Keywords: Adsorption isotherms, diagnose, phosphorus fertiliser SUMMARY

Transcription

1 Agronomy PHOSPHOR~S ADSORPTON SOTHERMS AS A DAGNOSTC CRTERON FOR PHOSPHORUS FERTLSER REQUREMENT FOR SUGARCANE R. Villegas, M. Lopez and A. Cabrera nstituto Nacional de nvestigaciones de la Cana de Azucar, Ministerio del Azucar Cuba SUMMARY Soil samples coming from 116 harvests of 31 experiments on brown and vertisol soils to study the effects of phosphorus fertilisation on sugarcane were used in this study. Phosphorus adsorption isotherms were determined for representative samples from each experiment and the results were adjusted by use of the Freundlich equation. Phosphorus buffer capacity (PBC) and sorbed phosphorus at a 0.2 ppm P concentration of an equilibrium solution (Po,,) were both adopted as sorption ratios. Both ratios were significantly and negatively correlated to relative yields of cane. Such figures, it is claimed, may serve to predict the response of sugarcane to phosphorus fertilisation. A significant and negative relationship was found between sorption ratio and assimilable phosphorus contents as determined by various methods. NTRODUCTON Studies of phosphorus fertilisation have high priority on sugarcane soils in Cuba since most P fertilisers come from abroad and their prices have increased markedly during recent years. Recommendations need to be made as to the right application for maximum production. n many sugar producing countries, critical levels of available phosphorus in the soil have been established, and on that basis recommendations for phosphorus fertilisation have been put forward (1>23921>20). On the other hand, phosphorus adsorption by the soil, which removes part of this nutrient from the soil solution, is a basic process which should be taken into account when any complete study of phosphorus fertilisation is undertaken. Phosphorus adsorption may be determined by different methods. A great range of solutions, concentrations, soi1:solution ratios and interaction times, are recommended in the literature; however, the most commonly used technique for determining soil phosphorus adsorption nowadays is that of representing experimental data as an adsorption isotherm (169733). The objective of the present work was to evaluate the reliability of adsorption isotherms as a diagnostic method for phosphorus fertilisation in sugarcane, based Keywords: Adsorption isotherms, diagnose, phosphorus fertiliser

2 i i R. VLLEGAS, M. LOPEZ AND A. CABRERA on the known relationships of adsorption ratios estimated from isotherms and relative 1 yield of cane; and also to estimate critical levels for these ratios and their relationship f to assimilable soil phosphorus. 141 MATERALS AND METHODS The soil samples came from the sites of 3 1 experiments carried out at various Experiment Stations belonging to the National Sugarcane Research nstitute of Cuba (NCA), located on brown and vertisol soils. Samples were always taken from the arable layer (0 cm to 20 cm depth), air-dried, milled and passed through a 1 mm sieve. Phosphorus levels applied in these experiments ranged from 0 to 200 kg P205/ ha, together with NK doses considered at least adequate for normal development of cane. Samples taken for analysis came from unfertilised plots just before planting. Experimental plot sizes ranged 48 to 64 m2. At harvest the cane was weighed in the field. Details of these experiments were published by Villegas20. Assimilable phosphorus extraction was determined according to Oniani18, Truog modified by HSPA (Humbert13) and ray-kurtzs No. 2 methods. Analytical methods are detailed in Table. TABLE. Main characteristics of the analytical methods used for determining assimilable phosphorus extraction. Extractive References Soil :solution Extraction solution (min.) NH4F0,03 N t HC 0,l N ~ra~-~urtz~ 1 :10 1 H2S04 0,02 N Truog modi. by 1 :10 30 HSPA (~urnbertl~) Phosphorus adsorption isotherms were determined by allowing 1 g aliquots of soil to come to equilibrium with 25 ml of CaC1, 0.01 N solution containing different quantities of Na2HP04, at ambient temperature. The phosphorus concentrations used were 1, 3, 5, 7, 10 and 15 ppm P and the time allowed to achieve equilibrium was 24 hours; after that the suspensions were centrifuged at 6000 r/min for 5 minutes and phosphorus in solution was determined by colorimetric means. Phosphorus lost from the solution was considered to have been sorbed by the soil. Experimental data were adjusted according to the Freundlich equation (Fitter and Sutton7): y = axb where y = sorbed phosphorus concentration (,u g P/g soil) x = phosphorus concentration in solution (,u g P/mL)

