effect of surface treatments on soil crusting and infiltration

SOIL T E C H N O L O G Y vol. 3, p. 241-251 Cremlingen 1990

E F F E C T O F S U R F A C E T R E A T M E N T S

O N SOIL C R U S T I N G A N D I N F I L T R A T I O N

H.v.H. van der Watt & A.S. Claassens, Pretoria

Summary

The effects of phosphogypsum and plant residue mulches on soil crusting and infiltration were studied on a Rhodic Paleustalf in Northern Natal, South Africa. Under field conditions infiltration rates were considerably higher than those obtained with a laboratory type rainfall simulator. Both ameliorants were effective in countering crust formation. Their effect continued over a growing season (4-5 months). Scanning electronmicrographs showed that under moist conditions microbial hyphae and residues were prominent in the surface crust. Extractable soil P built up to high levels in the phosphogypsum treated plots.

1 Introduction

Soil surface crusting is usually due to the action of raindrops and sprinkler irrigation, although depositional crusts (due to translocation and deposition of fine particles by flowing or standing water) are also recognised (SHAINBERG 1985). Soil crusts have important agro- nomic consequences. In particular, water infiltration and associated aspects such as runoff and efficiency of water use

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through storage thereof in the root zone are affected. In addition, seedling emergence is adversely affected by a surface crust (HADAS & STIBBE 1977).

It is known that the formation of surface crusts is dependent on soil phys- ical and chemical properties and on the energy and chemical composition of rain or water applied through overhead irrigation systems (AGASSI, MORIN & SHAINBERG 1985, BEN-HUR, SHAINBERG, BAKKER & K EREN 1985). In many of these studies laboratory-type rainfall simulators were used to create soil crusts, following which infiltration rate and runoff were measured continuously. The effects of texture, clay mineralogy, specific exchangeable cations and irrigation water composition on soil crusting have been studied by many authors in this way (MORIN, G O L D B E R G & SEGINER 1967, SHAINBERG 1985, LEVY 1988). The study of these effects under field conditions are, however, of greater agro- nomic significance, although more diffi- cult and time-consuming.

Various surface-applied soil ameliorants such as mulches, manure, gypsum and others will combat crusting and im- prove infiltration (FAO 1965). Their efficiency, required application rates and long-term persistence in a given situa- tion cannot be predicted with any degree of accuracy. In this study the re-

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242 Van der Watt & Claassens

pH CEC (H20) (cmol(+)/kg)

6.1 7.2

Exchangeable cations (cmol(+)/kg)

Ca Mg K Na

3.1 2.0 1.3 0.2

Sand (%)

24.25 0.25q).05 (mm) (mm)

21.4 36.1

Silt Clay ~%) (%)

0.05q3.002 <0.002 (mm) (ram)

21.5 22.0

Tab. 1: Some properties of the experimental site soil (0-30 cm depth).

sults obtained from a field experiment in which the effects of phosphogypsum and crop residue mulches on soil crusting were studied, are reported.

2 Materials and methods

The soil chosen for the experiment is one in which serious crusting problems have been encountered. The soil, located on the Makatini Experimental Farm (lati- tude 27°24 ' S and longitude 32010 , E) in Northern Natal, South Africa, is a deep red Rhodic Paleustalf (Hutton Form, Shorrocks Series in the South African Soil Classification System). Extensive ru- ral development for the indigenous pop- ulation is underway in the particular area which is eminently suitable for irrigation agriculture. Some relevant soil properties are given in tab.1.

Since both gypsum and mulches have been shown to be effective in combating soil crusting, they were used as surface treatments in a 3x3x3 factorial experiment designed to compare the long-term effects of such treatments. The treatment levels were as follows:

Gypsum: 0(Go), 2(G1) and 5(G2) ton/ha Mulch: 0(Mo), 4(M1) and 8(M2) ton/ha.

Each of the 9 possible combinations of these treatments was replicated 3 times.

Plot sizes were 20x10 m. The plots were initially ploughed conventionally, and thereafter shallow tilled so as to minimize incorporation of the mulch and gypsum. All plots received a dressing of 40 kg ha P as superphosphate initially. Since the gypsum used was a phosphogypsum containing 0.36% P, only plots receiving no phosphogypsum were fer- tilized with P thereafter, to the extent of 20 kg P applied at each new planting. The phosphogypsum supplied 7,2 and 18 kg P per hectare, respectively, at the two levels employed.

