Plants utilizing C-4 photosynthetic pathways have high photosynthetic rates, and high water-, potassium- and nitrogen-use efficiencies. Decarboxylating enzymes in two of the three main types of C-4 plants are activated by manganese (Mn), with NAD-malic enzyme (NAD-ME) showing an absolute requirement for Mn. In high pH soils, Mn may be unavailable for plants uptake. Since all carbon fixed in photosynthesis by NAD-ME C-4 crops must go through Mn-activated enzyme decarboxylation, Mn nutrition may be especially important for sustained yields. Applications of Mn in forms readily available for plant uptake may increase plant tissue Mn content, plant photosynthetic activity and crop yields. Two NAD-ME C-4 crop plants; pearl millet (Pennisetum glaucum L.), and purple amaranthus (Amaranthus hypochondricus L.) were grown on a Mexico sandy loam soil, soil test; pH 6.9, C.E.C of 8.7, OM 1.35 %, P=106 and K=264 in 2006. Plants received 0, 4.5, 9.0 or 18.0 kg/ha of elemental Mn. Plant photosynthetic rates and leaf stomatal conductances were determined with a LiCOR-6400 photosynthesis meter. Manganese fertilization increased leaf Mn concentration, leaf photosynthetic rates and stomatal conductance. Manganese fertilization also increased seed yield and seed P, Mg, Fe and Cu content. Manganese fertilization of NAD-ME crops like pearl millet and purple amaranthus grown on high pH soils may be important for increasing both seed yield and quality.