Abstract: Calcium loss from soil has dramatically increased for decades due to the increase of nitrogen input and/or the extension of acid rain,while calcium input into soil has declined substantially for the replacement of superphosphate by ammonium phosphate in fertilization,which intensified the imbalance of calcium input and output in agro-ecosystems,and needed to be solved in the near future for the sustainability of agriculture in such a country like China where the arable land resource is very limited.In recent years,the intensified soil acidification is mainly attributed to the root proton secretion stimulated by fertilization,and the nitrogen input from precipitation near industrialized regions promotes plant growth but results in other nutrients deficiency,which in turn acidifies soil and causes tree death.One of the most important mechanisms of saline soil bioremediation by sesbania is the activation of soil calcium by sesbania's proton secretion and the increase of soil calcium supply for subsequent crops.The present paper summarized the roles of calcium in plant tolerance to the stresses like acidosis,toxic metals,osmosis,ammonium toxicity,drought,extreme temperature (cold or heat shock),anoxia and pathogen infection,and the measures for maintaining soil calcium fertility.It was suggested that the production of ammonium phosphate should not be a pursued target for China's phosphorus fertilizer industry,and the roles of calcium in plant growth should be taken into account in fertilization experimental designs to make the experiments more accurate.

Key words: Calcium, Plant, Soil fertility, Stress, Stress tolerance