Answer: You can, and probably should, use only potting soil in your whiskey barrel. Soil from the garden can become overly dry in a container, and hard to rehydrate. Garden soil can also be extremely heavy, especially when wet.
After filling your container, be sure to water it thoroughly. The level will sink as the water helps pack the particles together. Add more potting soil until it's within a few inches of the top of the barrel, then plant your tomato starts.
The term "deep watering" means applying water slowly so it has a chance to percolate down and wet the entire root mass of the plant. You can water deeply by building a basin around each plant and flooding it 2-3 times, or you can use a soaker hose or drip irrigation. The important thing to remember is that just sprinkling with a hose will only wet the top inch or two of soil, leaving the deepest roots without moisture.
There is no absolute guideline for watering. How much and how often you apply water will depend upon the soil, the temperature and the plant's needs. Sandy soil literally sucks up moisture. Clayey soils are slower to absorb surface water, but they hold moisture for long periods of time. The only real way to find out if you're applying enough water to constitute a deep soaking is to dig a hole and see how far the water has penetrated.
Container gardens present a slightly different problem. I water my containers until water flows freely from the drainage holes. As long as you keep the soil from completely drying out, watering is fairly simple. But, if the soil gets too dry, water will flow out of the drainage holes even though soil in the center of the container is still dry. To combat this problem I immerse my pots in a slightly larger container of water. When air bubbles stop coming to the surface I know the soil is thoroughly wet and all the air pockets around the roots have been eliminated.
With your tomatoes in whiskey barrels it will be impossible to set the barrels in a larger container to water, so you'll have to use your own judgement in whether you're applying enough water to throughly wet the root masses. You can dig down to make sure the soil is moist at root level, and you can apply water slowly so it soaks in rather than spills out through the cracks. Let your plants guide you in frequency of watering. If they look wilted, or the color is slightly off, water. If they look happy and robust, water thoroughly once each week.
Good luck with your new garden!
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