37
Plant and Soil 192: 37–48, 1997. c 1997 Kluwer Academic Publishers. Printed in the Netherlands.
Calcium sulphate, phosphogypsum and calcium carbonate in the amelioration of acid subsoils for root growth M.C.S. Carvalho and B. van Raij1 Department of Soil Science, University of S˜ao Paulo, Caixa Postal 9, 13418-900 Piracicaba, SP, Brazil and Section of Soil Fertility and Plant Nutrition, Instituto Agronˆomico, Caixa Postal 28, 13001-970 Campinas, SP, Brazil. 1 Corresponding author Received 1 August 1996. Accepted in revised form 13 February 1997
Key words: Al toxicity, Ca deficiency, maize, phosphogypsum, root growth, subsoil acidity
Abstract The chemical barrier to root development existing in the subsoils of acid soils is a subject of increasing interest. In order to better understand the factors involved in the amelioration of subsoil acidity, the effects of calcium sulphate, phosphogypsum and calcium carbonate on the properties of the solid and liquid phases of subsoil samples and on the growth and nutrient uptake by maize (Zea mays L.) were evaluated. The soils used were two alic red-yellow latosols, two acric dusky red latosols and one alic dark-red latosol from the State of S˜ao Paulo, Brazil. A vertical split-root technique was used in a greenhouse experiment, with the plants initially grown in a small pot with 130 g fertile soil, which was introduced in a larger pot containing 2 dm3 of the subsoil samples. The treatments consisted of a control (C) and applications of calcium carbonate (CC), calcium sulphate (CS) and phosphogypsum (PG) at the rate of 10 mmolc Ca2+ dm 3 . CS and PG reduced soil acidity, but in a much smaller proportion than CC. Calcium carbonate reduced the activity of Al3+ because of the increase in pH. Total aluminum and calcium contents in the soil solution were much higher for the red-yellow latosols than for the other soils, indicating lower sorption of Ca2+ and SO24 in these soils. The activity of Al in the soil solution was decreased in different ways for the five soils, depending on the ionic strength and the formation of the ionic pair AlSO+ 4 and, in the case of PG, the formation of complexes of Al with F (AlF2+ , AlF+ 2 and AlF3 ). The subsoil samples presented severe restrictions for maize root growth and all three treatments were equally effective in increasing root development, which could be attributed to the supply of calcium in one of the acric dusky red latosols and a combined effect of the amendment in reducing the activity of Al and increasing the activity of Ca in the soil solution in the other soils. As a consequence the three treatments increased in the same manner water, N and K uptake from the subsoil and the dry matter production of maize. It can be concluded that, for the soils considered in this research, phosphogypsum is an effective amendment for acid subsoils containing low calcium or toxic aluminum contents. Introduction Calcium deficiency and aluminum toxicity are among the main factors limiting the productivity of crops cultivated on acid soils, which are predominant in Brazil, especially in the central "cerrado" and the Amazon regions (Olmos and Camargo, 1976; Ritchey et al., 1980). Liming, a practice largely used to neutralize the acidity of the plow layer, in general does not have a rapid effect in reducing the acidity of the subsoil,
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which depends on leaching of salts through the soil profile. The experiments reported by Gonzalez-Erico et al. (1979) and by Sumner et al. (1986) demonstrated that the deep incorporation of lime into the soil increased root development resulted in higher crop yields. However, deep incorporation of lime requires specific machinery and is expensive, which makes it unsuitable as a routine farm operation. On the other hand, the surface application of gypsum followed by leaching into acid subsoils has resulted in higher uptake of water and nutrients by plant roots
PIPS NO.:135737 (M) BIO2KAP plso6504.tex; 12/08/1997; 10:27; v.7; p.1
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38 (Alcordo and Rechcigl, 1993; Shainberg et al., 1989; Sumner, 1993). Such effects are generally attributed to the increase of Ca and to the reduction of toxic Al in the subsoil, considering that the excess aluminum as well as calcium deficiency are deleterious to root development (Ritchey et al., 1980, 1982). The reduction of Al toxicity in the subsoil by gypsum has been associated with a complex set of mechanisms, including: 1) precipitation of aluminum as the result of liberation of OH ions from oxidic surfaces by ligand exchange with SO24 , in a process named as “self-liming” (Reeve and Sumner, 1972; Sumner et al., 1986); 2) formation of insoluble basic Al sulphates (Alva et al., 1991; Hue et al., 1985): 3) formation of ionic pairs in the soil solution, such as AlSO+ 4 (Cameron et al., 1986; Mclay and Ritchie, 1993; Pavan et al., 1982) and AlF2+ (in the case of phosphogypsum), which are less toxic than the uncomplexed forms of Al (Cameron et al., 1986); 4) reduction of the activity of Al3+ ions in solution because of the increase of the ionic strength of the solution (Adams and Lund, 1966; Pavan and Bingham, 1982); 5) preferential adsorption of Al3+ and H+ over Ca2+ on the negative charges formed by the specific adsorption of SO24 (Sumner, 1993). Phosphogypsum seems to be more effective in reducing Al toxicity than pure calcium sulphate because of the presence of F , an anion that forms more stable complexes with Al than SO24 (Cameron et al., 1986). In cases of soils with very low contents of Al but with low contents of Ca, the supply of calcium is the main factor responsible for improved root development (Ritchey et al., 1982). Phosphogypsum, a by-product of the phosphoric acid industry, containing mainly calcium sulphate and small contents of P and F, is largely available in many parts of the world. Only in Brazil about 2.4 million tons are produced yearly (Freitas, 1992). Thus, it is necessary to increase knowledge on the use of phosphogypsum as a soil amendment for acid subsoils as an alternative to dispose of a bulky by-product of the fertilizer industry. Although in many cases gypsum applied to acid soils resulted in positive effects on crop yields, some papers report absence of effect (Alva and Sumner, 1990; Wright et al., 1985) or even a negative effect (Mclay and Ritchie, 1993). Such conflicting results suggest that the effect of gypsum in the amelioration of acid subsoils might be affected among other factors by still not well established soil properties. Especially the reaction of gypsum with soils requires a better understanding. Considering that the reaction of phos-
phogypsum in acid subsoils is in part due to the increase of Ca, one possible way to evaluate its beneficial effect in the reduction of Al toxicity, is to compare it with calcium carbonate applied in equivalent amounts of Ca. In this research the effect of calcium sulphate, phosphogypsum and calcium carbonate on chemical soil properties and soil solution composition for subsoil samples from five acid soils was compared and an evaluation was made on how these changes in soil properties affected root development and water and nutrient uptake by maize grown in the greenhouse.
Materials and methods Greenhouse trial Samples of the subsoil (0.5–1.0 m) were obtained from five uncultivated soils, represented by two alic redyellow latosols (LVal), two acric dusky-red latosols (LRac) and one alic dark-red latosol (LEal) of the State of S˜ao Paulo. The soils presented low Ca contents and variable contents of Al (Table 1). The samples were air-dried, ground and passed through a 2-mm sieve. The treatments were: control (C); reagent-grade calcium sulphate (CS); phosphogypsum (19% Ca, 15% S, 0.3% P and 0.6% F) (PG); and reagent-grade calcium carbonate (CC). Each salt was thoroughly mixed with a 2 dm3 soil volume at the rate of 10 mmolc Ca2+ dm 3 . All samples received a solution of KNO3 and NH4 NO3 , supplying per pot 28 mg NH4 -N, 28 mg NO3 -N and 78 mg K, uniformly mixed with the soil. The experiment was conducted in the greenhouse, cultivating maize and using a vertical split-root technique similar to the one described by Adams and Lund (1966), with plants grown initially in small pots containing fertile soil, which were later introduced in the two liter pots containing the treated subsoil samples. The experimental design was in random blocks with four replications. Eight seeds of maize (Zea mays L. cv ICI-868) were sown in a plastic cup (dimensions of 5 cm diameter lower base, 7 cm diameter upper base, and 5 cm height) filled with 130 g of a fertile soil sample, obtained from the plow layer of a reddish brunizem. To assure good plant development in the cups before transferring them to the larger pots containing the subsoil samples, 200 mg P (triple superphosphate) was mixed with the soil and amounts of 40 mg N, 30 mg K, 12 mg S, and 1.3 mg Zn, supplied as KNO3 , (NH4 )2 SO4 , NH4 NO3 and ZnSO4 , were applied in solution, divided in four
plso6504.tex; 12/08/1997; 10:27; v.7; p.2
39 Table 1. Selected properties of the soil samples used in the experiment
Clay (%) pH 0.01 mol L
1
CaCl2
Exchangeable cations (mmolc dm Ca2+ Mg2+ K+ Al3+ ECEC H+ +Al3+ (mmolc dm 3 ) Al sat. (%) SO24 -S (mmolc dm 3 ) NH+ 4 -N + NO3 -N (mg dm ∆pH (pHKCl - pHH2O )
3)
Soils LRac-2 LEal
LVal-1
LRac-1
LVal-2
Brunizen
33 4.03
68 4.40
52 5.53
58 4.32
40 4.10
n.d. 5.49
0.9 0.6 0.5 13.8 15.8
0.7 0.4 0.2 2.4 3.7
0.6 0.2 0.2 0.0 1.0
3.0 1.1 0.4 7.2 11.7
0.7 0.8 0.3 8.7 10.5
216.0 12.0 0.7 0.0 228.7
55.0 87.3 2.0 6.2 –0.6
43.7 64.9 0.3 4.2 –0.4
25.0 0.0 0.3 5.3 0.3
48.5 61.5 0.4 4.8 –0.8
50.6 82.9 1.1 6.2 –0.7
40.5 0.0 0.6 n.d. n.d.
