WO1986006061A1

WO1986006061A1 - Fertilization of crops

Info

Publication number: WO1986006061A1
Application number: PCT/AU1986/000093
Authority: WO
Inventors: Kathleen H. Bowmer; Warren Alexander Muirhead
Original assignee: Commonwealth Scientific And Industrial Research Or
Priority date: 1985-04-11
Filing date: 1986-04-11
Publication date: 1986-10-23
Also published as: AU5603486A

Abstract

Method for treating a crop by applying as a fertilizer an ammoniacal nitrogen source and an algicide or algistat which inhibits loss of available nitrogen.

Description

This invention relates to a new and improved method of fertilizing growing crops.

The yeild of many crops is limited by the supply of nitrogen in the soil. In view of this it is practice to add artifical fertilizing agents that supply nitrogen to soils.

In the case of ammoniacal nitrogen sources such as urea a high proportion of nitrogen is lost and not made available to the plant. It has been estimated that in excess of 50? of nitrogen added to soils is lost and this has been found to be particularly so in the case of wetland crops such as rice where land is flooded during the cultivation of the crop.

The object of the present invention is to provide means for reducing the loss of nitrogen added to soil by way of ammoniacal nitrogen sources. In accordance with the present invention it has been found surprisingly that if an .algacide or algistat is present in the soil with added ammoniacal nitrogen sources available nitrogen is increased. - Therefore in one form the present invention provides a method of fertilizing crops wherein there is applied to the soil an ammoniacal nitrogen source and an algacide or algistat. The ammoniacal nitrogen source and algacide or algistat may be applied together or in any order as will be further discussed in detail below.

In another form the present invention provides a fertilizing composition comprising an ammoniacal nitrogen source and an algacide or algistat. The fertilizer composition may be a mixture of an ammoniacal nitrogen source such as urea and a algacide algistat or may be comprised by the separate components for sequential application to the soil or for mixture prior to application.

The invention is not restricted to any theory on the mode of activity or action. However, it is believed that algae present in the soil contributes to the loss of available nitrogen. For example when urea is applied to flood water it is hydrolysed to ammonium ions at the soil surface. Nitrogen is subsequently lost by volatalization of ammonia and by de-nitrification as oxides of nitrogen. Alkaline conditions (that is high pH) favour the formation of volatile ammonia gas. De-nitrification occurs when ammonia is converted into nitrite and then nitrate salts in the aerobic zone in the sediment layers of soil. De-nitrification occurs in the deeper anaerobic zones with a consequent loss of gaseous nitrogen and the nitrogen oxides. It is believed that the pH of flood water associated with growing crops will change substantially throughout the day. The pH may vary by as much as 1.1 pH units between morning and afternoon. At' high pH's that are more likely to occur late in the afternoon ammonium ions are converted to the volatile ammonia form. The pH of conversion is about 9.3 and the effect of pH on the ratio of ammonium ions to ammonia may be illustrated by the following table:

pH NH3/NH4+ratio

7.0 10~^2*3 0.005

7.5 lO^"1'⁸ 0.016

8.0 10^"1*3 0.05

8.5 10^"0*8 0.16 ' 9.0 lO^"0,3 . 0.5

9.5 10⁺⁰-^{2 *} 1.6

It is also believed that dense populations of phytoplankton or algae tend to deplete the carbon dioxide present in water during photosynthesis with an associated rise in pH. Between about pH 6.3 and 10.3,. the equivalent of one proton, is consumed per unit of carbon fixed. It follows that areas of high algae content will tend to have a high pH and this will result in an increase in the loss of available nitrogen in the form of ammonia.

