Christine Blance, Trust Manager at the Gardens, explains.
We generally use a simple Top Crop > Bottom Crop > Legume Crop Rotation Cycle with a 4th Cycle of Green Crop used here and there. Green Crops such as Lupin, Barley, Mustard etc,,are used to improve soil nutrient levels and structure as well as protect against soil borne pests and diseases.
Top Crops here include leafy greens, brassicas and cops harvested above the ground, Bottom Crops are root crops ( carrots, parsnips, turnips, onions etc.,..) and Legumes are beans and peas .
The purpose of crop rotation is to conserve soil fertility – especially for intensive growing and to protect against pests and disease. It also reduces costs of inputs and is worthwhile to understand in order to keep alive our knowledge of traditional methods of maintaining soil fertility.
The process involves a continuous rotation of the crops raised on any given piece of land. The system we use at the gardens is illustrated in the diagram.
As usual there are all sorts of exceptions.
- Light top crop feeders such as lettuce, rocket etc., could go in after heavy top crop feeders such as brassicas.
- I often put in two quick lettuce crops one after the other in the summer – I just add a light dusting of blood & bone as a habit to all top crop plantings.
- Only one liming/year in the winter when there are more vacant beds available.
Key nutrients we are seeking to replenish are N, P & K – but also trace elements – Mb, Mg, S, Boron etc..
Obviously we can’t use some methods of crop rotation such as burning ( for potash) but can add untreated wood ash as a soil conditioner directly to the ground or into the compost system.
Animal manure is in my view essential at some stage of the cycle. Plants don’t easily get what they need from composted plant material alone.
Christine has been using this system at the Strickland St site since it was set up in 1999, so it works. For more information and variations try the Royal Horticultural Society’s website https://www.rhs.org.uk/advice/profile?pid=124