3)
n.d. = not determined.
applications at 2, 6, 10 and 12 days after germination. Four days after germination the plants were thinned out, leaving four plants per cup. Fifteen days after germination, the cups were cut off and the soil blocks were introduced in the 2 dm3 subsoil samples contained in the 2 liter pots with the treated subsoils. The upper surface of the soil from the cup and the pot were put at the same level. The plants were irrigated daily with distilled water, and the amount of water was monitored to assure the supply to reach about 80% of the field capacity. Thirty four days after germination, the tops of the corn maize plants were harvested, rinsed in distilled water, dried at 70 C, weighted and ground for chemical analysis. Samples of 100 cm3 of the subsoil were collected with a 2.5 cm3 diameter steel tube, taking one subsample of 25 cm3 of each quarter of the pot. These samples were used to determine root length by the technique described by Tennant (1975). The remaining soil was air dried, passed through a 2-mm sieve and kept for chemical analysis. Chemical analysis of soil and plant samples The pH was determined in water, 0.01 mol 1 CaCl2 and 1 mol L 1 KCl, using a 1:2.5 ratio and a 15 minute stirring period. The basic cations were extracted with a 1 mol L 1 neutral ammonium acetate solution; Ca and Mg were determined by atomic absorption spectrometry and potassium by flame photometry. Exchange-
able Al3+ was extracted with a 1 mol L 1 KCl solution and determined by titration with 0.025 mol L 1 NaOH solution. The effective cation exchange capacity (ECEC) was calculated as the sum of Ca, Mg, K and Al (Raij et al., 1987). The total acidity, H+ + Al3+ , was determined by a SMP buffer procedure described by Quaggio et al. (1985). Mineral N (NH+ 4 -N and NO3 -N) was extracted with a 2 mol L 1 solution of KCl and determined by the method of steam distillation of Bremner and Keeney (1966). Sulfate was extracted with a 0.01 mol L 1 Ca(H2 PO4 )2 solution and determined by a turbidimetric method (Tabatabai and Bremner, 1970). Plant samples were analyzed for total contents of N, P, K, Ca, Mg and S. Extraction and analysis of soil solution The soil solution was obtained by the method of the saturation extract with distilled water and vacuum filtration (Rhoades, 1982). The determinations made were: pH and electrical conductivity (EC); Ca, Mg, K, S and P by plasma emission spectrometry; Al by the colorimetric method of aluminon (Raij and Valadares, 1974); F by potenciometric measurement using a specific fluoride electrode (Adriano and Donner, 1982). With the values of pH and concentrations of the elements, the determination of ionic strength and the concentration of chemical species were determined using the computer program GEOCHEM (Sposito and Mattigod, 1980).
plso6504.tex; 12/08/1997; 10:27; v.7; p.3
40 Results Effect of the amendments on soil chemical properties The chemical analysis of the subsoil samples (Table 1) are typical of highly weathered soils, acid and with low contents of exchangeable bases. However, the aluminum contents are not very high and the high values of aluminum saturation are more a consequence of the low values of the effective cation exchange capacity. Such soils present variable electrical charges and the capacity to adsorb cations and anions at the same time (Raij and Peech, 1972). Soil LRac-2 is an extreme case of a highly weathered soil, with an insignificant ECEC and positive electric net charge, as shown by the pH in 1 mol L 1 KCl, which is higher than in water. This is consequence of a very low silica-alumina ratio of the clay fraction of this soil, which is mainly oxidic. Calcium was added to the soil at a rate equivalent to 10 mmolc dm 3 , corresponding to 1 t ha 1 of CaCO3 for a 20-cm deep soil layer. Calcium carbonate is a reference amendment for the practical neutralization of soil acidity, but cannot be incorporated into the subsoil in common liming practice. On the other hand, calcium sulphate leaches through the soil profile, affecting the subsoil. For these reasons, the effect of this salt on soil properties, soil solution composition and plant behavior, was compared with CaCO3 in this research. Comparing only the effect of the amendments of the pH in water, it seems that only CC had a significant effect in increasing its value and thus reducing soil acidity (Table 2). CS and PG promoted a significant reduction of the pH in water in the case of the three alic latosols, in the order of 0.2 to 0.3 units, practically not affecting the pH in water of the acric