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University of Manitoba Faculty of Agricultural and Food Sciences Department of Plant Science

Soil Phosphorus Dynamics in
the Glenlea Long-Term Rotation


Cathy Welsh collects a soil sample from the Glenlea Long-Term Rotation Study for her Master's work on soil phosphorus.

Phosphorus (P) management is a major challenge facing organic farmers. While P is continually exported from the cropping system in grain and forage crops, returning P to the system is more difficult. Studies from across western Canada and the world have shown that in many cases, available soil P is deficient on organic farms.

Available soil P is only a portion of the total P in the soil, however. Phosphorus exists in the soil in a variety of forms that range from highly available to highly unavailable to plants. It is important to understand how the different pools of P respond to depletion of the most plant-available fraction, as these less-available pools may provide P to the plant-available pool over time. It is also important to determine the role of crop rotation and crop management practices in depleting, replenishing, or facilitating the use of soil P reserves in order to develop cropping systems that use this resource in a sustainable manner.

Study Objective

The aims of this study were:

  • to determine if depletion of P observed on organic farms is a general depletion or only a reduction in the plant-available fraction.
  • to determine the role of crop rotation (grain-only vs. forage-grain), and application of composted beef manure in depleting, replenishing, and facilitating the use of soil P reserves.

Experiment Description

The soil phosphorus study was part of a larger ongoing trial, the Glenlea Long-Term Rotation Study. The Glenlea Study was established in 1992, 20 km south of Winnipeg, Manitoba, and compares the productivity and sustainability of annual (wheat - pea - wheat - flax) and perennial (wheat - alfalfa - alfalfa - flax) crop rotations under both organic and conventional (full input) management systems.

In fall of 2002, after the eleventh crop year, composted beef manure was applied to one-half of each plot in the perennial rotation at a rate of 10 t/ha (4.5 ton/ac), creating perennial rotations with and without manure addition. Other than the manure-compost in the perennial rotation, the organic plots received no fertilizers or pesticides. The conventional plots received fertilizer based on soil test recommendations and pesticides based on economic thresholds.

Soil samples were collected from all plots to a depth of 15 cm (6 in) in October 2004, after 13 years of the rotation study. Soil P in these samples was measured using two methods:

  • the modified Kelowna technique was used to measure "soil test P". This is the same technique used by many commercial soil test laboratories. Kelowna-P was later converted to Olsen-P using a mathematical equation. Olsen-P is the measurement used in the Manitoba Soil Fertility Guide.
  • a sequential extraction procedure, described in Table 1, was used to measure the size of 4 pools of soil P, separated based on availability to plants.
Table 1. Description of sequential phosphorus (P) extraction procedure and forms of P extracted.
Order of Extraction Extractant P form Availability to plants
1 water orthophosphate very high
2 sodium bicarbonate inorganic P weakly bound to aluminum and iron, and organic P weakly bound to soil organic matter high
3 sodium hydroxide inorganic P tightly bound to aluminum and iron, and organic P tightly bound to soil organic matter moderate
4 hydrochloric acid apatite-type inorganic P (rock phosphate) very low

The sizes of the different pools of P, as well as total extractable P, were compared between crop rotations, between management systems, and between perennial rotations with and without manure. Grain yields for years where crops were in common between rotations (flax and wheat) were also compared, as well as other soil parameters.

An estimated cumulative P budget was calculated for the 13 years of the study (1992-2004) to help clarify the roles of P removal and P addition in different crop rotations and input management systems. P removal was calculated from actual yield data from the Glenlea Study and estimated P content of harvested crops, based on reference values in the Manitoba Soil Fertility Guide. P additions were based on known nutrient content of fertilizers and manure added to the system.


Soil Test P

Plant-available P, or "soil test P" was affected by management system and by crop rotation. Available P was lower in organic systems than conventional systems and was lower in perennial rotations than annual rotations. Available P was especially low in the organic perennial system. P export from this system was highest of all systems, for two reasons:

  • alfalfa crops have very high P requirements and when alfalfa is removed from the system as hay, large amounts of P are exported from the system
  • grain yields were higher in the organic perennial system than in the organic annual system, resulting in greater P export from the perennial rotation.

Adding manure to the perennial rotations increased available P somewhat, although P levels were still much lower in the organic perennial rotation with manure than the conventional perennial rotation. Since manure was applied only once during the 13 years of the rotation study, the increase in available soil P as a result of manure application was quite small. More frequent application of manure would help to replenish soil P reserves.

Sequentially Extracted Fractions of P

Figure 1. Soil P extracted by water, sodium bicarbonate, sodium hydroxide, and hydrochloric acid in three crop rotations after 14 years of organic and conventional management.

Crop rotation and management system had a significant effect on the three more available fractions of soil P (water, sodium bicarbonate, and sodium hydroxide extractable). Trends in P concentrations in the first three fractions were similar to trends observed in soil test P — P levels were lower in organic systems than in conventional systems, and were lower in perennial rotations than in annual rotations (Figure 1).

