SOME ASPECTS ON HEAT UNIT CALCULATIONS FOR SOWING-PROGRAMS OF VINING PEAS
A simple heat unit system is based on knowledge and experience about the following points:
- Factory capacity, which determines the quantity of shelled peas a factory can utilize per unit time.
- Mean yield of shelled peas per variety during the season. This item in relation to the factory capacity will define the acreage which ought to be harvested per unit time. In general the yield may vary during early and late periods of the season. Such a variation must be taken into account as well.
- Average heat unit requirement for the pea varieties. This part gives the basis for the calculation of sowing and harvest date.
- Average heat unit distribution during the season. This point will be the basis for all planning since calculations have to be made from average climate conditions when looking ahead.
- Number of heat units during the present season.
A careful registration must be done on the number of heat units that come day after day during the present season.
This gives indication for sowing-days, as in simple heat unit system there shall be as many heat units registrated between sowing-days as there are on average between the days during the harvest season.
A heat unit system is based on the assumption that there exists a strong relation between temperature and the development of the pea plant. This is the case during good light conditions and in the temperature region from 7–28°C. In Northern Europe and specially in Scandinavia these conditions are common during the pea-growing period.
A simple heat unit system is a good basis for planning, although for an accurate calculation several factors have to be taken into account. In order to increase the accuracy of prediction, the following points must be considered:
- Corrections for local factors. A study has to be made on local conditions in the region. In relation to a standard area, the fields may show small differences in development, which ought to be established empirically. These local corrections include factors such as soil type, surface colour, altitude, humidity, light climate, etc., which are relatively constant year after year.
- Corrections for the effect of sowing-time. In general the heat unit of