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Mix in compost, compost andMix in compost, compost and more compost. Lots of recycling centers have compost for free. And if you haven't got enough compost, just add any dead organic material as it becomes available (like dead leaves as a mulch, some dried manure etc). It will eventually break down, and improve the soil quality. In terms of your neighbor's cats in the garden: Some readers say to try spraying a solution of a little cayenne and water.  It's nontoxic to the animals, doesn't hurt the plants, cats hate the smell. Reapply after rain. You could also try placing moth balls around the edges of your garden. There are also commercial cat repellents that you can get at garden centers or online at pet supply stores and gardening supply centers. Make sure that the repellent that you get is safe for any children, plants, pets, or other animals etc. that may be around. 

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