The amount and type of fertilizer for use on blueberries depends on your soil type (is it clay like ours), whether have you added a lot of peat or sand, age, and size. You will want to see how the plants are maturing, what has been their yield and look at the color of the leaves. The ph level will affect all of the nutrients that the plant is getting. You may need to test your soil to see how much iron the plants can get according to the ph level. Iron deficiency is found when soil pH rises above 5.3 to 5.5.
According to the Agriculture Exchange office, blueberry plants do not respond well to nitrate forms of nitrogen. They also do not respond to high levels of nitrogen. Therefore, several modest applications of the ammonium form of nitrogen are recommended. If the soil pH is trending toward higher than desired levels, ammonium sulfate should be considered as the nitrogen source as it will reduce soil pH as well as supply nitrogen.
Truthfully, we have basically been very organic and used hardly any fertilizers. Blueberry plants do very well in our area as we get lots of sunlight and plenty of rain in the spring. We have a lot of clover growing between the rows, which draws the bees to the plants and enriches the soil. They must be very hardy plants as we have done well up until this past year.