The reduction in the moderately available (sodium hydroxide extractable) fraction of P in organic systems suggests that as highly available P became scarce, plants began to access the less available P. Some of this P may have partially replenished the more available fractions, and some may have been used by plants more directly.

The unavailable fourth fraction (hydrochloric acid extractable) was not affected significantly by crop rotation or management. Since the fourth fraction of P was not reduced, even when the other fractions were, it appears that this highly unavailable fraction of P was not being depleted — at least not yet. Further study is required to determine whether unavailable P reserves will be affected by crop rotation or management system in the future.

When all four P fractions were added, the total extractable P ranged from 259 to 345 parts per million (Figure 2). In contrast, soil test P for the same systems ranged from 9 to 38 ppm. While total extractable P was still lower in the organic perennial system than the other systems, the differences between systems were not as extreme as in soil test P.

Adding manure raised soil P levels in available and unavailable fractions, but this increase was not statistically significant. Again, more frequent application of manure would likely increase soil P levels over time.

Estimated P Budget

Calculating a cumulative P budget for the duration of the study emphasized the trends observed in measurements of soil P levels. P removal was greatest in perennial rotations and was higher in conventional systems than organic systems (see Table 2 below). The cumulative P balances were negative for organic systems and positive for conventional systems.

Table 2. Estimated cumulative (1992-2004) phosphorus balance at the Glenlea study.
Rotation Management
Cumulative P Removal Cumulative P Addition Cumulative P Balance

---------------------------- kg P / ha ----------------------------

Annual Conventional 107.26 139.88 32.61
Annual Organic 52.05 0.00 -52.05
Perennial Conventional 160.07 230.56 70.49
Perennial Organic 117.92 0.00 -117.92
Perennial with compost Conventional 160.56 255.02 94.46
Perennial with compost Organic 118.83 24.46 -94.37

Differences in the cumulative P balance between organic and conventional systems were much larger than observed differences in soil P levels. While the reasons for this discrepancy are not clear, using estimated crop P concentrations to calculate P removal may have been a contributing factor. Measuring the P content of harvested crops and using those values for calculating the P budget would have given more accurate results.

P Limitation in Organic Systems

The difficulty of replacing P is one of the major concerns in organic systems. Without added P from external sources, P reserves will eventually be depleted. However, the onset of P limitation in organic systems depends on three major factors:

  1. the rate of P removal through crop harvests
  2. the ability of plants to access unavailable soil P reserves, if present, and
  3. the rate of replacement of P removed from the system.

Rate of P Removal

Alfalfa hay crops remove much more P from the system than annual grain crops, due to alfalfa's high P requirements. As a result, the perennial organic system became P-limited after approximately 12 years of production. The annual organic system, on the other hand, was not yet P-limited after 13 years or production, and based on annual P removal rates, could be expected to continue another six years before becoming P limited.

Accessing Unavailable P Reserves

When large amounts of unavailable, or recalcitrant, P are present in the soil, P limitation in organic systems may be delayed by promoting the cycling of unavailable P into plant-available forms. Processes that make recalcitrant P more available to plants include mobilization of unavailable forms by plants such as legumes and buckwheat or microbes such as Penicillium bilaiae, the active ingredient in JumpstartTM.

Colonization of plant roots with endomycorrhizal fungi can also help to delay the onset of P limitation, since endomycorrhizae can help plants absorb P from the soil.

Replacement of P

While mobilization of phosphorus reserves may delay P limitation in organic systems, replacement of nutrients taken up by crops is required to make these systems sustainable in the long term. In this study, the one-time application of manure-compost increased available P slightly in the perennial rotations. Applying manure-compost more frequently would help replace the P removed by the crops.

Conclusions and Recommendations

  • Organic crop production can deplete available soil phosphorus (P) over time if P is not replaced.
  • Perennial hay systems remove large amounts of P from the soil, largely due to alfalfa's high P requirements.
  • Annual grain systems do not remove as much P from the soil but may be lower yielding if N supply through green manures is not adequate.
  • Adding composted livestock manure increased available soil P levels somewhat. More frequent application of compost could prevent P depletion in organic systems.
  • Reserves of unavailable P may gradually replenish reserves of available P, but at a rate that is too slow to keep pace with P uptake by plants.
  • Helping crops access reserves of unavailable P with practices such as promoting mycorrhizal fungi may delay the onset of P limitation in organic systems.
  • For long-term sustainability, replacement of P removed by crops is necessary.

Further Reading:

Thiessen Martens, J. 2008. Where has all the phosphorus gone? Organic Agriculture Centre of Canada.

Welsh, C.M., 2007. Organic crop management can decrease labile soil P and promote mycorrhizal association of crops. M.Sc. Thesis. University of Manitoba, Winnipeg, Manitoba.

Welsh, C., M. Tenuta, D. Flaten, C. Grant, and M. Entz. 2006. Organic Crop Management and Soil Phosphorus. Better Crops, vol. 90, no. 4, pp. 6-7.

Copyright and Liability

This page created August 2008.

The production of this webpage was supported in part by funding from the Advancing Canadian Agriculture and Agri-Food (ACAAF) program of Agriculture and Agri-Food Canada.