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by Jon Cloud


Farmers, your soil’s level of acidity (pH) and calcium content help determine whether the application of rock phosphate will bring you bumper crops. Prior to applying rock phosphate, take into consideration 1) the rate at which the phosphorus is able to be dissolved and 2) the rate of phosphorus released once it has been placed on your soil.;

The rate of application of Carolinian 0-30-0 rock phosphate is 250 pounds per acre. This application rate should occur every other year on soils with low to moderate phosphorus levels (under 15 parts per million) and a soil pH which is acidic. Be sure to have a soil test performed after the second application to check your phosphorus level before making a third application. It is important to note that the availability of rock phosphate favors low soil pH (acid soil), low exchangeable calcium, and low concentration of P in soil solution.

When a particle of rock phosphate is dissolved completely in your soil, it releases ten units of calcium and six units of phosphorus. This means that when you apply your rock phosphate to the land, you are actually putting on 1.7 times more calcium than phosphorus. This is beneficial in that both calcium and phosphorus are introduced in one shot. A soil’s ability to separate the calcium and hold it promotes the availability of rock phosphate to the plants. Thus the soil acts as a sink for the calcium that is released by the rock phosphate. This ability of the soil to separate the calcium from the phosphorus in the rock phosphate is even higher when the calcium percentage in your soil is low.

You also need to watch the pH of your soil. Liming acid soils with any calcium-containing material such as limestone, lime or gypsum always results in a reduction of rock phosphate’s ability to become available to the plants. This is because both your soil’s pH and exchangeable calcium are raised upon liming. That means the soil becomes less acid and you get less of the phosphorus which is already in your soil and less phosphorus from the rock phosphate you spread after liming.

When there is an abundance of calcium in the soil, whatever rock phosphate you apply to the soil, the soluble phosphorus will first join with the available calcium to form a stable mineral. Only the phosphorus which is left over after the calcium bonding will be available to be taken up by the plants. When your soil test indicates over 5% organic matter, the activity level of all creatures in the soil is high enough to assist in breaking the bond and making phosphorus available to the plant.

Therefore, it is important to check the acidity level and calcium content of your soil before you apply your rock phosphate or gypsum.


Jon Cloud works with organic farmers and will help you interpret your soil analysis. For more information, call him at (416) 762-0940.



Copyright 1996. Jon Cloud.


Reprinted with permission. All rights reserved.

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