The average annual rainfall on the Makatini Experimental Farm is 620 mm/year, occurring mostly in the summer months. Irrigation was sched- uled to compensate for evapotranspira- tion during rainless periods. The practice was to irrigate every 5 7 days, applying 3 0 4 0 mm water by an overhead sprinkler system; the application rate was 5 6 ram/hour. The irrigation water used was of a very good quality, having an electrical conductivity (EC) of 28 mS/m and sodium adsorption ratio (SAR) of 1,25 (mmole/liter) l/2.

The main objective of the experiment was not to study the effect of the ameliorants on yield, but their effect on crust formation, the permanence of the crust and on soil infiltration and soil chemical alteration. Two crops were sown and

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Surface Treatments, Crusting and Intiltration 243

harvested per year; to date the crops were maize, cotton, field beans and up- land rice.

Infiltration rates were measured on undisturbed sub-plots one meter square in size. Measurements were usually made immediately after planting and application of the ameliorants, and shortly after harvest. Infiltration rate and runoff were measured using a portable rainfall simulator specially developed for this study. The rain simulator, using various sizes of flat spray type nozzles (e.g. Spraying Sys- tems Co. Fulljet Type H-U80200), could generate 2 to 5 mm diameter drops. Drop sizes were measured using the flour pellet technique (HUDSON 1971). Measure- ments were made using 4.5 mm median diameter drops emitted from a height of 2.5 m at a water line pressure of 80 kPa. The nozzle was oscillated by an arm driven from an electric motor with an adjustable relay switch; thus application rates from 40 to 130 mm/h could be obtained. For the tests reported here, an application rate of 100 mm/h was used. The terminal velocity of 4.5 mm drops falling through air from a large height is 9.10 m/s, according to SEGINER (1965). The initial velocity of drops emerging from a nozzle at 80 kPa pressure is estimated at 12.6 m/s, hence they will be deceler- ated by the drag force of the air. Using data and equations given by SEGINER (1965), the velocity at impact of drops in our tests was estimated at 11.5 m/s. Thus the apparatus used produced rain having somewhat greater energy than natu- ral rain with the same median drop size. The test plots used for infiltration measurements were 1 m x l m and bounded by 100 mm high steel plates. Uniformity tests showed a high degree of uniformity of water application; for 25 subplots of the 1 m 2 test plot the coefficient of vari-

ation of measured rainfall was usually less than 5 per cent and often only 2 per cent. During a test, runoff was funnelled to a measuring cylinder below ground level and infiltration rate calculated as the difference between application and runoff rates. The data from three replicate infiltration runs were used to cal- culate average infiltration rate vs. cumulative rain curves for each of the 9 treatments.

The plots were regularly sampled to depths of 5, 10, 20, 30, 60, 90 and 120 cm. All samples were subjected to a detailed chemical analysis, but only the Bray-2 extractable phosphorus and saturation extract electrical conductivity data will be reported here. Surface crust samples were collected and examined microscopi- cally using a Jeol JSM-840 scanning electronmicroscope and stereomicroscope.

3 Results and discussion

3.1 Infiltration rate

It was found that under field conditions much higher infiltration rates were measured than under laboratory conditions using the rainfall simulator described by MORIN et al. (1967). This was the case even though our field rainfall simulator produced larger median drop sizes than the laboratory model. Under laboratory conditions the final infiltration rate (FIR) of the disturbed soil prior to the establishment of the experimental plots, calculated from an equation proposed by MORIN & BENYAMINI (1977), was 2 mm/h. Under field conditions the low- est measured FIR recorded to date was 10 mm/h. The difference is probably due mainly to surface roughness of the field plots. No attempt was made to obtain the same degree of surface smoothness

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70

E E :,Mo(~)

40 0 ~.- GoM2(a )

"..~ (3

~ 2 0

] 0 GoMo(b ) =

I O . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0 10 20 30 40 50 60 70 80

Cumu la t i ve ra in ( r a m )

Fig . 1: Comparison of infiltration rate vs. cumulative rain curves Jor selected plots measured at two times: (a) Early in the season (recently disturbed surface) and (b) after harvest (crust-topped, undisturbed surface). Each data point is the average qf three replicate determinations.

