CHANGES IN SOIL ACIDITY DEPENDING ON AMELIORATION MEASURES

The long-term use of high mineral fertilizer rates, aimed at intensifying the plant production, causes the process of soil acidification, resulting in the decrease of the yield of grown plants, due to increased contents of aluminium and manganese and decreased contents of organic matter and basic cations in the soil. In order to eliminate or reduce these harmful effects, trials were set up and different materials for the neutralization of acidity and the excess of mobile aluminium were used: bentonite, zeolite, crude phosphates, MgO and CaO. The trial was set up on the pseudogley type of soil with an extremely acid reaction (pH/MKCl about 3.55) and a high content of mobile aluminium (over 37 mg/100 g soil). The obtained results show that the best effect on the decrease of all forms of soil acidity was exerted by the use of the amelioration measure CaO+MgO, with a ratio of 5:1. The application of crude phosphates and zeolite produced the weakest effect. Furthermore, the amelioration measure mentioned reduced mobile aluminium to 1.87 mg/100 g soil, and as regards three more variants (bentonite+CaO and both CaO rates) the content of aluminium was within the limits of successful tolerance for maize (about 10 mg/100 g soil).


I n t r o d u c t i o n
Too high soil acidity is a serious obstacle to the economical plant production management on most soils.Thus, Kamprath (1970) found aluminium to be a dominant cation in the adsorptive complex of the leached soils in the northeastern areas of the U.S.A., as well as in the tropical and subtropical ones with pH in H 2 O being 5 or less than that.Hardy et al., (1991), found mobile aluminium content to be the highest in the low containing pH top soil layer being by 23-100% higher than that in the sub-plowed one.This was likely to be the result of the calcium and magnesium leaching from the top to deeper soil layers so that on the highly containing mobile Al soil, the young plants sparsely rooted and decayed.
The major problems related to plant production management on the pseudogley type of soil are considered to be its rather poor physical, chemical and biological properties.Of the chemical ones, the most expressed seem to be soil acidity, higher aluminium and manganese contents, deficiency of alkaline cations in adsorptive complex and a lower content of available phosphorus (Okiljević, 1982;Dugalić, 1997;Radanović and Predić, 1997).Only by using agro-amelioration measures with calcification as the most significant one can these drawbacks be overcome.
Acid soils occupy large areas in the country and have even been increased by intensifying plant production along with long-term application of high mineral fertilizers amounts, thereby leading to a stronger acidification, higher concentration of soluble aluminium and manganese and to a lower content of organic matter and alkaline cations, as well.Thus, Stevanović et al., (1987) pointed out that fertilization with mineral nutrients and without manures had lowered soil pH even by 0.15 to 0.18 units in the five-years' time.Also, some biogenous (N, P, Mo) elements may have become less soluble and some others (Ca, Mg, B, Zn, Cu Co) more leached.
Stevanović et al., (1992) mentioned that not only lower yield and quality, but also extreme decay of the crops growing on the soils with pH being below 4.2 might occur, which was confirmed by Rajković et al., (1987) and Stevanović et al., (1994).Therefore, aiming at elimination of such drawbacks, calcification every 3-5 years, in the amount of 2-7 t/ha CaCO 3 , coupled with mineral and organic fertilizers, is strongly recommended.All the lime materials (crushed lime, saturated silt, Thomas phosphate) containing over 60% carbonates may be utilized for calcification.
The aim of this paper was to examine the effect of certain acidity neutralization materials on the changes in some chemical properties of the soil, primarily on the decrease in acidity and the content of mobile aluminium.

Materials and Method
The experiment was conducted in the village of Teocin (Western Serbia), on the pseudogley soil type, at three locations having similar chemical properties (tab.1).The variants studied were carried out using the random block design method with three replications.
Prior to setting up of the experiment, the soil had been chemically ascertained as pseudogley of the extremely acid reaction having unsuitable chemical properties (in the first place, high content of mobile aluminium).Appropriate amelioration measures were applied afterwards to determine how they affected these soil properties.
The experiment was set up using the following treatments: The maize hybrid NSSC 201 was used as a test plant.Every year, before the setting up of the experiment, soil samples were taken from the plowed up layer of the soil (0-30 cm) from all the variants with three replications each and chemical analyses were made (tab.1.).The same was conducted after the completed maize vegetation season (tab.2.).The samples were taken by shovel, with 5-6 individual ones taken per average one being appropriately distributed per plot surface (14m 2 ).
The chemical soil analyses were used to determine: active (pH in H 2 O) and substitution acidity (pH in M KCl) -potentio-metrically and mobile aluminium content -aluminon-acetate method.
The data were processed using the analysis of variance of the two-factorial experiment and statistical significance of the differences using LSD test.
The dependence between the individual chemical properties of the soil was determined by the coefficient of correlation and regression.

