ABSTRACT
Acid soils constrain agricultural production on perhaps 25% of the world’s crop-lands [1], including 101 million ha in the semiarid tropics and 262 million ha in the subhumid tropics [2]. Acid soils cover more than one third of sub-Saharan Africa [3], and Latin America alone includes more than 500 million ha with a soil pH less than 5.0 [4]. Stress under acid soil conditions may be due to hydrogen ion concentration per se or to deficiencies or toxicities of minerals that result from low pH. This chapter focuses on hydrogen ion effects and specifically addresses constraints that acid soils place on symbiotic relationships between plants and soil-dwelling microorganisms. These relationships are important to agriculture because they provide the plant with necessary nutrients that are often deficient in soil.
