Boron adsorption and desorption in soils and influence of fertilization intensity

Boron adsorption-desorption processes were invistigated in 11 soils and composted manure according to the Langmuir and Freundlich isotherms. The data obtained showed that these processes have been well described by these isotherms (r ranging from 0.902*** to 0.999***). Adsorption capacities of soils varied from 23.98 to 102.04 mug B/g soil. Significant relationships were observed between these adsorption capacities and some of the soil characteristics, mainly clay content (r = 0.730**), organic matter content (r = 0.615**) and cation exchange capacity (r = 0.852***). The adsorption capacity of composted manure was 312.50 mug B/g manure and represents 3 to 13 times that of the studied soils. Incorporation of manure to soils increased their adsorption capacities. Comparison of desorption to adsorption isotherms showed that there was a hysteretic effect for all the substrats studied. However, this effect was more important for the soils than for the composted manure. The desorption index for the composted manure was 1.5183, a value not far from 1, which indicates the reversibility of the adsorption process. The mean desorption indices observed varied from 1.9771 to 4.7639 for soils. Significant relationships were observed between these indices and some of the soil characteristics, of which organic matter content (r = -0.787**) and cation exchange capacity (r = -0.709**). The incorporation of composted manure to soils caused an important decrease of their desorption indices.