Soil Test Note – No. L002

Dr. Stanley L. Chapman, Extension Soils Specialist and Phil L. Tacker, Extension Agricultural Engineer

 

 

 

LIMING CROPS ROTATED WITH RICE

 

Introduction

Lime applied on acid silt loam soils can increase soybean yields up to 10 or more bushes per acre.  Similar yield increases may be achieved by wheat, corn, and sorghum on very acid soils.  But, too much lime (liming to a pH above 6.5) may decrease rice yields.  Together with lime-containing well water, lime application can cause zinc deficiency in rice, especially near the water inlet.

 

This publication applies only to light-textured soils irrigated with well water currently or in the recent past – not to clay soils, in any case, nor to light-textured soils irrigated entirely from streams, lakes, or reservoirs.

 

Most Arkansas well waters contain lime in widely varying amounts.  As a further complication, lime deposits unevenly as the water crosses a field.

 

The highest rate of lime in well water are deposited on the upper half of the field nearer the well in the water flow path.  This conclusion is based on findings of University of Arkansas researchers as illustrated in Figure 1.

 

Figure 1.

 

The farther water flows over the field from the inlet, the less lime is deposited on the soil.  Thus, the lower half of a field more distant from the well receives smaller amounts of lime.  On the upper half, lime may accumulate excessively for rice, while on the lower half, soil acidity may build up to a harmful level for soybeans and other crops.

 

Managing to Avoid Over-Liming for Rice

To prevent nutritional problems resulting from lime application, take special soil and water samples as follows and let the results guide you.  This sampling is in addition to normal sampling to represent the majority of the field.

 

1.                  Special Soil Sampling (See Figure2 and 3, next page)

 

a.    Take a soil sample from a minimum of 10 places covering about 2 acres near the point where water enters the field (the inlet area) and mix them together.  This sample should represent the highest concentration of lime which has been deposited by the water.

 

b.      Take another soil samples from a minimum of 10 places covering about 2 acres near the point where the water drains from the field (the outlet area) and mix them together..  This sample should represent the least concentration of lime deposited by the water.

 

c.       It may also be desirable to collect soil samples from the middle of the field for comparison.

 

Figure 2 and 3

 

2.         Well Water Sampling 

Get water tests from the University of Arkansas as follows:

 

a.          Get water bottles and information sheets from the county Extension office.

 

b.          Start the pump and let it run until the system has been completely flushed – at least 15 minutes – before taking a sample.  Ideally, take the sample while the field is being irrigated.  Fill bottle to overflowing and cap tightly to  avoid air space.

 

c.          Take the water sample and information sheet to the county Extension office the same day you sample the well.

   The longer the water stays in the bottle, the more the minerals tend to settle out, resulting in inaccurate analyses.

 

If you don’t collect the sample in an Extension water bottle, be sure to provide the county agent with at least 1˝ quarts of water as soon after sampling as possible.

 

Results of water tests are entered into a computer program that predicts soil changes or cropping hazards, associated with the water quality for different rice and soybean rotations.

 

Lime Recommendations

Recommendations for field crops rotated with rice are based on the lime rates in the following table.  These rates are considered safe for rice, provided the area is uniform in soil pH.

 

The lime rates in the table are for lower parts of the field regardless of the calcium content of the water, but only for the upper part when well water contains less than 3 milliequivalents of calcium per liter (60 ppm).

 

Recommended Lime for Crops Rotated with Rice

 

Upper Part of Field

 

Topsoil pH

Lower Part

of field

Irrigation Water

Calcium 60 ppm or above

Irrigation Water

Calcium below 60 ppm

 

-------- tons of limestone per acre --------

5.6 and above

5.3 to 5.5

5.0 to 5.2

4.9 and below

0

1

1.5

2.5

0

0

0

0

0

1

1.5

2.5

*Application of 0.4 ounces of actual molybdenum as a soybean seed treatment can help avoid nitrogen deficiency.  Mo is especially needed in the legume nitrogen fixing process when the soil pH is less than 6.0.  Mo is recommended up to pH 7.0.

 

 

Additional Precautions

 

1.       Where needed for rotation crops, apply lime immediately after the rice crop – as far in advance of the next rice crop as

      possible.  Distribute lime evenly, and mix well with the soil.  “Wind-rowing”

or overlapping can raise the soil pH too high and induce zinc deficiency in rice.

 

2.       If fields test between pH 5.5 and 6.0 take soil samples again in three years, especially if reservoir water or low-lime well water is used.

 

3.       As an extra precaution against over-liming for rice, take detailed samples within an area that tests below pH 5.5, especially where well water has been previously used.  This is increasingly important with increasing frequency of rice in the rotation and increasing irrigation of soybeans and other crops.

 

4.       Remember that well waters are highly variable.  Some cause soil pH to increase.  Others cause soil pH to decrease.  If soil pH remains below 5.6 after being watered from a given source, apply lime.

 

5.       When reservoir water is used for several years after well watering has been discontinued, subsoil sampling (6-12 inches) is good insurance against over-liming for rice.  The pH of the subsoil may remain high even though the topsoil pH may have dropped.  If the subsoil pH in a part of a field tests pH 5.8 or higher, use only 1 ton of lime per acre when lime is recommended.

 

Summary

 

Correction of strong soil acidity is essential for profitable production of crops rotated with rice.  But careless liming can be disastrous for rice.  Before applying lime where rice will be grown:

 

  1. Take soil samples from the water inlet and outlet areas in each field.
  2. Get irrigation water tested. 
  3. Do not apply lime on areas testing above pH 5.5. 
  4. Remember that soil in different parts of the same field can vary widely in pH. 
  5. Water from similar wells may still have differences in lime and salt concentrations. 
  6. Where needed for rotation crops, apply lime immediately after the rice crop – not just before the rice crop. 
  7. Distribute lime evenly, and mix it well with the soil.  “Wind-rowing’ or overlapping can induce streaks with zinc deficiency in rice.
  8. Precautions herein do not apply to clay soils or to stream, lake, or reservoir water. 

 

The Arkansas Cooperative Extension Service offers its programs to all eligible persons regardless of race, color, national origin, religion, gender, age, disability, marital or veteran status, or any other legally protected status, and is an Equal Opportunity Employer.

 

 

 

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