How often should I change the soil and gravel mixture in my tomato ring culture system?
Bill McLeod, by email
Stefan says: A ring culture bed comprises a trench dug in the greenhouse floor, approximately 45cm (1½ft) wide and deep, lined with a heavy gauge plastic sheet and then filled with gravel. On top of the gravel stand bottomless plastic pots, 20cm (8in) in diameter. The bottomless pots are filled with John Innes No 3 potting compost and the plants potted in them in the usual way at six weeks of age.
Once they’re established, tomato feed is poured into the pots while the main watering of the plants takes place in the gravel. But, you have undermined the value of the method by having a mixture of gravel and soil in the ring culture bed. This will allow wilt fungi to develop and these could bring about a failure of the crop.
Dig it out and replace the mixture with gravel alone.