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Phosphate rock: a viable option for organic farmers?

By Melissa Arcand

Soil phosphorus depletion by crop removal is an important issue for organic farmers. Phosphorus is an essential element for promoting root growth and in energy capture, especially in cold soils, and for nitrogen capture by legumes such as alfalfa or soybean. Conventional phosphorus fertilizer is not allowed in organic production because of the synthetic acids used to make the phosphorus soluble. Is phosphate rock a good option for organic farmers?

Plants can access phosphorus only once it has been dissolved and released into the soil solution. The availability of phosphorus improves in soils with a pH below 6 and low calcium – characteristics that are uncommon in most Ontario and Prairie soils, but more common in the Maritimes.

Phosphate rock is a generic term for any rock that contains phosphorus minerals. The properties of the rock including the amount and solubility of phosphorus can vary greatly depending on where the rock comes from and how it was originally formed. The low and unpredictable availability of phosphorus from these materials makes them difficult to use in Canada.

Careful crop selection may help address solubility problems on high pH soils. Crops, such as rape, white lupin and buckwheat, have been found to make phosphorus more available by acidifying its root zone.

To study this further we evaluated the effect of five different phosphate rocks on phosphorus uptake by buckwheat at the University of Guelph. The study was conducted over two field seasons on two organic farms in Southwestern Ontario and then one conventional farm where we compared the phosphate rocks to mono-ammonium phosphate (11-52-0) and triple-super-phosphate (0-45-0) fertilizers.

Phosphate rocks and fertilizers (on the conventional farm) were applied at 100, 400, and 800 kg phosphorus per hectare after measuring the total phosphorus content of each rock source. The phosphate rocks tested were: Spanish River Carbonatite (Ontario), Kapuskasing (Ontario), Tennessee brown (Tennessee), Pebbled (North Carolina), and Calphos (Florida). Buckwheat was planted on all plots and was harvested after 7 weeks of growth, just prior to seed-set.

Buckwheat dry yields ranged from 2.7 to 4.0 tones per ha, which is considered to be normal. Applications of the phosphate rocks did not increase yield at any of the rates. Phosphorus fertilizer also did not increase yields on the conventional farm, even with low levels of soil test phosphorus, suggesting that phosphorus may not have been a limitation to the buckwheat crop.

We were also interested in finding out if the rock amendments would increase the concentration of phosphorus in the plant tissue. When the plants are ploughed in they will decompose and make the phosphorus more available to the next crop. We found that Calphos at all application rates and Pebbled at the high rate of application increased phosphorus uptake in the buckwheat. Tennessee, Kapuskasing and Spanish River Carbonatite did not increased phosphorus uptake of buckwheat on any of the field sites.

These differences are most likely related to differences in the characteristics of the amendment and where they originated. Rocks from the Southeastern US (Pebbled, Tennessee, and Calphos) contain P minerals that are more soluble than the Ontario rocks (Spanish River Carbonatite and Kapuskasing).

The particle size of the phosphate rock can also have an impact on solubility. Phosphate rocks with a small particle size, such as Calphos, have an increased surface area that may come in contact with roots. Calphos comes as soft aggregates of clay-sized particles, while the other rocks have a coarser, sandy texture. The Pebbled and Tennessee phosphate rocks likely would have been more effective if they were finer.

We only saw a buckwheat response when 7 to 8 tonnes per ha of phosphate rock were applied. Before deciding whether to use phosphate rocks on your farm, find out how much soluble phosphorus is contained in the rock as well as the grain size of the rock. Despite the potential promise of using phosphate rocks, the application costs may be too high to make their use a viable option.


Melissa Arcand is a Consultant for the Organic Agriculture Centre of Canada. Please send comments or questions by phone to 902-893-7256 or by email to oacc@dal.ca.


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Posted December 2007

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