Hi,
There several reasons why the lower leaves on tomatoes turn yellow. It can be from over or under watering, viruses, or nutrient deficiencies.

In this case, the veins are still green which the surrounding leaf tissue is turning yellow. This is called inter-veinal chlorosis.(University of Connecticut Extension.) This means that the leaf does not have enough magnesium to complete the photosynthesis process. This can be due to general lack of magnesium and calcium in the soil, or it can be that the soil pH is too high or low, making the nutrients unavailable to the plant. 

First, I would simply fertilize with a tomato specific fertilizer. 

Second, I recommend taking a soil sample into the Yamhill County OSU Extension service office, or going to one of the farmers markets in the area. The Master Gardeners at any of these locations can test the pH of your sample and give you recommendations.

May your plants soon flourish and provide you with a bountiful harvest.