The optimum soil pH range for forages encompasses various contributing factors. Legumes have a higher pH requirement than the grasses. The legume requirement for a higher optimum pH is due somewhat to the availability of micro-nutrients but is more strongly related to the rhizobia activity in the root nodules. The rhizobia have a higher pH requirement for nitrogen fixation than the plant has for growth. Legumes also tend to be more sensitive to aluminum toxicity which is reduced as the pH increases. Within grasses, the warm-season grasses are more tolerant of low pH values than the cool-season grasses.
One can cover a lot of ground, or perhaps a multitude of sins, with the following generalizations:
Legumes -- aim for 7, keep it above 6
Legume/grass mixtures -- aim for 6.5, keep it above 5.8
Grasses -- aim for 6, keep it above 5.2
One should keep in mind that although the warm-season grasses grow well at a lower pH value than legumes and cool-season grasses, there are other reasons to maintain a pH of about 6.0. Some of our problem weed species are much more competitive at lower pH values. Also, over-seeded winter annuals, especially clovers, require a higher pH for optimum growth and production. Finally, remember that nitrogen fertilizer is a major acidifying force in pastures and that high nitrogen rates can rapidly decrease the soil pH.
