First off I don't think you can assume anything. And, if you don't know what was put there, you have no way of knowing how long it will be there.
Try gardening in another area.
Ann

Ann is right you can't assume.
The big question is - is anything growing there now?
I think I would give it a try. Try some radishes or some of fast germinating cool weather plant. Have an established plant you are willing to risk? Even moving something already in the yard.

If it doesn't work you will know to look for another spot.

How long will depend on what was used. Some products simply stay in the soil for much longer than others. If plants are growing there again that soil may be safe to dig into and look closely at and adding lots of organic matter might aid in dissipating what was used.

I would dust the area with compost and organic fertilizer, then cover it with several inches of a chipped tree limb mulch, then put a soaker hose set at a trickle for a couple weeks. That process will get the soil microbes going better than anything else, IMHO. The soil microbes will do the job of remediating the poisons. They cannot get rid of heavy metals, but other poisons should decompose in a healthy soil.

joyfuljan: Why not ask the landlord what he used to "poison it"? (And, I agree with you, that is about as idiot a statement as you'll generally hear.) Of course, organic certification requires the land be free of chemicals for a period of three years (whether this satisfies YOU is a matter of choice). My guess would be that he used Roundup, or other similar herbicide--every chemical-head's choice for "spraying down weeds". Conventional "no-till" growing begins with doing such.

Try planting a couple of bean seeds -- they're especially sensitive to Clopryalid (sp?), a common herbicide that's popping up in municipal composts as it's being widely used by contractors. Otherwise, getting the microherd going, as dchall said, is brilliant and will most help, I think. Also, planting daikon radishes (deep deep roots) will help add organics to the soil; tring wormies might help too.

I realize I may be treading on toes by bringing this up in an organic gardening forum, but most listed herbicides breakdown in the soil rather rapidly. Even RoundUp, perhaps the most commonly used herbicide by homeowners, has a soil half-life of only a couple of weeks. One of the few exceptions, chlopyralid, has already been mentioned and its residual effects target only certain plant families - doing the very simple bioassay of planting a bean seed will confirm its presence in the soil. But it is used primarily in weed 'n feed formulations and is not marketed residentially as a broad spectrum herbicide.

Otherwise, if the "poisoning" took place two years ago, you can be relatively certain that the soil is now free from chemical contaminates that would hamper plant growth. Now how you feel about the long term effects of these products in the soil, their influence on the current biomass and your own personal organic philosophies will determine how you proceed. As mentioned, adding compost will go a very long way in remediating any remaining soil problems, if any.

In addition, many of us have eaten plenty of stuff from the stores that has been herbicided/pesticided.

Sure, ask him what he used and how much, but don't sweat it. Chances are good that you've been exposed to far worse at some dinner party or other where the hosts used conventionally grown produce. But we don't worry about that! And rightly so!

I'm sorry, Gardengal, but Round-up's toxic affects (more important IMHO in this case than its halflife) lasts a lot longer than a couple of weeks. Some tests have shown halflives of up to 200 days with persistence occuring up to 3 years.

http://www.mindfully.org/Pesticide/Roundup-Glyphosate-Factsheet-Cox.htm

"A report from The United States Environmental Protection Agency states that Glyphosate is 'extremely persistent under typical application conditions'. It is one of the most residual herbicides, with studies in Sweden showing that one application can last up to 3 years. "

"Two separate studies in Sweden have linked exposure to Glyphosate to Hairy Cell Leukemia and Non Hodgkins Lymphoma. These types of cancers were extremely rare, however non-Hodgkins lymphoma is the most rapidly increasing cancer in the Western world. It has risen by 73% in the USA since 1973. Another study has found a higher incidence of Parkinson disease amongst farmers who used herbicides, including glyphosate.

Other studies show that Glyphosate and commercial herbicides containing Glyphosate cause a range of cell mutations and damage to cell DNA. These types of changes are usually regarded as precursors to cancer and birth defects."
http://www.geocities.com/opaq2001/glyphosate.htm

Just because something's on the shelves and has been around for a couple of years doesn't mean that it's benign. Cola has just been proven to reduce bone density in women! Instead of rationalizing unnecessary exposure, trying to reduce exposure to bad things is always a good idea.

There is a great deal of misinformation being spread over the internet, the Cox article, a non-peer reviewed and highly misleading piece, being one of the most frequently repeated. Exposure to certain pesticides in large enough quantity over long enough time can be problematical, but it's important to base statements on fact, with specificity as to what pesticide is being discussed, and with a good understanding of all the evidence available. Non-hodgkins lymphoma has not been generally linked to Roundup.
Wikipedia citation: "The US Environmental Protection Agency, the EC Health and Consumer Protection Directorate, and the UN World Health Organization have all independently concluded that glyphosate is not carcinogenic. Despite this, opponents of glyphosate-based pesticides frequently claim that glyphosate is linked to cancer, citing the research of Hardell and Eriksson (Cancer 1999 85:1353Â1360). Although this paper showed a link between glyphosate and lymphoma, this link was not statistically significant and was within the realm of random variation. The authors themselves concluded "definite conclusions cannot be drawn for separate chemicals, such as MCPA and glyphosate, from the multivariate analysis"".
In fact, Roundup has been classified as showing evidence of non-carcinogenicity. This cannot be said for approximately half of all known substances, natural or synthetic.

As for its persistence, halflives of 200 days are found in biologically inactive soils, such as those frozen for long periods of time. Gardengal is correct in stating that typical halflives for temperate climate soils are measured in weeks. Roundup will break down completely into natural substances.

Sometimes when I drive into work and pass 100+ acre "brown" fields (which were green the previous day) I have to wonder what's down in the shallow wells that surround these fields? This practice of herbicide nuking is an annual event and I'm glad that my well isn't nearby. Good luck with your decision joyfuljan.

The manufacturers of these products are the perpetrators of many myths themselves. What they test is the active ingrediant, not the product that is sold to you and when since all of these products have an active ingrediant and "inert" ingrediants which together react in a synergetic way and produce something much more potent, the place to look for information about a product is not the manufacturer but an alternative site such as the Northwest Coalition for Alternatives to Pesticides.

Here is a link that might be useful: NCAP

I worked for Chiron, emeryville campus. We were doing testing on drugs for non-hodgkins lymphoma treatments. While some studies posted on the web say "safe" and some studies say "not safe" and most literature says "inconclusive", the majority of our test and control subjects were people with pre-existing stage 2 or 3 cancers, mostly farm workers. All of them listed round-up or the commercial versions thereof.

"inconclusive" means just that. 3/4 of a cup of the stuff will kill you. Even if it doesn't do a damn thing, what about interactions with any of the other 200+ new chemicals we have in our bodies? Those haven't been tested.

Those people were in a lot of pain. Most of them came to us as a last resort. NHL incidence is up 700% in the US in the last 5 years, mostly in farm workers. Let's not confuse 'inconclusive' with 'safe'. Sorry to ruffle feathers.

Do a websearch into terra preta, bio-char & agri-char. Also you'll find several videos on YouTube... Just as charcoal absorbs toxicity and even heavy metal poisoning from human body, so charcoal helps soil detox and thrive. There are a number of companies now working with this approach, learned from ancient Amazon peoples. Also it's something "the little guy" can do-it-yourself at home to offset modern poisons.