The citrate buffer has a range of action from pH = 3 to pH = 6.2. The calcium cation binds very poorly to the citrate, and chelation is also very pH-dependent. Soil is a buffer system. Humic acids complex calcium cations, and this is also pH-dependent. Just measure the pH after mixing with the buffer and everything will be explained. If the pH is significantly different than expected, you need to increase the buffer capacity of your citrate buffer.