T-Sum is a method to determine when to make the first application of nitrogen fertilizer in spring. The ‘T-Sum’ value is the accumulated mean daily temperatures (in ° C) above zero, starting on January 1 (below-zero temperatures are ignored). For example, if the mean daily temperatures for a 5-day period were 6, 3, 0, 1, and -4°C, the ‘T-Sum’ total is 10. The ‘T-Sum’ concept assumes that rate of spring growth is related to accumulated mean temperature.
Why Do We Need T-Sum?
Research carried out first in the Netherlands and the UK, then confirmed in coastal BC, demonstrated that grass crops respond well to spring fertilizer that is applied when T-SUM is between 200 and 300. New manure management guidelines in BC recommend that early-spring manure application on grass be delayed until T-Sum reaches 200. In coastal BC, T-SUM 200 is generally reached between mid-Feb and mid-March. Note that a study conducted by the Pacific Agri-Food Research Centre at Agassiz showed that fertilizer application should not be delayed for late maturing grass varieties.