latosols. However, the pH measured in 0.01 mol L 1 CaCl2 increased significantly for the treatments with CS and PG for all soils, with values varying from 0.05 to 0.23 units whereas the increases varied from 0.27 to 0.45 for the CC treatment. This indicates that the so-called “salt effect” on the pH measured in water can lead to erroneous conclusions in gypsum studies, for reasons described in a recent review by Sumner (1994). The reduction of soil acidity by the CS and PG treatments was in this case confirmed by the significant decrease of exchangeable and total acidity (Table 2). Thus calcium sulphate neutralized subsoil acidity, asserting the pioneer observations made by Reeve and Sumner (1972) in soil columns and by Ritchey et al. (1980) in the field and more recently by several other authors (Alva et al.,
1990; Black and Cameron, 1984; Pavan et al., 1982; Raij et al., 1988) The effect of gypsum on the amelioration of subsoil acidity depends not only on the neutralization of acidity. In the case of soils as in this paper, with low calcium and aluminum contents, representative of enormous areas in Brazil and perhaps other countries, gypsum has considerable effect on the decrease of aluminum saturation (Table 2), which is an important factor for root development. In the present case, with the incorporation of only 10 mmolc dm 3 of Ca2+ as gypsum, the reduction of aluminum saturation is remarkable and not much different from the values achieved with calcium carbonate. Much of it is a “dilution effect” of Al with the Ca salts. The values of extractable Ca2+ , SO24 -S, K+ , NH+ 4 -N and NO3 -N at the end of the experiment (Table 3) indicate the effects of the amendments on the soils and the differential depletion promoted by plant growth. Potassium and nitrogen were equally depleted from the amended soils as compared to the control treatment; the nitrate ion was completely removed, indicating that even in the soils studied this ionic form presented high mobility. The depletion of K and N from the subsoil samples indicates that CS and PG were as effective as CC in reducing the effect of the chemical barrier to root growth. In the samples of the control treatment, more nitrogen and potassium remained in the soil. Soil LVal-1, which was the most acidic, had also the highest amounts of potassium and nitrogen left behind, both for the untreated and amended soil samples. Changes in soil solution The values of pH, electrical conductivity, ionic strength and the total concentration of Ca, Mg, K, SO4 and F in the soil solution after cultivation are shown in Table 4. The treatment with calcium carbonate resulted in higher pH and lower aluminum concentration due to the neutralization of soil acidity; the basic cations are also low because of the low ionic strength of the solution. The treatments CS and PG resulted in decrease of the pH, except for soil LRac-1, for which an increase was observed. The higher ionic strength is the result of much higher contents of elements in solution, and even the Al concentration is increased in the two red-yellow latosols, confirming the observation of other authors (Ismail et al., 1993; Pavan et al., 1982). There is, however, a remarkable difference among soils, with the highest ionic strength for the alic red-yellow latosols
plso6504.tex; 12/08/1997; 10:27; v.7; p.4
41 Table 2. Values of pH (H2 O and CaCl2 ), exchangeable Al3+ , total acidity (H+ +Al3+ ) and aluminum saturation of samples of the subsoils, after the pot experiment of corn, as a function of the treatments: control (C), calcium sulphate (CS), phosphogypsum (PG) and calcium carbonate (CC)a
Treatments
LVal-1
LRac-1
Soils LRac-2
LEal
LVal-2
pH em H2 O C CS PG CC
4.82 b 4.51 c 4.49 c 5.08 a
5.15 c 5.19 bc 5.21 b 5.50 a
6.14 b 6.18 b 6.18 b 6.55 a
5.28 b 5.02 c 5.05 c 5.51 a
4.98 b 4.61 c 4.64 c 5.30 a
pH em CaCl2 0.01 M C CS PG CC
4.07 d 4.12 c 4.16 b 4.34 a
4.42 c 4.65 b 4.65 b 4.87 a
5.57 c 5.80 b 5.78 b 5.94 a
4.34 c 4.52 b 4.53 b 4.67 a
4.09 c 4.28 b 4.30 b 4.53 a
Al3+ (mmolc dm C CS PG CC
13.7 a 12.0 b 10.8 c 6.6 d
2.3 a 1.6 b 1.5 b 0.0 c
0.0 0.0 0.0 0.0
7.1 a 5.6 b 5.6 b 2.3 c
8.6 a 6.7 b 6.5 b 2.7 c
54.4 a 51.1 b 51.3 b 38.3 c
44.8 a 42.0 b 41.8 b 33.8 c
24.9 a 25.0 a 24.9 a 22.0 b
48.2 a 45.2 b 45.1 b 36.1 c
50.9 a 48.0 b 48.9 b 36.4 c
85 a 47 b 45 b 35 c
51 a 11 b 11 b 0
0 0 0 0
61 a 27 b 26 b 14 c
80 a 34 b 33 b 20 c
3)
H+ + Al3+ (mmolc dm C CS PG CC Al sat. (%) C CS PG CC
3)
a Treatment
means followed by the same letter within each parameter within each soil are not significantly different by Tukey test at 5%.
