Compost Induced Soil Salinity: A New Prediction Method and Its Effect on Plant Growth

Though composts contain nutrients and can improve soils, there is widespread concern among growers in arid and semiarid regions about their salt content. We have modified an established soil hydrology model to predict the electrical conductivity (ECe) of soil-compost mixtures. The model was validated using three different soils mixed with nine different composts. This method predicted the ECe of the soil-compost mixture with a mean error of 2.7±0.2 percent. To evaluate the impact of compost salinity on plant growth, greenhouse pot experiments were conducted on lettuce, tomato, and blueberry plants amended with nine different composts. Using model predictions, compost application rates were adjusted to create soil mixtures with salinity levels similar to those associated with 10% and 25% rates of yield reduction, as reported in the scientific literature. Results indicate that compost salinity at very high rates without leaching decreases plant growth rates in a manner similar to other sources of soil salinity. However, in all cases, plant growth rates of lettuce, tomato and blueberry were significantly increased relative to the fertilized control suggesting that the benefits of compost use outweigh the possible negative influence of compost salts. At typical agricultural application rates, salinity added with compost amendments is unlikely to negatively impact plant growth.