THE METHOD OF IRON-HYDROXIDE PAPER SOIL T EST TO DETERMINE PLANT-AVAILABLE PHOSPHORUS IN SOIL

The suitability of different, older and newer methods to determine pla nt-available phosphorus applied in the Czech Republic, was assessed in a set of 18 soils by markedly different agrochemical characteristics (Tab. 1). The following methods were used to evaluate the soil reserve with phosphorus: Egner (Javorsky et al, 1987), Olsen (Olsen et al., 1 954), Mehlich 2 (Mehlich , 1978), EUF (Nemeth , 1971), KVK-UF (Matula , Pirkl, 1988), intensity factor (0.01 M CaCl2) (Zamjatina, 1969), sor ption index of phosphorus (Bache, Williams, 1971). These methods were compared with the method of iron-hydroxide impregnated filter paper so il test whose procedure was derived from the studies by Menon et al. ( 1988) and Sharpley et al. (1992). To compare with other methods of soi l tests, the weighed sample of earth was increased from 1 g to 5 g. Wi de-neck extraction vessels of the volume 300 ml, two strips of testing paper of a size 5 x 10 cm were used in tests. The volume of extractio n solutions was 200 ml. Short-time vegetation trials were established with a set of soils in climabox with a testing crop - spring barley, t he Zenit variety. The light and temperatures regimes of cultivation we re as follows: day - 16 hours, 20-degrees-C; night - 8 hours, 15-degre es-C, at photosynthetic efficient radiation of 500 muEm-2 s-1. Cultiva tions were completed after 28 days by the harvest of above-ground mass which was immediately dried at 65-degrees-C up to the constant weight and analyzed for nutrient content by routine procedures (Javorsky et al., 1987). The suitability of a soil test to assess the phosphorus re gime of soil was estimated on the basis of its relation to phosphorus content in plant. Characteristics of phosphorus regime of soils used a re presented in Tab. II. The iron-hydroxide paper soil test method exc eeded significantly all other tests applied, what is evident from Tab. III and from the correlation field in Fig. 1. Soil 13 only left the l ine from point field of dependences where a significant overestimation of availability of phosphorus for plant by soil test was found out. T his soil was remarkable by extreme agrochemical characteristics, high KVK (cation-exchangeable capacity) value, extreme saturation of calciu m and exchangeable potassium content (Tab. I). In discrepancy of soil test with plant phosphorus content could participate difficulty in tra nsport of phosphate into rhizosphere under the conditions of extreme K VK value and soil saturation with calcium. At the same time, physiolog ical substance of lower phosphorus uptake due to the following consequ ential interactions is not eliminated from the collaboration: excess o f potassium --> deression of calcium uptake --> state of cellular memb ranes --> lower performance of uptake mechanism of phosphorus. The pos itive of the iron-hydroxide-impregnated filter paper soil test compare d with other methods is that the correlation field was not spread with an increase in the value of soil supply with phosphorus. Rather its n arrowing was recorded. In the case of the method after Egner (Fig. 2) the soils 7,13, and 18 particularly, leave the line from the correlati on field. The soils in question are all with a high saturation by calc ium, with probability of phosphorus bond in apatites. In this case aci d leaching solution overestimates the possibility of phosphorus releas e from apatite forms than is the real availability for plants.The tigh tness of correlation in the method after Olsen (Fig. 4) is lowered mai nly by slightly alkaline soils (13 and 17) where phosphates bound to c alcium prevail with the dominant form of octo-calcium phosphate and hy droxyl apatite, available to plants only with difficulties. In the met hod after Mehlich 2 (Fig. 4) overestimation in soil 13 was found out o n one hand, and on die other hand, underestimation of soil supply with phosphorus in case of soils 4, 8, 1, and 3, in which no general reaso n for explanation was considered. Correlation field of methods EUF(b) (Fig. 5), intensity factor (Fig. 6), and KVK-UF (Fig. 7) am similar to one another. Overestimation appeared in particular in soils with extr eme low value of cation-exchangeable capacity (soils 2,6,12,14, and 15 ). Similar, this group of soils leaves the line from correlation depen dence in case of sorption index of phosphorus (Fig. 8). Phosphorus reg ime of soil is dependent, to a high degree, on clay contents (Bache, 1 977). As the KVK value is in a close relation to soft soil fraction co ntent - clays, in the KVK-UF method this value was used for the correc tion of determined phosphorus values. A sum of the KVK value and phosp horus content after the KVK-UF method improved significantly the tight ness of correlation, particularly in the field of higher soil supply w ith phosphorus (Fig. 9). After this correction the KVK-UF method came to the front position in the set of methods tested.