and, on the other extreme, much lower values for the soil LRac-1, indicating considerable ion adsorption The relative distribution of the main chemical species of Al and Ca in the soil solution (saturation extract) of the subsoil samples after maize cultivation is shown in Table 5. As a result of the pH increase, calcium carbonate modified the relative distribution of Al species, decreasing the total concentration and activity of Al3+ and increasing the proportion of the 0 hydroxylated forms AlOH2+ , Al(OH)+ 2 and Al(OH)3 . 3+ CS and PG reduced the proportions of free Al in relation to total Al (Alt ), due to the association of the ions Al3+ with SO24 and F (only for PG), forming
+ 2+ 0 the ionic pairs AlSO+ 4 , AlF , AlF2 and AlF3 . In the treatment CS, for example, the percentage of AlSO+ 4 varied between 12.5% for LRac-1 and 50%, for LVal1, within the range obtained by Pavan et al. (1982) and Sumner et al. (1986). Even so, due to the high ionic strength of the soil solution, both CS and PG increased the activity of Al3+ in the alic LV soils. In the treatment with CS, for example, the activity of Al3+ increased from 4.03 to 7.89 for LVal-1, and from 1.92 to 3.91 for LVal-2. The presence of F in the saturation extract of the soils for the treatment with PG (Table 4) resulted in a higher capacity of this material in reducing the
plso6504.tex; 12/08/1997; 10:27; v.7; p.5
42 Table 3. Soil contents of extractable Ca2+ , Mg2+ and K+ , mineral N (NH+ 4 -N + NO3 -N) and S-SO24 determined in the samples of the subsoils, after the pot experiment with corn, as a a function of the treatments: control (C), calcium sulphate (CS), phosphogypsum (PG) and calcium carbonate (CC)a
Treatments Ca2+ (mmolc dm Control CS PG CC
LRac-1
Soils LRac-2
LEal
LVal-2
0.62 c 12.15 a 12.15 a 9.22 b
0.45 c 11.20 a 11.13 a 9.88 b
0.38 c 9.30 a 9.25 a 7.78 b
2.50 c 13.89 a 14.25 a 12.12 b
0.45 c 12.08 a 11.92 a 9.88 b
1.9 b 11.1 a 11.8 a 2.0 b
0.1 b 8.6 a 8.5 a 0.1 b
0.1 b 7.5 a 7.3 a 0.2 b
3.6 b a 10.6 a 10.2 a 1.1 b
1.0 b 10.6 a 10.2 a 1.1 b
1.20 a 0.55 b 0.55 b 0.55 b
1.10 a 0.48 c 0.48 c 0.62 b
0.50 a 0.25 b 0.30 b 0.35 b
0.95 a 0.52 bc 0.48 c 0.62 b
0.95 a 0.42 b 0.42 b 0.45 b
12.8 a 5.8 b 5.8 a 5.3 b
11.4 a 4.5 b 5.1 b 5.1 b
5.5 a 2.3 b 2.8 b 2.5 b
10.2 a 3.3 b 3.5 b 3.4 b
6.0 a 3.1 b 2.8 b 3.2 b
5.5b 0.0 0.0 0.0
6.2 0.0 0.0 0.0
2.7 0.0 0.0 0.0
3.6b 0.0 0.0 0.0
1.0 0.0 0.0 0.0
3)
SO24 -S (mmolc dm Control CS PG CC K+ (mmolc dm Control CS PG CC
3)
NH+ 4 -N (mg dm Control CS PG CC
3)
NO3 - N Control CS PG CC
LVal-1
3)
a
Treatment means followed by the same letter within each parameter within each soil are not significantly different by Tukey test at 5%.
concentration and activity of Al3+ in relation to CS (Table 5). The values of percentage of Alt that was complexed with F was high for soils LVal-1, LRac-1, LEal e LVal-2, although the concentration of F was at least 26 times smaller than that of SO4 . A similar observation was made by Alva et al. (1988) and by Moore and Ritchie (1988). The ratio between the activities of Ca2+ and Al3+ , represented by 1/2 log (aCa2+ ) - 1/3 log (aAl3+ ), believed to be indicative of the reduction of aluminum toxicity for roots (Sumner et al., 1986), decreased for the three amendments in the order PG > CS > CC (Table 5).
Plant development and nutrient uptake Water uptake, root length and dry matter yield, three measurements associated with plant development, were all significantly affected by the treatments and by the soil (Table 6). The lowest results were observed for soil LVal-1, which had the highest values of exchangeable aluminium (13.8 mmolc dm 3 ) and the highest aluminum saturation (87.3%), indicating that, at least for this soil, the rates of the amendments applied were not sufficient for maximum plant development. The three treatments were equally effective in enhancing root development. There was a significant
plso6504.tex; 12/08/1997; 10:27; v.7; p.6
43 Table 4. Values of pH electric conductivity (EC), ionic strength (I) and concentration of the ions Ca2+ , Mg2+ , K+ , SO24 and F in the soil solution of the subsoil samples after the pot experiment with corn, as a function of the treatments: control (C), calcium sulphate (CS), phosphogypsum (PG) and calcium carbonate (CC) Soils
Treatments
pH
EC (dS m 1 )
I
Ca
Mg
K (mol L
Al
SO4
F
1)
L Val-1
C CS PG CC
4.18 3.93 3.93 4.35
0.079 0.438 0.460 0.086
349 6798 7352 495
17 1548 1698 28
8 135 132 7
136 136 136 71
7 35 36 3
90 2136 2329 180
n.d. n.d. 7 n.d.
LRac-1
C CS PG CC
4.08 4.33 4.34 4.60
0.068 0.084 0.083 0.056
197 441 427 121
19 25 27 16
3 5 4 1
85 33 33 21
2 1171 1 0
31 n.d. 161 33
n.d. 6 n.d.
LRac-2
C CS PG CC
4.70 4.44 4.38 5.10
0.045 0.159 0.161 0.050
89 1456 1455 98
1 141 133 1
3 17 17 1
33 136 149 33
0 0 0 0
3 545 554 66
n.d. n.d. 6 n.d.
