Seasonal Variation in Soil Bulk Density, Organic Nitrogen, Available Phosphorus, and pH

Abstract

Scientists are being called on to measure and predict the effects of soil management and climate change on organic matter and other soil properties. The estimates and predictions generated from current conditions and short-term experiments will only be accurate if our measurement techniques produce data that represent actual soil properties during the period of prediction. Perhaps the least studied aspect of soil sampling is the possibility that the timing of samples might introduce large non-random errors. This study sampled replicated plots every month for 39 mo. Even after removing the measurement variation caused by surface soil bulk density fluctuations, variations in organic N, available P, and pH were always >10% of the mean. Much of the variation appeared to be temporally correlated in a seasonal cycle. In this experiment, with a 1-yr crop rotation, averaging 12 monthly samples allowed differences among soil treatments to be detected that were within 2 to 4% of the mean. When highly accurate estimates are desired, researchers need to consider combining multiple sample timings to overcome temporal variability.