Rice paddies are highly eutrophic often with dense algal populations or "slime". It is believe that by use of the present invention the algal content is reduced and that this results in a reduction in the volatalization of ammonia. By inhibiting algal photosynthesis by the use of an algacide or algistat pH can be retained at about neutral and nitrogen conserved in the ammonium form in the soil and available for use by the plant. A wide range of ammoniacal nitrogen source materials may be used in accordance with the present invention, as will be appreciated by those skilled in the art. Fertilizers containing urea are particularly suitable as a source of ammoniacal nitrogen and the formulations of urea commonly used as fertilizers may be employed in accordance with the present invention. The ammoniacal nitrogen source material may be mixed or used together with other fertilizers such as phosphates and other phosphorus containing materials. The choice of a suitable algacide or algistat is important. The algacide or algistat should be effective in the inhibition of algal photosynthesis. Preferably the aglacide will also have selective weedicidal properties whereby to reduce the weed population around the growing crop. It is also desirable to select an algacide or algistat that is long lasting and environmentally acceptable, that is to say, that will not enter the food chain and will not damage aquatic organisms and fish.

It is desirable that the concentration of the algacide effective for preventing photosynthesis of the algae be less than one tenth the acute LD50 concentration, that is a concentration to which fish are exposed for a period of one to four days. Further, the algacide should not accumulate in fish tissue at concentrations more than five times the surrounding water concentration or give pathological effects which could be deleterious. Preferably the algacide is effective in hard and turbid water. Turbidity can be a natural consequence of water quality or caused by wind induced stirring or puddling. Preferably also the algacide is suitable for application in water-run methods of dispersing water through.the crop. Otherwise there is a tendency for the algacide to be absorbed close to the inlet of the field.

Furthermore the algacide is preferably effective on the range of algal flora contributing to the photosynthesis and pH fluctuation including phytoplankton epiphyton and periphyton filamentous algae and stoneworts such as chara and nitella. It will be appreciated that not all algacides and algistats will be effective against all algal flora that contribute to photosynthesis. It is preferable to obtain and use an algacide that is effective against most flora or alternatively to use a combination of algacides and algistats- that together contribute to the appropriate reduction in photosynthesis levels. Preferably also the algacide selected should be effective in controlling photosynthesis for a period of between 1 and 5 weeks. Preferably the first order half life in water and sediment is between 1 and 4 weeks. Preferably persistence is not excessive or the algacide may be deleterious to non-target organisms when water is drained or to subsequent or farming enterprises. Preferably concentration in water declines to an acceptable level such as about 0.1 parts per million within 4 weeks of application. The concentration levels which are acceptable will depend on the activity of the compound on non-target organisms as will be appreciated by those skilled in the art.

Importantly the selected algacide or algistat^"- in the concentrations necessary for effective activity in controlling photosynthesis does not damage the crop itself. A particularly suitable algacide is that known as

TERBUTRYNE. Terbutryne is-the common name for the compound 4-ethylamino-2-methylthio-6-t-butylamino-l,3,5 triazine. Terbutryne is available as a wetable powder herbicide under the trade mark IGRAN supplied by Ciba-Geigy Australia Limited and is available in granule form under the trade mark CLAROSAN supplied by Ciba-Geigy Australia Limited

Terbutryne is a very effective algacide at concentrations which do not damage aquatic organisms and fish. Terbutryne is used in the control of submerged weeds and algae in fresh water. The wetable powder is used in selective weed control in wheat. When applied to rice crops in a concentration of about 0.2 kg/hectare, with a water depth of about lOcms terbutryne is useful in the control of algae. It has a first order half life of between 9 and 20 days which means that it will maintain an effective pH control for a period of about 1 to 2 weeks. Terbutryne is effective both as an algacidal agent in that it kills existing algae as well as an algistatic in that it prevents the growth of algae after application. Previous studies indicate that use of urease inhibitors, delays the appearance of ammoniacal nitrogen in floodwater by slowing down the hydrolysis of urea. However, such inhibitors. do not reduce the maximum concentration in floodwater.

It has been found that a urease inhibitor can be used in conjunction with an algicide in accordance with the present invention to maintain ammoniacal nitrogen levels in the water at both higher concentration and for larger periods. Thus, the invention provides for the addition of a urease inhibitor. A number of examples illustrate the present invention.

