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How do the major and minor elements in soil used for agriculture and even the home garden affect each other? For example, does too much potassium cause more aluminium to dissolve from the soil and in turn toxicity of the plant occurs?

I am incensed that you would put this topic in the "not quite science" forum!

There are several interactions between plant nutrients that can be considered:

1) Too much of one nutrient may block the uptake of another nutrient;

2) The deficiency of one nutrient may prevent a plant from utilizing another nutrient;

3) An imbalance of nutrients in the soil can change soil chemistry so that some nutrients are in a form unavailable to plants.

Aluminum becomes soluble in soils at fairly low pH, and K is not what soil scientists consider an "acidic cation," so I don't think the K vs. Al scenario you mentioned would make much sense. [Note to those with a clue about chemistry: soil scientists refer to certain cations as "acidic" and "basic," even though this idea does not fit into any accepted acid-base system.]

Some of the more well-known interactions include:

N & S -- deficiency in one prevents utilization of the other (excess of one may also result in a deficiency of the other);
N & K -- similar interaction as N & S;
Cl can inhibit N uptake, but this may be a result of too many salts in the soil;
High pH can render Zn, Fe and others unavailable to plants;
P and many of the micronutrients (like Zn, Fe, Mn, etc.) form insoluble compounds in soil, rendering both P and the metal unavailable, this is more of a problem in calcareous soils.

K may retard the uptake of Ca and Mg, but I've heard different stories on this. The mechanism for uptake is different for K vs. Ca & Mg -- K having active uptake, while Ca and Mg are taken up passively.

Surely this sort of thing is in your text books? All this is from memory, so I would double check any *fact* that I've posted here.

I took some botany/plant physiology classes too many years ago, but I could probably answer the acid/base question. Are you still interested?

I'd love to talk about this further, but for now let me just say that hosting a strong compliment of microbes will "buffer" a lot of changes in the nutrient status, and pH of the soil.

Have you seen the Terra Preta thread:
http://www.scienceagogo.com/forum/ubbthreads.php?ubb=showflat&Number=8943#Post8943
on amendments to marginal soils?

~SA

Plants can be grown Hydroponically in a solution of water containing the necessary plant nutrients, but soil does provide a plant with things like protection from adverse temperatures, nutrients, support, and a source of water.

Do you define dirt and soil differently?

I'm wondering if you could grow nitrogen fixing legumes hydroponically, or if they need the soil (bacterial interactions).