This post is a
review and summary of an article in Crops & Soils Magazine about managing
legacy phosphorus (P) in agricultural soils and their immediate local
environment, including runoff into water bodies, accumulation in the soil, and
factors that affect availability to plants.
What is Legacy Phosphorous?
“Legacy phosphorus is an evolving concept useful in the
responsible management of crop nutrition. A cumulative balance of inputs and
outputs is part of responsible management of plant nutrition and 4R nutrient
stewardship. This cumulative balance can be used to compare fields that have
differed in their historical inputs and productivity and identify where current
replenishment, deficit, and surplus rates are appropriate. In addition, the
legacy concept is also useful for situations in which soil testing is impractical.”
Some definitions of legacy
phosphorus in scientific literature are given below:
· “The P
accumulated in soil through human activity, based on the definition of legacy
as something received from the past or carried over from past actions” (Turner
& Kim, 2024).
· “The P
within the environment (e.g., sediments, water bodies, soils) resulting from
historic human activity, excluding geogenic P stores” (Shober et al., 2024).
· “The P
that exists in soils and catchments as a result of either past
anthropogenically released rock phosphate-derived P, or of human impacts on P
fluxes in aquatic systems, including non-fertilizer P sources (e.g., sediment
loading via erosion)” (Margenot et al., 2024).
Legacy P is understood to be
the P that is left over in the soil year-over-year. Legacy P is both beneficial
as an available fertilizer and detrimental when soil erodes and the P enters
local water bodies and their sediments as a component of runoff. Manured farm
plots often result in a surplus of P.
Phosphorous Cumulative Input/Output Balance
When evaluating legacy P, it
is important to consider the cumulative input/output balance of additions and
removals. In many of the regions of the world with high crop yields, there is a
surplus resulting in legacy P. This means that over time, more phosphorus is
added than is removed. Measuring P levels is not easy and requires extensive
soil testing. Legacy P is both beneficial and potentially detrimental. The goal
in managing it is to keep sufficient levels of legacy P without losing it to
the local environment, where it can’t be taken up by the plants it was designed
to aid.
“On a national scale, 82% of the world’s countries have
a historical cumulative surplus of P since 1961 (FAO, 2024). While the soils
may be at a cumulative surplus, these countries also account for 87% of current
removal of P by crop harvest. Maintaining a surplus P balance supports the
current high levels of cropland productivity.”
The graph below shows the
average input/output nutrient balance for phosphorus over time. The plateau of
P surplus since the early-mid 2000s can be seen as well as the balancing of
inputs and outputs that was reached in the mid-2010s.
The graph above, however, is
just an overall average. The real situation is that farm fields differ
drastically in their amount of P surpluses and deficits. The graph below shows
the broad distribution of soil test P levels in U.S. cropland. For the U.S. as
a whole, there is a considerable legacy P surplus, but some farm areas still
have deficits of P.
“Describing legacy P in soils is complicated by the fact
that both natural and human-induced factors are at work. Phosphorus in most
soils originates as calcium phosphate minerals in the parent material. As soils
weather, the forms of P as phosphate bound to iron and/or aluminum and/or
organic matter become more prominent.”
Some of the legacy P is not
available to the target crops, but over time, it does replenish bioavailable P.
This means that the surplus is adding to availability, albeit slowly, and that
it can achieve a balance of addition and removal while still providing optimum
crop yields. (N.Barrow, et. al., 2021) showed that phosphorus added to cropland
has a buffering effect on soil pH. The effects of soil pH on plant roots can
increase plant uptake of phosphorus. Some highlights and conclusions of the
paper are shown below.
The Crops & Soils article
explains some of the conclusions of Barrow et. al, as follows:
“First, added phosphate slowly penetrates soil particles
through solid-state diffusion, increasing their negative charge and reducing P
sorption and soil P-buffering capacity. A second benefit is that this diffusive
penetration eventually slows and stops. The third benefit is that the natural
processes transforming soil P to less soluble forms are slowed. Thus, over
time, a legacy of surplus P input to the soil allows crops to be sufficiently
fertilized with amounts of P no greater than those removed by the previous crop.”
Unfortunately, these chemical
processes also increase the risk of loss of P in drainage water. When the
buffering capacity is reduced, so is the capacity to retain P. Soils with low P
buffering capacity can be identified via soil testing and would not benefit
from too much added P, especially manure. Those soils should also be targeted
with conservation practices for decreasing soil erosion.
Phosphorus in the Context of 4R Nutrient Management
The 4R program of
agricultural nutrient management includes the 4Rs: right source, right rate,
right place, and right time. Past sources of phosphorus, such as bat guano and
bones, are in limited supply, and there are efforts to extract P from manure in
a more pure and concentrated form. The most common source of phosphorus now is
mined rock phosphates that are often made into granules.
“The main source, fertilizer P, is manufactured from
phosphate rock through acidulation followed by processing into granular or
fluid forms. Phosphorus fertilizers are often ammoniated, meaning that nitrogen
(N) is added to the product (i.e., DAP or MAP).”
The right rate of P to apply
to a given soil is determined by soil testing. There is a critical rate, above
which soil can draw enough P from existing stores to balance the P being
applied. Below that critical rate, more P is applied than is removed, since
less of the legacy P is available to the plants/ Soils should be managed to
keep legacy P near that critical rate where P input and output are balanced.
This method of optimizing P is known as the ‘Build and Maintain’ approach. The
goal is to keep an optimal amount of P available, without losing much of it
through drainage and runoff, keeping legacy P in the soil where it can
gradually become available. The time of application is important, especially
for the likelihood of runoff. Thus, application of P, especially if it is
broadcast on top of the soil, should ideally be in seasons when there is less
chance of runoff events. Placement of the P fertilizer is important as well. If
it can be placed below the soil surface, then it is less vulnerable to the
initial runoff that can carry away P applied on top of the soil. Early-season
applications of P often involve adding it with the seeds as they are
planted.
The Crops & Soils article
concludes with the following recap:
“Legacy P is an important consideration when developing
4R nutrient management plans. A cumulative balance of inputs and outputs is
part of responsible management of plant nutrition and 4R nutrient stewardship.
This cumulative balance can be used to compare fields that have differed in
their historical inputs and productivity and identify where current
replenishment, deficit, and surplus rates are appropriate. In addition, the
legacy concept is also useful for situations in which soil testing is impractical.”
References:
Managing
legacy phosphorus with 4R nutrient stewardship. Tom Bruulsema and Leanna Nigon.
Crops & Soils Magazine. Volume 58, Issue 8. August 18, 2025. Managing
legacy phosphorus with 4R nutrient stewardship | Science Societies
Effect
of phosphate sorption on soil pH. N.J. Barrow, Abhijit Debnath, and Arup Sen.
European Journal of Soil Science. September 8, 2021. Effect
of phosphate sorption on soil pH
Phosphate-solubilising
microorganisms mainly increase plant phosphate uptake by effects of pH on root
physiology. N. J. Barrow and Hans Lambers. University of Western Australia. Phosphate-solubilising
microorganisms mainly increase plant phosphate uptake by effects of pH on root
physiology - the UWA Profiles and Research Repository
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