Why are the leaves on my tomatoe plant yellow

[it's perfect]

What isthe cause/causes and what can i do to keep the plant healthy etc.

[malcolmx]

either waterlogged or run out of feed if dry water until moist then water again with tomorite Iwater and work a small sprinkling of growmore into pots if more than 4 trusses now showing small toms pick out tops if not 4 then pick out top above last truss with little toms

reducing day length will mean toms will not ripen if flowers have yet to form toms toms already formed will probably ripen if well fed if a lot of leaf growth reduce leaves as well so toms are not shaded