Example 1 Test for algicidal activity (reflected in control of pH rise in the afternoon and reduction in photosynthetic oxygen production) .

Floodwater had been applied to rice 2-3 months previously and the crop was well-emerged from the water when the experiment began. Microplots of about 0.3m were inserted to isolate the floodwater. There were 3 treatments each of 3 replicates.

In the first treatment regimen the microplot was not disturbed and no additions were made. In the second treatment regimen no chemicals were added but the microplot was puddled. in the third treatment regimen nitrogen in an amount of 80 kg ha^" was added as urea and phosphorus in an amount of 18 kg ha^" was added as phosphoric acid. Six days later terbutryne in an amount of 0.2 mg L^" was added to the floodwater in granular form. The water was 7-10 cm deep. The temperature range was about 20-32°C. A dense felt like algal mat was present in the third treatment regimen, when observed just before the addition of terbutryne. The mat was held together by filaments of Spirogyra and Oscillatoria, and contained massive numbers of diatoms and bacteria. Diel fluctuation of pH developed in all the treatment regimens and was most pronounced in Treatments 2 and

3 as shown in Figure 1. It will be noted that the addition of terbutryne on the sixth day had the effect of reducing pH overall and in addition damping the fluctuations in pH. Oxygen levels were measured and it was found that in the microplots following the third treatment regimen oxygen levels were lower then in the other microplots. This illustrated an effective control of photosynthesis. Example 2

In this example the association between prevention of pH rise and reduction of loss of ammonia was demonstrated by measuring ammoniacal nitrogen (ammonium ions plus ammonia; (NH.⁺ + NH,)) remaining in the water. This example differs from example 1 in several respects, but in particular, the biological system being treated was considerably different. There was no algal mat at the soil surface, or visible evidence of algae in the water. Consequently the terbutryne would only be successful if it were algistatic (preventing the growth of algae), whereas the previous example was a test of its algicidal properties.

As in example 1 the floodwater had been applied to the rice several months earlier, and the crop was well-emerged. Fertiliser and terbutryne were added together.

2 Microplots (about 3 m ) were treated in duplicate.

The water was 7-10 cm deep. Water temperatures ranged from about 12 to 30°C. Either flowable or granular formulations of terbutrynewere used at 0.2 mg L~ of active ingredient.

Treatment Fertiliser/* N P No. algicide kg ha^"1 kg ha^"1

1 No fertiliser 0 0 or algicide

2 N 80 0

3 N + P 80 20

4 N + P + CLAROSAN 80 20

5 N + P + IGRAN 80 20

* Fertiliser phosphorus (P) added as phosphoric acid; fertiliser nitrogen (N) added as urea.

In this experiment diel pH fluctuations developed rapidly in fertilised plots without algicide but the afternoon pH rise was prevented or severely depressed by both CLAROSAN and IGRAN treatments for at least 9 days. A frothy scum-like algal bloom dominated by diatoms and bacteria occurred in the plots fertilised by both N and P while the water remained clear for more than 7 days in the algistat treatments.

Fluctuations in pH and the visible development of an algal bloom dominated by Oscillatoria spp. occurred in the plots fertilised by nitrogen alone, while the algal flora in the control unfertilised plots was dominated by the filamentous green alga, Spirogyra spp.

The visible development of algal blooms and diel pH fluctuations in the non-algicide treatmentswas parallelled by late afternoon supersaturation with oxygen indicating the role of algal photosynthesis in controlling pH. In the algicide treatments oxygen concentration in the floodwater was maintained at 2-5 mg L~ , indicating suppression of photosynthesis.

Urea hydrolysis was rapid, with a first order half life of 18.4 to 19.5 hours and was not significantly affected by the presence of terbutryne. As the urea hydrolysed, ammoniacal nitrogen concentrations increased in the water.

Ammonia loss was reduced where the algistat was used to control pH, as indicated by chemical analysis of residual ammoniacal nitrogen (ammonia - nitrogen plus ammonium nitrogen) in the floodwater.

The ammonia gas concentration in equilibrium with the water was calculated using the equation.

