Tomato plant leaves are yellowing - why?

[shivvy]

Why are the leaves on my tomato plant (in the greenhouse and planted in compost from the compost heap) turning yellow? Is it anything to do with under or over watering? I have looked at other similar question on answerbank but one says that it could be a magnesium deficiency and the other says that it could be a fungal problem. How do I know which it could be and therefore know how to treat it?
Thanks

[oldernowiser]

My guess is that there's too much nitrogen in the compost, because its not well-rotted enough. Too much nitrogen can cause yellowing as well as too little - it acts like a weedkiller.

[shivvy]

Thanks oldernowiser (great name by the way!).
The plant has been doing really well up until the last week or so when it seems to be gradually getting worse. If there is too much nitrogen in the soil what can I do to counteract it?

[lajohn]

Too much water also causes yellowing of leaves.

[shylock73]

Try a soil test kit to determine what`s required. Meanwhile add some tomato liquid feed.

[Lonnie]

Buy some Epsom Salts, and sprinkle round the base of the plants.