Question
Each year I buy new potting soil for pot planted tomato plants.  I usually discard about 1/2 of last year's soil and mix in new soil to the remainder.  Is this a good practice?  Also should throw away the used soil?  I thought of spreading it into the lawn.

Thanks, John

Answer
Roots need air, or more accurately, oxygen - to grow well. The soil must have spaces to provide this air. After a while, a soil becomes compacted; the air is given up and the soil solids pack down and the soil is said to be compacted.
Farmers plough the land and we 'fluff up' the soil in our plant pots to recharge the soil with air spaces. This is what you hope to achieve when you repot your tomato plants.
I would recommend that you renew all of the potting mix. Spread the old soil on the lawn. It will be improved over time.