Western Canadian Rock Phosphate Incubation Study
By Elston Solberg
Phosphate (P2O5) is a vital element for plant growth and generally considered the second most important plant nutrient after nitrogen. The base source for most phosphate fertilizers is rock phosphate. Direct application of rock phosphate has been demonstrated to be effective for providing plant available phosphorus in acidic soils but rock phosphate effectiveness is known to decline as soil pH increases because some acidity is required to convert the phosphate within rock phosphate to a water soluble plant available form of phosphate.
Sulphur is generally referred to as the 4th macronutrient and is becoming increasingly deficient in western Canadian soils. Typical fertility programs are currently removing more P, S and K than what growers are adding meaning that the deficiency of these 3 nutrients will continue to increase. The application of elemental sulfur (ES) has been shown to improve plant available SO4-S sulfur, although, it has been demonstrated that plant available sulfur can be improved by blending ES with various types of compost material as has been done with the product Bio-Sul. Rock phosphate is a certifiable organic crop input. Numerous research studies have indicated that phosphate availability in neutral to slight alkaline soils can be improved by blending rock phosphate with other organic soil amendments such compost, sulfur, sulfur compost blends and other elemental and bacterium blends.
This study compares plant available phosphate and plant available sulfur using a check treatment and 9 other various combinations of rock phosphate, compost, ES, and Bio-Sul on a pH neutral (pH of 7.1), phosphate deficient (4 ppm M3 extraction), western Canadian soil in a laboratory environment after a 2 week, 4 week and 8 week incubation period. See Table I below for study details.
Overall, phosphate availability was maximized with (from best to lowest):
- Bio-Sul + rock phosphate
- ES + Compost + rock phosphate
- ES + rock phosphate
- Compost + rock phosphate
- rock phosphate
- Compost + ES
- Check or Nil treatment
See Table II below for all study results.
The data indicates that on a pH neutral, phosphate deficient western Canadian soil, that the rock phosphate application doubled phosphate availability (M3 extracted) after 8 weeks or by a factor of 1.2x to 2x (over the check and three time periods) and that the use of ES and rock phosphate tripled phosphate availability after 8 weeks or by a factor of 1.8x to 3x.
The addition of compost to the rock phosphate and ES further increased phosphate availability by 50% after 8 weeks or by factors of 1.7x to 4.5x. The use of rock phosphate with Bio-Sul improved phosphate availability over compost, sulfur and rock phosphate an additional 44% after 8 weeks or by a factor of 3.3x to 6.5x (over check and three time periods).
Plant available sulfur was maximized with the use of Bio-Sul followed by the compost and ES, ES alone and the check. As expected, rock phosphate appears to have zero to very limited impact on available sulfur while Bio-Sul/ES had a high degree of influence on phosphate availability due to the acidic nature of sulfur.
The lower water solubility of rock phosphate versus commercial fertilizer can be beneficial because phosphate is available over a longer period of time and not prone to phosphate leeching (as is a concern with commercial fertilizers).
On pH neutral and alkaline soils rock phosphate should be blended with other crop inputs such as Bio-Sul or ES to maximize phosphate availability.
These rock phosphate blends provide another option for growers who wish to address their long-term P and S crop needs.
Table I: Soil Inputs and Incubation Conditions
Table II: Results