I bought a golden nugget mandarin Seedless tree.
it was about 3ft high about 5 gallon. planted in the back yard there is a lot of sun afternoon and eve. there are other oranges trees about 40 ft away and an lemon tree also about 40ft away.since winter the leaves are greenish yellow
its's been in the ground for over a year but has never flowered how do they bud?
what is the best fertilizer?
has a lot of leaves what can i do to help it a long?

Don, In my experience with growing citrus trees, they are finicky and a pain. But IMO, they are completely worth the trouble.
Winter temps will cause leaf drop and some leaf yellowing, they are more than likely last year's growth, there is nothing you can do to stop it, it's nature. Looking at the leaves, can you see the green veins? If you can, it's an iron deficiency. You can purchase (depending on where you are) something called Copperas from Hi-Yield or chelated iron (being the best option since it is water soluble) Chelated iron is a bit more expensive, but better used by the tree since the roots soak it up quickly and it doesn't get washed away like Copperas and other powdered/granulated nutrient supplements do.
The other citrus do not affect your "baby" tree. At the moment I can't find the information on whether or not your golden nugget needs a pollenizer, although most mandarins are self pollinating.
In order to bloom, most citrus need some sort of stress, temp dropping being the best method.
The best fertilizer would be 6-6-6 or 8-8-8, you could even use 10-10-10 in a pinch. When purchasing fertilizer, look for one with as many micronutrients as possible. If you haven't done so, now would be the perfect time to fertilize the tree.

I've had my citrus trees in the ground for two years now, the first year they flowered and set fruit, but the only one that held the fruit was my thompson pink grapefruit, sometimes the tree is too small to hold on to the fruit or there is another reason for drop; i.e.: sickness, weather conditions, insufficient pollination, too many fruit on the tree compared to size.

Hope that helps, have a photo of your tree you can post?

MCKH

Actually, it is quite common for the leaves of young citrus trees planted outside in the ground to become a light green color during the winter months. I doubt that the dull green color is due to a deficiency of Iron. When the soil cools down citrus roots lose much of their ability to supply nutrients to the tree. Until at a soil temperature of 55.4F (which is absolute zero to citrus roots), the trees roots stop all function. The reason the leaves turn light greenish/yellow is from the lack of nitrogen. When the weather warms up your tree's leaves will again darken. You should never fertilize a tree under three years old, with 10-10-10. For the age of your tree you should apply 7.5 to 15-lb. of 6-6-6/year divided over 4 applications, or 5.5 to 11-lbs. of 8-8-8 per year divided into 4 applications. Or in other words 1.9- 3.75-lbs. of 6-6-6 per each application. Spread the fertilizer in a circle that is twice the diameter of the foliage. Also, don't let the soil become completely, or mostly dry during the summer months. Probably the reason this tree has not produced blooms is due to it care.