Allen and others,
Colorado River Water is notoriously high in bicarbonates and carbonates, namely in association with Calcium. Without going into much detail, these are chiefly responsible for the high pH or alkaline condition - which can be a real problem at the levels you're describing. Another concern with these compounds is their tendency to clog irrigation components like low flow emitters. For example, as irrigation water dries around these orifices, insoluble calcium carbonate (lime) will precipitate in deposits. Adding an acid to your irrigation water will help combat this alone.
I would shoot for a pH between 6 and 7. This is the range at which - in most soil conditions - the majority of 17 plant essential nutrients are present in forms usable to plants, and the range in which micronutrients and other metals are least likely to pose toxicity. But even at 7.5, the nutrients most likely to be deficient, N-P-K, are little changed. There is direct toxicity, and reactions with other elements that occur at more extreme pH values. Iron, for example, becomes highly soluble at pH values below 4. This translates to more iron present in solution for reaction with other elements. At low pH, Iron is more likely to form insoluble complexes with phosphorus, making that primary nutrient less available.
At high pH, Ca and Mg does the same with phosphorus.
Buffers are compounds or materials that sponge potential acidity or alkalinity, thereby preventing wild swings in pH. A mineral soil, with some clay present, can act as a good buffer in a growing media.
I would not recommend adding more or less fertilizer to compensate, as this may lead to a host of other nutrient problems and issues with excess soluble salts. Treat the disease and not the symptoms. That being said, there are some acid and base forming fertilizers. Changing the pH with an injection system is easy, as long as you choose equipment that's resistant to acid corrosion. The most common acids used are phosphoric (at Rancho Soledad), urea sulfuric, citric, and acetic(vinegar). I use household vinegar now, but am open to change. The rates will vary by acid and starting pH of your water. Keep adding small doses until you reach the desired pH range.
As far as agaves and aloes are concerned, most should do fine in the 6-7 range, though some of the limestone species could tolerate higher. Most grow on raised slopes where the conditions that lead to severe alkalinity are mitigated. Even field tested soil of Agave utahensis growing in limestone read 7.8. I think calcium availability is most important for these types.
To answer your question on seeds; seedlings will generally tolerate higher acidity than mature plants, and will tolerate fewer soluble salts. Germination could also benefit from fewer soluble salts.
Blah, Blah, Blah...
Matt
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