Rick, its been a while (in the 60s), but if I remember correctly, it came out of an old rose book at the time. Also, I believe it was printed on the side of the boxes (the old orange and blue cartons of REXALL drug stores back then and the recommended use was 8 ounces/gallon for every 4 sq. ft. of rose space. Now, roses and lilies are two different characturs and i'm not sure if Mg in excess can be toxic with lilies, but I do know that it is one of the quickest depleting essential elements in a lily garden--because we like to use fast drainage soil, burn our stems and so on, as is the GOOD POINT you made about assumed Mg loss and the 3 in. rainfall.
Plants must consume an awful lot of it because its at the center of every chlorophyl molecule and the instant there becomes even the slightest deficiency of it--the yellowing shows up, especially in the lower, older leaves first. Mg is highly mobile within the plant itself and it will pull Mg from the older bottom leaves in favor to the younger more productive ones when needed so when one sees temporary chlorosis in the top leaves--it often goes away because of plants ability to utilize its resources efficiently. Brian talks about Casa Blanca--Thats a pretty big plant. A group of three or four might consume a pretty good amount. My experience with Casa Blanca is that its very prone to a blochie (sp) appearance because it grows so fast that all the necessary ingredients of 'green' can't keep up.
But Mg deficiency is not the only cause of Chlorosis. As Rick stated, Iron (Fe) deficiency is the next big one, although thats not lost as fast in a garden as Mg is. Edit added: pH,too, plays an important role with the uptake of many essential micro nutrients, but most problems we solve unknowingly by adding good compost. Chlorosis can be very complicated and complex with many causual factors or as simple as a newly planted bulb that is having a difficult time adapting to its new setting while the one right next to it is 'just lovin it'.
Rick, your right--it's epsom salt, not epsom salts. I think the two are used so interchangably by the general public that I even I go back anf forth without even realizing it anymore.