3 AGRONOMY 1 Sorption ratios estimated here were: Phosphorus buffer capacity (PBC), as the slope of the adsorption isotherm between 0.3 and 0.2 ppm P in solution, and sorbed phosphorus at 0.2 ppm P in solution (Po,,), obtained by evaluating the Freundlich equation for x = 0.2. Critical levels of sorption ratio were estimated by applying the Cate and Nelson6 mathematical method to the relationship between relative yield of stalks (NK yield 70 NPK yield) and sorption ratio. A confidence level of 1% was considered when estimating linear correlations among sorption ratio and relative yield. RESULTS AND DSCUSSON Sorption ratios Phosphorus buffer capacity (PBC)- PBC values found for the soils studied here are presented in Tables 1 and 111. This index ranged from 69 to 403 ml/g with a mean of 130 ml/g. The importance of PBC for characterising the availability to plants of P in the soil has been demonstrated by many 17,15,2912. t is considered to be one of the main properties of soil phosphorus which characterises the dynamic relationship between labile P in the solid phase and P in solution, the latter being the only source of supply for plants (White and ~eckett,~). PBC will vary according to P concentration in solution, measured as the slope of the isotherm as previously explained. This makes it difficult to compare results reported by different workers. However, it has been conclusively demonstrated that ECF ,7-59,O 89,4 40,6 94,8 Mean 116,8 47,2

4 R VLLBGAS, M. LOPEZ AND A. CABRBRA 143 / TABLE ll. Assimilable phosphorus, sorption ratios and relative yields of cane on Vertisols Assimilable phbsphorus (ppm P205) Sorption ratios Experiment code PPF - 5 PPF-16 PPF-17 PPF- 18 PPF - 20 PPF- 21 PPF - 26 PPF-27 PPF - 28 PPF - 40 PPF - 43 ECF- 9 ECF- 11 ECF- 12 ECF- 13 ECF14 ECF- 19 ECF - 22 MF -90 MF -96 MF -101 MF -106 MF -109 Bray-Kurtz Troug Oniani PBC Relative (ml1g.s.) (pgp1g.s.) yield, % 24,6 21,4 54,8 53,7 61,2 30,O 17,3 39,3 24,6 37,4 58,6 34,6 45,7 29,7 40,8 30,2 100,7 68,2 112,8 75,5 110,6 72,O 137,3 141,3 102,8 119,6 94,5 35,l 118, ,l 154,6 110,l 85,9 48,6 160,3 38,3 44,5-314,8 20,5 27,8 18,9-29,9 Mean PBC represents the resistance to change of the P concentration in solution which in turn is known to be inversely related to the ease of movement of the labile P from the solid phase into solution (Helfordl1). This implies that the higher the PBC, the lower the labile P content of soils. t can be observed in Tables 1 and 11 that the highest figures obtained for PBC were for vertisols, which correspondingly showed the lower assimilable P content. Sorbedphosphorus at a 0.2 ppm P - This index has been reported to be very useful for comparing different soils and also as a diagnostic criterion for adequacy of phosphorus fertilisation (4,9314). values found in the soils studied are also shown in Tables 1 and 111. This index ranged from 27 to 138 p'g P/g.s. with a mean of 56,LL g P/g.s. These results are within the ranges reported by FOX^^'^ for Hawaiian and Colombian soils and Pope19 in Brazil; although these authors report-

