I have read that tomatoes should be placed in a different area each year, due to the risk of diseases building up in one spot of soil.
Apparantly, this also applies to potatoes. Are they interchangeable (meaning that I shouldn't put tomatoes in the spot where last years potatoes were)?
So I am wondering how many different areas are needed? Meaning, How many seasons/years does a previously used spot need, before it is safe to put tomatoes there again?
If I rotated around three areas, would that be enough? Or four?
Is there anything else that shouldn't go in last season's tomato area?
Absolutely. Don't put tomatoes where potatoes were grown last year and vice versa.
Four would be ideal. There are all kinds of websites and books out there that describe the proper four crop rotation techniques.
You can start here:
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click on the How To Guides and choose Crop Rotation. That'll give you the basics. I agree with his advice on potatoes. Keep them separate altogether. I do that by planting them in containers with new potting soil every year. The used soil goes onto the lawn in the fall. Apparently, old potato soil is welcomed in the lawn. (Don't try the tire technique he mentioned - the spuds only rot in the tires. You can do the same thing by building a raised bed from wood or cinder blocks).
It depends on soil, moisture, high and low temperatures. It depends on whether you pull the plants after the season or leave the roots in the ground. If your soil is sandy and freezes in the winter, you can rotate a lot less than if you have a soil that stays moist through the year. It depends on the general health of the soil (compost, microfauna, etc.). It depends on the disease. Cabbage clubroot can persist 15 years.
Currently, I have a 1.5 years rotation in my tomatoes (that is, I rotate some spots, and I reuse some spots) and I see no diseases. I do the short rotation because I want the best possible access to the cages, other pick-once vegetables occupy the more remote parts of the garden. I use drip irrigation, mulch, and I make sure that wayward branches are pulled into the cage as soon as possible, all practices that cut down on disease. My soil is very sandy and, if not mulched, is dry at the surface within an hour of a rainstorm. I only saw a few diseased tomatoes a few years back, all Costoluto heirlooms, planted in a spot that had never grown tomatoes before. I have since learned that this particular heirloom has become very disease-prone in the last decade or so. The other heirlooms thrive.
So, in regard to your problem, you will have to see what works in your case. There are other considerations that can shorten your rotation. Is this spot more limed than another? Is this spot sunnier? Then lime-loving and sun-loving veggies will go there more often. I have two spots in my garden where the same veggies are grown every year. The rest I rotate. Generally, in rotation you try to have plants as unrelated as possible so that diseases can not thrive on successive years. Potatoes and tomatoes are both solanaceae, and likewise you should not plant cabbage and tatsoi (brassica and mustard) in the same place.
On the other hand, being bestowed with a profound ignorance of growing tomatoes, I have grown tomatoes in the same spot in amended clay soil, for eight years and I can't discern any difference in vigor, or fruit.
And though I would prefer to rotate to lower risk of diseases, I have very limited space that gets enough sun. So my amended sandy soil has had tomatoes in roughly the same spot for 20 yrs.
I plant only disease resistant plants & haven't had any wilt, fungus, or critter problems. [this year I stole some lawn far from the 'garden' to plant some Brandywines-- If I like them I'll probably keep planting them there until trouble shows up-- then I'll find another 8x8 spot.
We're blessed with hard winters here & that might mitigate some things somewhat. Or maybe I'm just lucky.
Count your blessings. You are both lucky and blessed. You didn't say were you live but for the majority of us tomato growers we have continual problems with blight and other diseases.
what is the best way to ammend soil each year and avoid disease. I'm in Minnesota so the ground definately freezes, and I'm limited to the number of boxes I have. If I was to rotate, what's the best veggies to follow tomatos?
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