In concluding this discussion, I would like to emphasize again that there are many gaps in our knowledge of the phosphorus cycle. And it is obvious that these gaps need to be filled if we are to maintain soils at a high level of fertility and use phosphate fertilizers efficiently. Many aspects of our knowledge are still in the descriptive stage. We need better quantitative information on the amount, and availability of the various phosphate compounds present in soils, both organic and inorganic. Such information is essential to the improvement of present methods of estimating the phosphorus need of soils, particularly of soils high in organic matter and those neutral and alkaline in reaction. It is also essential to the establishment of improved soil and crop management practices. Increasing attention should be given, in particular, to the organic phosphorus compounds, their composition and their transformations in relation to plant availability. We need to know more about the dynamic, short-time changes in the cycle resulting from the activities of soil microorganisms, and the effect of various soil conditions on such transformations. Present indications are that the answers to some of these problems are not far off. Much progress has been made in recent years, and the outlines of the picture are much clearer. Moreover, radio-active phosphorus offers particular promise in establishing the nature and extent of the transformations in the phosphorus cycle, as well as in determining the role of various soil phosphorus compounds in plant nutrition. Isotopic forms of nitrogen and carbon have already been used successfully in giving us the beginnings of a new concept regarding the nitrogen and carbon transformations in soils. Together with the use of radio-active phosphorus they should make possible a much better understanding of the relation between the nitrogen, carbon and phosphorus cycles in soils and of soil organic matter chemistry generally. And finally, I would emphasize that the important advances made recently have come from pooling skills of various specialists. These results make it clear that future progress lies along that line.