This paper is one of those rare gems that provide you with decades of scientific knowledge distilled into a very readable and inspiring format. Jim Barrow started doing research at the interface between soil chemistry and plant nutrition in 1954 when, as a new graduate, he was employed by CSIRO in Australia.
Here, he summarizes what he has learned about phosphorus (P), and why some of our views on it are outdated. He addresses two main questions: (1) Why do farmers reapply phosphate? The short answer is that phosphate effectiveness decreases with time and it is therefore necessary; (2) Why does P effectiveness decrease with time? The major reason for this decline is that phosphate ions diffuse and penetrate into the reacting soil surfaces, which changes their properties.
Readers will also learn a lot about terminology and methodology used in the study of P in soils and how the process of knowledge generation took place over decades, leading to more comprehensive hypotheses and models. This work has implications for some of the most widely held theories in soil science. These theories are largely based on the notion that soil phosphate exists in discrete compounds with iron, aluminium and calcium. The work summarised here indicates, however, that such compounds do not exist under most common soil conditions. Barrow suggests that long-term fertilized soils no longer lock up or fix applied P.
It is thus only necessary to feed the plants, not the soil, i.e. annual doses (if they are needed at all) should be much smaller. Failure to appreciate this is the cause of P losses from farmland and consequent pollution of water bodies. He also suggests that for long-term fertilized soils, current soil tests do not give information on two important characteristics: the decrease in P buffering and the slowing of the penetration.
This paper provides much food for critical and innovative thinking. Enjoy it.
One Response
An important and thoughtful addition to Dr. Barrow’s excellent paper, which reflects a lifetime of profound experience and scholarly contribution. This adjoinder not only enhances the ongoing dialogue but also connects the depth of his insights with the evolving landscape of soil science. Below, a random thought on possible directions for the future exploration is suggested:
Continuous phosphorus (P) fertilization influences soil microbial activity and soil reaction, gradually altering the dynamics of P availability. Over time, the prominence of water-soluble P, bicarbonate-extractable P (Olsen P), and ammonium fluoride-extractable P (Bray P) decreases. Instead, the release of P from moderately labile and stable pools becomes more significant. Under such circumstances, developing a Phosphorus Saturation Index (PSI) could be a valuable tool for monitoring and managing soil P levels. This index would consider critical factors such as time, organic matter content, clay fraction, and fertilization rates to assess soil P saturation.
A well-calibrated PSI could help evaluate the P-application holiday, minimize potential risk of P leaching into water bodies, particularly in soils with high organic matter or clay content, where P accumulation poses environmental concerns. By identifying critical thresholds of P saturation, this index could provide a science-based framework for sustainable P management, balancing agricultural productivity with environmental protection.