A = C/U+10 exp [0.09018 + 2729.92/T-pH) where A is a aqueous ammonia (NH_,) concentration in the water; C is ammoniacal (NH. + H₃) nitrogen concentration in the water mg L^~ ) and T is temperature (°K) (Denmead, Freney and Simpson, 1981).

The aqueous concentration is the driving force in loss of ammonia from the water, and at any time gives a comparative measure of propensity for gaseous ammonia loss.

(The actual loss, assuming that the water is well mixed, depends on wind speed, temperature and specific site conditions). Terbutryne maintained the equilibrium ammonia gas concentration at lower values^' than treatments without terbutryne for about 6-7 days,

Treatment Yield t ha^"1

1 No fertiliser or algicide 5.3

2 N only 7.2

3 N + P 8.2

4 N + P + Clarosan 8.2

10 5 N + P + Igran 7.9

LSD (P = 0.05) 2.0

These results are illustrated in figures 6 to 11 hereto.

15 Example 3

Test for algicidal activity on the stonewort, Nitella, and late-season application.

2 Two plots of 9 m were marked out (but not isolated by microplots) in an unfertilised flooded rice field, in

20. mid-March, near Griffith, New South Wales. The soil surface was completely covered by a dense layer of the algal macrophyte, Nitella. Terbutryne was added as granular

CLAROSAN to one of the plots to give a concentration of 0.2 mg

L^" of active ingredient in about 10 cm depth of water.

25 Oxygen concentration and pH were measured before treatment and on the day after treatment.

This crop was sufficiently large to strongly shade the floodwater (the photosynthetically-available-radiation was reduced to 25? of its incident level at midday), and no

30 fertiliser had been used, yet photosynthetic activity was still evident as demonstrated by a rise of pH and oxygen cencentrations. Day Time Plot* pH 02

( 24-h +/- (mg/L )

Clock )

15 . 55 9. 0 8. 2

9.1** 8. 2**

2. 09 .15 8.6 7. 9

7.4 2. 5 12.10 9. 8 14.0

7.6 7.0 15. 35 9.8 12.1

7. 5 5. 8

* +/- Terbutryne as CLAROSAN (0.2 mg L active ingredient.

** Terbutryne was added immediately after taking the readings.

The temperature ranged from 24-29°C concentration in untreated plots in the afternoon.

These results are given in figure 2. They show that terbutryne prevented the mid-afternoon pH rise and also maintained the oxygen concentration at a low level demonstrating that photosynthesis was effectively inhibited.

Example 4 Comparison of potential algicides in a large plot field trial, test of pH control, ammoniacal-N retention, and toxicity to crops.

This experiment was performed early in the growing season. Fertiliser-nitrogen (50 kg ha^" ) as urea together with phosphorus (27 kg ha ) was applied together with the appropriate algicide only 3 days after introduction of floodwater. Water depth was about 10 cm, and the seedling rice (which had been drill sown about 6 weeks before was about

5-10 cm high. Algicide and urea were diluted and mixed together in a tank of water then introduced into the rice flood water through the submerged nozzles of a spray boom as it was pulled along over the plots at 1 s^" . This application method simulates the .even mixing obtained when algicides are introduced by dripping into the flood water with fertiliser, as the water is introduced onto the rice, and avoids the uneven distribution which might occur with a compound which is strongly adsorbed, such as copper sulphate.

2 Plot size was 6 x 15 m (90 m ), and the plots gave a water depth difference of about 3 cm between the 'shallow' and

'deep' ends of the plots. There were 4 replicate plots for each treatment; and subplot treatments were rice varieties m7 and Calrose. Water temperatures were typically 14-30°C.

