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SWEEP Report #23

Processes Involved in Mobilization of Phosphorus in
Different Farming Systems in Southwestern Ontario:
Nutrient Levels in Plant Tissues and Soils

A. Abboud, Resource Efficient Agricultural Production (REAP) - Canada, Ste. Anne de Bellevue, Quebec

Executive Summary

Evaluation Summary (Tech. Transfer Report Summaries)

View/Download Final Report [675 KB pdf]

Associated SWEEP/LSP Research



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Completed: March, 1991

Key Words:

organic farming, conventional farming, rock phosphorous, tillage, green manure, compost

Executive Summary

The effects of the application of different types of phosphorus-containing fertilizers (rock P, super phosphate and compost) were examined on six different farms in order to identify some of the critical parameters for the mobilization of phosphate. The farms (three organic and three conventional) provided a gradient of conditions ranging from long-term organic to moderately intensive conventional. Three other experiments examined how crop, soil, and residue management practices on a long-standing organic farm affect mobilization of native and applied phosphate. The main findings of these four experiments are summarized below:

Responses to different phosphorus fertilizers in organic and conventional farming systems

Two methods of extracting soil P were compared: anionic resin and sodium bicarbonate. A significant positive correlation between the two extraction methods was found. However, only the resin extraction method was sensitive enough to detect statistically significant differences between the fertilizer treatments.

Applications of super phosphate and compost each resulted in significantly higher (p < 0.05) soil P (resin) than in controls in 3 out of 9 comparisons. On all farms, the values for soil P (resin) in compost and super phosphate treatments were numerically higher than those in controls. There was no statistically significant response to rock P.

Percent of P in one month old barley plants was significantly correlated with soil P (resin). There was no correlation of percentage P in barley leaves at the heading stage with soil P. Over all farms, barley grain yields were significantly correlated to the percent of P in one month old plants. However when farms were considered individually this correlation was not always significant.

Percent of P in barley plants was not significantly affected by the fertilizer treatments except in one organic farm. Grain yields were significantly affected by the treatments only in one (conventional) farm.

Mycorrhizal infection tended to be lower on the super phosphate treatment on all farms. There was no indication that the percent of mycorrhizal infection was highest on the organic compared to the conventional farms. Similarly, there was no indication of either root or soil phosphatase activities being higher in one of the two types of farms.

Effect of three tillage/straw treatments on soil P and crop nutrition

Three tillage treatments were imposed on a field after barley was harvested: mouldboard ploughing, discing both with barley straw removed and discing with barley straw retained. In the second year three P-containing fertilizers (as in experiment 1) were superimposed on the main tillage treatments. Soil fertility and nutrient contents of two different crops that followed these treatments were examined.

The disc + straw treatment increased soil P (resin) one year after the tillage treatments were imposed. As in experiment 1, no significant differences were found when soil P was extracted with sodium bicarbonate. Grain yields of maize, soil phosphatase activity and P concentration in maize grains were also higher under the disc + straw treatment. Compost and super phosphate increased soil P but no effect of rock phosphate was observed.

Our data indicates that the tillage/straw regime by itself can improve P nutrition.

Effects of green manuring and rock phosphate on P nutrition of different crops

Different green manures (faba beans, oil seed radish, buckwheat and weeds) were established in plots that had received 0 or 180 kg P2O5/ha as rock phosphate.

Rock phosphate increased soil P (resin) levels in 1988 and 1989. Green manures had no effect on increasing the levels of soil P when rock P was applied. Oat plants accumulated more Ca when buckwheat was the previous crop.

Effects of carbon additions on nitrogen fixation and P nutrition of hairy vetch and following crops

Three levels of oat hulls (0, 5, 10 t/ha) were incorporated into the soil with two levels of rock phosphate (0 and 180 kg P2O5/ha and hairy vetch planted in the summer of 1989. In 1990, maize was planted and the residual effects of 1989 treatments monitored.

Both levels of oat hulls significantly decreased levels of soil nitrate in June 1989, improved nitrogen fixation (acetylene reduction) and vetch biomass production as well as decreased weeds. Total uptake of N, P, K, Ca was improved by the addition of oat hulls. The amount of nutrients carried into the system by the oat hulls were smaller than the surplus of nutrient accumulated in vetch biomass under the effect of 5 t oat hulls/ha. This indicates that oat hulls stimulated growth by a mechanism other than addition of nutrients.

Evaluation Summary

(From Technology Transfer Report Summaries - A. Hayes, L. Cruickshank, Co-Chairs)

Mobilization of phosphate was examined on six farm management systems, ranging from long term organic to moderately intensive conventional. Three other experiments examined the affect of crop, soil and residue management, on mobilization of phosphate on a long term organic farm.

The experiments covered the following topics:

  1. Responses to different phosphorous fertilizers (0-46-0, compost, rock P) in organic and conventional farming systems.

    • There did not appear to be a pronounced difference between farming systems in response to the different types of P fertilizers.

  2. Effect of three tillage/barley straw treatments on soil P and crop nutrition. The field was divided into 9 strips which were moldboard plowed (no straw), disced (no straw), disced (barley straw). In 1988 four treatments were superimposed onto the main treatments, (no fertilizers, superphosphate, rock phosphate, and compost).

    • The straw disced in treatment increased soil P one year after the treatment compared to no straw disced or plowed.

  3. Effects of green manuring and rock phosphate on P nutrition of different crops. The treatments were rock phosphate and no rock phosphate with the following green manures: oilseed radish, buckwheat, faba beans and weeds.

    • Rock phosphate increased soil P levels while green manuring did not increase soil P levels.

  4. Effects of carbon additions on nitrogen fixation and P nutrition of hairy vetch and following crops. The treatments were oat hulls incorporated and no oat hulls, with rock phosphate and no rock phosphate.

    • Addition of oat hulls improved the total uptake of N, P, K, and Ca in vetch. There was no response in sunflowers.


The following concerns were not covered in the report.

  • Variation which occurred within the organic and the conventional farms.

  • What effect does soil type have on the availability and uptake of P?

  • Is there really a difference between treatments (sodium bicarbonate showed no differences) or was the positive response shown by the resin test used to illustrate a difference?

  • Why was the straw not worked in with the plow, as part of the treatment?

  • Weed suppression by oat hulls (5 & 10 t/ha) could better explain the nutrient uptake by vetch.

The sodium bicarbonate test for phosphorus reflects what is happening in Ontario soils more accurately than the resin test.

Associated SWEEP/LSP Research:

  • SWEEP Report #14 - The Effect of Terraces on Phosphorus Movement

  • SWEEP Report #35 - Nutrient Distribution and Stratification Resulting from Conservation Farming

  • SWEEP Report #53 - Phosphorus Movement in Soil as a Function of Phosphorus Solubility and Reactivity

  • SWEEP Report #60 - The Effect of Conservation Tillage Practices on the Losses of Phosphorus and Herbicides in Surface and Subsurface Drainage Waters

  • SWEEP Report #45 - Management of Farm Field Variability. III. Effect of Tillage Systems on Soil and Phosphorus Loss

Future Research: ( ) indicates reviewers suggestion for priority, A - high, C - low.

None required.




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Created: 05-28-1996
Last Revised: Thursday, May 19, 2011 02:20:17 PM