From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Summer 1999, No. 5


Why do agronomists say a phosphorus soil test every three to four years is adequate when they recommend sampling soil nitrogen every year? The answer is simple, but the explanation is more complex.

Soil test nitrogen (i.e. nitrate-nitrogen) is mobile and moves with the soil water, so its levels can fluctuate greatly from one year to the next. In contrast, phosphorus is immobile, so it moves very little within the soil profile.

In the absence of plant growth and soil erosion, soil test phosphorus would change very little from year to year. But, as with nitrogen, soil test phosphorus declines due to crop removal, although crops remove less phosphorus than nitrogen. Also like nitrogen, some phosphorus is released from the organic matter as it decomposes. But phosphorus released from soil organic matter behaves differently than released nitrogen. Phosphorus is strongly held in the soil and quickly interacts with other soil minerals, which affect its plant availability. It doesn’t matter if the phosphorus originates from organic or inorganic sources, it behaves the same in the soil.

Buffer power is the big difference between soil nitrogen and phosphorus. Buffering simply refers to the ability to resist change. Applied to phosphorus, it’s the ability of the soil to maintain a certain amount of phosphorus in soil solution. A soil with a high buffer power is like a well that doesn’t run dry. Plants remove phosphorus ions from the soil solution, but they are quickly replaced from ions adsorbed to clay, from dissolving minerals or from organic matter. The capacity of the soil to re-supply phosphorus to the soil solution is the buffer power.

Routine soil tests do not measure the buffer power of the soil. They only measure the nutrients in solution and a fraction of the nutrients that may come into solution. Soils have different properties that affect their phosphorus buffering ability and the rate of phosphorus replenishment. Amount and type of clay minerals, organic matter, and other soil constituents are all important.

Buffer power increases with increasing clay content and increasing organic matter. A finer-textured soil with high organic matter is better able to keep enough phosphorus in soil solution for optimum plant growth than a sandy-textured, low organic matter soil and keep it there longer. This has big implications for soil testing and phosphorus fertilizer management.

Buffer power determines how quickly soil test phosphorus changes and the quantity of phosphorus required to change the soil test. Soil test phosphorus is usually easier to change on coarse-textured soil than on medium or fine-textured soil. Soils with high buffer power need more phosphorus fertilizer to increase their soil test level than soils with low buffer power. However, they usually do not need to be fertilized as often to maintain the soil test level.

Typically, 14 to 22 pounds of phosphate per acre are required to increase a phosphorus soil test by 1 part per million. Special soil testing procedures can determine the buffer power of a soil, but keeping good records of historical soil tests, crop removal, and fertilizer/manure application provides valuable information about the buffer power of a soil.

We can’t change the soil’s natural buffer power, but if we understand it, we can use that information to do a better job managing phosphorus.


For more information, contact Dr. Terry L. Roberts, Western Canada Director, PPI, Suite 704, CN Tower, Midtown Plaza, Saskatoon, Saskatchewan, Canada S7K 1J5. Phone: (306) 652-3535. E-mail:
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