Treatment Algicide Active : Ingredient No. in floodwater*

(mg L^"1 ) ^*

1 no addition 0 2 copper sulphate 2.0 (as Cu) 3 terbutryne 0.1 4 terbutryne 0.2 5 terbutryne 0.4 6 diuron 0.4 7 simazine 3 0 8 endothal-amine 0.2 (as acid) 9 simetryn 0.1 10 dichlorophen 5.0

* Assumes depth of 10 cm Biological composition and algal flora

The algal growth differed markedly in this experiment. The water was highly-turbid on introduction, but rapidly clarified, as an algal "slime" grew at the sediment surface. The slime was composed of cyanophyta (Lyngbya P.P group) together with much opaque abiotic material - possibly clay colloids and gels. In the control, non-algicide treatments, the algal mat developed bubbles during the day, and lifted off the sediment surface in palm-size clumps during the afternoon, settling again at late evening. The algal mat was present, but less well-developed in plots where the algicides were successful in controlling pH and photosynthetic oxygen production and because of the reduced formation of gas bubbles, the algal slime did not lift off the surface of the sediment.

Figure 3 shows the diel change in pH measured on the 4th day after treatment. Measurements were taken at intervals of 2 hours for treatments numbered 1 to 10. As noted in the previous examples the pH increased through the day, and reached a maximum in late afternoon (in this example about 18.30 hours) .

The algicides fell into 4 distinct groups based on effectiveness of pH control measured by maintenance of low pH in late afternoon on day 4 after treatment.

Effectiveness Treatment Algicide Cone'n No. (mg L^"1)

Good 5^d Terbutryne 0.4 a 2^cd Copper sulphate

• ( 3^C Terbutryne 0.1

Moderate Terbutryne 0.2 6^C Diuron (j 7^C Simazine

Poor f 9^b Simetryn tιo^b Dichlorophen

Ineffective 8^a Endothal-amine

The daily development in pH fluctuations is shown in Figures 4a to 4h and the following table.

Algici de Days after appli .cation of urea and/or algicides

Treatm ent aj ^• 1 day 2 day 3 day 4 min*. max* min max min max min max***

1 8.46 9.20 8.96 9.66 8.99 9.69 9.34 10.04^a

2 8.48 8.76 8.59 8.84 8.50 8.76 8.56 8.92^cd

3 8.30 8.64 8^'.36 8.70 8.32 8.69 8.56 9.00^c

4 8.30 8.52 8.36 8.64 8.38 8.68 8.50 9.04^C

5 8.34 8.44 8.43 8.58 8.27 8.50 8.36 8.72^d

6 8.40 8.56 8.47 9.73 8.46 8.78 8.56 9.08^C

7 8.33 8.78 8.58 8.79 8.44 8.88 8.92 9.12^C

8 8.46 9.08 8.90 9.52 8.91 8.54 9.30 10.02^a

9 9.34 8.90 8.66 9.12 8.52 9.14 8.94 9.56°

10 9.62 8.84 8.70 9.25 9.77 9.34 9.08 9.74°

LSD**a lgicide 0.19 0. 22 0. 24 0 .24

Time 0.076 0. 13 0. 12 . 0.15

Algici de Days after application of urea . and/or algicides

Treatment day 5 day 6 day 7 day 8 No. min max min max min max min max

1 9.26 10.22 9.29 10.12 9.12 10.07 8.78 9.82

2 8.66 9.16 8.76 9.44 8.90 9.70 8.86 9.62

3 8.60 9.24 8.80 9.50 8.82 10.00 8.88 9.96

4 8.71 9.36 8.89 9.64 9.01 9.92 8.99 9.92

5 8.43 8.84 8.42 9.08 8.59 9.51 8.71 9.77

6 8.68 9.32 8.84 9.75 9.00 9.90 8.74 9;43

7 8.84 9.30 9.76 9.56 9.02 9.66 8.98 9.86

8 9.39 10.26 9.40 10.10 9.10 10.08 9.04 9.96

9 9.11 9.82 9.24 10.03 9.27 10.11 9.03 9.81

10 9.29 9.96 9.34 10.03 9.20 10.06 8.84 9.84

LSD** algicide 0.32 0. 49 0.41 0.45 Time 0. 15 0 . 31 0. 20 0 . 28

* Minimum measured at 08.30 h, maximum at 16.30 h ** Least significant difference (P = 0.05) *** Figures followed by the same letter are not significantly different (P = 0.05).

