Alpha transparency not working Before you tag this as an 'already asked' question , please read all of it.
It is a different case than other similar questions.
I am making a farm scene.However , a problem occur when i render it.
Here is an image of it.


As you can see from the image , the plants have a black layer covering it .
But when i zoom out , the alpha work correctly again. 
Here is the image.

The node for the crops look like this.

What i want it to look like is the view in material tab .Here is the picture of the scene in material mode.

I just made a simple , smaller size and similar scene blender file. Here it is if you want to check it yourself and solve the problem.It is only 5mb size.Here's the link - https://www.mediafire.com/folder/7fm3j77t24l4m/Help (Please download the textures along with it)
Thanks to chebbou for the link , i managed to find the solution to this .
Just go to render section and go to light paths . There will be min and max there . And if i am correct , default is set to 8. I turned both of it up to 120 and the problem is fixed ! Cheers.