Answer: In most cases a sparse root system reflects inadequate soil, either in terms of fertility or texture. I would suggest you run some basic soil tests and see if anything is out of balance there. Over feeding is no better than underfeeding and overfertilizing can actually burn plant roots, so it is better to run some tests and see what if anything you need to be supplementing.
You might also want to work in some organic matter such as compost and also, if your soil is clay based, some sand to try to improve its structure. A good soil structure with ample organic matter as well as adequate drainage will hold both air and water, both of which are needed for healthy roots. You want the soil loosened enough that the roots can penetrate it easily, and the sand and organic matter will help keep it loose.
Transplanting technique can also affect performance in the garden. Do not allow the plants to become rootbound with tight roots inside a too-small pot, this can stunt them. Take care to wait to plant until the soil has warmed up in the spring. A cold soil can stunt them, too.
When you do plant them into the garden, loosen the root ball a bit and direct the roots outward into the surrounding soil so they can easily expand away from the original potting soil mix. Keep the soil evenly moist but not sopping wet while they become established -- roots will not grow into dry soil.
Last but not least, remember geraniums like full sun and will languish in the shade. I hope this helps you trouble shoot, best of luck this spring!
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