Crop rotation: by season instead of by year?

Question

I have 3 raised beds in a small 30 m2 greenhouse, this is the first year I have it so I don't have much experience. I've been reading about crop rotation, and was wondering why you can't or shouldn't plant the same crop every year. Can't you just rotate the crop each season?

For example, in one bed, I have 16 plants of tomatoes, plus some radishes and arugula in another row that I didn't harvest so I can get some seeds. Lets say when the plants stop producing and start dying, I plant (just an example) some onions, garlic, or whatever would grow well here in the autumn/winter season (pretty dry but not too cold).

I would also add a few cubic meters of compost before spring. And btw, the raised beds are on top of the native soil. I made the retaining walls with wood and filled them with the same soil I got from leveling the terrain, plus about 1/5 of compost. There's no separation or insulation on the bottom.

Wouldn't this already help produce a healthier/more fertile soil since I already rotated the crops on winter plus amended the soil with compost?

I'm asking this because the tomatoes bed seems to be perfect for them. They are growing so big and have a lot of fruit, the other two beds receive a bit more shade so I don't want to end up with a bad tomatoes crop next year if I move them to a different location.

Answer 1

The thing with tomatoes is not so much rotation for nutrients, but rotation to avoid disease.

A complete cycle of the seasons is better for defending against disease than just one season. If you plant the same thing in the same bed for the same season it grows best in, chances are diseases and pests "know" that.

So you skip an entire year, to make the diseases and pests "think" there aren't tomatoes there any more.

But what the heck? Give it a try, and see what happens. You do keep records, right? (If you don't, you'll never know!)

Answer 2

You can certainly grow several crops in succession in one year, exactly the same as gardening out of doors (assuming your climate is suitable, of course).

But many diseases that are specific to particular plants have evolved to survive in the soil until the next annual growing season for those plants, so you need to break that cycle by not growing the same thing every year. This doesn't only apply to tomatoes, but also to things like club root virus affecting brassicas.

Also some crops like peas and beans add nutrients to the soil by fixing nitrogen from the air, so you want to rotate them to let other crops benefit from that.

As a rule of thumb, don't grow any crops that are annuals (the plant grows and dies within one year) and have a long growing season (say 12 weeks or more) in the same place two years running. Don't worry too much about quick growing plants like salad crops. Perennial plants obviously stay in one place for as long as you want to keep them - eventually they will start to produce less, but whether "old age" starts to kick in after 2 or 3 years (for example strawberries) or 100 years (for example fruit trees) depends on the plant species!