If you're planning on seeding new alfalfa into ground that has been in row crops for many years, it’s wise to get a "special" soil test before planting.
New fields of alfalfa often are planted where alfalfa has not been grown for many years. Seedlings usually emerge well, but sometimes growth slows and they can begin to appear yellow. That's because the surface layer of many of these fields has an acid pH. While this acid layer may only be a few inches deep, it can be severe enough to reduce the ability of alfalfa roots to absorb nutrients from the soil. More importantly, it can prevent nodules from forming on the alfalfa roots. When this happens, seedlings are unable to produce their own nitrogen and become nitrogen deficient and unthrifty.
To avoid this problem, before planting gather two types of soil samples — one at a normal 7- or 8-inch depth and one only 2 inches deep. Have a soil-testing lab run all the usual tests on the deeper sample — phosphorus, potassium, pH and so forth — and test the shallower sample only for pH.
If the pH of the 2-inch sample is below 6.2, you need lime. If the pH of the normal sample is above 6.2 and more than one-half point higher than the shallow sample, you need only about half the usually recommended amount of lime because your subsoil contains much less harmful acid.