APPLICATION OF CALCIUM-ALUMINUM-BALANCE (CAB) INDICES IN ACID SOILS FOR EVALUATING WINTER WHEAT RESPONSE TO CALCIUM-BEARING FERTILIZERS

Radosław Witczak, Jean Bernard Diatta
Department of Agricultural Chemistry and Environmental Biogeochemistry, Poznań University of Life Sciences, Poland

ABSTRACT

Field trials were established in 2006 at Gluszyna Lesna (52°18' N, 16°55' E), a 300 hectares Agricultural Farm, near Poznan. Winter wheat, variety Tonacja was the test plant. Three kinds of calcium-bearing fertilizers i.e., (i) CaO – 80% CaO, (ii) CaO + MgO – 60/20 (% basis) CaO/MgO, (iii) CaCO₃ – 52% CaO, were applied at four rates: 0, 500, 1000, 1500 kg CaO·ha^-1 at the first decade of September 2006. Nitrogen was applied once as NH₄NO₃ at tillering at the rate 120 kg·ha^-1. Soil samples were collected at the depths 0-20 cm at harvest. Soil chemical analyses involved: pH (1 mole KCl·dm^-3), exchangeable aluminum (Al_ex), exchangeable base cations (Ca_ex, Mg_ex, K_ex and Na_ex, extracted by 1 mole CH₃COONH₄·dm^-3 (pH 7.0). Since soils were acid, the effective CEC (CEC_e) was then obtained by summation of extractable acidity assessed in 1 mole KCl·dm^-3 and exchangeable base cations. From these, indices such as Ca_sat (Ca saturation); Al_sat (Al saturation) and CAB (Calcium-Aluminum Balance), were calculated and applied for testing winter wheat (yield, mainly) response to calcium and aluminum interactions under acid and very acid soil conditions. Results have revealed, that part of CAB indices shifted towards aluminum for both calcium-bearing fertilizers, i.e., CaO and CaO + MgO, as a result of Al prevalence over Ca in investigated soils. This implies the worsening of plant growth conditions related to the shortage of adequate calcium levels and hence relatively low grain yields. It appeared also, that grain yield remained still relatively appreciable, (CAB raised up to 1.00) for both cases i.e., CaO and CaO + MgO. The effect of CaO + MgO on plant growth was mediated by nitrogen and magnesium. The application of CaCO₃ raised CAB indices above 1.0 and even up to 1.4, which was the highest among these three fertilizers. Of additional value was the response of winter wheat yield to calcium and aluminum levels expressed as Ca_sat, Al_sat. It was shown, that irrespective of applied calcium-bearing fertilizers i.e., CaO, CaO + MgO, and CaCO₃, grain yield increased along with Ca saturation levels. However the opposite trend was observed in the case of Al_sat, where the increase in aluminum saturation led decidedly and systematically to yield decrease being the lowest at Al_sat ca 35–40% (CaO, CaO+MgO) and 25% (CaCO₃).

Key words: acidification, calcium-bearing fertilizers, winter wheat, calcium-aluminum-balance (CAB), aluminum saturation (Al_sat), calcium saturation (Ca_sat).

INTRODUCTION

Acidification of arable soils is one of the main factors limiting the agricultural use of soils in Poland as well as in the world. Acid cations (H, Al, Mn, Fe), mainly aluminum exert negative effects on crop plant at relatively high levels. This results in the reduction of the growth and size of root biomass [8]. All processes and effects, that worsen physical, chemical and biological proprieties of soils, and therefore influence negatively on their fertility are termed degradation. Degradation-based changes considerably reduce soil productive capacity, making impossible the achievement of maximum and sustainable yields [3,4]. One of the characteristics of soil chemical degradation under acid conditions is the occurrence of free aluminum species in soil and the resulting disturbance of root growth of some plants [11,19]. Furthermore it should be mentioned, that high concentrations of active aluminum in the soil solution impairs the efficient absorption of phosphorus by plants due to precipitation processes [14,15]. The accumulation of hydrogen and aluminum ions is related to the displacement of most alkaline cations such as calcium, magnesium and potassium from the soil sorptive complex to the ambient solution. This may lead to the leaching of these elements and hence favors the emergence of the secondary phase of soil acidification [4]. Moreover the increased leaching of displaced alkaline cations induces significant chemical changes within the soil complex [10]. Hydrogen and aluminum cations are antagonists to alkaline cations (Ca²⁺, Mg²⁺, K⁺) and make difficult their uptake by crop plants. This generally results in nutrients deficiency as induced by aluminum [9].

