Nullis
Moderator
I beg to differ. All rain water I have collected tests between 003 and 011 ppm after falling from the atmosphere, flowing down the roof, through the gutters (filled with debris) into the bucket. I don't entirely disagree that when it rains mineral deposits which are scattered and concentrated throughout the soil surface dissolve and/or flow where they might be more useful, but I strongly disagree with the second statement I put in bold. Minerals/trace elements should not cost a 'small fortune'. There is always molasses (they make it specifically for plants, too) which has an exceptional mineral content and can be used in all water applied for that purpose as well as to lower pH... then there is Earth Juice Microblast which costs maybe $15 for a good sized bottle and lasts quite a while.
On another note, those cheap pH/moisture meters totally suck. They are not typically accurate and always tell you that your pH is 7 or something. Run-off probably isn't the best indicator, either. For one thing you have to water properly, and slowly, to ensure that what comes out isn't the exact same thing that went in. You also have to realize that if you are watering properly what comes out could be quite different than what is left in the soil solution.
Good soils have cation exchange capacity, and straight sphagnum peat has some CEC as well but something like humus has a much higher CEC. Cations are ions meaning they have an electrical charge, cations are positively charged (anion is negative). Most plant available nutrients are cations (Ca++, Mg++, K+, NH4+, Cu++, Fe++, et al.). Humus, clay and peat to a lesser degree have oodles of negatively charged 'ports' that attract these cations and hold onto them so they don't wash away. However, while humus and clay like cations they consequently push the anions away.
Therefore cations would seem to have a much better tendency to stick around in the soil, whereas the anions are more likely to wash right out of the soil (out with your run-off) with the exception of phosphates which have a tendency to react with other things in the soil and precipitate from the solution yet remain immobile.
Once you understand what CEC is about and realize the Ca and Mg are cations you might begin to see why it works so well in keeping pH up around 7. Many seem to think the lime is a buffer (because that's what they heard) but really it's the soil/humus/media with appropriate CEC. When properly limed the media should resist any significant changes in pH in part because its exchange sites are saturated with it. When more acid (H+) comes along it gets exchanged with the Ca++ and Mg++ from the lime.
Back to checking pH, I only test run-off once in a while to make sure it isn't way way off. There are also inexpensive testing kits that come with capsules which are specifically for testing soil.