Results and Discussion
The pseudogley, on which studies proceeded, is characterized with extremely acid soil reaction, which, depending on the research year, ranged from pH/H 2 O =4.46 to 4.61 and pH/KCl from 3.52 to 3.58 (tab.1).So highly active and substitution soil acidity is accompanied by high hydrolytic acidity (H=20.07-22.42 meq/100 g soil), low content of adsorbed alkaline cations (S=4.6 -5.2 meq/100 g soil) as well as by low level of soil saturation with alkali (V=17.55-19.94%).
Extremely acid soil reaction brought about a higher mobility of Al 3+ ions, the content of which ranged from 37.9 to 41.,7 mg/100 g soil.Such a high content of mobile aluminium is considered toxic to cultivated plants so that certain crops are likely to completely decay on the highly mobile aluminium containing soils.The amelioration measures were observed to differently affect the changes in the soil active acidity (tab.2.).Relating to the control variant, with the average pH in H 2 O being 4.61 pH units, the lower crude phosphates and zeolite rates increased soil acidity by 0.05 pH units being statistically non-significant.All the remaining materials initiated the fall in the active soil acidity, on the average by 0.02 -1.0 pH units.The lime used alone or combined with other materials suited acidity neutralization best, with soil acidity being decreased by 0.3 -1.0 pH units or 0.4-21.5%.
As far as study years are concerned, the change in the active soil acidity was the most pronounced in 1997, when in all the variants, excepting bentonite+CaO, the highest average pH values were accomplished for the property under consideration, which was largely favoured by the most suitable environmental conditions of that year, with enough and suitably distributed rainfalls, especially over the summer months.This enabled the acidity neutralizing materials to dissolve properly, which did not hold for 1998, with rainfall sum being lower than the long-term average, hence the lower average values of the active acidity in all the variants.Just as being with active soil acidity, the lower crude phosphates rates (3.58 pH units) had also the weakest effect on altering substitution acidity (tab.2.), with a minor decrease related to the control.The highest average value was recorded on the variant with combination CaO+MgO of ratio 5:1 (4.54 pH units), i.e. the increase of 26.5% compared to the control.The other variants with lime also incited the increase in soil pH by 6.1-19.2%,allowing for no statistically significant differences.Inasmuch as one compares the values between the variants with solely bentonite or zeolite used with those to which lime was added, a positive effect of an added lime could be noticed.From tab. 2., one can see that the average values of the substitution soil acidity tended to increase in all the three years, with use of all the acidity neutralizing materials, excepting both crude phosphates rates, which was in agreement with results of Jurić et al. (1986).The variations in the accomplished average pH values between the materials inducing lower soil acidity were statistically highly significant.
Comparing the average values of the active and substitution soil acidity, one can notice that the difference between these values over all the three years and on all the variants was higher than 1.0 pH units.This might seem to be the first indication of greater mobile Al amounts in such soil, which was proved through soil analyses.It was also detected that the content of mobile aluminium was higher than 10 mg/100 g soil on the variants with pH/KCl lesser than 4.0 pH units, being in conformity with the results obtained by Nikodijević (1964), according to which the content of mobile aluminium ranged from 13-45 mg/100 g soil on the soil with pH/KCl<4.0pH units.The soil mobile aluminium content changes are directly linked with those in soil pH.In other words, once the remedial measures had reduced soil acidity, the mobile aluminium content tended to decrease, as well (tab.3.), which was highly noticeable with the effect of the four remedial measures being, as follows: combinations CaO+MgO (5:1) and bentonite+CaO, with substitution soil acidity value being over 4,0 pH units, while that of mobile aluminium was decreased to 1.87 and 3.90 mg/100g soil and in both lime rates, as well, with pH recorded to be around 4.0 with mobile aluminium reduced to around 10 mg/100g soil.These data once more point at the inevitable use of lime, either alone or in combination with some other neutralizing materials, aiming at a lower toxic effect of aluminium.This was strongly corroborated by numerous authors (Brković, 1993;Dugalić, 1997;Predić et al., 1997;Marković, 2000).
The content of mobile aluminium at the level above 10 mg/100g soil was likely to adversely affect plant growth, primarily its root system (Avdonin, 1976;Durman, 1985;Jakovljević et al., 1991).Root ability to take up plant nutrients and water from soil was thus weakened, particularly over the first growing phases, causing plants to lag behind in their growth and, subsequently, produce poorer yield (Bošković-Rakočević Ljiljana, 2001).
The existence of the negative linear dependence between the active and substitution acidity of soil and mobile aluminium content, is confirmed by scatter graph (Fig. 1) as well as by the highly strong correlation level (r = -0.,879(pH/H 2 O) and r = -0.906(pH/KCl).It is also worth mentioning that, for both acidity types, linear curve has had an approximately the same slope (coefficient values are nearly identical), but varied for the constant (around 1.2 pH units).From the graph 1., one can see that with pH in H 2 O above 5.0, and that in KCl above 4.0 Al content is below 10 mg/100g soil, but higher than that with lower pH values, which may depressively affect the growth and development of some plants.

C o n c l u s i o n
The research results point at the following conclusions: Of all the amelioration measures, those with lime used either alone or combined with other materials intended to acidity neutralization, proved to be the most effective.
Most of the amelioration measures incited the decrease in active and substitution acidity of soil.The use of CaO+MgO of ratio 5:1 displayed the best possible effect on the decrease in all the soil acidity forms.However, crude phosphates and zeolite had the poorest effect in this regard.
Combinations CaO+MgO, of ratio 5:1, reduced mobile aluminium to 1.87 mg/100g soil, while on the other three variants (bentonite+CaO and both CaO rates), Al content was within the boundaries that could still be effectively tolerated by maize (around 10 mg/100g soil).

Graph 1 .
-Dependence of soil mobile aluminium content on pH Bentonite, zeolite, crude phosphates, MgO and CaO were used to neutralize soil acidity and an excess of mobile Al.The materials were spread manually over the soil surface and introduced together with NPK fertlizer (600 kg/ha, 15:15 :15) over the preceeding soil preparation, every year.