Controlling soil water content in fertiliser dissolution experiments

Phosphorus lost in runoff from agricultural land leads to the enrichment of surface waters and contributes to algal blooms. Fertilisers are one source of this P. To compare the water available P of different fertiliser formul ations in the laboratory it is necessary to control environmental condition s, temperature, relative humidity and soil water content, prior to simulati ng rainfall. Two chambers were designed in which relative humidity and soil water content were controlled using salt solutions. An initial design comp rising a sealed chamber with three layers of soil samples over a salt bath was found to be inferior to a single layer design. The changes in water con tent of soil samples were used to test the single layer chamber in a consta nt temperature environment (15 degrees C) using a saturated KCl solution (9 0% relative humidity). Based on the final soil water content of the samples , the spatial variation within the chamber was within tolerable limits. The single layer chamber was used for a simulation experiment comparing the wa ter available P of two commercial fertilisers. Using a saturated resorcinol solution (95% relative humidity) soil samples were equilibrated at 15 degr ees C for 21 days, fertiliser added, and the water available P measured up to 600 h after fertiliser application. The results indicate that the amount of water available P was related to the fertiliser compound and exponentia lly related to the time since fertiliser application. It was concluded that the single layer chamber is suitable for controlling relative humidity and soil water content in trials such as these where the water available P of fertilisers are being compared.