Systematic Approach to Diagnosing Fertility Problems in Soils of Sri Lanka

Diagnosing soil fertility problems is essential to supply plant nutrients in a complete and a balanced manner. We evaluated and validated a systematic approach of diagnosing fertility problems based on optimum values established elsewhere for 32 cultivated soils representing different agroecological zones of Sri Lanka. Soils were analyzed for available ammonium nitrogen (NH4-N), phosphorus (P), potassium (K), copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), calcium (Ca), magnesium (Mg), sulfate - sulfur (SO4-S), and boron (B) concentrations, and based upon these analyses we calculated nutrient application rates. Molybdenum (Mo) and calcium carbonate (CaCO3) application rates were calculated based on the active acidity of soils, while those of P, K, S, Cu, Mn, Zn, and B were adjusted considering their respective fixation capacities. An optimum treatment providing nutrients at optimum concentrations was validated in the greenhouse using sorghum as the indicator plant. The optimum treatment was compared with 13 other treatments of individual nutrients or CaCO3 consisting of either omissions (optimum treatment minus a particular nutrient) or additions (optimum treatment plus a particular nutrient). The treatments were replicated four times. All soils were deficient in N and B, while most (30 out of 32) were deficient in P, K, and S. Deficiencies of Ca, Mg, Cu, Zn, and Mn were less common (<75% of soils), while all soils had sufficient Fe. Fixation abilities for nutrients varied widely. Significant (P < 0.05) yield reductions in minus treatments were observed for N and P in 100% of soils; K, S, and CaCO3 in 50-60% of soils; Ca in 38% of soils; and Mg, Cu, Zn, B, and Mo in less than 20% of soils. Plus treatments gave significant yield reductions occasionally, probably due to toxicities and nutrient imbalances. In 11 soils identified as deficient in K, Ca, and B, dry-matter yields were significantly lower in the optimum treatment than the respective minus treatment, indicating that bringing the nutrient concentration up to the optimum level reduced the yields, signifying the need to validate the optimum levels used for these nutrients. This approach of combining soil analyses and fixation studies could be successfully used to diagnose fertility problems; however, the results indicate the need to adjust the optimum concentrations of K, Ca, and B before using them in a fertilizer recommendation program.