Soil pH is only an indicator of acidity or alkalinity. Soil pH measured in water changes according to the ratio of water to soil used in the analysis. Most labs in the US use a 1:1 water to soil mixture in which to measure pH. When the water: soil ratio increases, measured soil pH also increases.
Regardless of the water: soil ratio used, as soil pH goes lower, soil acidity increases. At about pH 5.5 aluminum in the soil begins to solubilize and becomes more available as aluminum with three positive charges, or elemental aluminum in the soluble form. As soil pH goes much below 5.5, solubilized aluminum becomes more toxic to plant roots, burning the growing point of these roots and preventing root extension into the soil for water and plant nutrient uptake. Solubilized aluminum also precipitates plant available phosphorus in the soil, making it unavailable to plants.
Leaving acidity at a pH 5.1 in the soil surface risks crop yield loss because of reductions in water and nutrient uptake. Also, a pH of 5.1 in the surface soil can have the effect of increasing subsoil acidity. You need to apply limestone to this soil regardless of what your local agronomist thinks. Lime this soil to pH 6.2 for most crops. If you should want to grow alfalfa, lime the soil to pH 6.8 to 7.0.