No Treatment Gypsum Mulch

(ton/ha/crop)

GoMo 0 0 GoM i 0 4 GoM2 0 8 GjM0 2 0 GIM1 2 4 GIM2 2 8 G2Mo 5 0 G2MI 5 4 G2M2 5 8

T a b . 2 : Final (one hour) the fourth crop cycle.

FIR at planting (mm/h)

22.2 27.8 33.0 27.2 48.0 46.3 40.8 62.9 64.1

FIR at harvest (mm/h)

10.1 24.2 23.6 22.1 33.2 40.4 30.0 33.2 53.5

infiltration rates (F IR) of plots at beginning and end of

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Surface Treatments, Crusting and Int~ltration 245

under field conditions as is the practice for laboratory infiltration rate determinations. For example, in the latter case large clods are crushed and the sample is usually passed through a 4 mm sieve. It was also found that for field infiltration runs the M O R I N & B E N Y A M I N I (1977) equation mentioned above, could not be fitted to our data. For prac- tical reasons our runs were terminated after one hour. Possibly, if runs are of longer duration, the equation could then be used. Our F I R values reported here thus are not the calculated values, but field values obtained after one hour of continuous rain application.

Field infiltration rates were furthermore dominated by the immediate past surface treatment and cultivation his- tory. Recently cultivated soil without any added amelorants (control plots) had rel- atively high F I R values when measured shortly after planting. However, towards the end of the season when a firm crust had been established, FIR 's on those same plots were markedly lower. An example of these observations is given in fig.1.

A comparison of the effects of the various surface treatments on infiltration rates after four crop cycles, i.e. at a time when earlier treatments would also have an effect, indicated that the gypsum only treatments, at the levels employed, con- sistently succeeded in countering crust formation more effectively than the cor- responding mulch only treatments ("cor- responding" referring to the application levels which were of course not the same for gypsum and mulch) (tab.2). Thus the data of tab.2 show that the G2M0 treatment has a somewhat more favourable effect on infiltration rate than the GoM2 treatment. The G1M0 and GoM1 effects are however, very similar. Furthermore,

combinations of gypsum and mulch had even more favourable effects than any one of the ameliorants alone. The values reported in tab.2 are the averages for three replicate infiltration runs (on one each of the replicated field plots). Un- fortunately, under field conditions, the replicates differed rather widely and a statistical evaluation of the data was not attempted. The inferences drawn from the data are thus subject to this limita- tion and the observance of trends.

The favourable effect of the ameliorants on an "older" (end-of-season) crust was surprising. Thus, at harvest the GzM0 treated plots had a three- fold higher F I R than the untreated plots. Similarly, the G2M2 plots had F I R rates five times higher than the untreated plots. Although all FIR 's decreased towards the end of the season, both ameliorants succeeded in facilitating a very much improved soil infiltrability.

These results have an important bear- ing for long-term recommendations taking cognisance of economic factors, availability of plant rests for mulches and effects of continued large gypsum applications on soil chemical properties. It is noteworthy that despite the fact that or- ganic residues decompose rapidly in the prevalent hot, humid climate and moist soil conditions, the beneficial effect of a 4 t/ha mulch is still significant at the end of the growing season (+4 months after application).

3.2 Microscopic studies of crusts

Both surface and sectional views of crusts, using a stereomicroscope and scanning electronmicroscope, indicated distinct differences in crust structure for the various treatments. The following were observed:

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Photo la: Untreated soil, surface view.

Photo 1 b: Gypsum treatment (5 t/ha), surface view.

Photo lc: Untreated soil, fresh crust, sectional view.

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Surface Treatments, Crusting and Infiltration 247

i :

?

Photo ld: Gpysum treatment (5 t/ha), fresh crust, sectional view.

Photo le: Untreated soil, old crust, sectional view.

Photo If: Gypsum treatment (5 t/ha), old crust, sectional view.



i ~ ..... :~i~ ~ ~̧ '~

Photo lg: Untreated soil, old crust, surface view, showing microbial slime.

