A pot-culture technique was used to calculate relative yields for N, P, K, and S as an index of availability of these nutrients in soil from under mesquite [Prosopis juliflora (Swartz.) DC] trees and from adjacent open areas on a desert grassland site. Arizona cottontop [Trichachne californica (Benth.) Chase], plains bristlegrass (Setaria macrostachya H.B.K.), and bush muhly (Muhlenbergia porteri Scribn.), all native perennial grasses which grow abundantly under mesquite, were used as test plants.
Relative yields for N were up to 15 times higher in soil under mesquite trees than in nonmesquite soil, whereas the level of total N was only three times greater. The soil under mesquite trees apparently provides a more favorable environment for supply of N than nonmesquite soil.
Relative yields for S in mesquite soil were three times greater than in nonmesquite soil and were proportional to the difference in total S between soils, indicating that S is supplied more slowly than N under mesquite trees.
Phosphorus showed a slight, although significant, difference in availability between soils, but total P was the same in the two soils. A threefold higher level of organic matter in soil under mesquite probably contributes to increased availability of P.
Differences in total potassium and its supply to test plants were not detected between the two soils.