SOIL COMPACTION STUDIES AT PONGOLA

Proceedirlgs of The South African Sugnr Teckizologists' Associatio~zJurze 1971 SOIL COMPACTION STUDIES AT PONGOLA By M. A. JOHNSTON and R. A. WOOD South African Sugar Association Experiment Station Abstract The effects of soil compaction and various cultivation treatments on the growth of ratoon cane were studied in two experiments on a Makatini sandy clay at Pongola. Despite significant compactive effects on soil bulk density and macroporosity, no reductions in yield due to compaction were recorded. Restorative treatments following compaction sometimes improved the soil physical condition without, increasing yield. Certain subsoiling treatments adversely affected the development of the cane. Utilization of moisture under all treatments from considerable depths, and regular irrigation are thought to be the main factors responsible for preventlg a decline in yield. Introduction At the 1964 S.A.S.T.A. Congress a symposiumg was held to discuss the compaction of soils, following the increased use of infield transport in the industry, and its possible effects on sugarcane growth. In a pilot trial at Mount Edgecombe (Anon') compaction was shown to cause a serious reduction in yield. Consequently a formal trial was then carried out on a Windermere clay loam in collaboration with the Tongaat Sugar Company (Anon2). Unfortunately the cane was so severely affected by drought that resultant yields were low and variable. A 9% reduction in yield due to compaction was not statistically significant. This trial was followed by one in 1965 at Chaka's Kraal on a Waldene sandy clay loam (Anon3"3"). Although a 14% reduction in vjeld occurred when the <oil was 'compacted whilewet, this was not significant due mainly to lack of uniformity of the site. In order to obviate problems such as soil variability and drought, it was decided to conduct a wellreplicated trial under irrigated conditions on the uniform Makatini sandy clay at Pongola, previously shown to be highly compactible. of six rows 11 m long and 0,9 m apart, while nett plots at harvest had four rows 8,5 m long. The trial was fertilized according to standard field practice. Compaction was effected by two passes over each interrow by a tractor and trailer loaded with approximately five tons of cane. Compaction in the wet and dry state was carried out when topsoil moisture contents corresponded to 0,2 and 15 bars respectively. Restorative treatments were as follows: (a) cultivation to 20 cm with a multitine cultivator, (b) subsoiling with singletine subsoilers to 38 and 76 cm, (c) subsoiling to 38 cm with wing attachments fitted to the implement (see Fig. 1). Experimental Methods A plant crop of NCo 376 was first grown and harvested on the trial site and uniformity data were recorded. Compaction treatments were then applied to the first ratoon crop. First ratoon The trial was designed to study (a) the effect on FIGURE 1: (a) Subsoiler with wing attachments; and (b) its effect on the soil. cane growth of soil compaction in a wet and dry state, (b) the ameliorative effect of various tillage treatments following compaction. Control plots re Rainfall received during the season was 572 mm ceiving no compaction and the various restorative and the site was irrigated with 1 120 mm of water treatments were also included. using an application of 75 mm per stand on a mini The experimental layout was a 4 x 4 lattice design mum cycle of 21 days. (16 treatments, 4 replications). Each plot consisted Cylindrical gypsum soil moisture blocks were

