The effect of some silicate slags on the utilization of soil and fertilizer phosphorus

When certain silicates are applied to phosphorus‐deficient soils they improve crop growth. The effects of waste slags containing calcium silicate from iron and steel manufacture on crop yields and on phosphorus uptakes were investigated. In a pot experiment on barley grown on acid soil, ordinary blast‐furnace slags gave smaller increases in yields than equal weights of calcium carbonate or ‘falling’ haematite blast‐furnace slags. Slags made by special steel‐producing processes were more active than calcium carbonate and gave higher yields both with and without added phosphate. In field experiments ‘falling’ haematite blast‐furnace slags behaved in the same way as ground limestone and did not promote extra uptake of phosphorus by the crops. When no fertilizer phosphorus was applied, steel‐furnace slags were more active than ground limestone, and gave higher yields of swedes containing more phosphorus. When superphosphate was applied, steel‐furnace slags were still somewhat superior to limestone. Both ‘falling’ blast‐furnace slags and the steel‐furnace slags were active liming materials. Steel‐furnace slags had the additional property of increasing the amounts of phosphorus taken up by the crops. The reactions between silicates and soil phosphorus compounds require further study. Methods of characterizing products which promote the uptake of soil phosphorus by crops are needed.