Evaluation of Various Methods for the Routine Determination of 'Available' Phosphorus Against the Phosphorus Supplying Power Of Soils And Phosphorus Response of Crops from Pot Experiments
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In this study, the relative phosphorus availability status of a range of twenty four mainly acid, Kenyan soils (three Acrisols, six Andosols, six Ferralsols, three Luvisols and six Nitosols) was assessed by determining both their phosphorus supplying power and their response to added phosphorus from greenhouse experiments using Serena sorghum (Sorghum vulgare Pers.) as the test crop. These dat3 were then used to determine the best chemical methods for the routine measurement of 'available' phosphorus, both fDr the whole range of soils, and for different groupings of. the 50ils. The phosphorus supplying power was determined by repeatedly cropping the soils, towards. phosphorus exhaustionover a period of 308 days. The responses This experiment is referred to as the exhaustion experiment to added phosphorus were assessed over a period of 50 days using a double-pot technlque in which thp phosphorus was added in a 'complete nutrient • solution' to the lower pot, that was separated from the soil in the upper pot, but still accessible to the plant's roots. In the controls, the minus phosphorus nu t r i en t solution was applie d tot he lower pot. Selected plant parameters were then determined from these two experiments to reflect the available phosphorus' levels of the soils. Extractable phosphorus in the soils was determined using nine extraction methods i.e. Mehlich, Bray II, Olsen, Saunder, Legg, resin, 0.2N H2S04, M/500 CaC12 and mineralisable phosphorus. The extractable phosphorus values were then correlated with five plant parameters i.e. dry This experiment is referled to as the response experiment. weight of tops, X1 and total phosphorus up Lake of tops, X2 from the exhaustion experiment, and percentage relative yield - dry weight (whole plant), Y2, percentage relative yield - dry weight (tops), Y4, and percentage relative yield - phosphorus uptake (tops), Y6, from the response experiment. From these correlations the extraction mp.thod~that 9 avet h e b est in d ice S 0 f 'a v ail ab 1e' p h 0 s ph a r u s lJJere determined. Different soil tests gave the highest correlation coefficients for different plant parameters in the simple regression analyses for all soils for example: resin extractable phosphorus correlated best with percent relative yield - dry *** weight (tops), Y4 (r = 0.862 ) wheread Bray II extractable phosphorus correlated best with percentage relative yield - dry weight (whole plant), Y2 (r=Oo8l3 ). Olsen method correlated best with the plant parameters from the long time exhaustion experiment, whereas the resin meted W2S generally the short - term, response experiment. All the soils gave a response and wen therefore all deficient in phosphorus. Higher correlation: coefficients were found than those obtained by other workers who found , values not exceeding 0.52 accounting for only 25% of the variation in available phosphorus, whereas in this study correlation coefficients of 0.684 to 0 *** 0862 were obtalned accounting for 47 - 74% of the variation in available phosphorus. Saunder, Mehlich and indices of organic phosphorus generally gave low correlation coefficients. The low correlation coefficients with the indices of organic phosphorus suggests organic phosphorus is not an important source of available phosphorus under continously moist conditions in greenhouse experiments. Another experiment designed to give wet and dry cycles showed that under those conditions ph2sphorus uptake is significantly increased presumably because of organic matter mineralisation and release of inorganic phosphorus for plant uptake. Multiple and partial linear regression analysis in which Quantity, Intensity and organic phosphorus parameters were correlated plant parameters showed that the Intensitv and organic'phosphorus terms diJ not greatly increase the correlation coefficientc for the better chemical extractants viz. Olsen, H2S04, resin. The partiql correlation coefficients showed that the Intensity term is more important in the shorter, response experiment than in the longer exhaustion experiment. When the five plant parameters viz. X1, X2, Y2, Y4 and Y6 were combined and correlated against each soil test in turn for all the tmenty four soils, reSin, H2S04 and Olsen methods were best (correlation coefficients of 0.9.31 *** ; 0.869 *** and 0.862 *** respectively). The partial correlation coefficienta showed that V2 and X2 accounted for most of the variation in extractable phosphorus values. The sofls were grouped into five groups on the basis of their physical and chemical properties viz. pH, ApH (ioe. difference between pH in water and pH in calcium chloride), percentage carbon levels, mineralogy and Oil the basis of their FAD - UNESCO legend classification. Higher correlation coefficients were generally obtained when the soils were separated out into groups than when all the soils were considered together and different extractants worked best for different groups of soils and for different plant parameters. The most discriminating plant parameters from the short-term response experiment (50 days) i.e. V2 & V6 gave the best correlation coefficients when sbils were grouped on the basis of clay mineralogy. More attention was hO2 over paid to the two plant parameters which gove the high~st degree of discrimin3tion bet~een soils and :he lowest coefficients cf variations i.e. X1 and X2. These two paramet~rs reflect availability of phosphorus and how it influenCES dry matter production over long periods of time (241 or 170 days respectively) .ullder conditions of intensive phosphorus uptake. For these two plant parameters, grouping of soils on the basis of percentage carbon gave the highest set of correlation coefficient values. Thus the Olsen method gaVE 0.955 and 00932 for the low (L2%) and high (>208$) percentagE carbon groups respectively, whereas S04 gave a value of 00870 for the medium <2.0 - 2.8%) percentage carbon group Using theSE two extractants 91%, 76%, 87% of the variation in available phosphorous as reflected by the X2 parameter were accounted for the low, medium end high percentage carbon groups, If all the soils are considered together, the best extractant was the Olsen method which accounted for only 66% of the variation in the X2 parameter. Since in these experimental conditions soils were moist i.e. no wet-dry cycles, the best Qhemical extraction methods determined, probably largely reflect the availability of inorganic Goil phosphoruso It is suggested that the higher cnrrelation coefficients obtained when the soils were separated on the basis of percentage carbon are due to the three percentage carbon groups reflecting differences in soil forming process ,hence in the nature of their soil phosphorus. Thus the different groups require different chemical extractants to determine their available phosphorus levels.