Acid soils are generally reported to be "symptomatic" in terms of Ca shortage. This is expectable, since hydrogen as well as aluminum induce a direct Ca leaching, which in turn create negative acidification-nutrient absorption-crop yields imbalance [15]. Most frequently, it has been reported about the soil saturation of either Ca or Al. Assumptions are based on commonly accepted approaches, that any raise in Ca saturation of the soil sorptive complex induces favorable growth conditions and hence better plant yielding. The reverse has been agreed to occur in the case of aluminum saturation. Therefore a question arises on the fact how satisfactory yields may be harvested under acid and very acid soils where aluminum saturation prevails?

Soil acidification results in aluminum (Al), hydrogen (H) and at times manganese (Mn) depression of plants. Soil acidification decreases base saturation and solubilises Al of soils. The research reported by Sucoff [22] has demonstrated that the growth of honeylocust (Gleditsica triacanthos L.) was negatively correlated to Al and positively correlated to increasing Ca concentration in the soil solution. Moreover, the applied ratio Ca/Al best described the plant response to available Al [6]. Since Al excess in soil occurs in the absence of adequate base saturation (mostly Ca), increasing the Ca saturation will eventually reduce and possibly eliminate Al complications. Therefore once the Al concentration or its ratio to others cations is reduce by Ca additions, then added Ca can have the growth stimulating effects.

It appears, then sufficiently necessary to find out any relationship or even ratio for calcium versus aluminum especially in acid soils. Calcium-Aluminum-Balance (CAB) enables the most precise assessment to be made of the effects of soluble Ca and Al, together, on root growth [17]. The CAB indices are typically related to the chemical status of the soil environment. It is suggested, that when CAB > -0.81, therefore crop plants grow in a healthy environment [5]. It means that CAB indices shift towards the prevalence of calcium over aluminum. In case of significant amounts of aluminum, indices tend to lower progressively. Strong and even extreme chemical-induced aluminum degradation may occur at CAB < -1.21.

Soil incorporation of calcium via liming practices is generally agreed to counteract acidity, which seriously impairs the growth of crop plants. Winter wheat belongs to these plants exhibiting relatively weak tolerance to acidity effects inducing yield depression under extreme acid conditions. Therefore it was assumed, that liming should improve winter wheat growth in one hand and the incorporated calcium is intended to alleviate aluminum phytotoxicity and further stabilize winter wheat yield.

The purpose of the current paper was to elaborate biogeochemical indices for controlling acid soils-based degradation. Additionally a detailed analysis of the direct effects of calcium and aluminum as well as their interactions expressed as CAB (Calcium-Aluminum-Balance) and how they shape winter wheat grain yield was performed.

MATERIAL AND METHODS

Characteristics of experimental fields and agricultural practices
Experimental fields were established in 2006 and lasted up to 2007 at Gluszyna Lesna (52°18' N, 16°55' E), a 300 hectares Agricultural Farm, which has been receiving for more than 20 years waste waters and diluted sludge from the Wielkopolska Potatoe Industry in Lubon (Wielkopolskie Przedsiębiorstwo Przemysłu Ziemniaczanego w Luboniu). Waste waters and diluted sludge were sprinkled within the framework of the project entitled: Agricultural use of waste waters and diluted sludge from potatoes processing. Table 1 reports briefly the volumes and appropriate concentrations of total nitrogen (N), phosphorus (P) and potassium (K) incorporated yearly to soils, with special attention to huge amounts of potassium.

Experimental designs and agrochemical soil properties
Field trials were settled on soils belonging to the agronomical category covering the range from IV to V classes. Winter wheat was the test plant. Calcium-bearing fertilizers were as follows: (i) CaO – 80%CaO, (ii) CaO + MgO – 60/20% CaO/MgO, (iii) CaCO₃ – 52% CaO. They were applied at four rates: 0, 500, 1000, 1500 kg CaO·ha^-1. Nitrogen was supplied once at the rate 120 kg·ha^-1 as NH₄NO₃.

Soil samples were collected at the depth 0-20 cm, at winter wheat harvest. They were air-dried at room temperature for 4 days, crushed to pass a 1.0 mm screen and stored in plastic bags before chemical analyses. Chemical analyses concerned soil pH (1 mole KCl·dm^-3), exchangeable aluminum (Al_ex) according to the Sokolow method [16,21]. The effective cation exchange capacity (CEC_e) was obtained by summation of 1 mole KCl·dm^-3 extractable acidity and exchangeable base cations (Ca_ex, Mg_ex, K_ex and Na_ex) extracted by 1 mole CH₃COONH₄·dm^-3 (pH 7.0) as described by Thomas [25]. Analyses were performed in duplication and exchangeable base cations were determined by the FAAS (Flame Atomic Absorption Spectrometry) method (Varian 250 Spectra Plus).

