Proceedings of The South African Sugar Technologists' Association — June 1970 121
OPTIMUM IRRIGATION LEVELS FOR CANE UNDER
Ex. Mg(m.e. %) 3.1 Min. N p.p. m. initially 12
Min. N p.p.m. after incubation 24 Soil physical data measured on soil adjacent to the experiment are given in Table II.
The crop was harvested at approximately 12 months in plant, 1st and 2nd ratoons in November 1967, 1968 and 1969. The variety was N:Co.376.
The following levels of fertilizer were applied to all plots (kg/ha.)
N P205 K20
Plant 157 168 56 IstRatoon 179 84 — 2ndRatoon 179 84 — During the course of the experiment, stalk height was measured weekly on 10 stalks in each plot of one replication; i.e. 40 stalks per irrigation treatment.
Regular smut and flower counts were carried out on all plots at appropriate times.
Results Level of Irrigation Cane Yield
The effect of increasing levels of water from 37 % to 84% of Pan was a virtually linear increase in cane
yield as shown in Fig. 1 and Table III. This amounted to 11.7 kg cane/m3 of water applied (1.33 tons cane/
acre inch water).
The additional yield increment obtained between 84% and 100% of Pan was slight. It can be seen that the variable treatment AB gave appreciably poorer results than the remaining treatments in the plant and 1st ratoon crops when the factors applied were 100%
in summer and 50 % in winter. This treatment is clearly undesirable; however in the 2nd ratoon when the applications were 84% for 8 months and 60% for the last 3$ months, cane yields were similar to those of the 100 and 84% Pan treatments.
Cane Quality
Table IV and Fig. 2 show the effect of irrigation level on the sucrose content of cane.
The highest sucrose content was obtained with a moderate-severe degree of stress (53 to 68 % of Pan) while both wetter and drier treatments resulted in significantly lower sucrose content. The variable treat- ment AB again gave poor results in the plant and 1st ratoons, but in the 2nd ratoon, its sucrose content was approximately the same as those of the best treatments.
Table V and Fig. 3 show the effects of irrigation level on cane quality for the 2nd ratoon.
TABLE H Soil moisture determinations
Depth (mm.) 0-152 152-305 305-457 457-610 610-762 762-914
Field capacity (in field) %
17.2 19.6 20.2 20.6 20.3 19.0
Wilting point (15 bar) %
6.9 11.1 9.1 12.6 12.6 14.3
Bulk density
1.48 1.46 1.51 1.50 1.55 1.54
Available moisture mm./152 mm.
23.2 23.3 20.9 18.2 18.1 12.4
Cumulative (mm.)
23 46 67 86 116 104
TABLE m
Effect of irrigation on cane yield (MT/HA)
Pan Factor P
IR 2R Mean
100 141.1 169.6 137.2 149.3
84 138.9 160.5 139.7 146.3
AB 114.6 139.5 134.0 129.3
68 114.8 117.1 124.6 118.8
53 98.9 86.2 89.3 91.4
37 91.2 46.4 58.1 65.2
L.s.d.
5%
9.0 10.7 8.2
—
c.v. %
5.3 8.1 10.5
—
TABLE IV
Effect of irrigation on sucrose % cane Pan Factor
P IR 2R Mean
100 14.1 12.8 13.8 13.6
84 14.0 13.1 14.4 13.8
AB 13.5 12.2 15.1 13.6
68 14.3 13.7 15.2 14.4
53 15.1 13.0 15.2 14.4
37 14.0 11.8 14.6 13.5
L.s.d.
5%
0.7 0.9 0.8
—
C.V. % 5.5 6.2 6 2
—
TABLE V
Effect of irrigation on cane quality (2nd ratoon)
Pan Factor Sucrose % cane Brix % cane Fibre % cane
% Purity E.R.S. % C.
100 13.8 16.1 13.9 85.7 11.7
84 14 4 16 6 13.1 86.9 12.4
AB 15 1 17 2 13.6 87.3 13.0
68 15 2 17 4 13.0 87.8 13.3
53 15 2 17.5 12 8 86.4 13.1
37 14 6 17 4 12 6 83.6 12.3
L.s.d.
5 % 0 8 0 8 0.9
— 0.9
C.V. % 6.2 3.4 7.6
— 5.6
N.B. E.R.S. % C. (Estimated recoverable sugar % cane)
= S - 0.451 (B -S)i — .077 F where S = Sucrose, B = Brix and F = Fibre content of cane obtained by direct analysis.