LEal
C CS PG CC
4.45 4.39 4.38 4.54
0.067 0.124 0.144 0.061
123 941 997 121
3 69 105 2
2 16 19 2
33 46 46 8
3 2 2 1
35 374 366 46
n.d. n.d. 6 n.d.
LVal-2
C CS PG CC
4.40 4.22 4.18 4.86
0.071 0.413 0.390 0.078
239 6807 5953 350
5 1540 1318 14
3 147 133 6
71 161 149 33
4 19 19 2
90 2140 1874 141
n.d. n.d. 6 n.d.
n.d. – not detected.
difference due to CC, as compared to CS and PG, for the most acid soil LVal-1 but for the other soils no difference was observed. The higher root length in the amended subsoils increased water uptake and dry matter yield, with higher average values for CS and PG, indicating that, in this case, gypsum was more effective than CC in alleviating the constrains of subsoil acidity on plants. The effectiveness of the presence of more roots in the subsoil samples and also the specific effect of the elements added with the amendments is further demonstrated by the concentrations of Ca and S in the plant dry matter and by the uptake of N and K. (Table 7). The calcium contents were significantly higher in all cases as a result of the treatments, and higher than CC for CS and PG for three soils. The S contents were much higher for the alic soils, maintaining relation with the higher contents in solution (Table 4). The absorption of N and K was higher for all treatments, indicating
that the fertilizer added to the subsoil was transferred to the plant as a consequence of the higher root density.
Discussion The reduction of the pH in water of the subsoils treated with CS or PG is the result of two opposite reactions that occur between CaSO4 and the surface of soil particles (Alva et al., 1990): 1) Ca2+ dislocating H+ and Al3+ (which suffer hydrolysis, liberating H+ ); and 2) SO24+ dislocating OH , in a ligand exchange reaction. Thus it is to be expected that the overall effect of these reactions on the pH will be dependent of the magnitude in which they occur in each particular case. In the alic latosols, such as the ones used in this study, H+ originated from the hydrolysis of Al3+ surpasses the release of OH , resulting in pH decrease. When the pH was measured in 0.01 mol L 1 CaCl2 , keeping
plso6504.tex; 12/08/1997; 10:27; v.7; p.7
44 Table 5. Relative distribution of the main chemical species of Ca and Al and activity ratio of Ca2+ and Al3+ in the saturation extract of the subsoil samples after the pot experiment with corn, as a function of the treatments: control (C), calcium sulphate (CS), phosphogypsum (PG) and calcium carbonate (CC) Treatments
Al3+
Alt(1) a (mol L
AlSO+ 4
1)
Al-OHb
Al-Fc
aAl3+d (mol/L
(% of Alt )
Cat a 1)
Ca2+
CaSO4
(% of Cat )
aCa2+d (mol L
1/2 log(aCa2+ ) 1/3 log(aAl3+ ) 1)
LVal-1 C CS PG CC
6.6 36.3 34.7 3.2
74.2 47.7 37.1 60.6
7.7 50.0 39.8 12.5
18.0 3.8 2.7 26.2
20.5 -
4.03 7.89 5.70 1.54
17 1549 1698 28
98.6 85.0 84.1 97.2
1.4 15.0 15.9 2.8
15.7 928.4 994.9 24.2
0.396 1.185 1.247 0.629
LRac-1 C CS PG CC
1.6 1.0 0.9 0.0
83.3 62.5 9.7 -
2.4 12.5 0.2 -
13.6 30.5 0.2 -
98.4 -
1.00 0.50 0.01 0.00
19 25 27 16
99.5 97.3 97.5 99.4
0.5 2.7 2.5 0.6
17.0 22.2 23.1 15.1
0.615 0.780 1.348 -
LRac-2 C CS PG CC
-
-
-
-
-
-
1 141 133 1
100.0 93.1 93.1 99.3
0.0 6.9 6.9 0.7
1.0 110.8 103.4 1.3
-
LEal C CS PG CC
2.9 1.7 1.8 1.1
62.8 51.2 1.7 49.6
2.8 18.8 0.6 3.5
34.6 29.7 1.2 46.9
97.0 -
1.61 0.64 0.02 0.51
3 69 105 2
99.7 94.9 95.1 99.0
0.3 5.1 4.9 0.1
2.7 57.1 86.3 1.9
0.148 0.943 1.534 0.245
LVal-2 C CS PG CC
3.7 19.1 18.6 1.9
60.9 44.9 32.6 22.5
6.7 45.6 30.9 3.7
32.4 3.7 5.6 7.7
30.9 -
1.92 3.91 2.89 0.35
5 1540 1318 14
98.5 84.9 85.9 97.8
1.5 15.1 14.1 2.2
4.2 926.9 814.7 12.7
0.217 1.286 1.302 0.703
a
Alt and Cat = total concentration of Al and Ca, respectively. Al-OH = AlOH2+ + Al(OH)+ 2 + Al(OH)3 . c Al-F = AlF2+ + ALF+ + AlF . 3 2 d aAl3+ and aCa2+ = activity of Al3+ and Ca2+ , respectively. b
constant the effect of salt on the hydrolysis of Al3+ , an effective increase of the pH was observed for the soils treated with CS or PG. Since CaSO4 is a neutral salt, its effect on the reduction of soil acidity can be partially explained by the mechanism of “self-liming”, proposed by Reeve and Sumner (1972), involving a ligand-exchange reaction, in which OH is replaced by SO24 on the surface of the hydrous oxides of Fe and Al, followed by precipitation of Al3+ as Al(OH)3 in the soil solution. In this case, the reduction of aluminum saturation for the treatments CS and PG was associ-
ated not only with the increase of exchangeable Ca2+ (Table 3), but also with the reduction of exchangeable Al3+ . The lower concentrations of Ca2+ and SO24 in the soil solution of the acric LR soils and the LE soil indicate that these soils have higher adsorption capacity for the ions, in comparison to the alic LV soils, probably a consequence of the higher clay contents in the first group (Table 1), associated to low silica to alumina ratio (data not shown), indicating higher contents of iron and aluminium hydrous oxides in the clay fraction.