The following indices were used for evaluating winter wheat response to applied calcium-bearing fertilizers:

where:
CEC_e – effective cation exchange capacity = (EBC + TA), cmol₍₊₎·kg^-1,
EBC   – exchangeable base cations (Ca_ex, Mg_ex, K_ex and Na_ex), cmol₍₊₎·kg^-1,
TA     – Total Acidity (H⁺ + Al_ex ), cmol₍₊₎·kg^-1,
Ca_sat   – Ca saturation, %,
Al_sat – Al saturation, %,
CAB – Calcium-Aluminum Balance.

Graphs and computations were made by using Excel^® Sheet facilities.

RESULTS AND DISCUSION

1. Pre-trial chemical properties of soils
Data reported in Table 2 showed, that the site before trials settlement was extremely acid (pH 3.9–4.0) throughout the whole investigated profile. These conditions are by far not favorable for the formation of any satisfying plant biomass as well as the establishment of grain yield. This is supported by relatively high levels of exchangeable aluminum (Al_ex), which varied from 58.9 to 71.5 mg·kg^-1. Such concentrations may potentially impair the normal growth of arable crops and particularly winter wheat considered as moderate to very sensitive [15,18]. Interestingly, the levels of exchangeable calcium (Ca_ex) decreased progressively with the depth, whereas the pattern was reverse for both Al_ex and exchangeable potassium (K_ex). Furthermore the content of the latter one was very high and fluctuated quite within the same ranges as Ca_ex.

The high level of K_ex is attributed to the long-term application of potatoe waste waters, which were the basis of the secondary source of soil chemical degradation. Therefore the application of calcium-bearing fertilizers was intended to improve soil pH, mitigate aluminum phytotoxicity and most specifically to widen the Ca_ex/Al_ex ratio. The next very important effect to be related to Ca incorporation into these soils results in the widening of the Ca_ex/K_ex ratio for avoiding K-based nutritional disorders as compared to Ca_ex/Al_ex for Al-based phytotoxicity mitigation.

2. Grain yield and geochemical indices
2.1. CaO-based Ca_sat, Al_sat and CAB indices
It was assumed that winter wheat adequate growth and further yielding should be closely dependent on the proper levels of Ca in soils. This was verified by the application of different calcium-bearing fertilizers. As reported in Fig. 1, the progressive saturation of the soil sorptive complex with Ca induced a concomitant increase of grain yield as affected by CaO application. Two Ca stimulating areas may be pointed out: Ca_sat about 21% with the lowest grain yield, i.e., 2.70 t·ha^-1 and Ca_sat about 47% with the highest grain yield, over 3.90 t·ha^-1. It appeared practically that the increase of Ca saturation as twice (from 21 to 47%) evoked a grain increase of ca 1.0 t·ha^-1. This supports the assumption that calcium saturation within the range 20–50% may be a deciding factor in regulating soil chemical properties controlling plant growth as well as yielding [1,2,7]. Wheat growth under such extreme soil acidity may not be lean solely on Ca boosting effect. The application of nitrogen at the rate of 120 kg·ha^-1 was intended to assist plants in the assimilation of nutrients such as magnesium, phosphorus as well as micronutrients and most specifically Ca, too. Nitrogen may have directly activated the photosynthesis process, which was intensified, and resulted in a growth in plant assimilates inducing the establishment of a noticeable yield.

The raise in aluminum saturation (Al_sat) of soils induced a progressive decrease in grain yield biomass (Fig. 2). This effect was obviously outlined and implies, that relatively low Al saturation may not directly hamper wheat growth. Data outcome the Figure 2 showed, that the lowest yield (2.8 t·ha^-1) was harvested at the Al_sat ca 35%, whereas the highest one, i.e., 3.85 t·ha^-1, at Al_sat ca 7%. According to Rayburn [20], aluminum at high saturation levels displaces alkaline cations, Ca most mostly, which in turn is basically responsible for stimulating plants growth. This explained the results obtained in the current field trials, where the primary levels of calcium were scarce enough compared to Al negative effects. It may be most importantly pointed out at the "smooth" decrease of yield along with Al saturation. This could be attributed to the alleviating combined effects of nitrogen and incorporated Ca from calcium-bearing fertilizers.

On the other hand, agronomically as well as physiologically, nitrogen added as NH₄NO₃ may potentially counteract Ca effect by simultaneously inducing that of Al. The verification of this assumption was undertaken via assessing geochemical interactions of these elements.