TABLE VI
Effect ef irrigation on tons sucrose/hectare
Pan Factor P
IR 2R Mean
100 19.92 21.65 16.02 20.15
84 19.38 21.04 17.32 20.19
AB 15.46 16.98 17.44 17.54
68 16.38 16.00 16.48 17.12
53 14.92 11.19 11.69 13.21
37 12.77
5.57 7.21 8.94
L.s.d.
5 % 1.50 1.58 1.38
—
C.V. % 7.3 10.7 11.8
—
N.B. 2nd ratoon data is tons recoverable sugar/hectare, but mean is of tons sucrose/hectare on all crops.
— June 1970 123
*LE V
cane quality (2nd ratoon)
68 15 2 17 4 13.0 87.8 13.3
53 15 2 17.5 12 8 86.4 13.1
37 14 6 17 4 12 6 83.6 12.3
L.s.d.
5 % 0 8 0 8 0.9
— 0.9
C.V. % 6.2 3.4 7.6
— 5.6
ugar % cane)
re S = Sucrose, B = Brix and F = Fibre ilysis.
LE VI
in tons sucrose/hectare 68
16.38 16.00 16.48 17.12
53 14.92 11.19 11.69 13.21
37 12.77
5.57 7.21 8.94
L.s.d.
5 % 1.50 1.58 1.38
—
C.V. % 7.3 10.7 11.8
—
gar/hectare, but mean is of tons sucrose/hectare Proceedings of The South African Sugar Technologists'1 Association
Increasing irrigation from 37% to 53% of Pan resulted in increased sucrose % cane while the brix remained constant. Consequently there was a large rise in purity up to a peak at 68 % Pan. Sucrose, brix and purity all dropped steadily with irrigation levels increasing above 68 % Pan up to 100% Pan. The fibre content showed a steady increase produced by increas- ing levels of irrigation with a marked increase at 100%
Pan level. Recoverable sugar % cane was maximum at 68% Pan, with significantly lower values with both wetter and drier treatments.
Sucrose Yield
Table VI and Fig. 4 show the effect of irrigation levels on sucrose yield/ha..
There was a linear increase from 37 % to 68 % Pan, with a curvilinear response up to 84% Pan and no further yield increase with 100 % Pan. Treatment AB was appreciably poorer than the others in plant and 1st ratoon, but in the 2nd ratoon it gave virtually identical results to the 84% Pan treatment.
Efficiency of water use
The return in yield of cane or sucrose per unit ir- rigation water applied is obviously of tremendous importance. However, the most efficient use of water may be regarded as the highest yield per unit of total water (rainfall plus irrigation). Table VII and Fig. 5 show the effect of irrigation on the yield of sucrose in kg per m3 total water.
In all cases the intermediate levels of 68 or 84 % Pan were the most efficient in terms of water use. However there were different trends in the various seasons.
During the very dry 1st ratoon, the efficiency of water use in the driest treatments was extremely low, whereas in the wettest season (Plant) the efficiency of
TABLE VII
Efficiency of water use (kg sucrose/m" total water) Pan factor
P IR 2R Mean
100 0.97 1.09 0.83 0.96
84 1.04 1.18 1.04 1.09
AB 0.88 0.98 1.11 0.99
68 0.99 1.09 1.20 1.09
53 0.99 0.92 1.05 0.99
37 0.94 0.58 0.79 0.77
water use of these treatments was much higher. In all seasons, the efficiency of water use was poorer at the highest irrigation level, evidently due to less efficient use of rainfall.
Stalk Population
The effect of irrigation on stalk population is shown in Table VIII.
There was no effect of irrigation on population in the plant crop, but in both ratoon crops, increasing levels of irrigation produced increasing numbers of stalks, especially in the 1st ratoon when the rainfall was very low. It was noteworthy that these effects were more pronounced in the burnt plots, where moisture stress was more acute than in the trashed plots, and it may be concluded that a fairly severe degree of moisture stress resulted in a significant reduction in stalk population, while a moderate stress (down to the AB treatment) had no effect.
Stalk Height
The effect of irrigation on stalk height is shown in Table IX.
TABLE VHI
Effect of irrigation on stalk count (thousands per hectare)
Pan Factor P
IR 2R
Mean
100 132 170 160 154
84 129 163 159 150
AB 132 169 160 153
68 134 159 157 150
53 129 156 155 146
37 135 149 150 145
L.s.d.
