Gardening is expensive, what with buying all the plants, pots and potting soil. You can wind up spending the equivalent of your holiday budget every season if you don’t watch out. There’s one way to save on potting soil, at least, and it benefits the environment, too: Mix your own substrate. Start off by taking either garden soil or unpolluted topsoil as your basis. This should make up around 50% to 60% of the mixture. You can also use the soil from last year’s pots or window or balcony boxes as a basis. Then add compost – either your own, free from the garden, or something you bought cheaply …