I have a liming question. Last year was my first year gardening since I was a kid. The soil started out with a pH of 5.5, I added the recommended amount of lime to get it to 6.5, but when I tested it recently prior to planting for this year, it came back at 8.0. So what did I do wrong? And how do I fix it?
The soil is a silt loam with about 1/3 sand, 2/3 silt, and only a negligible amount of clay. When I started last year, I measured the pH myself several times at 5.5, using a Nasco kit; this number was later confirmed by a professional soil test from the local state university.
I used Pro-Select Prill Limestone and followed the recommendations on the side of the bag. When the starting pH is 5.3-5.6, and the target pH is 6.3-6.5, these call for 65 lbs of CaCO3 per 1000 sq ft for sandy loam. (I have silt loam, but I followed the recommendations for sandy loam, which call for the lowest amounts of lime except for very sandy soils.) This works out to about 11 lbs of CaCO3 for my 175 sq ft of beds, or 13 lbs of prill limestone (which is 87% CaCO3). I actually added only 7.5 lbs, or a bit more than half the recommended amount.
The only other things that I added were ~1/2" of compost with a tested pH of 7.0, alfalfa meal, phosphate, and crushed granite. Additionally, I was just looking at a test of our well water from a couple of years ago, and its pH is 8.0 (has high levels of calcium, too).
So where did I go wrong? Is it possible that one season's worth of watering with pH 8.0 water could have affected the soil pH so much? Should I have made some allowance for the pH-altering effect of the other soil amendments? If so, how does one go about calculating that allowance? Any other ideas?
Are you confident in you sample? How big of a sample did you take, how did you select the portion used for the test, how many tests did you do? Were both samples dry? Was the soil to liquid ratio constant? A jump from 5.5 to 8 at your application rate doesn't sound accurate, and pH is not necessarily homogeneous throughout the soil.
The pH of soil solution is only a small fraction of overall acid/base balance in the soil compared to CEC and mineral reserves. I am not confident that irrigation with pH 8 water could so quickly change your cation saturation in the soil, but it might affect your test results, since the test likely uses a water extraction of the soil, maybe if you took the sample after an irrigation--but I have no experience with hard groundwater. None of your other ammendments suggest pH change to me.
Paul Cereghino- Stewardship Institute Maritime Temperate Coniferous Rainforest - Mild Wet Winter, Dry Summer