Piracicaba, Brazil E.C. BRASIL EMBRAPA, Belém, Brazil W.B. SCIVITTARO EMBRAPA,
Pelotas Brazil
Abstract. A pot experiment was carried out under greenhouse conditions at the Centro de Energia Nuclear na Agricultura, Piracicaba (SP, Brazil), to evaluate the phosphorus availability of different phosphate sources in five Amazonian soils. The soils utilized were: medium texture Yellow Latosol, clayey Yellow Latosol, very clayey Yellow Latosol, clayey Red-Yellow Podzolic and very clayey Red-Yellow Podzolic. Four phosphate sources were applied: triple superphosphate, ordinary Yoorin thermophosphate, coarse Yoorin termo-phosphate and North Carolina phosphate rock at P rates of 0, 40, 80 and 120 mg kg-1 soil. The dry matter yield and the amount of P taken up by cowpea and rice were correlated with the extractable P by anionic exchangeable resin, Mehlich-1, Mehlich-3 and Bray-I. The results showed that the extractable P by Mehlich-1 was higher in the soils amended with North Carolina rock phosphate. Irrespective of the phosphorus sources used, the Mehlich-3 extractant showed close correlation with plant response. The Mehlich-3 and Bray-I extractants were more sensitive to soil variations. The Mehlich-3 extractant was more suitable in predicting the P availability to plants in the different soils and phosphorus sources studied.
1. INTRODUCTION
The Amazon region presents great geological diversity, where sedimentary, metamorphic and magmatic rocks are encountered. This fact combined to a high geomorphological variation, determine the existence of many types of soils. Among the soil taxonomic units, yellow Latosols and red yellow Podzolics are the predominant soils of the region. Approximately 88% of the Amazon soils are characterized as poor in chemical soil fertility, with very low nutrient contents. Phosphorus (P) is the most limiting element for plant growth (1 to 9 mg g-1 available P, by Mehlich 1, and <200 mg g-1 total P). To increase crop productivity, the need for P fertilization is imperative. The utilization of phosphatic fertilizers in this region, however, is still restricted, due to high costs of P fertilizer supplies (product and transport) and limited investigations on the subject.
The application of water-soluble phosphate fertilizers, in spite of P being totally available, promotes further acidification in acid soils with consequent dissolution of Al and Fe oxides, accelerating P-fixing processes. An alternative to reduce these processes is the utilization of low-cost P sources, with low acidification capacity, and satisfactory solubility, such as thermophosphate and North Carolina phosphate rock.
Pepper growers have used the thermophosphate in the region. One of its advantages is the acidity correcting effect, which reduces P fixation. Rock phosphates are another alternative that have better efficiency in acid and low P-content soils. The North Carolina rock phosphate, a fluorapatite, has been reported as a highly efficient product [1, 2]. Considering transport costs of the products, the cost ofthis fertilizer is almost the same as PR locally produced in Brazil.
151 The rational and economical utilization of P fertilizers requires the understanding of P dynamics and its interactions with the soil. In addition, an accurate determination of “available” P is required in order to diagnose the soil P status and consequently to recommend the necessary practices for obtaining maximum yield.
The Mehlich 1 method is routinely utilized to determine available P in approximately 90% of the laboratories in the Amazon region [3]. It was introduced without previous studies to evaluate its efficiency. Paradoxically, this method has been considered not suited for acid soils, because it extracts more soil P bound to Ca, and only small proportions of P bound to Al and Fe. Considering that acid soils are rich in Fe and Al oxides and caolinite, as occurs in the majority of Brazilian soils, the predominant P reaction products in the soil are aluminum and iron phosphates. Therefore, this extractant would not be the correct option for the determination of soil available P. Furthermore, the Mehlich 1 extractant solubilizes unaltered rock phosphate residues, overestimating P availability in soil fertilized with PR.
According to Tomas and Peasle [4], the Bray 1 extractant can be recommended for acid soils and also for the soil fertilized with PR. The extracting reaction occurs by its acidity and complexation of Al-P and Fe-P by fluoride.
Since 1983 in the Sao Paulo State, Brazil, the official method for soil P testing is the resin method [5].
