Phosphorus Levels In Colorado Soils

Categories low, medium, high, and very high are based on crop response.

Evaluating phosphorus in the soil relative to P fertilizer application is an important part of any P test. According to the ammonium bicarbonate-DTPA test that is used at the Soil, Water, and Plant Testing Lab at Colorado State University, the critical levels for determining fertilizer application rates have been set at 0-3 ppm P for the low category, 4-6 ppm P for the medium category, 8-11 ppm P for the high category, and greater than 11 ppm P for the very high category.

Each category was determined by calibrating crop response to nutrient rate experiments in the field. The steps in calibration studies are 1) determine the critical levels which are the numbers below which there is a fertilizer response, 2) make comparisons between predicted responses based upon the critical levels and field observed responses, and 3) determine the availability indices and amount of nutrient needed to obtain the maximum yield. The low category indicates that with a maximum level of P fertilizer applied, there is a high probability of crop response to the added P fertilizer. In the medium category, there would be a moderate probability of response to added P. In the high category there would be a low probability of response, while in the very high category there likely would be no crop response to additional fertilizer.

In the fiscal year of July 1, 1998 to June 30, 1999, the Lab analyzed many surface soil samples sent in by farmers. Of the soils received, 29.4% were in the low category (0-3 ppm P), 26.4% in the medium category (4-7 ppm P), 14.1% in the high category (8-11 ppm P), and 30.1% in the very high category (>11 ppm P). The percentage of soils testing high or very high in P amounted to 44.2% of the total.

Since a large percentage of soils analyzed were high in P, it is important that agricultural soils are tested routinely for phosphorus to avoid adding unnecessary P fertilizer.

by James Self

Phosphorus Quiz:& True or False?

1. Phosphorus does not move through the soil.
2. High pH soils are infinite sinks for phosphorus.
3. Agricultural runoff does not contain enough P to cause water quality problems.
4. Erosion control will stop phosphorus losses in runoff.
5. Soil test P level is a good indicator of P loss hazard.
6. Manure rates should be based upon N loading.
7. We don't know enough about agricultural P losses to implement solutions.

Answers

1. FALSE. P can move down through soils.  Most of the time, P moves with runoff and erosion.  BUT P can leach in soils with low P-fixing capacity, high percolation, or high soil test P.  Organic P seems more prone to leaching than inorganic P.



2. FALSE. Any soil can be saturated with P.  The more lime (or calcium carbonate) a soil has, the more P can be fixed, BUT this fixation is not infinite. When soils are saturated with P, then P becomes more mobile.



3. FALSE. We sampled 38 irrigated fields sampled in Colorado in 1998 – 1999, and found total P levels in runoff that ranged from 0.1 - 1.8 mg/L. Total P thresholds are 0.05 mg/L in lakes and 0.10 mg/L in streams.



4. FALSE. Runoff P is also a concern.  Most P moves attached to soil particles that move in the erosion process, BUT there is still a substantial fraction of soluble P that can move in runoff water.  Therefore, erosion control will substantially reduce P loss, but will not stop it completely.



5. TRUE, BUT. . . Increasing soil test P increases runoff P, BUT the relationship between soil test P and runoff P is different for each soil type. Transport factors such as slope and irrigation practices have a big influence on this relationship.



6. TRUE AND FALSE. When manure rates are based on N loading, overapplication of P occurs. If this is repeated year after year, soil test P levels climb, AND more P runoff will occur.



7. FALSE. We know enough about the P problem and how P moves, that we can act now.  Let's not wait till the problem is so severe that we have no choices.