262 Proceedirzgs of The South African Sugar Techrtologists' AssociationJune 1971 manufactured according to the method of Pereira et ulhnd placed under four replications of each treatment. The blocks were installed halfway between the cane rows and centre of the interrow at depths of 30, 90, 150 and 210 cm, and readings were taken at weekly intervals. Undisturbed soil cores were taken at the centre oe the interrow in each plot in the experiment, between 08 cm and 3038 cm so that any physical changes due to compaction could be studied. Sampling was carried out shortly after treatments had been applied and again after harvest. Stalk height measurements and shoot counts were taken at regular intervals during the early period of growth. Second ratoon When results from the first ratoon crop showed the compaction treatments to have had little effect on yield, niore severe treatments were applied to the second ratoon. Compaction was effected by five passes of a tractor plus loaded trailer (five tons nett) over (a) both interrow and cane row and (b) interrow only. Both treatments were applied to the soil when wet, topsoil moisture content corresponding to a suction of 0,3 bars. Restorative treatments were (a) cultivation to 20 cm and (b) subsoiling to 38 cm. The winged sub TABLE I yields (tc/ha) of first ratoon treatments soiling treatments from the first ratoon were retained to test for any residual effects. The new treatments were rerandomized upon the old, and the experiment was analysed as a simple random design. Field measurements and soil sampling were carried out as in the first ratoon. Total rainfall received was 446mm, while a further 890 mm was applied by irrigation. Results 1. Yield data First ratoon Results showed no significant evidence of any treatment effects or of any interactions (Table I). The only effect which approached significance (P=0,05) was an apparent depressing effect due to winged subsoiling when compared with normal subsoiling to 38cm. For this reason plots which had received winged subsoiling were left as residual treatments in the second ratoon in order to see if cane yields would recover. Second ratoon Despite the more severe compaction treatments applied no significant differences in yield between treatments were obtained (Table 11). No residual effects from treatments which had previously been subsoiled with wings were apparent. Restorative treatments No restorative treatment (1) Cultivated to 20 cm (4) Subsoiled to 38 cm (2a) Subsoiled to 38 cm with "wings" (2b) Subsoiled to 76 cm (3) S.E. (body of table) L.S.D. (P=O,O5) I 1 Conlpaction treatments No Conipacted Compacted compaction ;;c; 3 1 (0) (OfW) (OfWfD) ' TABLE I1 yields (tc/ha) of second ratoon treatments Compaction treatments Restorative treatments No restoration (0) Cultivated to 20 cm (I) Subsoiled to 38 cm (2) S.E. (body of table) L.S.D. (P=0,05) No compaction (0) 132 121 123 125 Complete compaction (1) 127 125 5 5,4 15,7 9,6 Interrow compaction (2) 128 127 127 129 125 125. Kesidual treatments No conlpaction winged subsoiling Wet compaction winged subsoiling

Proceedings of The South African Sugar Technologists' AssociationJune 1971 No Compacted 3~ * No,,, Compacted wetno restoration compactionno restoration compactionsubsoiled with wings wetsubsoiled with wings I I I I I I I Nov. Dec. Jan. Feb. Mar. Apr. 4 1968 1969 w FIGURE 2: Progressive height growth of four treatments in the first ratoon crop.

264 Proceedirzgs of The South African Sugar Technologists' AssociotionJune 1971 Dec. Jan. Feb., Mar. FIGURE 3: Progressive height growth in the second ratoon of each compaction treatment averaged over all restorative treatments.

