Information from 1996 Missouri Rice Research Update.

Control of Algae in Rice Field Soils1

Steven Hefner, Ray Archuleta, Andy Kendig, and Gene Stevens2

Waterseeding rice culture is a viable alternative to Missouri rice producers who continue to battle red rice infestations. This production practice usually involves early flooding of zero slope fields to suppress red rice weed germination. Fields are then planted by broadcasting pre-sprouted rice seeds into the floodwater. Although early flooding suppresses red rice, it can promote algae formation. Algae proliferation is often detrimental to rice stand establishment, especially in fields with high inoculation levels.

Control methods for algae control are limited at best. Draining fields is usually not an option because of loss of agrichemicals in floodwater. Draining fields after algae mats have formed can weigh emerging seedlings downward. The absence of floodwater also promotes emergence of weed species (e.g. sprangletop, barnyardgrass, red rice). Applications of granular copper compounds have been used with mixed results. Successful algae control with copper sulfate seems to be linked with early scouting and application.

Although never tested for rice production, a double salt of ammonium sulfate and aluminum sulfate (commonly called alum) has properties that could inhibit algae growth. Alum is frequently used in waste water facility plants to precipitate solids in water. Recent research in Arkansas showed that alum applied to litter in chicken houses reduced ammonia gas levels. Alum applied to chicken manure also reduced phosphorus solubility and runoff into streams when manure was applied to pastures.

A greenhouse study was initiated on March 11, 1996 at the University of Missouri Delta Center, Portageville, MO. The objective of the greenhouse study was to screen possible algae control strategies. Treatments which show merit can be further tested both in the greenhouse and in the field.

Materials and Methods

Five gallon glass tanks (8 x 16 inches) were filled with 4-6 inches of soil (description) from a problematic rice field. Seventeen unreplicated treatments (Table 1) were evaluated for algae control.

Nitrogen fertilization varied by source, quantity applied, and application timing. Tanks 1-7, 9-12, 15, and 17 received the preflood equivalent of 90 lbs N/a as urea. Tank 14 also received urea but at a lower N quantity (45 lbs N/a) and a delay application timing (postflood at seedling emergence).

Tanks 8 and 16 received ammonium sulfate as the N source. Preflood equivalent of 90 lbs N/a was applied to Tank 8, while Tank 16 received 45 lbs N/a applied into floodwater at seedling emergence. Tank 13 received liquid aqua ammonia (ammonium hydroxide) knifed preflood into the soil. All urea and ammonium sulfate N treatments were broadcast onto dry soil and immediately flooded to a depth of two inches.

Table 1. Treatments included in the initial screening experiment.

Tank          Treatment          Tank          Treatment
1          Earthtec 5 lbs/a       10        Alum
                                            100 lbs/a preflood

2          Earthtec 8 lbs/a at    11        Copper Sulfate
            rice emergence                  + Bolero
3          Earthtec 8 lbs/a at    12       Command preflood 
            2 days post flood               0.5 lb a.i.

4          Copper Sulfate 5       13       Aqua Ammonia preflood
            lbs/a preflood                   

5          Copper Sulfate 5       14       Urea at rice emergence
            lbs/a preflood                  

6          Copper Sulfate 8       15       Untreated check
            lbs/a at rice emergence     

7          Copper Sulfate 8       16       Ammonium Sulfate at  
            lbs/a at 2 days                 rice emergence
            post flood
8          Earthtec 5 lbs/a       17       Copper Sulfate +
            preflood                        water pH 2.0

9          Alum
            1000 lbs/a postflood

All copper sulfate and Earthtec* applications were applied with a pressurized backpack sprayer. Earthtec* is a liquid copper sulfate formulation. Alum (aluminum ammonium sulfate) applications were broadcast preflood onto soil in tank 9 and into an algae bloom in floodwater in tank 10. Tank 17 was acidified after flooding with one sulfuric acid application. An arbitrary scale was used to visually rate tanks for algae growth at least every other day (Table 2). The pH level of floodwater was monitored at least every other day with a Cardy Twin pH B-113 portable pH meter.

