and third year, as Arad reactive rock phosphate (RRP). ¶ 35 kg ha –1 of P broadcast on the soil surface in the first and third year, as triple superphosphate (TSP).

Table 4. Soybean yields as affected by phosphates broadcast and phosphate mixtures applied to the seed furrow. Treatments No P† RRP§ TSP¶ LSD (P < 0.05)

Yields Year 1 Year 2 Year 3 ————————– kg ha–1 ————————– 2964b‡ 2912b 2953b 3030b 3095ab 3140ab 3328a 3133a 3281a 285 165 202

† No P broadcast on the soil surface. ‡ Means followed by the same letter within a column are not significantly different at P < 0.05. § 35 kg ha –1 of P broadcast on the soil surface in the first and third year, as Arad reactive rock phosphate (RRP). ¶ 35 kg ha –1 of P broadcast on the soil surface in the first and third year, as triple superphosphate (TSP).

was adsorbed and remained in soil, mainly on uppermost soil layers, which was also observed in the present experiment. Under no-till, the soil maximum P fixation capacity may be decreased (Pavinato and Rosolem, 2008), which would favor the soluble phosphate and explain the higher P availability observed in our experiment as well as the lack of interaction. Triticale yields were increased by broadcast TSP (Table 3) even with the low efficiency of phosphate fertilizer when broadcast on the soil, which favors high P adsorption due to the high surface contact with soil particles (Chien et al., 2009; Ghosal et al., 2003). Furthermore, P has very little mobility in

Fig. 2. Soybean yields as affected by the proportion of soluble phosphate applied in the soil, averaged over treatments with and without phosphate broadcast. *, ** significant (P < 0.05 and 0.01, respectively); vertical bars show the standard error (n = 12)

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tropical weathered soils and when applied to the soil surface without incorporation, it may be less accessible to plant roots (Novais and Smyth, 1999). Franzini et al. (2009b) observed in a greenhouse pot experiment with corn and soybean that P efficiency of TSP was higher when the fertilizer was more localized than when mixed in the soil, while the reverse would be true for a reactive phosphate. Triticale, as well as wheat, has many roots near the soil surface (Furlani Junior et al., 1992) and can take up more P from the soil surface layers, provided that there is enough moisture. As the accumulation of organic matter in this soil layer is high, P fixation would be minimized (Pavinato et al., 2009), which favors the use of soluble phosphate. Moreover, as the reactive phosphate was not mixed into the soil, its reaction was impaired, at least in the first year. Phosphorus application resulted in higher P concentrations in grains of triticale compared with the absence of phosphate fertilizer, while the difference was significant only in the first year (Table 3). Although no difference was observed in P concentration in triticale plants due to P sources, P export in grains was higher with TSP. This was a result of a higher grain yield, implying that fertilization with soluble P allowed for a better nutrition of plants (Table 3). This increased export confirms the higher efficiency of TSP (Ghosal et al., 2003). It has been observed that yields obtained with RRP may be similar to yields obtained with soluble P sources after a few years, particularly under no-till (McLaughlin et al., 1981; Choudhary et al., 1996). Lopes (1998) reported efficiencies between 60 and 70% for a RRP in the first year when broadcast to annual cereal crops, besides a longer residual effect compared with TSP. In the present experiment it was observed that the residual effect of the natural RRP was not enough to achieve such high triticale yields similar to those obtained with TSP up to the third year. There was no interaction of broadcast and furrow-applied P on soybean yields, as discussed for triticale. In the first year soybean responded to broadcast TSP and there was no response to the RRP (Table 4), and there was no difference between P sources in the second and third years. In these years, soybean leaf P and P export in grain followed yield responses (Table 5). Interestingly, in the first year, when soybean responded to broadcast soluble phosphate, leaf analysis showed no significant difference in P concentrations (Table 5). In the following years, both phosphates, soluble and reactive, resulted in higher P leaf concentrations. A similar response was observed for P exported in soybean grains. Also, in the first and third years no significant difference was observed in P leaf concentrations due to the phosphate mixtures (Table 5). In the second year, leaf P concentrations increased up to 20% of soluble phosphate in the Agronomy Journal  •  Volume 106, Issue 4  •  2014

Table 5. Phosphorus leaf concentration and exported in soybean grain as affected by phosphates broadcast and phosphate mixtures applied to the seed furrows. Treatments P broadcast No P† RRP§ TSP¶ LSD (P < 0.05) P seed furrow 0TSP/100RRP 20TSP/80RRP 40TSP/60RRP 60TSP/40RRP 80TSP/20RRP 100TSP/0RRP LSD (P < 0.05)

Concentration Year 1 Year 2 Year 3 ————————— g kg–1 —————————

Export Year 1 Year 2 Year 3 ————————— kg ha–1 —————————

5.7 5.4 5.6 0.5

4.3b‡ 4.8a 5.0a 0.4

5.6b 6.4a 6.9a 0.6

16.8 16.6 18.7 2.2

12.2b 14.6a 15.2a 2.1

16.5b 20.1ab 23.6a 3.7

5.4 5.6 5.5 5.5 5.6 5.9 0.6

4.3b 4.6ab 4.7a 4.8a 4.8a 4.9a 0.4

6.2d 6.2d 6.7a 6.4c 6.2 d 6.6 b 0.5

16.2b 17.1b 17.0b 17.2ab 17.8ab 19.0a 1.9

12.5d 13.3cd 13.4bcd 14.8ab 14.2bc 15.8a 1.5

18.2 18.5 22.2 20.8 20.6 21.3 4.1

† No P broadcast on the soil surface. ‡ Means followed by the same letter within a column are not significantly different at P < 0.05. § 35 kg ha –1 of P broadcast on the soil surface in the first and third year, as Arad reactive rock phosphate (RRP). ¶ 35 kg ha –1 of P broadcast on the soil surface in the first and third year, as triple superphosphate (TSP).