plso6504.tex; 12/08/1997; 10:27; v.7; p.8
45 Table 6. Root length, water uptake and dry matter production of the above-ground parts of the corn plants, as a function of the treatments: control (C), calcium sulphate (CS), phosphogypsum (PG) and calcium carbonate (CC)a Soils Treatments
Water uptake (mL pot Control CS PG CC Means Root lenght (cm dm Control CS PG CC Means
3
Top dry weight (g pot Control CS PG CC Means
LVal-1
LRac-1
LRac-2
LEal
LVal-2
Means
1472 b 1594 a 1593 a 1619 a 1669 C
1538 c 1885 a 1912 a 1734 b 1768 B
1712 b 2036 a 2019 a 1972 a 1935 A
1548 c 1931 a 1969 a 1786 b 1808 B
1717 b 2000 a 2018 a 1940 a 1919 A
1597 c 1889 a 1902 a 1810 b
196 c 399 b 438 b 549 a 396 C
222 b 630 a 653 a 591 a 524 B
390 b 658 a 698 a 642 a 597 A
298 b 583 a 616 a 536 a 508 A
283 b 605 a 656 a 584 a 532 B
278 b 575 a 612 a 580 a
4.85 b 5.81 a 5.99 a 6.09 a 5.68 C
5.03 c 6.99 a 6.84 a 5.75 b 6.15 B
6.22 c 7.54 a 7.50 a 6.91 b 7.04 A
5.9 c 6.77 ab 6.97 a 6.33 b 6.29 B
6.12 b 7.27 a 7.51 a 6.97 a 6.97 A
5.46 c 6.85 a 6.99 a 6.41 b
1)
soil)
1)
a Average values followed by the same minuscule letter in the columns and the same capital letter in the line, within each determination and each soil are not significantly different by Tukey test at 5%.
The low concentrations of Ca2+ and SO24 in solution for soil LRac-1 treated with CS or PG (Table 4) indicate the “adsorption of salt” (Alva et al., 1991), which consists in the simultaneous adsorption of Ca2+ and SO24 as consequence of the occurrence of coexisting positive and negative electrical charges, as demonstrated by Raij and Peech (1972) for similar soils. In soils treated with gypsum, an increase of the adsorption of Ca2+ , as well as of other cations is possible because of the increase of negative charges generated by the specific adsorption of SO24 (Marcano-Martinez and McBride, 1989). The significant increases of total Al in the soil solution of the alic LV soils treated with CS and PG suggest that the effect of ion exchange of Ca2+ in solution with exchangeable Al3+ , increasing Al in solution, surpassed the effect of the ligand exchange or, in other words, the ion exchange reactions overcame the reactions of adsorption and precipitation. On the other hand, in the solution of soil LRac-1 the pH
increase indicates the predominance of the effect of “self-liming”. The amount of CaSO4 added, equivalent to 10 mmolc dm 3 , appears to have been insufficient to reduce Al3+ in the soil solution of the alic LV soils, which presented the highest values of exchangeable aluminium (13.8 and 8.7 mmolc dm 3 , respectively) and Al saturation above 82% (Table 1). A similar effect was observed by Ismail et al. (1993) in an ultisol from Malaysia with 72% aluminium saturation that received 2 t ha 1 gypsum. The chemical conditions existing in the untreated subsoil samples severely restricted root development of maize plants (Table 6). In the LRac-2 soil the increase in root growth was a consequence of Ca supply, since this soil did not contain exchangeable aluminium because of the high pH value, but presented very low Ca content (Table 1). Ritchey et al. (1982) demonstrated that in acid subsoils from the Brazilian cerrado, Ca2+ contents of the order of 0.2 to 0.5 mmolc dm 3 limited root development of corn, soybean and
plso6504.tex; 12/08/1997; 10:27; v.7; p.9
46 Table 7. Concentration of P, S, Ca and Mg in dry matter and N and K uptake by corn, as a function of the treatments: control (C), calcium sulphate (CS), phosphogypsum (PG) and calcium carbonate (CC)a Treatments
P (g kg Control CS PG CC
LVal-1
LRac-1
Soils LRac-2
LEal
LVal-2
3.0 a 2.5 b 2.5 b 2.3 b
2.9 a 2.2 bc 1.9 c 2.4 b
2.3 a 2.0 ab 2.1 ab 1.8 b
2.7 a 2.0 b 2.0 b 2.3 ab
2.5 a 1.9 b 2.0 b 1.8 b
0.7 b 1.6 a 1.6 a 0.9 b
0.7 a 0.8 a 0.8 a 0.7 a
0.5 c 0.8 a 0.8 a 0.6 b
0.7 a 0.8 a 0.8 a 0.8 a
0.7 b 1.6 a 1.6 a 0.7 b
2.9 b 5.6 a 5.4 a 5.3 a
2.7 b 5.7 a 5.4 a 5.3 a
2.1 c 5.7 a 5.9 a 3.8 b
3.6 c 5.5 a 5.7 a 4.7 b
2.4 c 5.5 a 5.3 a 4.5 b
3.0 a 2.3 b 2.3 b 2.2 b
2.8 a 1.8 c 2.0 bc 2.2 b
