Aluminium and iron oxides affect the soil structure in a long-term mineral fertilised soil

Purpose The influence of iron and aluminium (hydr)oxide on soil aggregation is still not sufficiently understood. Therefore, we undertook a study on the effects of Al and Fe forms on soil structure and their mutual relationships in sandy soil under long-term fertilisation experiments near Skierniewice (Poland). Materials and methods In this study, we examined three treatments of soil under long-term fertilisation: in 94- and 41-year- experiments: (1) control-without mineral fertilisation (Co); (2) mineral fertilisation (NPK); (3) mineral fertilisation + calcium (CaNPK). Treatments in the 25-year experiment included the following: (1) farmyard manure and no mineral fertilisation (FYM); (2) farmyard manure + mineral fertilisation (FYM + NPK); (3) farmyard manure + mineral fertilisation with calcium (FYM + CaNPK). We analysed parameters of soil structure such as dry-sieved aggregate and water-stable aggregate contents as well as calculating soil structure indices. Total contents of Al and Fe and their free oxides were determined. Results and discussion The results showed that in 94-year experiment, for NPK and CaNPK treatments, the content of dry-sieved macro-aggregates (DSA(ma)) in the 5-3- and 3-1-mm size classes was significantly higher by 144 and 192% as well as by 37 and 36%, respectively, compared with that in the Co treatment. In comparison with the control, a lower percentage of macro-aggregate destruction (by 50%) and crusting index (Ic) (by 16%) was seen during the treatment with NPK fertilisation with added Ca. In the 41-year experiment, the structure coefficient significantly decreased after NPK and CaNPK fertilisation. In the 25-year experiment, the mineral fertilisation in combination with FYM did not affect individual size classes of DSA(ma). Values of Ic significantly decreased after the application of FYM in both NPK and CaNPK treatments. Overall, the highest number of correlations between Al and Fe free oxides and the soil structure was in the 94-year experiment, followed by the 25-year experiment, and the lowest number of significant correlations was in the 41-year experiment. Conclusions Fe and Al oxides might act as important agents for stabilising mainly the higher size classes of DSA(ma). On the other hand, Fe, Al and their oxides did not have effect on the smaller size classes of DSA(ma), particularly in the 94-year experiment. Overall, the application of mineral fertilisers with farmyard manure could accelerate the positive changes in the structure of sandy soil, even, in particular, through Fe oxides.