Bernardo van Raij

Disponibilidade de fósforo em solos avaliada por diferentes extratores

The objective of this paper was the comparative study of the most important extractors of soil phosphorus. It presents a literature review of papers that deal with methods of extraction for the evaluation of phosphorus availability in soils. The methods considered were: anion exchange resin, Olsen, Bray 1, Bray 2, Mehlich 1, Troug, Egner, water, 0,01M CaCl2, iron hydroxide impregnated filter paper (Pi) and the determinations of the E and L values. The comparison between results of soil analysis by different methods of phosphorus extraction in soils and phosphorus uptake by plants was initially made considering the coefficient of determination (r2) for several extractants. The following average values and corresponding numbers of articles in which the method was considered were obtained: resin, 70% (34); E value, 68% (16); L value, 65% (8); Olsen, 54% (48); Bray 1, 50% (42); Mehlich 1, 46% (25); Egner, 44% (9); Bray 2, 42% (19); Water, 42% (15); Truog, 38% (13); CaCl2, 36% (13), and Morgan, 32% (13). The comparison of several methods, considering the pairs of results (r2) obtained for the extractants, that were tested together through linear correlation and average contrasts (t student test), indicated that the resin method was statistically superior to the other methods. The resin method presents the following favorable features: (a) highest values for the determinations coefficients for the correlations between P uptake by plants and soil P by the different methods, as reviewed in 72 papers; (b) it can be used in both acid and alkaline soils, which is not the case for the other most commonly used methods; (c) it reveals adequately the effect of liming on the increase of P availability in soils for crops, which is not the case of Mehlich 1, Bray 1 and Olsen; (d) it does not overestimate P availability in soils treated with rock phosphates, as is the case of acid extractans; (e) it is the method with the best theoretical background.

Multivariate calibration applied to a highly interfering chemical system: the simultaneous spectrophotometric determination of aluminium and iron in plants using xylenol orange and partial least-squares regression.

Abstract A partial least squares (PLS) calibration model was developed for the simultaneous spectrophotometric determination of Al and Fe in plant extracts by using an excess of xylenol orange as the cromogenic reagent. Experimental conditions were established to reduce interferences, decrease system complexity and produce a robust procedure that could be used for routine analysis. The spectra should be recorded from 2 to 4xa0h after mixing the reagents. Ethanolic solutions were used to improve the formation of the Al-XO complexes. The best calibration model was obtained using the PLS-2 algorithm after mean centering the data with five latent variables. Under the experimental conditions adopted, only Zn was found to be a potential interfering species with the plant extracts used. Validation of the proposed spectrophotometric method was performed by running nine plant samples from the 1997 International Plant-Analytical Exchange (IPE) which were also analyzed by ICP-AES for further correlation. The proposed procedure showed to be useful for prediction of Al and Fe values from 12.5 to 2500xa0mgxa0kg −1 in plant dry weight basis.

Extraction of phytoavailable trace metals from tropical soils by mixed ion exchange resin modified with inorganic and organic ligands

Abstract Chelating agents and organic acid ligands readily solubilize trace and heavy metals in soils. We compared Fe, Mn, Cu and Zn extracted from 16 soils by a mixed ion exchange resin (Amberlite IRA-120+ Amberlite IRA-400) modified with six simple inorganic and organic acid ligands with a conventional chelating agent (DTPA) and bioassay. The mixed cation–anion exchange resins were saturated with Na-salts of chloride (R–CHL), bicarbonate (R–BIC), fluoride (R–FLU), acetate (R–ACT), citrate (R–CIT) and tartrate (R–TAR). Solubilization and adsorption of Fe and Cu from the soils by resin–ligand systems decreased in the order R–CIT≥R–FLU≥R–TAR=R–ACT>R–CHL=R–BIC reflecting differential stability of Fe- and Cu–organic/inorganic complexes. The R–CHL solubilized and adsorbed more Mn and Zn than other resin–ligand systems because of the relatively low pH maintained by the R–CHL-soil suspension. Extraction with DTPA gave similar amounts of Fe and Mn as resin–ligand systems, but DTPA extracted two to three times more Cu and Zn than resin. Resin-extractable Fe, Mn and Cu correlated with the corresponding DTPA-extractable metal contents, while both resin- and DTPA-extractable Cu correlated with the bioassay results. The R–ACT and R–TAR extractable Zn correlated more strongly with the bioassay results than DTPA-extractable Zn. Mixed ion exchange resin is apparently feasible for characterizing plant available Fe, Mn, Cu and Zn in soils.

