I don't know what the nutrient content of your soil is, so I can't give you a scientific answer. I've always used regular garden fertilizer, the cheap stuff that you buy in big bags, like 10-20-20. But be warned: if you give the trees a lot of fertilizer and water, they will grow at a brutal rate, and they will have a lot of space between branches. You may be able to make them denser with proper pruning. Someone on this forum should be able to tell you more about that.

Is there a reason you think you need to fertilize? Are the trees struggling, look off-color? Have you had your soil tested to see if you have any nutrient deficiencies?

If you have relatively decent soil, there is typically little need to have any sort of routine fertilization program for trees and shrubs. Most soils contain a full range of nutrients in sufficient quantities. Your best move is to mulch or topdress regularly with a layer of organic matter - compost, wood chips, bark, etc. These will decompose gradually and release nutrients to supplement those that can dwindle over time.

Fertilizing simply to speed growth is not recommended. It generally results in weak wood and/or lush growth that is more attractive to insect predators or prone to diseases. Leylands (note spelling) grow fast enough once fully established - about 3-5 growing seasons - that this should not be a concern.

Leylands do grow very fast! I have 2 that I planted to form a bit of a barrier/transition from the very back of my backyard to the greenbelt that it butts up to. One interesting thing to note- soil makes a huge difference! The 2 we planted are essentially in raised beds with trucked in topsoil (can't recall if I added compost). They have grown spectacularly. Below this area, we planted 4 or 5 additional Leylands on the property line. These trees were planted in very poor soil (mostly rock), with just some compost in the holes as amendments. A couple of those trees died, and the others, while living, are still very small, although they've been in the ground at least 4 years. They're only about 5 feet tall and very spindly. In other words, hope your trees are planted in good, rich soil and you will have a fine privacy barrier in no time.

The less happy ones could be malingering because of the soil amendments and the effects these have on water movement into and out of amended planting holes. Are they mulched? Lack of mulching can place newly installed plants at a serious disadvantage in our dry summers.

Inherently vigorous Leyland cypress may split apart or topple under ordinary conditions, as well as overwhelm less-than-parklike spaces. Deliberately inducing rank growth from these is not necessarily desired.

Here is a link that might be useful: Planting Fact Sheet