2.0 a 1.8 a 1.7 a 1.8 a
2.8 a 2.3 b 2.3 b 2.5 ab
2.7 a 2.0 b 2.0 b 1.8 b
57.9 b 77.6 a 78.1 a 78.5 a
50.0 b 75.3 a 72.6 a 70.9 a
63.7 b 73.6 a 77.2 a 75.5 a
55.0 b 69.6 a 71.5 a 70.9 a
63.3 b 73.0 a 74.1 a 73.2 a
34.0 b 60.5 a 63.5 a 61.5 a
35.0 c 65.0 a 63.4 a 54.6 b
56.8 c 68.7 a 70.7 ab 63.4 b
42.2 b 65.5 a 65.4 a 61.4 a
49.5 b 69.1 a 68.9 a 64.6 a
1)
S (g kg 1 ) Control CS PG CC Ca (g kg Control CS PG CC
1)
Mg (g kg Control CS PG CC
1)
N uptake (mg pot Control CS PG CC
1)
K uptake (mg pot Control CS PG CC
1)
a
Treatment means followed by the same letter within each nutrient within each soil are not significantly different by Tukey test at 5%.
wheat. For the other soils a single factor cannot be indicated for the favorable effect of CS and PG, since the increase in root growth was associated with a complex set of factors discussed before, including: the reduction of the Al3+ activity in LRac-1 and LEal soils; the increase of the complexes AlSO+ 4 and Al-F (in the case of PG), which are less toxic than Al3+ (Cameron et al., 1986); the increase in Ca2+ activity. Alva et al. (1986)
demonstrated that the increase of the concentration of Ca reduced the toxicity of Al for the roots of soybean, alfalfa and sunflower grown in nutrient solution. The smaller effect of CS and PG on the roots of corn for soil LVal-1, as compared to the other soils (Table 6) was probably due to the increase of Al in solution caused by the higher ionic strength, resulting that the formation of the complexes AlSO+ 4 and Al-F (FG) was
plso6504.tex; 12/08/1997; 10:27; v.7; p.10
47 not so effective in reducing the activity of Al3+ as for the other soils. Although CC was more effective in reducing Al toxicity for roots in the alic LV soils, the higher concentration of nutrients in solution for the treatments CS or PG (Table 4) contributed for similar accumulation of dry matter for the three treatments. The lower Ca plant contents for the treatment with CC in three soils (Table 7), as compared with the treatments with CS or PG, can be explained by the lower concentration of the element in the soil solution (Table 4). However, the relation with Ca content in the soil solution (Table 4) is not clear. In the case of S content, the higher accumulation observed for soils LVal-1 and LVal-2 (Table 7) is clearly related to the much higher contents in the soil solution (Table 4). Considering that the amounts of N and K supplied to the maize plants before introducing the smaller pots in the larger pots containing the subsoils, were of 40 and 30 mg, respectively, higher absorption values were associated with the capacity of roots in penetrating and absorbing these nutrients from the subsoil. Thus it was proved that, for the conditions of the experiment described, gypsum favors the uptake of nutrients of subsoil, as was shown before by Raij et al. (1988). This is a fact of considerable practical importance, considering that leaching losses of nitrogen in the humid tropics can probably be reduced by enhanced root development in the subsoil, which can be achieved with gypsum applications.
Conclusions The samples of the subsoils studied presented severe restrictions for root development of maize plants due to calcium deficiency and/or aluminium toxicity. Calcium sulphate and phosphogypsum modified in a similar way the soil reaction, increasing the pH measured in calcium chloride solution, the contents of extractable calcium and reducing the total and the exchangeable acidity, although in lower proportion than calcium carbonate. Even so, calcium sulphate was as effective as calcium carbonate in promoting adequate conditions for maize root development in the five soils studied. Therefore, phosphogypsum can be considered a good amendment for subsoils that present toxic levels of aluminium and/or calcium deficiency.
Acknowledgements The second author wishes to thank the Brazilian National Research Council (CNPq) for support of his research activities.
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