Rice (Oryza sativa L., cv. L202) seeds were subjected to both a 24 hour wetting and subsequent 24 hour drying period prior planting. Rice was planted on March 15, 1996 at a seeding rate of 52 seeds/tank. Warm temperatures and a shallow flood accelerated seedling emergence. Plants were out of the water in 3 days. Stand counts were made on March 21, 1996. Whole plants were collected, oven dried (110 F for 2 days), and weighed for biomass measurement comparisons.


Greenhouse conditions were optimum for algae proliferation. Daytime soil temperatures approached 95F and overnight low temperatures were 80F. Within 30 hours of flooding, algae had developed at least 50% of the soil surface in half of the tanks. Tanks receiving either copper compounds or postflood N management had less initial algae growth than the untreated check with preflood urea. The algae colony was identified as a mixture of Eucaryotic algae containing a filamentous Ulothrix species and two flagellated species, Euglena and Chlamydomonas3.

Table 2. Visual rating scale for algae growth.

Rank Description
1    clean tank 
2    25% of the soil surface covered with algae
3    50% of the  soil surface covered with algae
4    66 % of  soil surface covered by an algae 
5    83% of soil surface covered by an algae 
6    100% of soil surface covered by an algae 
7    algae floating on water surface
8    33% of  water surface covered by algae
9    33 - 66%  of  water surface covered by algae
10   100% of the water surface covered by algae

Copper sulfate dissolved into solution and Earthtec* performed at the same level of effectiveness. Granular copper sulfate is somewhat difficult to dissolve into solution. Preflood application timings were lesseffective than postflood applications. Tanks receiving both preflood applications of N and 5 lbs/a of copper sulfate slowed algae growth from 3 to 5 days. Acidifying the floodwater to an initial pH of 2.0 extended control of algae with copper sulfate to 7 days.

Increasing the quantity of copper sulfate to 8 lbs/a and applying into floodwater before the algae colony floated to the surface extended control to 7 days. The copper sulfate applications were made two days prior to planting. These applications at 8 lbs/a of copper sulfate equivalent decreased rice stand counts for both Earthtec* (16 plants/tank) and copper sulfate (6 plants/tank). Plants that did emerge were chlorotic even though preplant N had been applied.

Nitrogen fertilizer stimulated algae growth. Tanks receiving preflood urea or ammonium sulfate, but no copper containing compounds experienced immediate algae pressure. Delaying the N until rice seedlings emerged slowed algae growth. Subsequent fertilization further stimulated algae proliferation. No differences in algae growth were noticed between urea and ammonium sulfate N sources. Knifing aqua ammonia into the soil was superior to broadcasting either urea or ammonium sulfate on the surface. The aqua ammonia tank had the most plants emerging (42) of any in the test.

Both preflood and postflood applications of alum were included in the test. The preflood application at 100 lbs ammonium alum was not effective, but some inhibition of algae growth was observed. Applying 1000 lbs/a postflood into an floating algae mat was highly effective.

The postflood application of alum lowered the floodwater pH from 8.2 to 4.2, The algae mat soon settled from the water surface to the soil surface. It is unclear if the algae control experience from this treatment was the result of the pH floodwater shift, the physical sedimentation of the algae bloom, or possibly a deprivation of available phosphorus brought about by the aluminum in the alum.


Copper sulfate compounds were somewhat effective at controlling algae growth under accelerated conditions. Soil applied preflood applications were less effective than applications into floodwater. Delaying N until rice emergence was also effective at slowing algae growth. However, postflood applications of urea are also terribly inefficient for a production basis. The alum treatments show merit for further testing.

At the onset of the experiment, we were anxious about getting the algae to grow in the greenhouse. Warm conditions were established to enhance algae growth. A replicated test is now underway in a cooler environment more representative of seasonal May temperatures. This study includes copper sulfate sources, quantities and timings. Floodwater pH levels and alum quantities are also being examined. Field test are planned for the upcoming season.

1The authors express gratitude to Gaylon Lawrence Farms for financial support of this project.
2Research Associate, University of Missouri Delta Center; Soil Conservationist, Natural Resource and Conservation Service; Extension Weed Scientist and Extension Soil and Crop Production Scientist, University of Missouri Delta Center.
3Algae identification curtesy of Dr. Philip Colbaugh, Texas Ag. Exp. St., Texas A&M University System, Dallas, TX.

* registered trademark

Back to Missouri Rice Farm Research Reports