5%"
9 8 8
—
C.V. % 3.0 4.8 5.3
—
TABLE IX
Effect of irrigation on stalk height (m).
Pan Factor P
IR 2R Mean
100 2.62 2.40 2.19 2.40
84 2.55 2.39 2.11 2.35
AB 2.26 2.05 2.07 2.13
68 2.35
1.84 1.87 2.02
53 1.97 1.17 1.19 1.44
37 1.83 0.90 0.95 1.23 N.B. Heights were taken from the last weekly measurement
before lodging became excessive.
Increasing levels of irrigation produced a curvilinear response in height growth; the 100% Pan treatment resulting in very slight additional stalk length over 84% Pan treatment.
Lodging
Table X shows the effect of irrigation on lodging.
There was a significant increase in lodging between the 84% and 100% Pan treatment, and this may account for the very small increase in cane yield produced by the additional water application, and also the reduction in sucrose content with the 100%
Pan treatment. No lodging was observed with the two driest treatments, while it was negligible at the 68 % Pan level.
Flowering
Virtually no flowering was observed in the plant and 1st ratoon crops, but there was a certain amount in the 2nd ratoon. Number of flowers per acre was as follows:
100% Pan 84 AB 68 53 37
18 23 9 3 5 2
There was evidently an increase in flowering with increase in irrigation.
Smut
No effect of irrigation on the incidence of smut could be detected; in the 2nd ratoon the counts ranged up to 670 per hectare.
Burning vs. Trashing
The overall responses to irrigation discussed in the preceding section were frequently modified by the trash management treatment, and these effects may be observed in Figs. 1, 2 and 3, the average values of 1st and 2nd ratoons being taken.
Cane Yield
As shown in Fig. 1 and Table XI, the response to irrigation with burning was substantially greater than with trashing.
At the lowest level of irrigation, (37 %), trashing gave a mean yield increase of 30 tons/ha over burning; this increment declined with increasing irrigation until there was no difference at 84% Pan; while the burnt plots appeared to give higher yields of 8 tons/ha at 100 % Pan. The higher yield with burning was probably due to the higher stalk population.
Cane Quality
The sucrose content of the burnt plots was in general higher than that of the trashed plots (highly significant in the 2nd ratoon). This is shown in Fig. 2 and Table XI from which it may be seen that the burnt treatment had a lower sucrose content at the driest irrigation level, but higher at all other levels, especially at the intermediate level (68%) and at 100% Pan.
Table XII shows the effect of trash management on cane quality in the 2nd ratoon.
Sucrose, brix, fibre and E.R.S. % C. were all significantly higher (P<.01) with burnt than with trashed cane; purity was also somewhat higher. In the case of fibre, this difference was consistent over all levels of irrigation; with sucrose, brix, purity and TABLE X
Effect of irrigation on % lodging Pan Factor
P IR 2R Mean
100 61 71 94 75
84 34 52 72 52
AB 26 1 78 35
68 4 1 24 9
53
OOO
0 37
OOO
0
L.s.d.
5%
15 15 24
—
C.V. % 95 67 31
—
Proceedings of The South African Sugar Technologists' Association —
TABLE XI
Some effects of burning vs. trashing (Mean 1R & 2R) Pan Factor
Tons Cane B per ha. T Sucrose B
% Cane T Tons B Sucrose/ha. T Stalk count/ B hectare T
100 157.2 149.6 13.6 12.9 21.2 19.3 174 157
84 150.0 150.2 13.9 13.6 20.8 20.4 171 151
AB 138.4 135.1 14.0 13.4 19.2 18.0 174 154
68 114.7 127.0 14.8 14.2 17.0 18.0 165 150
53 82.6 92.9 14.2 14.0 11.7 13.0 161 149
37 37.5 67.0 13.0 13.4 5.0 9.0 148 150
Mean 113.4 120.4 13.9 13.6 15.8 16.3 166 152
TABLE XII
Effect of burning vs. Trashing on cane quality (2nd Ratoon)
Sucrose % Cane Brix % Cane Fibre % Cane Purity % E.R.S. % C.
Burning 14.96 17.28 13.78 86.6 12.85
Trashing 14.46 16.80 12.56 86.0 12.43
L.s.d. (1 %) 0.34 0.32 0.55 0.38
E.R.S. % C, the difference applied to all levels of irrigation except 37 % Pan.