5 144 AGRONOMY ed higher maximum values than those presented here. The highest figures here recorded relate to vertisols indicating that these soils have higher P sorption than the other soils tested. Evaluation of sorption ratios The performance of sorption ratio as an index of P demand was evaluated by studying the correlation between sorption ratio and relative yield in data for 116 harvests. Relative yield has been used because it excludes other factors influencing plant production (Cate and Nelson6). TABLE V. Correlation coefficients between sorption ratio and relative yield due to phosphorus fertilization (n = 31). Linear Determination Correlation Coefficient Sorption index Coefficient (r) (R2) P0,2( tlg P/g.s.) -0,72** 0,52 PBC (ml1g.s.) -0,69** 0,48 n Table V, linear correlation coefficients between relative yield and sorption ratio are shown. A significant negative relationship was found for both ratios; hence they may be regarded as reliable for predicting sugarcane response to phosphorus fertilisation. Critical levels of sorption ratios The relationship between relative cane yield and sorption ratio in each experiment was used to estimate the critical value by the Cate and Nelson6 method. Figures obtained are presented in Table V. TABLEV. Critical levels and determination coefficients of sorption ratios in the soils studied (n = 31). Sorption ratios Determination Coefficient PBC (ml1g.s.)

6 R. VLLEGAS, M. LOPEZ AND A. CABRERA 145 levels found. n contrast, an important response must be expected above critical levels. However, when ratios found for experiments PPF 20,21 and 37 are observed, it becomes evident that the values were above critical levels, and in consequence, the response to P fertiliser was also reflected in relative yields of 93%, 94% and 91070, respectively (see Figures 1 and 2). CL= 54 4 p g Plg s SORBED PAT 0,2 pprn P N SOL (fl g Plgs) FGURE 1. Relationship between relative cane yield and sorption ratio based on P (see Table V). When assimilable phosphorus was evaluated as a diagnostic criterion in these experiments, it was found that in spite of assimilable P being above the critical level of 22 ppm P205 obtained through the ~ ra~-~urtz~ No. 2 method, reported by VillegasZ0, there was no response to P fertilisation. This result pointed to the necessity of considering not only the P availability but also the P sorption capacity of the soil and those physical and chemical process determining dynamic exchange of P between the solid phase and the soil solution. Correlation between sorption ratio and assimilable P Relationships between assimilable P content determined by Oniani, Truog modified by HSPA and Bray-Kurtz methods and sorption ratios were calculated. Results are shown in Table V. t may be seen that a significant negative correlation exists, meaning concisely,

7 146 AGRONOMY the higher the assimilable P in soil, the lower the sorption ratio. However exceptions may occur, such as those previously mentioned of experiment PPF 20, 21 and 37. The highest correlation coefficients were recorded for the Bray-Kurtz method. This was in agreement with previous reports of Villegas20 who found that this method was more stable giving also more reliable predictions of P requirements in sugarcane, compared with other methods PHOSPHORC BUFFER CAPACTY (PBC) FGURE 2. Relationship between relative cane yield and sorption ratio based on PBC (see Table V). TABLEV. Linear correlation coefficient among assimilable phosphorus results and sorption ratios in the soils studied. Sorption Methods of determination of P index (y) Bray-Kurtz Truog Oniani r = - 0,57** r = - 0,49** r = -0,50** PBC n = 63 n = 45 n = 63 x represents P205 in ppm

