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6 Better Crops/Vol. 90 (2006, No. 4)

The inability to add fertilizer P to an

organic farming system, as currentlydefined, limits the ability of soils to

balance other nutrients, such as nitrogen(N) and sulfur (S), and to achieve yieldgoals. While soils are well known to be ableto supply some P to crops even after manyyears of production and no P additions,the amount supplied is often insufficientto optimize crop yields. Also, many organicgrowers are hard pressed to find an ad-

equate supply of organic manure to sup-ply P to their production fields.

Soil testing for P estimates the soil Psupply available for plant uptake. How-ever, these analytical methods do not takeinto account those less available forms ofsoil P which are known to be quite signifi-cant in many soils, especially clay soils. Asa result, these questions are often asked:

Is the depletion of soil P in organic

cropping systems a general depletion, orjust a reduction in plant-available P?

What impact does annual crop removalof a nutrient like P have on the stable, re-calcitrant fractions of soil P?

A cropping systems study initiated atthe University of Manitoba in 1992 evalu-ated the impact of crop rotation and pro-duction inputs (herbicides and fertilizers)on crop yield, weed dynamics, energy use,and soil fertility. The project is located ona Rego Black Chernozem (Udic Boroll),with a textural analysis of 12% sand, 32%silt, and 55% clay. Soil organic matter con-tent is 5.5%. The three cropping systems

M A N I T O B A

Organic Crop Management

and Soil PhosphorusBy C. Welsh, M. Tenuta, D. Flaten, C. Grant, and M. Entz

Long-term organic production systems show a deficiency of plant-available phos-phorus (P), but not total P. High crop removal of P with alfalfa had the greatestimpact on soil P.

included wheat/dry pea/wheat/flax, wheat/

alfalfa/alfalfa/flax (no manure), and wheat/alfalfa/alfalfa/flax (composted manure). Apreviously cropped area was also restoredto native tall grass prairie with no crop re-moval.

The conventional production systemsreceived fertilizer and herbicide treat-ments, while the organic production sys-tems received no herbicide or fertilizer ad-ditions. The amount of P applied annu-

ally in conventional systems ranged from0 to 27 lb P

2O

5/A. After 12 years (1993-

2004), soil samples were collected and soilP (0 to 6 in.) was evaluated. Soils wereevaluated using a modified Hedley frac-tionation procedure, including P removedwith water, sodium bicarbonate, sodiumhydroxide, and hydrochloric acid. Total Pwas also determined using inductivelycoupled plasma. The P balance was esti-

mated using average P content of cropsharvested (Soil Fertility Guide forManitoba), measured crop yields, and an-nual fertilizer and manure additions.

The P balance calculated for the croprotations showed a P deficit when no fer-tilizer P was added to the organic systems,and a surplus when fertilizer P was addedto the conventional systems (FigurFigurFigurFigurFigure 1e 1e 1e 1e 1).The large P deficit for the organic forage-grain production systems reflects the highlevel of nutrient removal with alfalfa pro-duction and harvest. This P deficit wasmuch larger than the organic grain produc-tion system, where only grain was removed

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from the cropping system and all straw re-turned. The P surplus recorded for the con-ventional forage-grain systems reflects Pfertilizer additions in excess of estimatedcrop removal at this location.

The soil P analysis revealed that themanagement system used in the study hada much greater impact on soil P levels thanthe cropping system (FigurFigurFigurFigurFigure 2e 2e 2e 2e 2). The levelsof plant-available P in the soil sample frac-

tions (water, sodium bicarbonate and so-dium hydroxide) were significantly(p