Sucrose Yield
Fig. 3 and Table XI show that at the lowest irriga- tion level, trashing produced 4 tons sucrose/ha more than burning (highly significant in both crops); this difference decreased with increasing levels of irrigation until at around 80 % Pan the yields were the same. At 100% Pan, burning resulted in 2 tons sucrose/ha more than trashing (significant in 2nd ratoon only).
Table XIII shows the similar effects for recoverable sugar/ha for 2nd ratoon only, and from this table it should be noted that treatment AB (burnt) outyielded all other treatments.
TABLE Xm
Effect of burning vs. trashing on recoverable sugar yield and efficiency of water use (2nd ratoon)
Pan Factor
Tons Sugar B per hectare T Efficiency of B water use T (kg/m3)
100 17.2 14.9 0.89 0.77
84 17.3 17.3 1.04 1.04
AB 18.0 16.9 1.15 1.08
'68 16.0 16.9 1.17 1.24
53 11.1 12.3 1.00 1.11
37 5.3 9.2 0.58 1.01
Mean 14.2 14.6 0.97 1.04
Efficiency of Water Use
As shown in Table XIII and Fig. 6, trash conserva- tion resulted in overall higher efficiency of water use, expressed in kg recoverable sugar produced per ms total water. This improvement was very marked in the drier treatments, but disappeared at about 84%
Pan, whilst burning gave a higher efficiency at 100%
Pan. The highest efficiency of all was obtained with 68% Pan treatment trashed, followed by the same level, burnt.
— June 1970 125 Stalk Count
From Table XI it may be seen that at the lowest irrigation level, burning tended to produce fewer stalks.
However at all other levels, burning resulted in signifi- cantly more stalks, giving about 14,000 per hectare more over all levels of irrigation. The decrease in number of stalks with decreasing levels of irrigation was much more marked with burnt cane than with trashed cane, due to the severity of moisture stress.
Lodging
There was no difference in lodging between burnt and trashed treatments in 1st ratoon but in the 2nd ratoon, the burnt treatment showed significantly less lodging (<.05) than the trashed treatment: 41.6 and 48.0 % respectively. This effect was especially marked with the AB irrigation treatment.
Discussion Cane Yield
The effect of increasing levels of irrigation was to produce a very marked and linear increase in cane yield from 37% to 84% of Class "A" Pan evaporation (net application). There was virtually no further response in cane yield to increasing the irriga- tion level up to 100% Pan. This may be related to the significant increase in lodging (from 52 to 75 %) which occurred between the 84% and 100% Pan treatments;
other work in Rhodesia (Anon 1969) has shown a very marked reduction in cane yield of N:Co.376 with lodging.
This response to irrigation was more marked with burnt cane than with trashing, and there was an increase up to 100% Pan with burnt cane, the yield being some 8 tons/ha greater than with trashing at this level of irrigation. This may be accounted for by the increased stalk population which was some 17,000 stalks/ha greater with burning than with trashing at the 100% Pan level. Retaining the trash evidently caused the smothering of a large number of stalks. By contrast in the driest treatment (37% Pan), trashing tended to produce slightly more stalks than burning, because of severe moisture stress in the latter treatment.
Cane Quality
Maximum sucrose content was obtained at around 68% Pan, this being more marked with burnt than with trashed cane. There was a pronounced drop in sucrose content at the 37% Pan level; however the brix with this treatment was similar to that of the 68 % Pan, and it is evident that conversion of sucrose to non-sucrose solids occurred with severe moisture stress.
This resulted in a marked decline in purity with the dry treatments. With the wet treatments, a drop in sucrose content was accompanied by a drop in brix, resulting in a much smaller drop in purity.
The drop in sucrose with high levels of irrigation may be due to several factors: (i) dilution by a higher moisture content, (ii) more vigorous vegetative growth resulting in lower sucrose accumulation and (iii) the effect of lodging which caused considerable decrease in sucrose content of N:Co.376 (Anon 1969).
Sucrose % cane was higher in the burnt plots than in the trashed plots at all levels of irrigation except the driest. This result is contrary to the data of Thompson
(1965) but may possibly be explained as follows; (i) the reduced lodging in the burnt plots resulted in a higher sucrose content (ii) the burning caused actual desiccation of stalks at harvest with consequent con- centration of sucrose content.
The increase in fibre content with increasing levels of irrigation was surprising, as a drop might have been expected. However other work (Thompson et. al. 1967) has shown similar trends.
