or Fe-oxide impregnated paper (Menon et al. 1989, Chardon 2000) have also been used as a P-s<strong>in</strong>k to determ<strong>in</strong>e available P <strong>in</strong> a wide range <strong>of</strong> soils. Given <strong>the</strong> wide range <strong>of</strong> methods to test for different P fractions <strong>in</strong> soils, it is very important to select <strong>the</strong> right test for specific soils and target fractions. The soil properties affect<strong>in</strong>g selection <strong>of</strong> <strong>the</strong> appropriate P test and recommended methods are outl<strong>in</strong>ed <strong>in</strong> <strong>the</strong> Table 9. Da Silva and Van Raij (1999) reviewed different methods <strong>of</strong> measur<strong>in</strong>g <strong>the</strong> available P content <strong>in</strong> agricultural soils. The methods considered were: anion exchange res<strong>in</strong>, Olsen, Bray 1, Bray 2, Mehlich 1, Troug, Egner, water, 0,01M CaCl2, iron hydroxide impregnated filter paper (Pi) and <strong>the</strong> determ<strong>in</strong>ations <strong>of</strong> <strong>the</strong> E and L values. Details about <strong>the</strong>se methods are given by Kuo (1996). Average values <strong>of</strong> <strong>the</strong> goodness <strong>of</strong> fit for regressions between <strong>the</strong> soil test method and P uptake <strong>of</strong> plants and correspond<strong>in</strong>g numbers <strong>of</strong> articles (<strong>in</strong> brackets) <strong>in</strong> which <strong>the</strong> method was considered were obta<strong>in</strong>ed, showed <strong>the</strong> follow<strong>in</strong>g descend<strong>in</strong>g order: res<strong>in</strong>, 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); CaCl3, 36% (13), and Morgan, 32% ( 13). The results show that <strong>the</strong> anion exchange res<strong>in</strong> method was superior to o<strong>the</strong>r methods. It has <strong>the</strong> additional advantage that it can be used for both acid and alkal<strong>in</strong>e soils, has probably <strong>the</strong> best <strong>the</strong>oretical basis, and does not overestimate <strong>the</strong> amount <strong>of</strong> available P as may be <strong>the</strong> case when <strong>in</strong>soluble P forms get dissolved by acids extractants. Neyroud and Lischer (2003) compared <strong>the</strong> methods used for soil P availability across Europe which <strong>in</strong>volved 16 different methods on 135 soils from 12 countries. Each European country was us<strong>in</strong>g its own method for <strong>the</strong> determ<strong>in</strong>ation <strong>of</strong> <strong>phosphorus</strong> availability to plants, toge<strong>the</strong>r with an appropriate <strong>in</strong>terpretation scheme <strong>of</strong> <strong>the</strong> P <strong>status</strong> and fertilizer recommendations. The amount <strong>of</strong> extracted P decreased <strong>in</strong> <strong>the</strong> order Ptotal > Poxal. > PAL > PMe3 > PBray > PAAEDTA, PDL, PCAL > POlsen > Ppaper strip, PAAAc, PMorgan > PH2O, PCO2, PCaCl2. Isotopically exchangeable P was also measured. Even though all <strong>the</strong> methods reacted <strong>in</strong> <strong>the</strong> same way to <strong>in</strong>creas<strong>in</strong>g amounts <strong>of</strong> added P <strong>in</strong> several trials, <strong>the</strong>re were wide differences between results obta<strong>in</strong>ed with different methods. The <strong>in</strong>terpretation schemes for P <strong>status</strong> suggested that about 50 % <strong>of</strong> <strong>the</strong> tested soils were P-deficient. Sibbesen (1983) ranked different P-tests on <strong>the</strong> basis <strong>of</strong> data from 29 published papers <strong>in</strong>to three groups: best group: anion-exchange res<strong>in</strong> method; <strong>in</strong>termediate group: 62
water, and sodium bicarbonate methods; worst group: all acid methods <strong>in</strong>clud<strong>in</strong>g acetate buffer, lactate buffer, citric acid, Bray-1, Truog and Bondorff methods. Fig 13 : Frequency distribution <strong>of</strong> soil samples aga<strong>in</strong>st double lactate soluble P concentrations <strong>in</strong> soils from different land uses. Values obta<strong>in</strong>ed for <strong>forest</strong> soils are very low 63
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