Additional phosphorus intensity analysis needed to identify phosphorus requirement on acid sandy soils with high Olsen-P contents

Abstract

Meeting the crops requirement for phosphorus (P) is crucial to reach the yield potential. Therefore, it is essential to determine soils´ ability to supply the crop with P to identify soils in need of P fertilization and to know where P fertilization is redundant. Thirty-two field experiments with increasing rates of mineral P fertilizers (0, 7.5, 15, 30 and 60 kg P ha−1) to spring barley were carried out from 2020 to 2022 on sandy soils developed on marine or non-marine deposits in Jutland, Denmark. Soil P extraction methods representing both intensity (P extracted with demineralized water, P-water or 0.01 M calcium chloride, P-CaCl2) and quantity tests (P extracted with 0.05 M sodium bicarbonate, Olsen-P) were evaluated by their relationship with relative yields. The examined soils had pH values between 4.6 and 6.7 and total P contents between 305 and 1309 mg P kg−1 soil. Yield increased after P fertilization on twelve soils in Northern Jutland (relative yields < 90 %). Yield responsiveness occurred in all three experimental years despite differences in precipitation and temperatures among the years. Olsen-P contents were not related to yield response, and all responsive soils had moderate or high soil P status according to the Olsen-P test (>20 mg P kg−1 soil). Contents of P-CaCl2 were significantly related to the relative yields (R2 =0.39), and slightly more of the variation in relative yields was explained by the P-CaCl2 to Olsen-P ratio (R2 =0.43). Ten of the twelve response soils were detected with the suggested critical threshold value of 0.02 for the P-CaCl2 to Olsen-P ratio. Soils responsive to P fertilization on older marine (Yoldia) and moraine deposits had higher contents of amorphous aluminium oxides determined by oxalate extraction than the non-responsive soils (+41 mmol mmol kg−1), whereas four lowland soils on younger marine (Litorina) deposits responded to P-fertilization despite low contents of oxalate extractable aluminium. We conclude that an additional P intensity test with CaCl2 extraction offered a much better identification of acid sandy soils requiring P fertilization than relying on the Olsen-P test alone.