Learn English – What does the proportion 1:3 mean in the following example

The Oxford Dictionary says:

The basic ingredients are limestone and clay in the proportion 2:1.

In the proportion of 1:1, though not used commonly, is correct grammatically. You can also say in equal proportions, in the 1:1 proportion, or in the proportion 1:1.

I think in the 1:1 proportion and in equal proportions sound more idiomatic.