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Onions should be planted in a sunny location. That means that the area doesn't get any shade from other plants, including trees or your house.  The best way to figure out which location is sunniest in your yard is to monitor it. Spend a day going outside every couple of hours. Notice which areas are sunniest throughout the day.  When choosing the location, pick the one that has the most sun for onions. Raised beds are structured beds with soil in the middle. They are usually bordered with wood, concrete, or wood bricks. They raise the surface of the bed above the ground.  Start by measuring out your bed. Four feet by four feet is fairly standard, as most people can reach to the middle of the bed. Make the ground even with a shovel or rake.  Get the wood you'll need. You'll need 4X4s as stakes for the corners. They should be a foot each. You'll also need 2X2s for the center stakes, four of them. Finally, you'll 2X6s for the sides. They need to be 4 feet long, and you'll need 8 of them.  Lay out your 2X6s on the square. Start by screwing one 2X6 into the side of the 4X4 post, flush with the bottom and the outer edge. Place another 2X6 on top of the other one in the same position but further up the post. It should be flush with the top and right up against the first one. Screw it in.  The next set of 2X6s should be added so that they go to the edge of the post but cover up the ends of the other boards. In other words, the first boards should run into the second, with the post sticking on the outside rather than the inside. Continue around the square until all the boards are joined. Once you've done that, measure across the square diagonally to make sure it's all square. If it's not, adjust the square by moving it slightly.  Add the other stakes. Hammer them into the ground up against the middle of each wall on the outside, then use deck screws to attach them to the wall. Finally, add in soil. Another option is raised rows. Raised rows are like raised beds, but they aren't structured like raised beds. Instead, you create them with just the soil.  When the soil is dry, spread biodegradable material on top of it, such as compost or rotting hay or grass. With a garden rake or rototiller, work it into the soil until the soil is loose and crumbly. Stake out how you want your rows to go. You want them to be about 4 feet wide or a little shorter if you can't reach to the middle. Between the rows, you need space to walk. If you want room for a wheelbarrow, make the in-between part at least a foot to two feet wide. Create raised beds by moving the soil from the paths into the raised beds. A rake is good for this purpose. You may need to use a shovel near the end. To keep the paths from growing anything, line them with newspaper (5 pieces thick). You can also use cardboard. Put mulch or wood chips on top. You can find soil kits at your local gardening store, or you can take a soil sample into your county extension office for testing. Your soil should be at between 6 and 6.8 pH.  You may need to adjust the pH of the soil. If you need to make it more acidic (down towards 6.8), you'll add powdered sulfur, aluminum sulfate, or iron sulfate. To up the pH (make it slightly more alkaline), add lime.  If you need help deciding how much, use a pH kit from your local university agriculture extension office, which will give you detailed instructions about what to add. Find your local extension at http://nifa.usda.gov/partners-and-extension-map?state=All&type=Extension. Onions need nitrogen to grow well. Therefore, always add nitrogen to the soil before planting. You can also work nitrogen in the soil in the fall to help for the next spring. The easiest way to add nitrogen is to use a fertilizer rich in nitrogen. Good sources of nitrogen include fertilizers, composted manure, blood meal, and other composted organic materials.
Find a good location. Try raised beds. Create raised rows. Check your soil. Add nitrogen.