The advantage of this method is that other elements, Ca, Mg and K, are also determined in the same extracts. Several studies carried out during the last 5 years have shown the superiority of this method over the others. Reports about its performance in soils fertilized with PR, however, are scarce.
The objective of this research was to evaluate four chemical extraction methods for assessing P availability in soils from the Amazon region, amended with phosphates of varying water-solubility.
2. MATERIAL AND METHODS
The five soil types, their characterization and the pot experiments run were described in previous work [6].
Soil samples were taken from the pots twice: first prior to cowpea seeding and second, before rice seeding, and analysed for available P, using the following methods: Mehlich 1 [3], Bray-1 [7], Anion + cation exchange resin [5] and Mehlich 3 [8].
To evaluate the P availability, data of soil extracted P were correlated against dry matter yields of cowpea and rice, considering fertilizers and soils separately. The regression curves of the dry matter yields and soil P contents were adjusted for each method, considering all the soils and P sources.
In order to evaluate differences between correlation coefficients obtained as a function of soils, sources and rates, the r-values were transformed to Z and statistically analysed by t test [9]. Linear simple correlations among soil P extracted by different methods were also determined.
3. RESULTS
Correlation results for plant response to different P sources versus P extraction methods are presented in Table I. The correlation with Triple Superphopsphate (TSP) was highly significant with all the extractants for both crops, which indicated that for water-soluble P sources any method gives a satisfactory estimation. With North Carolina phosphate rock (NCPR) fertilizer, the correlation coefficient reflected the specific behaviour of each method. The Mehlich 3 method presented good sensitivity in extracting available P in soils with phosphate residues having an apatite origin. The results confirm Mehlich’s [8] observation that acetic acid is a more moderate agent to solubilize apatite
152
in relation to equivalent mineral acids present in other extractants. The Mehlich 1 extractant did not present good correlation in soils amended with phosphate rocks, due to its high extraction capacity, which gave overestimation of P availability, as observed by Feitosa and Raij [10] and Raij [11].
TABLE I. LINEAR CORRELATIONS BETWEEN P EXTRACTED BY DIFFERENT METHODS AND DRY MATTER AND P PUTAKE OF COWPEA AND RICE PLANTS, FOR EACH APPLIED FERTILIZER
---Linear Correlation coeficient ( r )---
---Cowpea--- ---Rice--- Dry matter P uptake Dry matter P uptake
---Triple Superphosphate---
Resin 0.742* 0.755* 0.722* 0.898*
Mehlich 1 0.798* 0.758* 0.720* 0.901*
Mehlich 3 0.774* 0.809* 0.714* 0.926*
Bray 1 0.751* 0.844* 0.681* 0.889*
---North Carolina Phosphate Rock---
Resin 0.524* 0.467* 0.551* 0.582*
Mehlich 1 0.540* 0.521* 0.653* 0.739*
Mehlich 3 0.736* 0.700* 0.684* 0.797*
Bray 1 0.236n.s. 0.201n.s. 0.757* 0.871*
---Coarse Yoorin Termophosphate---
Resin 0.305* 0.256* 0.609* 0.697*
Mehlich 1 0.753* 0.728* 0.683* 0.757*
Mehlich 3 0.705* 0.690* 0.716* 0.797*
Bray 1 0.568* 0.547* 0.679* 0.759*
---Ordinary Yoorin Termophosphate---
Resin 0.683* 0.680* 0.627* 0.683*
Mehlich 1 0.790* 0.737* 0.716* 0.836*
Mehlich 3 0.825* 0.828* 0.741* 0.875*
Bray 1 0.826* 0.785* 0.750* 0.867*
* = significant at 5% level. n.s.= not significant.
Surprisingly the Bray 1 extractant did not correlate well with cowpea P uptake and dry matter yield in treatments with phosphate rock. The results disagree with those obtained by Raij and van Diest [12]
and Kaminski [13], as they reported that Bray 1 is one of most adequate methods for available P estimation. According to Smith et al. [14], the Bray 1 extractant is able to discriminate soils fertilized with phosphate rock from those with water-soluble phosphates. For the treatments with coarse Yoorin (Yoorin-c), both Mehlich 1 and Mehlich 3 gave good correlation with cowpea plant response.