Proceedi~lgs of The South African Sugrir Technologists' Associatio~zJulze 1971 26 5 2. Growth data First ratoon Stalk height measurements and shoot counts taken during the growing season did not reveal any noticeable effects of compaction on cane growth. The only treatments which caused a depressing effect on growth were those in which winged subsoiling was applied, as shown in figure 2, and to a lesser extent the deep subsoiling treatments. Lodging of the cane prevented reliable measurements towards the end of the season. Reduction in height due to winged subsoiling treatments is also confirmed by yield and stalk height data shown in Tables I and I11 respectively. Second ratoon Figure 3 shows that cane growth in terms of mean stalk height was reduced by the compa~tion treatments applied to the second ratoon. Complete comuaction had a more severe effect on mowth than Interrow compaction. These effects, thiugh clearly seen during the early stages of growth and shown in figure 4, diminished towards the end of the season. Table IV does, however, show a fairly consistent reduction in stalk height for complete plot compaction, and also a trend towards a higher stalk population in the compacted treatments compared to those not compacted. 3. Gypsum block data Despite the different compaction and restorative treatments applied to the first and second ratoons, patterns of moisture removal to depth were similar. Only the winged subsoiling treatments in the first ratoon showed somewhat pgorer utilization of moisture at all depths due to reduced cane growth. TABLE It1 stalk height (m) for each treatment in the first ratoon Restoration treatments No restoratior~ ( 1 ) Cultivated to 20 cm (4) Subsoiled to 38 cm (?a) Subsoiled to 38cm w~th wlngs (2b) Subsoiled to 76 cm 2,15 ( 3 2,33 Figure 5 shows the 'similar patternsof moisture FIGURE 4: The effect of complete compaction on cane rem.ova1 which occurred at all depths to 210cm growth, six weeks after treatment application. Restoration treatments TABLE IV stalk height and population for each treatment in the second ratoon I Compaction treatments I I No Wet Dry compaction compaction compaction (0) (W) ID) 2,36 2,40 2,36 Compaction treatments 1, No Complete Interrow Compaction Compaction ~ornrgtion 1 I Stalk Stalk Stalk Stalk Stalk Stalk heigh"' population "2 1 1 1 population h~ig? population (m) (X103/ha) (X103/ha) (X103/ha) No restoration (0) 2,22 2,06 2,07 Cultivation to 20 crn (I) Subsoiled to 38 cm (2) 1 2,13 1 150,O 1 2,04 1 162,O 1 2,12 1 157,9 Residual treatments No compaction winged subsoiling Wet compaction winged subsoiling under the three main compaction treatments jn the on the soil following wet compaction was indicated second ratoon. by a highly significant increase in BD and a corres 4. Soil physical data ponding decrease in MPS where no restorative treat First ratoon ment was applied. Similar but less marked effects Compactive effects. Table V shows that dry com were recorded where the soil was cultivated. paction had very little effect on bulk density (BD) At the 3038 cm depth, effects of wet c~mpaction and macropore (MPS). A marked compactive effect were far less important. However, a small but con

266 Proceedings of The South African Sugar Technologists' AssociatiorlJune 1971 sistent increase was noted in BD (significant at 0,05 level) when averaged over all restorative treatments. Restorative effects. (i) Cultivation appeared to have little effect on the soil physical properties. (ii) Of the tillage treatments subsoiling to 38 cm showed the greatest ameliorative effect. A highly significant decrease in BD and increase in MPS resulted at the soil surface when this treatment followed wet compaction. Similar but less marked effects were noted on the noncompacted and dry compacted plots. (iii) Winged subsoiling showed a tendency to compact the soil, but the effect was not clear since BD at the surface was lowered with wet com pac tion. (id., The onlv ameliorative effect of deep subsoiling wag shown at the soil surfacewith wet compaction where BD was significantly lowered and MPS significantly raised. Second ratoon Compactive effects. Table VI shows that compaction brought about marked changes'in physical pro TABLE V perties at the soil surface. Significant increases in BD were caused throughout the 08 cm depth by both compaction treatments and, with one exception. were accompanied by highly significant decieases in MPS. The soil at 3038 cm showed a slight but fairly consistent increase in BD due to compaction. However there were no statistically significant effects. Restorative effects. Cultivation and subsoilin2 caused small reductions in BD and slight increases in MPS in the compacted treatments, though none of these were significant. Subsoiling to 38 cm~in the second ratoon did not have the same.ameliorative effect as in the first ratoon. This is probably due to the unfavourably high soil moisture conditions which prevailed when subsoiling treatments were applied, thus preventing adequate shattering of the soil. Soil samples taken after harvest in both first and second ratoons revealed negligible changes in BD and MPS figures from those obtained soon after treatments were applied. Thus it would seem that little Changes in soil physical properties, following the application of compaction and restorative treatments (first ratoon) Bulk density (g/cc) Macropore space *(% by vol) Treatment No conlpac tlon 0 Compacted aet W Compacted dry D (o+w)(o+w+d) 08 cm depth No compactlon 0 Compacted wet W Con~pactcd dry D (o+w)(otw+d) No restoration (1) Cultivated (4) Subsoiled (2a) Subsoiled+ wings (2b) Deep subsoiled (3) S.E. trmt. means (4 plots) L.S.D. P=0,05 0,Ol 1,30 1,17 1,37 1,26 1,27 1,48 1,42 1,18 1,37 1,36 1,24 1,30 1,21 1,36 ( 1,25 / +0,37 0,ll 0,14 5,s 1,37 1,36 1,18 1,37 1,31 1,33 1,34 1,19 14,8 12,3 19,4 10,2 67 89 15,5 13,O 11,9 11,2 12,7 13,4 16,O 10,8 10,6 17,4 11,6 13,O 14,O 1 + 1,31, 3.7 58 20,7. 11,4 11,5 17,O 3038 on depth No restoration ( 1) Cultivated (4) Subsoiled (2a) Subsoiledf wings (2b) Deep s.~bsoiled (3) S.E. trmt. means (4 plots) L.S.D. P=0,05 0.01 121 1,28 1,13 1,27 1,18 1,25 1,31 1,21 1,36 1,26 1,21 1 1,28 1,21 1,33 1,19 1,24 1 1,23 1,30 1,17 1,31 1,22 1,22 1,31 1,18 19,5 14,4 18,s 12,4 15,9 15,l 10,9 19,9 14,s 71 14.1. 13,7 17,7 IS92 1 17,7 14,3 17,O + 0,37 + 1,66 0,ll 4,7 0,14 63 6,1 21,3 *Macropore space pore space occupied by pores of diameter>50 microns. 11,6 17,2 16,4 17,2

