Introduction:
In his paper to the Fertilizer Society in 1966 (Proceedings: 90) Hamamoto listed a number of materials in use or for possible use as slow-release fertilizers. A relatively small number of these have been manufactured on a commercial scale. This paper examines those which have been commercially manufactured, and reviews theft properties and field performances. Particular emphasis is placed on sulfur-coated urea (SCU), probably the most recent commercial product.

General Conclusions:
At present the markets for large scale sales of slow-release fertilizers have been found where premia are available for the crop grown or the saving on fertilizer application costs have been substantial. Only where the cost of the slow-release component has been diluted by the lower costs of other ingredients of a compound fertilizer have they been acceptable on lower value crops.

From the extensive trials work it is clear that a range of crops that have a high demand for nitrogen over a short period (such as maize, sorghum, wheat and barley) are unlikely to show yield benefits after the use of slow release fertilizers, though benefits such as improving the protein value of the grain are possible.

Sulfur coatings offer the possibility of lower cost slow release of nutrients. Additionally the evidence shows that it is possible to radically alter the pattern of nutrient release by varying the level and quality of the coating. If the value of the sulfur coating on an S fertilizer can be obtained it is probable that the other fertilizers components (N and K) can be marketed at a very small premium over conventional soluble fertilizers.

However, some disadvantages of sulfur-coated fertilizers are evident (List). Despite these constraints, the other side of the balance sheet shows much in favour of sulfur-coated fertilizers (List).

In conclusion, The sulfur-coating of fertilizers offers the best potential for the manufacture of slow release fertilizers with the ensuing advantages.

L H Davies, ICI Ltd., Agricultural Division, Billingham, UK.

64 pages, 6 figures, 18 tables, 227 refs.