The Calcium-Aluminum Balance (CAB) concept as illustrated in Fig. 3 visibly showed, that the naturally evoked aluminum effect was important enough to induce soil degradation, which further shifted the CAB indices below the value 0, i.e., indicating the geochemical prevalence of Al over Ca. Data reported by Gorban [5], decidedly stressed on the "normal" growth of "healthy" plants on soils with CAB values higher than -0.81, but growth processes were strongly impaired and most frequently plants were damaged at CAB < -1.21. Results of our field trails have revealed that yield depression occurred just with the range -0.40 < CAB >
-0.15. This range coincides with grain yield varying between 2.60 and 2.80 t·ha^-1, hence below profitability.

Appreciable (as compared to the former ones) grain yields were established when CAB > 0.25 and progressively up to the highest one, i.e., CAB = 0.90 corresponding, respectively to 3.75 t·ha^-1. The predominance of calcium in the sorptive soil complex should not be limited solely to the improvement of growth conditions as most frequently agreed. One of the key role of Ca is to counteract, by mass or equivalent basis, Al levels. This was pertinently outlined by CAB indices.

2.2. CaO + MgO-based Ca_sat, Al_sat and CAB indices
The application of CaO+MgO involved a simultaneous incorporation into soils of Ca and Mg at w/w of 60/20. Therefore, it should be considered this ratio as decidedly favorable for Ca geochemical effect towards Al, than attributable to Mg, which was strictly a physiological yield "improver or shaper" supported by nitrogen. Grain yield progressive increase as induced by Ca successive saturation (Fig. 4) appeared most pronounced if compared to the CaO-based Ca_sat (Fig. 1). The magnitude of these indices differed by the fact, that in the case of the Fig. 1, at Ca_sat = 19% the recorded grain yield was ca 2.70 t·ha^-1, whereas for the Fig. 4, respectively 22% and 2.95 t·ha^-1.

Since the rate of nitrogen remained similar for all treatments, therefore it should be assumed the presence of magnesium to prime the establishment of healthy stand and further the formation of appreciable yield, especially under very acid soil conditions [12,23,24]. Magnesium may have played an intermediary role in the neutralization of aluminum cations. This action along with that exhibited by Ca were the core of the geochemical processes which influenced growth. The highest grain yield was then observed at Ca_sat in the ranges 45–50%, similarly to CaO treatments. This implies, that calcium in the sorptive complex of soil, in half decides about plant crops, as compared to other alkaline cations.

The application of calcium-bearing fertilizer in the form of CaO+MgO also reveals, that grain yield progressively decreased along with increasing aluminum saturation (Al_sat) in soils (Fig. 5). Data outcome from this figure point out on the highest grain yield, i.e., 4.00 t·ha^-1 harvested just at Al_sat = 6% (similarly to CaO-based Al_sat). A quite 7-fold increase (Al_sat = 40%) in soil aluminum saturation induced a grain decrease of about 25% as compared to the highest one. The yield depressive effect evoked by aluminum was alleviated probably by presence of magnesium, which practically is intended to participate in the root cell build-up process [12,23,24]. Well established root biomass is a prerequisite of further and optimal nutrient assimilation, of which nitrogen was first targeted for the plant biomass formation. Therefore the yield response, which occurred under CaO + MgO based conditions supported well the fact, that in this treatment, the lowest recorded grain yield (3.00 t·ha^-1) remained even higher when compared with the lowest one (2.85 t·ha^-1) observed for the CaO-based Al_sat.

Interactions between aluminum and calcium (assisted by magnesium) as illustrated in Figure 6 clearly showed, that CAB – CaO + MgO based indices correlated strongly with winter wheat grain yield. These indices ranged between -0.20 and 0.85, and corresponded to 2.90 and 4.00 t·ha^-1, respectively. Yield depression was observed within a narrow range -0.20 < CAB > -0.05, which implies a worsening of plant growth conditions related to the lack of adequate calcium levels. Plant response and CAB related indices seem not confirming "normal" growth of "healthy" plants range reported by Gorban [5] for CAB > -0.81. By extrapolation, the grain yield harvested at CAB = -0.20 could decrease 3-4 times, indicating a deep Al-based chemical degradation, which strictly hampers plant growth. Such conditions have been partly exhibited in the case of CaO-based CAB indices, where the lowest yield, i.e., 2.60 t· ha^-1, corresponded with to CAB = -0.40. It should be kept in mind, that the alleviation of any negative aluminum effects under extremely acid conditions, as those currently investigated, should be undertaken progressively and firmly. Nitrogen must be managed with caution, but if applied should be supported by magnesium.

