Rainfall washes out, or leaches nutrients from the soil, thus lowering soil pH (called power or potential of hydrogen). Arizona receives very little rainfall, and so nutrients – mostly “salts” – remain in the soil to the point that there is too much salinity, and plants can’t grow.
Calcium is “the foundation of a good feeding program” (JR Mittleider), because it’s the main ingredient in plants’ cell walls, and so adding calcium improves soil fertility, and in doing so raises soil pH. There are two main types of calcium fertilizer, lime and gypsum. Lime raises soil pH, and is used in growing conditions having more than 20″ of annual rainfall. Gypsum does NOT raise soil pH, because it contains almost the same amount of sulfur – which lowers pH – as calcium, and so is used in growing conditions that receive less than 20″ of annual rainfall.
Gypsum comes in different particle sizes, from powder to “pebbles”. The size of the particles determines the solubility, and thus how quickly it is available to the plants. So, if you want/need instant availability you choose powder, but if you want availability over a long time you choose pebbles. 
