Comparison of Al P and Olsen P Test in Calcareous Soils in Croatia

The common use of soil-fertility control data in Croatia includes properties such as soil pH, organic matter content and plant-available phosphorus (P) and potassium (K) extracted by ammonium lactate (AL) solution. These data are commonly used for fertilizer recommendations for both field vegetable (Lončarić et al., 2003) and arable crops (Vukadinović et al., 1996). Also, the same soil data have been used on acid and calcareous soils as well as the same extractant. A suitable extractant should extract a proportionate fraction of plant-available P from soils differing in other properties; the procedure should work with reasonable accuracy and speed; and the determined soil test P value should be in correlation with crop growth response to P or P uptake (Kamprath and Watson, 1980). Acidic extractants have been found to be very effective in estimating available P in acidic soils but the same methods may not be appropriate for using on calcareous soils because of neutralization of the extractant acidity by soil carbonates (Indiati et al., 2002). In Croatia, use of the acidic Egner-Riehm-Domingo (ammonium lactate) soil P test (Egner et al., 1960) as an extractant for P fertilizer recommendation has been commonly practiced (Lončarić et al., 2003; Vukadinović et al., 1996) but the Olsen method (ISO, 1994a) recently became the standardized soil test for P in Croatia. It is generally accepted that acid extractants are more appropriate for acid soils and bicarbonate-containing reagents are more suitable for calcareous soils and the Olsen (NaHCO3) extraction could be better choice for calcareous soils in Croatia, therefore for soils in valley of the Neretva River, near Opuzen city too. Another approach to improve P fertilizer recommendations is modeling plant availability of soil phosphorus using empirical constants or soil P test data to describe the reaction of applied soluble P with the soil (Fixen and Grove, 1990). It has been proposed that soil properties such as pH, calcium carbonate content and soluble Ca can be included in models describing plant availability of soil P (Fixen and Grove, 1990). This could be very useful for specific soil type as extra salted soils with high carbonate and/or high dissolved salts content. The AL method is the most commonly used method for determination of soil phosphorus content and the AL method results are used for fertilizer recommendations for both field vegetable and arable crops in Croatia. However, in certain conditions these data may be insufficiently precise, especially for calcareous soils. In such cases, phosphorus recommendations based on AL P test could ISSN 1330-7142 UDK = 631.416.2