The T-sum calculation is used to estimate spring N fertilization timing for perennial ryegrass. Beginning January 1, calculate the average daily air temperature measured in celsius. Fertilize for early spring or late winter growth when 200 "units" have accumulated. Calculate the average daily air temperature by adding the maximum to the minimum, then dividing by 2. This assumes the measurement is made in degrees (°) C. If the air temperature is measured in °F, calculate the daily average air temperature, then convert to °C. If this system is to be used by a grower, suggest the investment in a °C thermometer. This might be easier for the grower than converting each average daily air temperature.

Other rules to follow are:

Do not add negative values. If the air temperature does not rise above freezing for a day, no credit is given for the day. If the minimum temperature is below 32°F (0°C), and the maximum above, but the average temperature is below 32oF (0°C), no credit for the day is given. Example calculations follow.

Date Minimum °F Maximum °F Average °F
Jan. 1 37 48 (37 + 48)/2 = 42.5
Jan. 2 26 48 (26 + 34)/2 = 30
Jan. 3 26 42 (26 + 42)/2 = 34

Convert Jan. 1 & Jan. 3 to °C
42.5 - 32 = 10.5 x 5/9 = 5.8 units
34 - 32 = 2 x 5/9 = 1.1 units

For further reading:

Kowalenko, C.G., S. Freyman, D.L. Bates, and N.E. Holbek. 1989. An evaluation of the T-Sum method for efficient timing of spring nitrogen applications on forage production in south coastal British Columbia. Can. J. Plant Sci. 69:1179-1192 (Oct. 1989).