Answer: There is growing evidence that it is important to have a healthy microorganism system in the soil for optimum plant growth. Whether adding these microorganisms will help is still in the research stages. If your soil is in good shape already, with lots of organic matter and healthy, abundant soil life, then it's likely that these microbes are already there. If it is in poor shape -- compacted, overtilled, etc. -- then the addition of these microorganisms might be of use, but only if you improve soil at the same time to provide them with a hospitable environment.
You can certainly experiment with the products, but I would focus my efforts on adding organic matter to the soil, such as compost, shredded leaves, grass clippings, etc. and on making sure you don't overtill, till when the soil is wet, or compact the soil.
The one exception would be inoculants for certain legumes. Leguminous plants form a symbiotic relationship with certain microbes to "fix" nitrogen. If those microbes are not present, they cannot do this. Since the microbes are very specific, adding them to the soil at planting time ensures they are present. However, many gardeners never add inoculants, and still get fine yields.
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