Effects of inorganic and organic amendments on soil phosphorus chemistry in calcareous soils

Organic amendments provide both plant-available nutrients and organic matter to soils, resulting in improvements to soil chemical, physical and biological quality as well as fertility. These amendments can directly affect the chemistry of soil phosphorus (P) by altering the amounts, distribution and transformations of various soil P fractions. The overall goal of this thesis is to understand the effects of inorganic and organic amendments on soil P chemistry in calcareous soils for establishing more effective P fertility management. Transformations of various P forms in the soils amended with either animal manure compost or inorganic P fertilizer and with or without plant growth were studied. The dynamics of P in calcareous soils and analytical techniques for quantifying and characterizing forms of inorganic and organic P in either calcareous soils or in animal manure composts were also studied to assist in designing appropriate experiments to evaluate plant P availability. The highlights of this thesis were (1) forms of P in animal manure composts were clearly different from those in inorganic P fertilizers, and mainly associated with calcium and magnesium, which were extractable with weak acids, (2) turkey litter compost increased weak acid-soluble inorganic P in soils, which transformed into plant available P with plant growth, (3) ryegrass took up P from the inorganic P extractable with a weak base extractant (NaHCO3) in all soils and from the inorganic P extractable with a weak acid extractant (NH4Ac) in the soils rich in P, (4) ryegrass promoted mineralization of organic P in the soils relatively deficient in P, and (5) the plant availability of P in turkey litter compost could be equivalent or greater from that in synthetic inorganic P fertilizers in the long term. The current soil P fertility management system in Ontario only considers NaHCO3 extractable inorganic P (Olsen P) as an indicator of plant available P. This may result in significant underestimation of the plant available P in soils where organic amendments, such as animal manures and composts, as they affect soil P chemistry differently from that of inorganic P fertilizers. This research has shown that weak acid extractable P should be considered as an alternative indicator of soil P fertility, especially in soils receiving organic amendments.