Proceedings of The South African Sugar Technologists' AssociationJune 1971 267 No compactionno restoration. I Complete compactionno restoration Interrow compactionno restoration 150. Rainfall + irrigation (mm) 100% 1 bar Weekly totals 0% 15 barst P 75 FIGURE 5: 1 Changes in available soil mqisture occurring under sugarcane (second ratoon) following three compaction treatments.

Proceediizgs of The Sou111 African Sugar Teckizolog~ists' Associatioi~June 1971 TABLE VI Changes in soil physical properties, following the application of compaction and restorative treatments (second ratoon) Bulk density (g/cc) Macropore space (% by vol.) Treatment Complete compaction 1 lnterrow 2 No restoration 0 Ciiltivated 20 ciii I Subsoiled 38 cm 2 I 1,3O 1 1,47 1 1,45 1 1 12,l 1 5,8 1 6,l 1 S.E. trmt. means (4 plots) L.S.D. P=O,OS 0,Ol natural amelioradon of the soil occurred during any one growing season. Discussion and Conclusions In both the first and second ratoon crops compaction under moist soil conditions was shown to change soil physical conditions significantly, increasing bulk density and decreasing macroporosity. As anticipated these effects were far greater at the soil surface than at depth. It is surprising, therefore, that no relationship between compaction and reduction in yield was found. There was every indication in the early stages of growth that yield reductions would be obtained, especially in the second ratoon.. The compactive effect produced by five runs over ' the row and interrow by a tractor and fully laden trailer was undoubtedly severe (see Fig. 6). This was subsequently confirmed by the difficulty experienced in penetrating the compacted soil when ameliorative treatments were applied. Thus water which was unable to enter the soil immediately tended to pond in the interrow until it was accepted by the soil. The gypsum block readings in both ratoons indicated that moisture was freely available and utilized to considerable depths under all treatments on this deep welldrained soil. It is felt that this factor, associated with regular irrigation, was mainly responsible for the absence of any decline in yield due to the various compaction treatments. Under a less favourable moisture regime or normal rainfed conditions it is considered likely that notable reductions in yield would have been obtained. Lack of yield response due to compaction has made it impossible to assess any ameliorative effect on yield of the various tillage treatments. However, the soil physical data from the first ratoon treatments did indicate that normal subsoiling to 38 cm had had the most beneficial effect. Acknowledgements The authors wish to thank the staff of the Pongola Field Station for their cooperation and assistance. Thanks are also due to Mr. E. A. von der Meden for his participation in the initial stages of the project, Mr. M. Murdoch for statistical analysis of the results and Mr. P. Moberly for his helpful advice. REFERENCES I. Anon. Rep. Exp. Stn. S. Afr. Sug. Ass. 1963164 p. 19. 2. Anon. Rep. Exp. Stn. S. Afr. Sug. Ass. 1964165 p. 19. 3. Anon. Rep. Exp. Stn. S. Afr. Sug. As;. 1965166 p. 18. 4. Anon. Rep. Exp. Stn. S. Afr. Sug. Ass. 1966167 p. 80... FIGURE 6: Application of the interrow compaction treat 5. Anon. Rep. Exp. Stn. S. Afr. Sug. Ass. 1967168 p. 70. ment (second ratoon). 6. Anon., 1964. Symposium on soil compaction. Proc. S. A decrease in infiltration late which must have Afr. Sug. Technol. Ass. 37: p. 144153. followed compaction was ineffective in reducing the 7. Pereira, H. C., Wood, R. A,, Brzostowsk~, H. w., and Hosegood, P. H., 1958. Water conservation by fallowwater supply to the cane roots, probably kcause the ing in semiarid tropical East Africa. Emp. J. exp experiment was situated on relatively level ground. Agric. 26: 213228.