mixture. The P export by soybean increased with the mixture of soluble phosphate up to 60%, although the difference was not always significant (Table 5). Therefore, there was a residual effect of the RRP for soybean, as observed in other studies (McLaughlin et al., 1981; Choudhary et al., 1996). This could be an indication that soybean would be more efficient in acquiring soil P than triticale, as rhizobium inoculation enhances P acquisition in soybean, especially in soils where Ca–P is the primary P source (Qin et al., 2011). The higher the proportion of soluble phosphate in the mixture the greater the soybean yields, with a linear response in the first 2 yr (Fig. 2) showing that the rock phosphate was not able to substitute for the soluble phosphate fertilizer. A similar result was observed by Oliveira Junior et al. (2011). In the third year the response was quadratic and the maximum yield was reached with 50 to 60% of soluble phosphate in the mixture, probably due to an accumulated residual effect of the rock phosphate, as suggested in other studies (McLaughlin et al., 1981; Choudhary et al., 1996). This value is similar to that obtained by Oliveira Junior et al. (2011), when phosphate mixtures were broadcast on the soil surface and incorporated, under a conventional cropping system. Hence, the mixture of RRP with a soluble P source was not enough to improve the response to the natural phosphate so as to achieve soybean yields as high as those observed with the soluble phosphate applied alone in the first year, as observed in other studies (Zapata and Zaharah, 2002; Oliveira Junior et al., 2011). Conclusions In cropping systems under no-till, the partial solubilization of reactive phosphate by acid generated from superphosphate hydrolysis in the soil, the early development and increased root growth in the presence of soluble P forms were not sufficient to enhance the effectiveness of RRP. Only after some time does the effect of this phosphate seem to be significant, showing that other soil P interactions, possibly microorganisms and organic matter (Pavinato et al., 2009), are affecting the process.

ACKNOWLEDGMENTS This research was funded by FAPESP (The State of São Paulo Research Foundation). References Buchanan, M., and L.D. King. 1993. Carbon and phosphorus losses from decomposing crop residues in no-till and conventional till agroecosystems. Agron. J. 85:631–638. doi:10.2134/agronj1993.00021 962008500030021x Chien, S.H. 2003. IFDC’s evaluation of modified phosphate rock products. In: S.S.S. Rajan and S.H. Chien, editors, Direct application of phosphate rock and related appropriate technology-latest development and practical experiences. Spec. Publ. SP-37. Int. Fertilizer Development Ctr., Muscle Shoals. p. 63–77. Chien, S.H., R.G. Menon, and K.S. Billingham. 1996. Estimation of phosphorus availability from phosphate rock as enhanced by watersoluble phosphorus. Soil Sci. Soc. Am. J. 60:1173–1177. doi:10.2136/ sssaj1996.03615995006000040031x Chien, S.H., L.I. Prochnow, and H. Cantarella. 2009. Recent developments of fertilizer production and use to improve nutrient efficiency and minimize environmental impacts. Adv. Agron. 102:267–322. doi:10.1016/ S0065-2113(09)01008-6 Choudhary, M., L.D. Bailey, and T.R. Peck. 1996. Effects of rock phosphate and superphosphate on crop yield and soil phosphorus test in longterm fertility plots. Commun. Soil Sci. Plant Anal. 27:3085–3099. doi:10.1080/00103629609369763 Correa, R.M., C.W.A. Nascimento, S.K.S. Souza, F.J. Freire, and G.B. Silva. 2005. Gafsa rock phosphate and triple superphosphate for dry matter production and P uptake by corn. Sci. Agric. 62:159–164. doi:10.1590/ S0103-90162005000200011 Franzini, V.I., T. Muraoka, H.M. Coraspe-Leon, and F.L. Mendes. 2009a. Eficiência de fosfato natural reativo aplicado em misturas com superfosfato triplo em milho e soja. Pesqi. Agropecu. Bras. 44:1092–1099. Franzini, V.I., T. Muraoka, and F.L. Mendes. 2009b. Ratio and rate effects of 32P-triple Superphosphate and Phosphate Rock Mixtures on corn growth. Sci. Agric. 66:71–76. doi:10.1590/S0103-90162009000100010 Furlani Junior, E., C.A. Rosolem, S.J. Bicudo, E.G. Moura, and L. Bulhões. 1992. Preparo do Solo e Sistema Radicular do Trigo. Rev. Bras. Cienc. Solo 16:115–120. Galvani, R., L.F.K. Hotta, and C.A. Rosolem. 2008. Phosphorus sources and fractions in an oxisol under no-tilled soybean. Sci. Agric. 65:415–421. doi:10.1590/S0103-90162008000400014 Ghosal, P.K., T. Chakraborty, B. Bhattacharya, and D.K. Bagchi. 2003. Relative agronomic effectiveness of phosphate rocks and P adsorption characteristics of an oxic rhodustalf in Eastern India. J. Plant Nutr. Soil Sci. 166:750–755. doi:10.1002/jpln.200321034

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Agronomy Journal  •  Volume 106, Issue 4  •  2014