Chemical properties of atoll soils in the Marshall Islands and constraints to crop production

The sandy carbonatic soils of Pacific atolls are considered infertile and poorly suited to agriculture. The Republic of the Marshall Islands is comprised predominantly of atolls, and there is scant information on the fertility status of the Marshall Island soils. We conducted a survey of the soils to quantify chemical properties and determine significant trends related to soil distribution and climate. A total of 116 surface (0–15 cm) soil samples from 13 atolls and 42 subsoil (15–45 cm) samples from five of the atolls were analyzed for pH, electrical conductivity (EC), organic carbon (OC), total nitrogen (TN), cation exchange capacity (CEC), and nutrient levels. A missing element study in the greenhouse on a typical soil from Majuro Atoll was conducted to rank soil nutrient deficiencies. There were some strong differences in soil chemical properties between the two depths, but not necessarily between the two soil series constituting the sampled soils. Soil chemical properties were not affected by a strong rainfall gradient running from the southern (≈ 4000 mm) to the northern (≈ 1350 mm) atolls, but human activity had a significant effect on some properties. Soils located near the center of islands tend to show higher concentrations of OC than soils located along the ocean exposed shoreline. The missing element study indicated that the soil was deficient in Cu, P, N, S, and K. Potassium was the most serious nutrient constraint whereas the micronutrients Fe, Mn, Zn, and B did not limit maize growth significantly in a greenhouse pot experiment. Interpreting soil test data on atoll soils, however, remains difficult for two reasons: (i) the soil tests have not been calibrated with crop growth, and (ii) standard soil tests may not be applicable to the unique physical and chemical properties of atoll soils.