This was a rather severe test of effectiveness because of the addition of. floodwater to compensate for percolation and evaporation losses on the third day after 0 treatment. (High percolation losses through the banks are an artifact of the small plot example), ϋnpredicted rainfall, in addition to the added irrigation water, increased water depth by 4-5 cm, diluting the algicide, and putting the rice seedlings under additional stress from submergence. 5 In this example, in contrast to previous experiments on this soil the early morning pH tended to increase within days after treatment, so that by days 3 and 4, the control non-algicide, plots were close to or above the pH of- the pK value of the ammonia and ammonium equilibrium (pH 9.25)1 20. Therefore, in windy conditions (which we.re prevalent in the experiment) it was anticipated that in the non-algicidal plots ammonia loss would occur very rapidly.

Ammoniacal nitrogen concentration

Control of pH by terbutryne increased the total ammoniacal nitrogen concentration in-the floodwater relative to the non-algicidal control. Crop phytotoxicity

Results for vegetative growth are as follows:

Yield Figures Quadrat size 0.6 x 0.5 m (0.3 m )

DRY WEIGHT (gm) PER QUADRAT

TRT CALROSE M7

NO . Treatment Date* Date

18.11.85 6.12.85 21.1.86 18.11.85 6.12.85 21.1.86

1 Control 1.1904 4.2472 78.1 1.2222 6.2459 72.0

2 CuS0₄ 1.4609 5.6177 74.0 1.4184 6.3042 73.3

3 Terb(low) 1.1203 3.8638 59.2 1.1570 4.1160 51.0

4 Terb( ed) 1.1408 3.2251 49.0 1.0832 3.2170 42.1

5 Terb(high)0.9138 2.9394 44.5 1.1408 3.5702 54.0

6 Diuron 1.1852 2.9001 56.2 1.3526 3.7745 48.1

7 Simazine 1.3455 1.8245 12.4 1.3981 1.1995 7.3

8 Endothal 1.6269 5.4866 63.2 1.3789 4.6228 57.2

9 Simetryn 0.9138 3.7766 58.7 1.0830 4.3607 48.7

10 Dichlorophen

1.2219 5.5123 69.1 1.3930 6.0261 52.1

PLANT NUMBERS PER QUADRAT

TRT CALROSE M7

No. Treatment Date Date

18.11.85 6.12.85 21.1.86 18.11.85 6.12.85 21.1.86

1 Control 61 40 60.5 74 68 62.75

2 CuSO. 67 84 74.0 71 72 55.75

3 Terb(low) 70 61 62.5 49 75 63.5

4 Terb(med) 65 47 56.75 48 52 56.75

5 Terb(high) 56 49 46.75 66 53 46.75

6 Diuron 65 54 55.5 85 63 60.5

7 Simazine 77 44 32.75 83 35 29.25 8 Endothell 80 74 73.0 82 63 70.75

9 Simetryn 57 68 62.0 69 71 63.5

10 Dichlorophen 58 71 69.5 86 80 67.0

YIELD FIGURES

Tmt CALROSE M7

No. Treatment Height Tiller Height Tiller

(cm) No. (cm) No.

1. Control 44.5 165.5 38.9 161.3

2. CuSo. 41.1 168.8 38.2 161.0

3. Terb (low) 40.7 136.8 35.1 126.3

4. Terb (med) 40.5 125.8 34.7 121.8

5. Terb (high) 38.6 116.5 37.2 129.0

6. Diuron 41.2 130.5 35.7 139.8

7. Simazine 34.3 79.3 27.9 60.0

8. Endothall 40.7 149.8 30.2 146.3

9. Simetryn 40.5 139.8 32.7 121.0

10. Dichlorophen 40.2 155.3 35.2 138.3

Phytotoxicity was evident in several treatments when measured early (2 weeks) after treatment with the general order of serverity summarised as follows:

Overall order of IR 34 severity of damage Phytotoxicity visual rating (% of leaves damages : Mean of 4 plots) _D= worst U0= least Treated 29.11.85 5.12.85 18.12.85 22.11.85

"Deep" "Shallow" "Deep" "Shallow" "Deep" "Shallow

Tmt Cal M7 Cal M7 Cal M7 Cal M7 Cal M7 Cal M7

1 Control 23 20 18. . 19 10 10 10 8 2 0 0 0

2 CuS0₄ 22 23 9 11 13 11 9 8 4 5 0 0

3 Terb 0.1 42 54 23 24 20 19 11 13 6 8 0 0

5 0.4 71 75 23 38 59 71 22 30 24 31 4 3 > 6 Diuron 61 71 30 34 56. 58 24 28 46 45 5 4

® 7 Simazine 88 86 40 ^' ^' 48 93 90 60 60 96 72 83 76 ® 8 Endothal 45 41 20 26 11 10 10 10 6 5 3 1

9 Simetryn 41 40 29 23 18 18 10 6 3 1 0 0

25 28 15 19 18 16 10 8 5 1 0 0

However to a large extent, the crop was able to compensate for early damage in later growth and tillering except for the most severe damage; notably that caused by simazine.

This .capacity for compensation of damage is illustrated for a 60-day harvest for variety M7 where plant numbers are reduced, in a trend with increasing terbutryne concentration, but quadrat dry weight vegetative bio ass was similar at low and high concentrations. Average figures (mean of 4 replicates) gave

Terbutryne Dry weight Plant Av. wgt

(mg L^"1) (g/quadrant) Nos g/plant

0 72.0 62.8 1.15

0.1 51.0 63.5 0.80

0.2 42.0 56.8 0.74

0.4 54.0 46.8 1.15

Thus, relative to the low concentration treatment, the plant numbers were reduced in the high concentration by 262, but the plant weight of the survivors increased by 452, more than compensating for the early damage.

The algicides showing both good control of pH and acceptable safety for crops were terbutryne at 0.1 - 0.2 mg L~ and copper sulphate.

Simazine at 3 mg L~ (used previously for pH control and ammonia retention, by Vlek et al. (1980), was moderately effective in controlling pH, but phytotoxic. Example 5

Comparison of pre-flood application of urea-nitrogen with post-flood treatments; and further comparison of the effect of terbutryne algicide in the floodwater treatment Treatments were: 1. Control; no fertiliser nitrogen

2. urea-nitrogen (65 kg ha ) added immediately before permanent flood

3. Urea added after permanent flood

4. urea and terbutryne (0.1 mg L-i) _{as IGRAN (}f_lowable formulation) added after permanent flood.

(All treatments received phosphorus fertiliser).

Each treatment was replicated in six blocks, each

2 plot being 15 m x 6 m (90 m ). Rice (cv. Calrose) was drill sown on and permanent water was introduced to a depth of approximately 10 cm about 6 weeks later.

In treatment 2, urea was applied to the dry soil surface immediately befor.e introducing the floodwater.

Treatments 3 and 4 were applied on the day following floodwater application, using a method to simulate application by dripping into the floodwater.

A heavy rainstorm 4 days after introducing the floodwater, stirred up the sediments giving highly turbid water. However, the water clarified readily. Algal growth was similar in character to that in. example 4. A thick cohesive skin-like slime (of algae, bacteria, detritus and clay particles) developed in all the plots, but was most pronounced in the fertilised non-algicide treatment. Bubbles formed underneath the skin during the day, and the clumps of the skin rose to the surface during the afternoon, and sank again in the evening. In the algicide treatments the skin stayed intact at the sediment surface until about 7 days after treatment, when massive numbers of zooplankton began to appear and the 'skin' began to disintegrate.

Phytotoxicity. In this example compared with example 4 the rice seedlings were larger when subjected to the stressful addition of permanent water. The following table shows that there was no difference in vegetative dry weight biomass about ιι weeks after treatment, between treatments 3 and 4

(floodwater fertilised treatments with an without terbutryne).

