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Understand the soil triangle. Try a quick texture test. Prepare a soil sample for thorough testing. Mix ingredients for a jar test. Mark the jar as the soil settles. Improve your soil with compost or natural debris. Consider manual soil adjustment. Deal with soil compaction.
Soil scientists divide the particles that make up soil into three categories. Sand particles are the largest, silt is somewhat smaller, and clay particles are the smallest. The ratio of these three types determines the type of soil you have, described in a chart called the "soil triangle." For most plants, you'll want to aim for a "loam," or approximately a 40-40-20 mix of sand, silt, and clay respectively. Succulents and cacti often prefer a "sandy loam" with 60 or 70% sand instead. Pick up a small clump of soil, from beneath the top surface layer. Moisten it, then try to roll it into a ball and flatten it into a ribbon. This quick and dirty method can detect major problems, based on the following diagnosis:  If your soil ribbon breaks before it reaches 2.5 cm (1 inch), you have loam or silt. (If it can't form a ball or ribbon at all, you have sandy soil.) If your ribbon measures 2.5 to 5 cm (1–2 inches) before breaking, you have clay loam. Your soil could probably benefit from more sand and silt. If your ribbon reaches more than 5 cm (2 inches), you have clay. Your soil will need major additives, as described at the end of this section. If you're still not sure about your soil, you can find out more accurate information with twenty minutes of work and a few days of waiting. To get started, discard the surface soil, then dig up a sample of your soil about 15 centimeters (6 in) deep. Spread it out on a newspaper to dry, and remove all trash, rocks, and other large debris. Break up clumps of soil, separating it as much as possible. Once the soil is dry, add it to a tall, large jar until the jar is ¼ full. Add water until the jar is ¾ full, then add 5 milliliters (1  tsp) of non-foaming dishwasher detergent. Seal the jar and shake it for at least five minutes to further break it into fragments. Let the jar stand for at least a couple days, marking the outside with a marker or tape at these intervals:   After one minute, mark the jar at the top of the settled particles. These are sand, which settle first due to their larger size. After two hours, mark the jar again. By now, most of the silt will have settled above the sand. Once the water is clear, mark the jar a third time. Soil with heavy clay can take a week or two to settle, while more loamy soils may reach a clear jar after a couple days. Measure the distance between marks to get the amount of each particle. Divide each measurement by the total height of the particles to get the percentage of that particle type. For example, if you have 5 cm (2 in) of sand and a total 10 cm (4 in) layer of particles, your soil is 5 ÷ 10 = 0.5 = 50% sand. If you discover you already have loam, there's no need to alter your soil. Clay soils greatly benefit from mature compost, as described in the soil nutrients section. Other natural additions such as dry leaves or grass clippings serve similar purposes. Old, weathered wood chips, twigs, or bark will increase water and nutrient retention, both by creating soil pores and soaking up materials for slow release. Ramial wood chips or chips from small branches are the most nutrient dense when it comes to improving soil. Avoid new wood, which can reduce soil nitrogen levels. If you have clay-heavy soil (more than 20% clay) or extremely sandy or silty soil (more than 60% sand or 60% silt), you can mix in the other types of soil to achieve a roughly even mix of sand and silt, and no more than 20% clay. This can be labor-intensive, but is faster than creating your own compost. The goal is to create a porous soil that can hold plenty of water, air, and nutrients.  If you have a commercial composting operation nearby, you can purchase compost in bulk, typically by the truckload. You can use this compost instead of making your own. Note that you should only use sand that is salt-free and very sharp.  Perlite, available from garden supply stores, is useful for all soil types but especially for clay soils, essentially acting as extra-large particles. Keep foot traffic and vehicle traffic to a minimum to keep the soil aerated. If the soil looks dense or crusted on top, use a pitchfork to turn over the soil and break up large clumps. For seriously compacted soil, use a machine tiller, or plug holes with a lawn aerator. Even if water retention is not an issue, densely compacted soil can kill off beneficial bacteria and fungi, and encourage harmful anaerobic bacteria.  Mixing in organic material also helps, as described in the section on soil nutrients. Daikon or tillage radishes, dandelions, and other plants with long tap roots can help prevent clumping and compaction.  Alternatively, you can follow "no-till" or "no-dig" gardening techniques to leave soil undisturbed, allowing it to form as natural soil does over a few years. Minimizing traffic is still recommended for this method.