Answer: Unfortunately the best time to improve the soil is prior to planting, and that would be by loosening it as far down as possible to at least 18 inches and then adding copious amounts of organic matter such as compost, rotted leaves, or aged stable manure and bedding, for example. Any additional amendments would be determined by running soil tests and interpreting the results as needed. Another approach would be to use raised beds or to mound the soil in the beds slightly so that you increase the area of improved soil for the plants' roots to use.
Since you have already planted and probably don't want to dig everything up, there are two things you could do. One would be to top dress with compost on a regular basis. Thsi will help to improve the soil over time. The other would be to make sure to use organic mulches which will break down and feed the soil over time, too. The mulch will also help keep the soil moister and cooler during the summer as well as help keep weeds from growing in the first place. Try to keep about a two to three inche layer of mulch at all times. Topping it up occasionally will have the added benefit of disturbing any weed seeds that might have blown in and be getting ready to germinate.
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