(1) burning incident to horticultural or agricultural operations of: (a) prunings from trees, bushes, and plants; or (b) dead or diseased trees, bushes, and plants, including stubble;
(a) prunings from trees, bushes, and plants; or
(b) dead or diseased trees, bushes, and plants, including stubble;
(2) burning of weed growth along ditch banks incident to clearing these ditches for irrigation purposes;
(3) controlled heating of orchards or other crops to lessen the chances of their being frozen so long as the emissions from this heating do not violate minimum standards set by the board; and
(4) the controlled burning of not more than two structures per year by an organized and operating fire department for the purpose of training fire service personnel when the United States Weather Service clearing index for the area where the burn is to occur is above 500.