"naturally rich" is correct and usable in written English.
You can use it to describe something that is bountiful or abundant in resources, materials, or qualities. For example, "The soil in the garden is naturally rich and fertile so plants grow easily."
While Congo is naturally rich, Rwanda is perennially poor.
The company claims its "night milk crystals" are naturally rich in melatonin.
Can you get enough from foods naturally rich in these nutrients or fortified with them?
But how does it compare with a classic French chicken or veal stock, also naturally rich in umami?
One of the world's longest-running conflicts, Angola's war has laid waste to a naturally rich and beautiful country.
Like common flax, or linseed, its seeds are naturally rich in the plant version of short-chain omega-3 fatty acids.
For a start, many of the world's major cities are located in prime locations that are naturally rich in species and still support a significant number.
The only naturally rich dietary sources are oily fish from the sea like salmon and mackerel, egg yolks, liver and fish liver oil.
Volcanic ash is a naturally rich fertilizer, and the campus is overgrown with plants, flowers and trees that have earned the campus its status as an environmental reserve and one of the "lungs" of Mexico City.
The reigning assumption in this nation, which sits atop some of the world's richest oilfields, has been that Venezuela is naturally rich and if it is getting poorer it is because someone in power is stealing.
Stinging nettles are my favourite plant: they're really rich in vitamin C, they're naturally rich in antihistamines, and I make rope out of them – they're a great string-maker, with fibrous stalks.