Proceedings of The South African Sugar Technologists' Assoc Discussion Mr. von der Meden: From Figure 4 it appears that the effect of compaction was more noticeable in the early stages. It is mentioned that soil samples were taken at time of harvest and I would like to know the analysis of these from the point of view of any change in soil physical properties. It is possible the cane roots improved soil conditions during the growing season. Mr. Johnston: The data we have showed little improvement in soil physical conditions, at time of harvest. Mr. Wilson: Could some of the effects be due not to compaction but to severance of roots by the wings of the subsoiler? Mr. Glover: The experiments were on ratoon crops, which already had an established root system, which would help plant survival and which could reduce differences which might otherwise have been found in the compaction experiments. Mr. Wood: We used a ratoon crop because we wanted to experiment with an established crop. I agree the subsoiler wing effect must have damaged roots, and in fact such #damage was visible but the plant did recover. Dr. Cleaslby (in the chair): Subsoiling can relieve compaction but it can do a lot a damage to roots, particularly if it is followed by drought. Mr. Moberly: This experiment indicates that compaction is not a serious matter under ideal moisture conditions. But the effects of overall compaction are certainly observable so our advice to growers would beif you have to compact under wet conditions then do so on the interrow and not on the row. Professor Snmner: Compaction is very much related to type of soil. This soil possibly had too much clay to be very much affected by compaction, but a sodic soil would have been far more affected and certainly one would expect a crust to form with a consequent decrease in infiltration capacity and retardation of shoot emergence. Mr. Wood: We tested the soil beforehand and it was found to be readily compactible. Mr. Johnston: There was only a slight slope on the site used for the experiment. Possibly with a steeper slope and consequently a greater runoff, results would have been different. Mr. van Schalkwyk: At what rate was water applied to the compacted areas and what was the effect of runoff? Under irrigated conditions I think subsoiling would be beneficial after compaction to improve infiltration. What was the moisture content of the soil when subsoiling was carried out? We use subsoiling extensively to eradicate cane. Have any compaction experiments been carried out on soils similar to those in Swaziland, where there are only twelve inches of top soil in many areas? Mr. Jdmston: The irrigation rate was 3 inches applied at 0,25 inches per hour. There was no runoff, water which was unable to enter the soil immediately ponding in the interrow until accepted by the soil. When the first ratoon was subsoiled moisture conditions were ideal but in the second ratoon they were less favourable, the soil being rather too moist, thus preventing adequate shattering. Mr. Wood: We do not have results for other types of soils, but certainly the depth of soil at Pongola influenced the results of this experiment. Mr. von der Meden: The trial at Chaka's Kraal was carried out on shallow soil and although results were variable, there was a fifteen per cent re'duction in yield. Dr. Cleaslby: The decrease in yield owing to compaction is possibly less than we expected, but it seems that more work should be done on shallow type soils. Mr. Boyce: Could not a cultivator be used to restore the soil instead of a subsoiler? Mr:' Johnston: A tillage operation has the most shattering effect on the top 30 cm of soil so a cultivator could be used.