2.3. CaCO₃-based Ca_sat, Al_sat and CAB indices
It can be hypothesized, that conditions for CaCO₃ dissolution were favorable due to the high level of acidity, which in one hand sustains a high activity of Ca²⁺ and on the other hand, appreciable concentrations of Ca(HCO₃)₂. The former one is very soluble in acid soils, hence the buffering properties of CaCO₃ seem to control the efficiency of Ca retained by soils (i.e., Ca_sat) and this process is time-related. Similarly to the former calcium-bearing fertilizers, CaCO₃-based Ca_sat indices have exhibited a well established relationship with the grain yield, irrespective of applied rates (Fig. 7).

The increasing saturation of soils by calcium ions progressively improved growth conditions, which shaped the grain yield from 2.80 t·ha^-1 at Ca_sat = 23% to 4.0 for Ca_sat = 53%. This means 1.20 t·ha^-1 or Ca_sat = 30%.

Chemical changes evoked by the incorporation of CaCO₃ and the resulting yield increase may be observed throughout the scatter of mean data (i.e. grain yield and Ca_sat) versus trend line. The comparison made with other treatments exhibited quite obviously yield response-induced by soil conditions after the application of calcium bearing fertilizers (CaO and CaO + MgO). This is in line with data reported in the literature [9,13,14] about the time-dependence of soil reactivity of CaCO₃, i.e., the slow release of Ca²⁺ and the simultaneous formation of HCO₃^- ions which are generally agreed to act as "smooth" plant growth stabilizer and next better yield formation.

The progressive trend and systematic grain decrease along with increasing aluminum saturation (Al_sat) as illustrated in the Fig. 8 is a proof of the harmful effect of Al, specially in light textured and acid soils as those under current investigation.

This was manifested, irrespective of the type of calcium bearing fertilizers, being applied. However high grain yield (4.0 t·ha^-1) was observed at aluminum concentration ca 4% Al_sat, which when raised 6-fold i.e., to 24% has decreased the yield to 2.80 t·ha^-1. A bulk comparison of all applied calcium bearing fertilizers at Al_sat ca 20% showed, that grain yield varied within a relatively narrow range, i.e., from 3.12 to 3.25 t·ha^-1, respectively for CaCO₃: 3.12; CaO + MgO: 3.20 and CaO: 3.25 t·ha^-1. This trend, however clearly established at this aluminum saturation level should not be considered as ultimate due to several geochemical factors, which for sure, may directly or indirectly shape yield size at different Al_sat levels. The role of magnesium under such unfavorable growth conditions should not be omitted.

The verification of assumptions mentioned for both Ca_sat and Al_sat was undertaken by using the CaCO₃-based CAB indices as reported in the Fig. 9. It appeared that the application of CaCO₃ raised the CAB indices above 1.0 and even up to 1.4, what has not been observed for other calcium bearing fertilizers. Most importantly was the lack of negative CAB values, indicative of a prevalent effect of aluminum. Such Ca-Al balance in the case of the current study implies that the acid neutralizing capacity (ANC) parameter commonly agreed for eliminating soil acidity should be treated with cautiously.

Calcium aluminum balance-based plant growth ranges established for the CaCO₃ treatment show the decisive prevalence of Ca over Al, since all CAB indices were positive. This quantitative predominance of Ca may be intended to boost winter wheat growth and the formation a relatively high grain yield. This was partly achieved since within the range 0.0 ≤ CAB ≥ 1.40, the recorded grain yield varied by 1.42 factor, i.e., from 2.80 to 4.00 t·ha^-1. This satisfying increase was a joint effect induced by CaCO₃ at rates from 0.0 to 1500 kg·ha^-1 and nitrogen (120 kg N·ha^-1).

CONCLUSIONS

1. Geochemical changes recorded under such extreme acid conditions were strictly related to the type of calcium-bearing fertilizers as well as to their rates. The same applied for winter wheat response, too.

2. Some of calcium aluminum balance (CAB) indices shifted towards aluminum for both calcium-bearing fertilizers, i.e., CaO and CaO + MgO, as a result of Al prevalence over Ca in investigated soils. CAB indices raised up to 1.00 for both calcium-bearing fertilizers whereas the application of CaCO₃ raised CAB indices above 1.0 and even up to 1.4, which was the highest among these three fertilizers.

3. Grain yield increased along with Ca saturation (Ca_sat) levels irrespective of applied calcium-bearing fertilizers. Calcium saturation varied within the range 19–54%.

4. The increase in aluminum saturation led decidedly and systematically to yield decrease being the lowest at Al_sat ca 35–40% (CaO, CaO + MgO) and 25% (CaCO₃).

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Accepted for print: 30.03.2010

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