Combined Use of Organic and Inorganic Nutrient Sources for Soil Fertility Maintenance and Replenishment

Abstract

The beneficial effects of combined organic and inorganic nutrients on soil fertility have been repeatedly shown, yet there are no guidelines for their management. Organic materials are not magic; many of their functions with respect to soil fertility are known. Organic materials influence nutrient availability (i) by nutrients added, (ii) through mineralization-immobilization patterns, (iii) as an energy source for microbial activities, (iv) as precursors to soil organic matter (SOM), and (v) by reducing P sorption of the soil. The challenge is to combine organics of differing quality with inorganic fertilizers to optimize nutrient availability to plants. Numerous field trials indicate both added benefits and disadvantages of combining nutrient sources. Increased nutrient recovery and residual effects are associated with combined nutrient additions compared with inorganic fertilizers applied alone. Unfortunately, for many trials there is lack of crucial information on the nutrient content and quality of the organic inputs. Trials are needed that link the quality of the organic material to its fertilizer equivalency and its effect on the longer term composition of SOM and crop yields. A systematic framework for investigating the combined use of organic and inorganic nutrient sources includes farm surveys, characterization of the quality of organic materials, assessment of the fertilizer equivalency value based on the quality of organics, and experimental designs for determining optimal combinations of nutrient sources. The desired outcome is tools that can be used by researchers, extensionists, and farmers for assessing options of using scarce resource for maintaining soil fertility and improving crop yields.