Resposta de cafezais adensados à adubação NPK

The NPK fertilization for high tree density coffee plantations, on an area basis, has been overestimated because it is derived from the recommendation for the traditional coffee blocks, on a tree basis. This research aimed at evaluating the NPK fertilization effects on two high tree density coffee plantations, one with the Mundo Novo variety e other with the Catuai variety, under comercial production. Two 1/2(4 x 4 x 4) factorial experiments were conducted in coffee plantations under full production, in Santo Antonio and Samambaia farms, both at Mococa, State of Sao Paulo, Brazil. In the first farm, the coffee variety was Mundo Novo and the spacing was of 2.0 x 1.0 m. In the second case, the variety was Catuai and the spacing was 1.5 x 1.0 m. The rates of nutrients applied were the following: nitrogen - 100, 200, 300 and 400 kg. ha-1 of N as urea; phosphorus - 0, 30, 60 and 90 kg. ha-1 of P2O5 as triple superphosphate; and, 0, 80, 160 and 240 kg. ha-1 of K2O as potassium chloride. The fertilization was split into four applications during the rainy season, starting in 1989. Four harvests were recorded for each location, from 1991 to 1994. At the Santo Antonio farm, the application of N reduced yields, which is unusual. In this case the initial soil sampling detect medium contents of P and high contents of K. The N leaf content was high for samples taken in 1992 and 1993. In the experiment at the Samambaia farm there was a significant effect of P fertilization, which is also unexpected. The results point to the need to fertilize coffee plantations taking into account the P and K contents of the soil and N contents of leaves. The results allow to conclude that coffee fertilization using standard NPK formulas, in higt tree density systems, can depress production when there is excess shade. Fertilizer needs should be determined taking into consideration soil analysis for P and K leaf analysis for N, and prescribing amounts to be applied on an area basis.

Efficiency of multinutrient extractants for the determining of available zinc in soils

The increasing use of the ICP-AES (inductively coupled plasma atomic emission spectrophotometry) as a multielement analytical technique has stressed the preference for the use of multinutrient extractants in routine soil analysis. However, few studies relating the extraction of zinc (Zn) from soil with such extractants and the absorption of the element by plants have been published. Two experiments under greenhouse conditions were carried out in order to determine the efficiency of some multinutrient extractants for the determination of available Zn in 44 soils from the State of São Paulo, Brazil, for corn and soybean. All soil samples were limed to increase the CECs base saturation at pH 7 to 70%. Twenty-two soil samples with DTPA (diethylenetriaminepentaacetic acid) extractable Zn concentration lower than 0.6 mg dm−3 received either the application of B, Cu, Mn, Zn or of these micronutrients excluding Zn. The remaining 22 soil samples, with Zn concentration above 0.6 mg dm−3, received application of three micronutrients (B, Cu and Mn). Macronutrients were applied to all pots as needed for crop development. Zinc was extracted from the soils before each planting using the extracting solutions, DTPA, Mehlich 1 (M-1), Mehlich 3 (M-3) and AB-DTPA (ammonium bicarbonate-DTPA) and determined by ICP-AES. The results showed significant correlations between plant Zn concentration and soil Zn concentration. The correlation values between soil-Zn and plant-Zn were of 0.74 (M-3), 0.73 (DTPA), 0.62 (AB-DTPA) and 0.61 (M-1) for corn, and of 0.71 (DTPA), 0.63 (M-1), 0.58 (M-3) and 0.46 (AB-DTPA) for soybean. Thus, the DTPA solution was the most efficient extractant for the determination of bioavailable Zn when both crops are considered together.

Rate processes of calcium, magnesium and potassium desorption from variable-charge soils by mixed ion-exchange resins

Abstract There is growing incidence of calcium (Ca), magnesium (Mg) and potassium (K) deficiencies in tropical soils dominated by variable-charge colloids. Management strategy to improve cationic balance and fertility of soils requires kinetic data to predict their release, and analytical methodology to assess phytoavailability. This study examines the rate processes and energy relations governing the simultaneous release of Ca, K and Mg from variable charge soils to ion-exchange resins. The desorption of Ca, K and Mg was adequately described by an apparent first-order rate, a film diffusion equation and an Elovich equation. Agitation of soil samples increased desorption rate two to threefolds, suggesting that film diffusion rather than chemical exchange reaction was rate-limiting. The magnitude of activation energy ( E a ) of desorption further supported a film diffusion mechanism. Compared to reports from temperate soils, the free energy Δ G * and enthalpy Δ H * of activation for desorbing Ca, Mg and K were low. The low E a and Δ G * suggested a more rapid release of Ca, Mg and K to ion-exchange resins from tropical soils than temperate soils probably because of the pre-ponderance of low-activity clays which have no interlayer binding sites for cations. The trend in Δ H * and Δ S * for desorption of Ca, Mg and K from the soils by the mixed resins seemed to indicate stronger binding energy for K than Ca and Mg.