Sucrose Yield
The interaction between trash management and irri- gation level on sucrose yield per hectare is of consider- able interest; while burning produced 4 tons sucrose/ha less with the driest treatment (37% Pan), it produced 2 tons more with the wettest (100% Pan); the crossover point being in the region of 80% Pan. With trashing, there was actually a reduction in yield through in- creasing irrigation from 84 to 100% Pan. It is possible from Fig. 3 to estimate the quantity of water saved by trashing at any particular yield level e.g. at a level of 10 tons sucrose per hectare, 100 mm more water were required with burning, but 20 tons sucrose/ha were
reduced by the same quantity of water, whether urning or trashing took place.
An economic assessment of the two systems of trash management can thus be made if the costs of water application and the savings in cost of harvesting burnt cane are known.
These results partially confirm the findings of Thompson (1965) who concluded that responses to trashing are mainly due to moisture conservation.
However, Thompson found no evidence of higher yields with burning than with trashing, and the results mentioned above seem surprising.
They are however confirmed by unpublished results from an experiment at Triangle, the data of which was kindly made available by Mr. J. Burton, Agronomist, Triangle Limited. In this experiment, three levels of irrigation were combined with burning vs. trashing in two crops (4th and 5th ratoons). Mean results in tons sucrose/ha are given in Table XIV.
TABLE XIV
Effect of irrigation level on sucrose yield (tons/ha.) Triangle Water applied (mm).
Burning Trashing Mean
1029 16 32 15.67 15.98
818 14.86 14.39 14.62
660 12.55 12.98 12.72
Mean 14.57 14.35
Efficiency of Water Use
The effect of irrigation and trash management on efficiency of water use are shown in Figs. 5 and 6. It can be clearly seen that stretching the available water produced an increasing return per unit of water only up to a certain point (usually 68% Pan), after which production per unit water was reduced. These con- clusions were similar whether cane or sucrose yields were taken.
In the 1st ratoon which received very low rainfall, the efficiency of water use with dry treatments fell to a very low level, presumably because the extensive
foliage mortality resulted in a period of foliage recovery after each irrigation before further cane could be pro- duced. This adverse effect is accentuated by burning and alleviated by trash conservation.
The AB treatment gave poor results in plant and 1 st ratoon; it is apparent that application of 100 % Pan in summer and 50% Pan in winter resulted in lower yields of cane and sucrose per hectare then would be expected from the number of mm water actually applied. It thus produced a low efficiency of water use and does not support a popular belief that savings of water may economically be made by reducing the pan factor in winter.
In the second ratoon the AB treatment was given a
"new look" and received 84% Pan for 8 months and a ripening treatment of 60% Pan for the last 3 J months.
This resulted in a relatively high cane yield and sucrose content to give the highest tons sucrose/ha, particular- ly in the burning treatment. Efficiency of water use was also high, and it is evident that this type of treat- ment gives a good combination of high sucrose yield per hectare and per unit of water applied.
There is evidence from Natal (Thompson 1969) that the potential evapotranspiration of lodged cane is appreciably less than that of upright cane. The reduc- tion in irrigation of the AB treatment at 8 months coincided with the onset of lodging, and it is probable for this reason that the efficiency of water use was high in this treatment. Similar results have been observed in a number of trials in Rhodesia (Anon 1970) where efficiency of water use has been increased by drying-off in lodged cane.
Conclusions
It can be seen that different levels of irrigation were best for different criteria:
100 % Pan gave highest yields of sucrose/ha with burnt cane.
84% Pan gave highest yields of sucrose/ha with trashed cane.
68 % Pan gave highest yields of sucrose per unit of water applied and also highest cane quality.
84/60 % Pan (AB treatment in 2nd ratoon), with burn- ing was probably the optimum treatment, as it achieved the highest yield of sugar/ha together with one of the highest efficiencies of water use.
In conclusion it must be reiterated that the above levels of irrigation were taken on a net basis, and for practical purposes should be multiplied by the follow- ing approximate factors :-
Sprinkler irrigation 1.2 Efficient furrow systems 1.2-1.4 Less efficient furrow systems 1.4-1.6
References
1- Anon, 1969. Lodging. Sugarcane Newsletter (Rhodesia) 15 p.6.
2. Anon, 1970. Ripening of cane and maturity testing. Sugar- news (Rhodesia) 2 p.7.
3. Boyce, J. P., 1969. First ratoon results of two irrigation experiments at Pongola. Proc. S.A. Sug. Tech. Ass.
43 : 35.
4. Thompson, G. D., 1965. The effects of Trash conservation on soil moisture and the sugarcane crop in Natal. Proc.
S.A. Sug. Tech. Ass. 39 : 143.
