A nutrient index quantifying productivity of agaves and cacti.
Plant productivity is influenced by water status, temperature, and photosynthetically active radiation, the effects of which provide an environmental productivity index (EPI), and by element levels in the soil. Based on previously determined growth responses of four species of agaves and eleven species of cacti, individual multiplicative indices are proposed for soil levels of five elements (N, P, K, B, and Na) to give a first approximation of the influence of these elements on the productivity of agaves and cacti; the product of the N, P, K, B, and Na indices was termed the nutrient index (NI). The rate of leaf unfolding from the central spike of folded leaves, which is highly correlated with EPI and productivity for agaves, was determined for Agave deserti at two sites in Arizona with similar values of EPI; leaf unfolding was 73% higher at the site whose NI was 74% higher, primarily because of a 47% higher nitrogen index. For Agave tequilana at ten sites in Jalisco, soil levels of N, P, K, B, and Na each varied 3-fold, leading to a 24-fold variation in the NI. These variations were positively correlated (r2 =0.96) with the more than 2-fold variation in the annual rate of leaf unfolding. Although the proposed NI is only an estimate of edaphic influences on productivity of agaves and cacti, it satisfactorily explained the variation in productivity with soil element level at various site for the two species tested.