8 R. VLLEGAS. M. LOPEZ AND A CABRERA CONCLUSONS PBF and Po., were found to be reliable criteria for predicting the response of cane to phosphorus fertilisation, since there was a very significant correlation among them and relative yield. The critical level of PBC and Po,, were assessed at 136 ml/g.s. and 54 pg p/g.s., respectively. The two criteria may be useful for explaining some exceptions found when trying to relate overall P content to the phosphorus fertility of the soil. There was a significant negative correlation among assimilable phosphorus determined severally by the Oniani, Truog modified by HSPA and Bray-Kurtz methods and the sorption ratios studied here. REFERENCES 1. Ayres, A.A. (1960). The growth of sugarcane as influenced by phosphorus. The critical range of soil phosphorus. Proceed. 10th Cong. of the SSCT, Hawaii, pp Barrow, N. J. (1967). Relationship between uptake of phosphorus by plants and the phosphorus potential and buffering capacity of the soil - an attempt to test Schofield's hypothesis. Soil Sci. 104: Barrow, N.J. (1978). The description of phosphate adsorption curves. J. Soil. Sci. 29(4): Beckwith, R.S. (1964). Sorbed phosphate at standard supernatant concentration as an estimate of the phosphate status of soil. Aust. J. Exp. Agric. Anim. Husb. 5: Bray, R.H. and Kurtz, L.T. (1945). Determination of total organic and available forms of phosphorus in soil. Soil Sci. 59: Cate, R.R. and Nelson, L.A. (1971). A simple statistical procedure for partitioning soil test correlation data into two classes. Soil Sci. Soc. Am. Proc. 35: Fitter, A.M. and Sutton, Y.C.D. (1975). The use of the Freundlich isotherm for soil phosphate sorption data. J. Soil Sci. 26(3): Fox, R.L., Plucknett, D.L. and Whitney, A.S. (1968). Phosphate requirements of the Hawaiian latosols and residual effect of fertilizer phosphorus. Trans. 9th nt. Congr. Soil Sci. Adelaida. 2: Fox, R.L. and Kamprath, E.J. (1970). Phosphate sorption isotherms for evaluating the phosphate requirements of soil. Soil Sci. Soc. Am. Proc. 34: Fox, R.L., Nishimoto, R.K., Thompson, J.R. and Pena, R.S. de la (1974). Comparative external phosphorus requirements of plants growing in tropical soils. Trans. 10th nt. Congr. Soil Sci. Moscow. pp Holford,.C.R. (1979). Evaluation of soil phosphate buffering indices. Aust. J. Soil Res. 17: Holford,.C.R. and Mattingly, G.E.G. (1976). Phosphate adsorption and plant availability of phosphate. Plant and Soil. 44: Humbert, R.P. (1970). El cultivo de la cana de azucar. 2nd. ed. Ed. Ciencia y Tecnica, nst. del Libro, La Haband, 787 p. 14. Juo, A.S.R. and Fox, R.L. (1977). Phosphate sorption characteristics of some benchmark soils of West Africa. Soil Sci. 124: Mattingly, G.E.G. (1965). The influence of intensity and capacity factors on the availability of soil phosphate. Tech. Bull. Ministry Agric. Fish Food. London. 13:l Olsen, S.R. and Watanabe, F.S. (1957). A method to determine a phosphorus adsorption maximum of soils as measured by the Langmiur isotherm. Soil Sci. Soc. Am. Proc. 21: Olsen, S.R. and Watanbe, F.S. (1963). Diffusion of phosphorus as related to soil texture

9 AGRONOMY and plant uptake. Soil Sci. Soc. Am. Proc. 27: Oniani, 0. (1964). Apridil podvishni svedinieni fosfora i Kalia is adnci bitiashki is krasnosiomnij i podsolisty pochv Gruzi (in Russian). 6: Pope, R. (1976). Use of soil survey information to estimate phosphate sorption by highly weathered soils. PhD. Thesis, Dept. Soils, North Caroline Univ. 110 p. 20. Villegas, R., Rubio, R. and Lopez, M. (1983). Calibracion del analisis fosforico y potasico del suelo para recomendaciones de fertilizantes en el cultivo de la cana de azucar. Presented at the Forum Cientifico de la Academia de Ciencias de Cuba, La Habana. 39 p. a 21. Wang, C.H. and Fang,. (1982). Taiwan Sug. Exp. Sta. Taiwan Annual Report. 50 p. 22. White, R.E. and Beckett, P.H.T. (1964). Studies on the phosphate potentials of soils.. The measurement of phosphate potential. Plant and Soil. 20:l Zambelle, E. Jr., Filho, J.O. and Rodella, A.A. (1981). Recomendacao de adubacao fosfatada para cana de acucar a traves de analise Quimico de solo. Brasil Acucareiro.