Joe, I have to ask. Where in Ohio do you live? Did you have the drought we have had in SW Ohio, or are you in part of the lucky north? Our rain deficit is near 14" now, and nothing did well here.

Karen

Usually, plants release allelopathic substances only while they're actively growing, and after the plants are removed, the allelopathy diminishes. So I'm guessing that it might be whatever else was in the soil -- were pesticides and herbicides used?

If you can get a free soil test from the local ag extension, do it.

Otherwise, I'd just add a whole bunch of good quality compost. Compost -- if you use enough -- can solve most soil problems on its own. Or, since winter is coming up, you can pile up compostables and do a version of lasagna gardening -- use whatever you have, garden waste, straw, pumpkins, etc.

Much research has shown that soils that once grew alfalfa can limit the growth of plants seeded in that soil compared to a soil that did not have alfalfa grown on it, allelopathic tendenecies. This generally lasts 4 to six weeks after the alfalfa has been turned in and digested, according to the research I've seen.

Research has shown that most of alfalfa's allelopathic tendencies are autotoxic - that is, the plant exudes a chemical that reacts over time with itself, not with other plants. Continuous cropping of alfalfa results in markedly diminished yields, as accumulation of the chemical (actually 2, ethylene and medicarpin) reduces the germination and seedling development of the alfalfa itself and apprently has little or no effect on other plants. Crop rotation is SOP for alfalfa farming.

Alfalfa is a very heavy nutrient drain however, and soils that have previously grown alfalfa can be deficient in both phosporous and potassium as well as of low pH. Low levels of P & K commonly produce stunting and poor growth. Testing to determine an accurate nutrient base from which to build is certainly in order.

Just as an aside, although I don't have anything to support this assumption, if alfalfa's allelopathic effects were that widespread with regards to other plants, it seems unlikely that it would be so highly regarded as a soil amendment or organic nutrient source or tea base.

Alfalfa does contain a natural plant growth hormone, triacontanol, that has the effect of boosting photosynthesis and increasing cell division, resulting in higher yeilds. Roses in particular respond very positively to triacontanol.

I think the difference here is probably more to do with the soil than with the alfalfa plant material itself. Alfalfa pellets, hay, etc make good soil amendments since the plant took up lots of nutrients. The soil left behind is depleted and perhaps there are effects that inhibit germination of seeds as well.