Treatment Terbutryne Phytotoxicity Vegetative

No. +/- rating [ % ) dry matter(g) "Deep" "shallow"

1. No addition - 10.8(10-15)** 4.2(0-5)** 132(26)*

2. Nitrogen before - 0 (0-0) 0 (0-0) 301(26)

3. Nitrogen in flood - 3.3 (0-5) 0.8(0-5) 177(11)

4. Nitrogen in flood + 2.5 (0-10) 0.8(0-5) 176(24) * Average of 6 plots, and coefficient of variation { % ) in brackets

** Mean and range of 6 plots pH control A continuous record of pH was obtained in selected plots from each of treatments 1, 3 and 4 as shown in figure 5.

Results show that, as in .previous experiments, the fluctuation in pH was reduced by the algicide, and the absolute values of pH maxima and minima were maintained at a lower value for at least 14 days, even though the sediments were stirred by a heavy rainstorm a few days after treatment.

Because of the very large influence of pH on gaseous loss of ammonia this results in the conservation of fertiliser. The present invention has been described with partiuclar reference to terbutryne as an appropriate algacide.

It should be appreciate that other algacides and algiastats may be employed. For example SIMETRYN and PROMETRYN are suitable. SIMETRYN is"

2,4-Bis(ethylamino)-6-methylthio-l,3,5-triazine may be used in conjunction with thiobencarb to control broad-leaved weeds in rice. PROMETRYN is

2,4-bis(isopropylamino)-6-methylthio-l,3,5-triazine. Chelates of copper are useful provided that the long term accumulation of copper in the environment is monitored.

The potassium salt of endothal is useful in the control of submerged weeds and the amine salt is useful in the control of algae, however, it is necessary to monitor toxicity to fish. It should be noted that the mere addition of greater quantities of an ammonium based fertilizer is insufficient in increasing available nitrogen. Volatilization losses increase as the ammonia concentrations increase.

It will be appreciated that the composition of the present invention may be applied at any stage before or during the growing of a crop when the nitrogen is required for that particular crop. A particular advantage of the present invention is that the composition can be applied in floodwater without the substantial nitrogen loss that would otherwise occur due to ammonia volatilization and denitrification. In the past large volatilization losses occur in wet seasons when the first application of the ammoniacal nitrogen source is applied to either the wet soil or into flood water. Minimizing these volatilization losses by the use of the present invention allows -greater flexability in the overall management of the crop. The nitrogen can be added when it is required. In the past urea application before permanent water flooding encouraged weed growth and reduced the effectiveness of the herbicides. The use of the algacide with the ammoniacal nitrogen source applied into flood water gives improved weed control as well as better nitrogen use. In Australia practice has been to apply urea to drill sown rice crops either immediately onto soil with a dry surface when the plants are between 4 and 6 weeks old and before permanent flooding or onto the flood water when the plants are about 10 weeks old. For aerial sown rice fertiliser is applied to the flood water when the plants are about 10 weeks old. Early application to the floodwater leads to losses through ammonia volatilization and denitrification as above indicated. By the use of the present invention these losses are reduced substantially. Preferably sufficient algacide is added to control the pH of the water over the effective period to less than 9 and more preferably less than 8.

Also it is preferred that the nitrogen in the soil is maintained at a level in excess .of 10 parts per million over a period of 4 days after application but more preferably in excess of 15 parts per million over a period of 4 days.

The present invention is also effective in controlling slime or algal epiphytes on plants which weaken and kill plants through shading and weighing them down. The invention may also be effective in reducing seedling mortality caused by high pH in the root zone induced by algal growth. Chemicals other than ammoniacal nitrogen sources are also hydrolyzed more quickly at high pH. Accordingly, the present invention is also effective in maintaining the effectiveness of such chemicals used in pest control.

It will be appreciated that many variations and modifications may be made within the scope of the present invention and the detail given herein should not be taken as restricting the present invention.