Answer: Rocky soil is probably why your carrots are giving you trouble. Try digging a trench for them and putting good soil there (with lots of sand), plant the carrots in the trench. Or build a raised bed and fill it in with top soil for your carrot crop. There is no practical way that I know of to keep the rocks from surfacing--it is caused by expansion and contraction of the freezing and thawing ground.
Some potato varieties produce smaller tubers than other varieties. The soil fertility can also affect tuber size. Potatoes prefer an acid soil with a pH of around 6.5 (confirm with a soil test). Mix some compost into the soil before planting and go easy on the manure, which can encourage potato scab. Since too much nitrogen will promote foliage growth at the expense of tuber growth, stick to fertilizers such as phosphate rock and greensand to provide nutrients.
Hilling potatoes encourages production of more tubers. When the plants reach about 6" tall, mound soil or mulch around the base of the plants within 1/2" of the lower leaves. Repeat this process several times throughout the summer.
Hope this information helps!
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