Proceedings of The South African Sugar Technologists Association —
5. Thompson, G. D., Gosnell, J. M.,and de Robillard, P. J.M., 1967. Responses of Sugarcane to Supplementary Irrigation on two soils in Natal. Expl. Agric. 3, 3 : 223.
6. Thompson, G. D., 1969. Personal communication.
Discussion
Dr. Thompson (in the chair): In table VII you have given the efficiency of water use for pan factor treatments 100, 84, A B, 68 and 53. The data show that these efficiencies vary from crop to orop, but I wonder if the differences are really significant? But at pan factor 37 the efficiency really does decline, and we have found a similar effect in our own work. The relationship between yield and water use appears to be linear until a certain point of moisture stress is reached. We think that mortality of otherwise har- vestable stalks then occurs and the efficiency conse- quently declines.
You have given an alternative explanation in that the foliage dies, irrigation water is applied, and a lot of it is then lost by evaporation.
In Table XI, giving some effects of burning and trashing, you have combined data for first and second ratoons. Did the first ratoon data, when rainfall was low, give a bigger effect due to trash than the second ratoon when rainfall was high?
Dr. Gosnell: The rainfall was also low for the second ratoon — it was only high in the plant crop, and the burning versus trashing effect was similar for both.
Mr. Boyce: In our drying off experiments we have also found that fibre percent increased with the wetter treatments.
How does Dr. Gosnell relate his pan factor method of irrigation to our method of minimum cycle during peak demand?
At Pongola we achieve maximum yield of sucrose per acre with a 21 day cycle, which indicates that rainfall cannot be ignored.
Dr. Gosnell: We cannot relate at all the two different methods of applying waiter.
Our method suits our particular conditions and re- quirements.
For design of new schemes, I think Mr. Boyce's method would be more suitable.
Dr. Thompson: Regarding your tons cane and tons sucrose per hectare, your 100 and 84% factors gave similar yields. If a direct relationship exists between yield and water requirement, did you get the same yield at 84 and 100 because the 84 made better use of rainfall?
Dr. Gosnell: That is partly the reason but it is also connected with the burning versus trashing inter- action. The burnt plots increased in yield from 84 to 100 but the trashed plots decreased in yield, possibly due to excessive lodging.
Mr. Browne: In Figure 5, efficiency of water use, the peaks of the graphs appear to move to the left
— June 1970 ' 127
and upwards from the plant crop to the first ratoon and second ratoon crop. Could compaction have had an effect here?
Dr. Gosnell: I do not know. As regards the plant crop there was a much higher rainfall, and the differences between driest and wettest treatments and of water use were much closer.
Mr. Moberly: Were drying off procedures followed with these treatments? The sucrose per cent cane is lower than one would expect with high water treat- ments.
Dr. Gosnell: No, the treatments were continued right up to harvest.
If you compare the 84% pan treatment through- out of the second ratoon with the 84 pan up to eight months, followed by drying off, substantially improved results are seen.
In this trial we did not want the confusion between drying off and irrigation effects, except in the one treatment AB.
Mr. du Toit: Regarding increase of fibre with irri- gation, the figures published for the industry every year by Mr. Perk puzzle me because they show a low sucrose at the beginning and end of the season and high sucrose at the peak in September, yet the fibre tends to fall at that time.
A period of drought does not apparently give high fibre but instead gives low fibre. With increased water, the photosynthesis appears to work more towards fibre than sucrose.
Dr. Gosnell's optimum use of water is at 68. Would he recommend this figure for a commercial irrigation scheme?
Dr. Gosnell: This would have to be considered for each case, and would depend on which was the more valuable — the land or the water.
Generally, a figure of about 84 would be recom- mended.
Mr. Boyce: Because of the soil type at Pongola we have tended to discount the applicability of our re- sults — we have found a crop feeding from as deep as eight feet.
However, Dr. Gosnell's results tend to confirm our results.
Dr. Gosnell: The soil depth for this experiment is about 30 inches and there is no effective root pro- liferation below this depth.
Dr. Thompson: We have been hesitant about extrapolating our Pongola results too far because we are working on such a deep soil that the crop may be getting water from depth that it would not get on your soil. On a shallow soil you appear to be getting similar trends for the different treatments.
Dr. Gosnell: They seem similar but our intermediate treatment is not similar to your 21 day treatment.
If we did not irrigate for 21 days in summer, our crop would be severely affected.