RATIONALE OF THE ECONOMY OF SOIL TESTING

Probably the cheapest and most widely used chemical analysis employed in agriculture, soil testing is often not properly appreciated for the advantages it has for economically sound fertilizer recommendation for crops. In fact, soil testing is not only useful, but it becomes indispensable for this purpose, considering that it is used to determine unfavorable soil reaction and nutrient deficiencies and excesses that affect plant growth. To demonstrate how soil analysis and the economy of fertilizer use are interrelated, a quadratic model was used to show the magnitude of economic returns under different conditions, using soil analysis as starting point. The parameters used in the quadratic response model used were the sufficiency index of a nutrient obtained by soil analysis, the expected or maximum yield, and the nutrient rate to achieve this yield. Using the sufficiency index of the nutrient in the soil, the cost of the nutrient unit, and the value of the crop unit as inputs for the model, profits and return to investment ratios were calculated for several conditions, covering the range of yield levels, fertilizer costs, and crop values found in agriculture. Profits are higher for nutrients applied when they are more deficient in soils, under conditions of higher maximum yields and also when crop value are higher. The results point to the importance of the proper evaluation of soil fertility and to the removal of yield limiting factors. On the basis of the concepts presented on the economics of fertilizer use, the following cases were discussed: fertilizer use in extensive low value crops; policies to increase crop production with fertilizers; fertilizer use under conditions of limited capital; fertilizer recommendation for precision agriculture; choice of adequate fertilizer blends; fertilization as a sound environmental practice.

Analytical methods and quality assurance

The proposed analytical protocol is applicable to potassium chloride and ammonium chloride soil extracts. It makes use of ethanol as a reaction accelerator. Full color development is achieved after a standing period of 2 hours and the absorbance measurements were performed at 555 nm, against the blank. Under these conditions the interference of other elements usually found in the soil extracts were not important. The calibration graphs, in the range from 0 to 1.0 mg L -1 Al +3 , were fitted by 2nd order polynomial regressions. The protocol was tested with 10 representative soil samples from the State of Sao Paulo (Brazil), containing exchangeable aluminum varying from 30 to 200 mg Al kg -1 of soil and variable amounts of iron. All samples were run in triplicate and the results averaged. The correlation among the proposed procedure and the measurements obtained by ICP-AES were [Al,XO] = 0.96[Al,ICP] - 7.38 with r 2 =0.9950 and [Al,XO] = 1.06[Al,ICP] - 6.43 with r 2 =0.9970, for the KCl and NH 4 Cl extractions. For absorbance readings above 0.9 or below 0.1 it is advisable to repeat the determinations by taking sample aliquots of 1.00 or 10.00 mL, respectively. This analytical approach proved to be appropriate for routine work.Abstract The proposed analytical protocol is applicable to potassium chloride and ammonium chloride soil extracts. It makes use of ethanol as a reaction accelerator. Full color development is achieved after a standing period of 2 hours and the absorbance measurements were performed at 555 run, against the blank. Under these conditions the interference of other elements usually found in the soil extracts were not important. The calibration graphs, in the range from 0 to 1.0 mg L‐1 Al+3 were fitted by 2nd order polynomial regressions. The protocol was tested with 10 representative soil samples from the State of São Paulo (Brazil), containing exchangeable aluminum varying from 30 to 200 mg Al kg‐1 of soil and variable amounts of iron. All samples were run in triplicate and the results averaged. The correlation among the proposed procedure and the measurements obtained by ICP‐AES were [Al,XO] = 0.96[Al,ICP] ‐ 7.38 with r2=0.9950 and [Al,XO] = 1.06[Al,ICP] ‐ 6.43 with r2=0.9970, for the KCl and NH4Cl extractions. For absorbance readings above 0.9 or below 0.1 it is advisable to repeat the determinations by taking sample aliquots of 1.00 or 10.00 mL, respectively. This analytical approach proved to be appropriate for routine work.