12 February 2018

50-year field trial supports more targeted phosphorus fertilization


After 50 years' research on the UCPH experimental farms, scientists have identified how differences in nutrient supply affect soil and crops. In particular, new knowledge about how phosphorus and soil fertility interact can contribute to transforming crop advice about phosphorus.

Frederik van der Bom examining the UCPH experimental fields. Only nitrogenous fertilizer was added for 30 years but since 1996 long term research has been conducted.

In his recently completed PhD, Frederik van der Bom laid out results derived from many years of research conducted in the experimental fields of the University of Copenhagen.

The new results help researchers draw important conclusions about nutrient supply, which they believe can be used to better target phosphorus use in agriculture.

Professor Lars Stoumann Jensen of the University of Copenhagen’s Department of Plant and Environmental Sciences specializes in nutrients and soil fertility. He is responsible for the experimental fields and is Frederik van der Bom’s PhD supervisor.

A "huge amount of statistical analysis” was required to make sense of the data compiled during many years of studies, according to Lars Stoumann Jensen. Frederik van der Bom has now completed the studies and correlated the many years of records, including climate data from the Danish Meteorological Institute (DMI) along with his own study data. Among other things, van der Bom’s dissertation addresses how many years of applying different levels of mineral and organic fertilizers has resulted in varying soil fertility and affects crop growth, yield and resistance. In particular, the study focused on the addition of phosphorus, potassium and nitrogen nutrients.

While analyses of extensive data sets have produced a range of new results, the two researchers highlight new knowledge about the effects of phosphorus availability as critical, because it could change how crop advice about phosphorus is given.

Low phosphorus levels may be acceptable

Traditional crop advisor-knowledge recommends that soil phosphorus levels (a measurement of the phosphorus available to plants) should be between 2 and 4 (expressed in mg/100 g soil). However, results from the experimental fields indicate that this is not necessarily always the case. These results demonstrate that phosphorus levels as low as 1-2 can produce high yields. Provided that a sufficient amount of phosphorus fertilizer is applied during a growing year, there is a definite response in terms of crop yield.

"Therefore, we challenge the general objective of phosphorus levels between 2 and 4. We believe that in many cases, one can manage with phosphorus levels between 1 and 2," says van der Bom. He adds: "but, it obviously requires that a farmer is carful about the timing of their tilling, fertilization and seeding. In more difficult soil, such as in colder regions, waterlogged or very sandy soil, it may well be that phosphorus levels need to be higher, perhaps as high as 3-5."

Raising phosphorus levels is expensive

Frederik van der Bom has also deduced how much extra phosphorus is needed on a yearly basis to raise the phosphorus level in soil by 1 unit: 35 kg P/ha/year for many years, in addition to the roughly 20 kg P/ha absorbed by crops annually.

Phosphorus is a relatively expensive nutrient if procured in commercial fertilizers and raising levels from 1.5 to 2.5 can come at a cost that might not be necessary according to the newly acquired knowledge. Frederik van der Bom explains that this is due to the fact that, although the application of P-fertilizer (in whatever form) increases the amount of readily available P to a small degree, the majority of added phosphorus becomes fixed in soil. Much of this ‘fixed’ phosphorus can be released at only an extremely slow pace, too slow to contribute to plant availability.

Sensitivity highest in first weeks after sowing

The analysis of the long-running study also demonstrates that low phosphorus levels or lack of phosphorus fertilization can be crucial for crop resistance to stress.

Frederik van der Bom correlated yield and climatic data for 13 of the experimental years during which spring barley was the predominantly grown crop. By doing so, he precisely identified when under-fertilized crops were more vulnerable to stress.

In particular, low phosphorus fertilization makes plants more susceptible to colder temperatures during the first 40 days after sowing. The need for fertilization is critical during this period.

"We have shown that it's more important to have good fertilization during cold years, and that this serves as a kind of insurance for stable yields," says Frederik van der Bom.

Targeted fertilization is becoming easier

Lars Stoumann Jensen, who has many years of experience in nutrient supply research and experimentation, explains that the results of this extensive study all point in the same direction: “It's very much about timeliness," as he calls it. For the sake of both the farmer's wallet and the environment, it makes sense to research and improve our understanding of how to limit phosphorus, potassium and nitrogen use to amounts that are absolutely necessary.

"This research is important for both our basic understanding of  phosphorus turnover and mobilization in soil, and for the finances and fertilization practices of farmers. There is a direct cost to yield when a farmer doesn’t apply enough nitrogen. But in the case of phosphorus and potassium, there remains uncertainty about proper fertilization. We simply don’t know how much phosphorus gets "saved in the bank" of soil and how much of it is accessible to plants. It's a bit like retiring without knowing one’s savings," says Lars Stoumann Jensen, who continues, "the significance of these results is that we can use them to better target fertilization using nutrients that are scarce and expensive."

The experimental field is unique

At the experimental field in Taastrup, the work continues to study plant and soil interactions during low and varying levels of soil fertility. Stoumann Jensen explains that others have now tuned in to the unique nature of these experimental fields. For example, UCPH, in collaboration with Novozymes/Monsanto Bioag-Alliance is testing a variety of new microbial products that stimulate crops’ ability to dissolve and absorb phosphorus reserves in soil.

"These experimental fields is one of the few locations in Denmark where no fertilizers have been used since the mid-1960’s. It is quite unique because University of Copenhagen is one of the only places where it is possible to test the effects of new crop varieties, fertilizers and other inputs in plant production under experimental conditions with well-defined differences in soil fertility. It provides more valid results and enables us to try things that wouldn't be possible elsewhere. In other words, we will have the chance to develop sustainable fertilization strategies for the future,” explains Lars Stoumann Jensen.