394 PLANT SOIL ENVIRON., 49, 2003 (9): 394–401 Phosphorus availability in hydromorphic soils of Eastern Croatia D. Petošić 1 , V. Kovačević 2 , M. Josipović 3 1 Faculty of Agriculture, University in Zagreb, Croatia 2 Faculty of Agriculture, J.J. Strossmayer University in Osijek, Croatia 3 Agricultural Institute, Osijek, Croatia ABSTRACT The phosphorus (P) availability was tested on hydromorphic soils located in the Sava valley. 480 soil profiles covering the area of 31 227 ha was analysed in our study. The plant available P was determined by the Ammonium-Lactate meth- od. The P availability in the surface layer (0–30 cm) is very low (up to 5 mg P 2 O 5 /100 g of soil) in about 30% of the tested agricultural land (9 440 ha), next 32% (9 897 ha) is in the range of a low P availability (from 5.1 to 10 mg), while only 17% (5 445 ha) has a good or very good P availability (above 20 mg). Especially high frequency of low P availability was found in vertic gley, amphygley and hypogley soils (total 8 680 ha or 28% of tested agricultural land). Keywords: phosphorus availability; soil test; hydromorphic soil; Eastern Croatia; ameliorative fertilization; field experiments The growth retardation of maize at the early growth stage was found, which is in close connection with dif- ferent types of nutritional unbalances under the influ- ence of phosphorus (P), potassium (K) and zinc (Zn) deficiency (Kovačević et al. 1988). The P deficiency was found mainly in hydromorphic soils. The P deficiency in hydromorphic soils is in connection with their more acid reaction, relatively heavier texture and more intensive P fixation capacity (Vukadinović et al. 1988, Voplakal 1994, Efimov et al. 2001, Mengel and Kirkby 2001). Routine soil test data (pH, humus contents, plant available P and K according the AL-method) as a source information for the adequate fertilization are available in about one fourth of total agricultural soils (160 000 ha) in Eastern Croatia (approximately 50% of soils at former state farms and only 4% of soils at small farms). For this reason the fertilization practice is based mainly on general needs of individual field crops instead of on a nutritional status in the soil (Kovačević and Bašić 1997). The development studies are the source of additional information concern- ing the nutritional status of agricultural soils in the re- gion. The intention of this study is the testing of available P status in hydromorphic soils of the eastern part of Sava lowland area based on data from studies made for demands of hydromelioration projects. MATERIAL AND METHODS Description of the area Part of agricultural land of three former state farms in the area of the eastern part of Sava valley (Croatian: Posavina) in Croatia (PPK Zupanja, Zupanja, AK Jasinje Slavonski Brod and PPK Nova Gradiska, Nova Gradiska) is covered in this study. In the period from 1978 to 1989 this area was drained (total 31 227 ha or 19.2% of agricultural soil of the region – the municipalities Zupanja, Slavonski Brod and Nova Gradiska). It is a narrow belt in the west-eastern direction, approximately 150 km long and between 5 and 10 km wide. The western and eastern borders are defined by the riv- ers Slobostina and Josava, respectively. The river Sava defines the area from the south and Psunj, Pozeska gora and the Dilj Mountains from the north. Geological and pedological characteristics The relief of this area is a typical lowland with the alti- tude between 80 and 100 m above sea level. The litholog- ical ground is mainly sediment of pleistocen-holocen age (Vidacek et al. 2001). There are three ways of dampening the soils as follows: by surface water, underground wa- ter and by their combination (Škorić et al. 1985). Domi- nant soil types are different types and subtypes of hydromorphic soils (Kovačević 1997). Some improve- ment of soils was made by hydromelioration in the peri- od from 1978 to 1989 (Petošić 1994a, b, 1998, Marušić 2001). However, after the regulation of water-air relations in the soils, some chemical properties including nutrition- al unbalances became limiting factors of field crop yields. Climatic characteristics Eastern Croatia, including the Sava valley, has a mod- erate continental climate characterized by low horizontal changes of temperature and a specific distribution of rainfall (more rainfall in the warmer part of the year, from April to September). The mean amplitude of temperatures (differences between mean monthly air-temperatures in