The resin method did not perform well for cowpea in both NCPR and Yoorin-c treatment. The low efficiency of this method could be due to the capability of resin to extract high amounts of P on higher pH (6.5-7.0). Probably Yoorin-c, which has an alkaline reaction, increased soil pH in the laboratory, favoring the resin extraction, without correspondent increase in P uptake by cowpea.
153 The residual effect (rice plant response) generally correlated better with extracted P, for the different phosphates studied, mainly in treatments with smaller correlation in the first crop. This indicated that having longer period for fertilizer/soil reaction, the extractants present higher efficiency in extracting P with regard to plant uptake.
For both crops, Mehlich 3 and Bray 1 showed better sensitivity to soil variation (Table II). Lins and Cox [15] evaluated the effect of some chemical properties of seven Cerrado soils, reported the superiority of Mehlich 3 over Bray 1 and resin methods. Piha [16], comparing the Mehlich-3 and resin methods in soil of Zimbabwe, observed that the two methods varied in efficiency depending on soil texture, though Mehlich-3 was considered more adequate for large variations of soil type. Freire et al. [17] observed good sensitivity of Bray 1 extractant to soils with different clay content. The Mehlich-1 was the least correlated method for all soils.
The correlation coeficients obtained as a function of soils, P sources and rates for Mehlich 3 and Bray 1 in cowpea plants were significantly superior to other methods. For rice plants, through the tendency was similar, the values were not significantly different.
The Mehlich-3 method correlated well with Bray 1 (Table IV). Wolf & Baker [18] and Gascho et al.
[19] reported similar results. The Mehlich-1, Bray 1 and Resin methods did not correlate well with each other, which was probably due to differences in extracting capabilities of each solvent.
TABLE II. LINEAR CORRELATIONS BETWEEN P EXTRACTED BY DIFFERENT METHODS AND RESPONSE OF COWPEA AND RICE IN DRY MATTER AND P UPTAKE, FOR EACH SOIL
---Linear correlation coefficient ( r )---
---Cowpea--- ---Rice---
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TABLE III. COMPARISON OF CORRELATION COEFFICIENTS OBTAINED BETWEEN DRY MATTER WEIGHT OF COWPEA AND RICE AND SOIL EXTRACTED P, IN FUNCTION OF SOILS, P SOURCES AND RATES (VALUES TRANSFORMED TO Z, T TEST)
---Cowpea--- ---Rice---
Method
R Z* R Z*
Resin 0.776 1.035A 0.657 0.787A
Mehlich 1 0.756 0.987A 0.553 0.623AB
Mehlich 3 0.659 0.791B 0.554 0.624AB
Bray 1 0.524 0.582C 0.581 0.664A
Z values in the same column followed by the same letter do not differ estatistically (p=0.05)
TABLE IV. LINEAR CORRELATION COEFICIENTS BETWEEN QUANTITY OF EXTRACTED P BY DIFFERENT EXTRACTANTS IN SOIL SAMPLES COLLECTED PRIOR TO COWPEA AND RICE SEEDLING
---Correlation coefficient ( r)--- Method Mehlich 1 Mehlich 3 Bray 1
---Soil analysed before cowpea seeding--- Resin 0.496** 0.623** 0.458**
Mehlich 1 1.000** 0.521** 0.308**
Mehlich 3 0.521** 1.00* 0.850**
---Soil analysed before rice seeding--- Resin 0.529** 0.614** 0.441**
Mehlich 1 1.000** 0.572** 0.328**
Mehlich 3 0.572** 1.000** 0.887**
* = significant at 5% level. n.s.= not significant.
4. CONCLUSIONS
1. The Mehlich-3 extractant presented good correlations with cowpea and rice response, irrespective of the P source utilized.
2. The Mehlich-3 and Bray 1 were the most sensitive to soil variations, and the correlation between the two extractants was highly significant.
3. The Mehlich 1 extractant gave the poorest correlation with both cowpea and rice plant responses.
ACKNOWLEDGEMENTS
Financial and technical support of the International Atomic Energy Agency (IAEA) under research contract BRA-7500 is greatly acknowledged.
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COMPARATIVE STUDY OF P UPTAKE AND UTILIZATION FROM P FERTILIZERS