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Organic crop management and soil phosphorus

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

March 5, 2008  By C. Welsh M. Tenuta D. Flaten C. Grant and M. Entz*

The inability to add fertilizer P to an organic farming system, as currently defined, limits the
ability of soils to balance other nutrients, such as nitrogen (N) and sulphur (S), and to achieve yield goals. While soils are well known in being able to supply some P to crops even after many years of production and no P additions, the amount supplied is often insufficient to optimize crop yields. Also, many organic growers are hard pressed to find an adequate supply of organic manure to supply P to their production fields.

Soil testing for P estimates the soil P supply available for plant uptake. However, these analytical methods do not take into account those less available forms of soil P, which are known to be quite significant in many soils, especially clay soils. As a result, these questions are often asked:

  • Is the depletion of soil P in organic cropping systems a general depletion, or just a reduction in plant-available P?
  • What impact does annual crop removal of a nutrient like P have on the stable, recalcitrant fractions of soil P?

Figure 1. Expected P balance for 1993 to 2004.

A cropping systems study initiated at the University of Manitoba in 1992 evaluated the impact of crop rotation and production inputs (herbicides and fertilizers) on crop yield, weed dynamics, energy use and soil fertility. The project is located on a Rego Black Chernozem (Udic Boroll) soil, with a textural analysis of 12 percent sand, 32 percent silt and 55 percent clay. Soil organic matter content is 5.5 percent. The three cropping systems included wheat-dry pea-wheat-flax, wheat-alfalfa-alfalfa-flax (no manure) and wheat-alfalfa-alfalfa-flax (composted manure). A previously cropped area was also restored to native tall grass prairie with no crop removal.

The conventional production systems received fertilizer and herbicide treatments, while the organic production systems received no herbicide or fertilizer additions. The amount of P applied annually in conventional systems ranged from zero to 27lb/ac P2O5. After 12 years (1993 to 2004), soil samples were collected and soil P (zero to six inches) was evaluated. Soils were evaluated using a modified Hedley fractionation procedure, including P removed with water, sodium bicarbonate, sodium hydroxide and hydrochloric acid. Total P was also determined using inductively coupled plasma. The P balance was estimated using average P content of crops harvested (Soil Fertility Guide for Manitoba), measured crop yields, annual fertilizer and manure additions.

The P balance calculated for the crop rotations showed a P deficit when no fertilizer P was added to the organic systems, and a surplus when fertilizer P was added to the conventional systems (see Figure 1). The large P deficit for the organic forage-grain production systems reflects the high level of nutrient removal with alfalfa production and harvest. This P deficit was much larger than the organic grain production system, where only grain was removed from the cropping system and all straw returned. The P surplus recorded for the conventional forage-grain systems reflects P fertilizer additions in excess of estimated crop removal at this location.

Figure 2. Total P concentrations in organic and conventional systems under different crop rotations.

The soil P analysis revealed that the management system used in the study had a much greater impact on soil P levels than the cropping system (see Figure 2). The levels of plant-available P in the soil sample fractions (water, sodium bicarbonate and sodium hydroxide) were significantly (p<0.05) lower for the organic management. The more plant-unavailable (recalcitrant) hydrochloric acid fraction was not different between cropping systems or management. This indicates that during the 12 year period of this study, it was the plant-available forms of soil P which were selectively altered most by management.

It is important to note that while the calculated P balance (see Figure 1) showed a P deficit for the organic grain rotation, soil fractionation of P indicated that it had similar soil P levels as the conventional production system (see Figure 2). This similarity in soil P levels after 12 years of grain crop removal and no P addition reflects the large buffering capacity of these clay soils. Only the organic forage-grain cropping systems show a difference in the plant-available forms of soil P as determined by the detailed Hedley fractionation procedure (see Figure 2) and the soil test results (see Table 1).

We speculate that the difference in both plant-available and unavailable forms of P in the soils of the organic and conventional systems may be showing some shift in P forms with time. Without replenishment of plant-available P in the organic forage-grain system, P will be the most limiting nutrient in the future. -end-

Table 1. Soil test P after 12 years.
 Rotation   Management
 Organic Conventional
 parts per million (ppm)
 Wheat-pea-wheat-flax 30a 38a
 Wheat-alfalfa-alfalfa-flax 9b 21a
 Wheat-alfalfa-alfalfa-flax-manure 14b 35a
 Prairie 35a 
Wheat (W), pea (P), flax (F), alfalfa (A), manure (M). Levels followed by the same letter are not different at p = 0.05.

*Ms. Welsh (e-mail: is a graduate student, Dr. Tenuta a soil microbiologist, and Dr. Flaten a soil scientist in the Department of Soil Science; Dr. Entz is an agronomist in the Department of Plant Science, all at the University of Manitoba. Dr. Grant is a Soil Scientist with Agriculture and Agri-Food Canada in Brandon, Manitoba.

Reprinted from Better Crops with Plant Food, with permission of International Plant Nutrition Institute (IPNI).


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