Introduction

To determine if a P application is necessary we estimate a field P balance and use a soil P test (Olsen P) if available.

P balance: The P balance is calculated as the difference between what you apply and what is removed by the crop. Most P is removed in the grain but if you remove straw then you are also removing some P. Below we ask you to estimate straw removal, grain yields and fertilizer P additions. We suggest taking the average of the past 5 years.

Using this P balance we assume that the only P added to the field comes from fertilizer and that the only P removed is that which is removed when harvesting the crop (i.e. the P in the grain and the straw-if straw is removed). Burning straw does not remove significant amounts of P from the field. Also little P is added or removed in irrigation water. Finally P is not readily leached in these high clay soils.

Soil test: For the soil P test it is important that you have the Olsen P test. Sometimes this is called the bicarbonate P test.

Important Notice: UC Rice On-line has made every effort to ensure the accuracy and reliability of phosphorus fertilizer application information presented and produced by this online tool. However, we cannot guarantee 100% accuracy for every and all situations. Thus, the information and results presented here should be viewed and used only as a guideline.

Should I apply P?

The P balance is calculated as the difference between what you apply and what is removed by the crop. Most P is removed in the grain, but if you remove straw then you are also removing some P. Below we ask you to estimate straw removal, grain yields and fertilizer P additions. We suggest taking the average of the past 5 years for all of these estimated values.

*entry required
Over the past five years, how many years did you bail straw?*
Grain yield (cwt/acre; 5 year average)*
Fertilizer added per year (lb P2O5/acre/year; 5 year average) *

How much P should I apply?

Please complete the fields below to estimate a maintenance phosphorus fertilizer application.

*entry required
Over a five year period, how many years did you bail straw?*
Grain yield (cwt/acre/year; 5 year average)*

When should I apply P?

P can be applied during land preparation and it should be lightly incorporated into the soil.

If algae is a problem you definitely want to incorporate it into the soil if you apply during land preparation. This will help minimize algae. Another alternative is to apply P between 20 and 30 days after planting. By this time the rice is above the water surface and algae will not damage the crop. Do not apply after 30 days post-planting (especially if your soil P values are low). You should hold water for 2 weeks after application to avoid P loss in water.

NOTE: P in a starter blend usually has some N. This N needs to be accounted for in your overall P rate.

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Calculations and Further Reading

Phosphorus input/output budget

This is designed for a rice only rotation (or rice fallow). If you are rotating rice with other crops skip down to maintenance P applications.

We use fertilizer equivalents for phosphorus (i.e. P2O5 instead of P). To convert from P2O5 to P use the following formula: P = P2O5 * 0.43.

P budget (lb/ac) = P2O5 inputs (lb P2O5/ac) – outputs (lb P2O5/ac in grain and straw)

Inputs are those from fertilizer addition. We base this on the average input over the past 5 years. We assume little is added from irrigation water and other sources (see below for further reading material to support this).

Outputs are the P removed in grain and straw at harvest. We assume little is lost in irrigation water or to leaching (see below for further reading material to support this).

For average straw removal we base the budget on the number of times straw has been bailed in the previous 5 years. We assume that the amount of straw is the same as the amount of grain (i.e. a harvest index of 0.5) and that baling removes ½ of the straw. We assume that grain contains 0.52% P2O5 (0.23% P) and that straw contains 0.21% P2O5 (0.09% P).

Grain P (lb P2O5/ac) = Grain yield (lb/ac) * 0.52/100
Straw P (lb P2O5/ac) = Grain yield/2 * X/5 * 0.21/100 (NOTE: X/5 is the proportion of years the straw was removed)
P removed (lb P2O5/ac) = Grain P (lb P2O5/ac) + Straw P (lb P2O5/ac)

Maintenance P application (applying the amount of P removed through grain and straw removal)

This is the “P removed” from the above calculations which is the amount of P removed in grain and straw at harvest.

For further reading, the data to support this type of budgeting can be found in:

Linquist, B.A. and M.D. Ruark. 2011. Re-evaluating diagnostic phosphorus tests for rice systems based on soil phosphorus fractions and field level budgets. Agronomy Journal 103:501-508.

For further reading on P inputs and losses in irrigation waters refer to the following:

Krupa, M., K.W. Tate, C. Kessel, N. Sarwar and B.A .Linquist. 2011. Water quality in rice-growing watersheds in a Mediterranean climate. Agriculture, Ecosystems and Environment 144:290-301.

Linquist, B.A., M. Ruark, R. Mutters, C. Greer, and J. Hill. (2014). Nutrients and sediments in surface runoff water from rice fields: Implications for nutrient budgets and water quality. Journal of Environmental Quality 43:1725-1735.