Photo 1 h: Gypsum treatment (5 t/ha), old crust, showing microbial slime and

__ hyphae.

Photo 1 : Scanning electromicrographs of crusts.

(a) The surface condition of a crust developed when gypsum was applied clearly differed from that of an untreated soil (photo la and lb). The photo shows crusts which had developed over a period of four months. The smoothness of the crust from untreated soil is apparent, while microstructure and roughness can

be seen on the crust from gypsum treated soil. Under the stereomicroscope recrystallized gypsum crystals of various sizes were observed. The accumulation of gypsum (and possibly other salts) on the crust surface due to evaporation could also be seen on a macroscopic scale in the field. Under mulch, in the absence

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Surface Treatments, Crusting and Inl~ltration 249

of gypsum, the surface condition of the crust appeared to approach that of untreated soil. All these observations are, of course, qualitative and perhaps prone to subjectivity due to the immense variability of structure on the microscopic level.

(b) Cross-sectional views of the up- per 0-2 mm of crusts also showed structural differences between gypsum treated and untreated soils. A greater degree of compaction is ev- ident in the case of untreated soil (photos lc to If). Movement of fine material through the zone investi- gated was not very clear, in con- trast to the observations of LEVY et al. (1988). The structural differences between untreated and gypsum treated crusts were more obvi- ous in the older than in the young crusts.

(c) The presence of a dense mat of fungal hyphae and other microbial structures was an important and striking feature of older crusts and were not observed in "fresh" crusts (photos le to lh). The effect of these microbial structures on crust stabilization and soil infiltrability is not known and requires further inves- tigation. They may not always be present since the examples shown in photos le to lh pertain to crusts formed during a wet season with ir- rigated rice as the crop.

4 Soil chemical properties

The exchangeable sodium percentage (ESP) and EC of the soil saturation extract as well as the SAR and EC of the irrigation water play an important

role in crust formation (e.g. SHAIN- BERG 1985). High ESP levels are not a problem in this particular case, and the exchangeable cation and anion composition of the soil will not be discussed herein. However, gypsum applications do increase the EC of the soil solution, and the increased electrolyte concentration counteracts chemical soil dispersion and hence crust formation. The EC values of the saturation extracts of samples from the test plots, to depths of 600 mm and after 2 years (4 crop cycles), are given in tab.3. The high EC in the up- permost 0-50 mm of soil will undoubt- edly have a favourable effect on aggre- gation/soil stabilization. The higher EC in the deeper layers is overwhelmingly due to dissolved CaSO4 which has moved down the profile (results not shown). Of course, the displacement of exchangeable nutrient cations such as Mg and K by the high solution concentrations of Ca is a factor to be considered when using gypsum as an ameliorant.

Since phosphogypsum was used in this study, it could be expected that soil P- levels would increase with continued use of this ameliorant. This was indeed the case. An example of the contribution of phosphogypsum to soil P is shown in tab.4. Although the mulch added to the soil should also contribute to soil P, and this is indeed observed for the Go treatment series and G2M2 treatment, the effect is not clear and consistent. The significant contribution of phosphogypsum to topsoil P is apparent. Taking account of the fact that initially soil P-levels were in the region of 1-3 mg P/kg soil, it is ev- ident that regular applications of 2 t/ha phosphogypsum can replace fertilizer P.



Electrical conductivity (mS/m) Treatment* 0-50 mm 50~100 mm 100-200 mm 20(~300 mm 30Oq500 mm

GoMo GoMi GoM2 GIM0 GIMI G1M2 G2M0 G2M1 G2M2

24.0 23.6 38.3 34.7 24.9 32.8 19.9 32.6 26.6 29.9 23.6 15.1 40.6 29.4 28.9 44.3 18.8 42.3 45.0 60.2 32.3 19.0 28.4 41.7 42.6 42.1 19.5 23.9 37.5 49.0

124.6 15.0 52.9 91.5 104.2 104.2 35.3 63.4 92.0 83.2 200.0 67.6 96.0 120.7 119.1

* Refer to tab.2 for explanation of symbols

Tab. 3 ' Electrical conductivity of saturation extracts of soil from test plots sampled to 600 mm depth, following ,four crop cycles and surface treatments.

Sources of P over 2-year period** Bray-2 extractable soil P (mg/kg soil) Treatment* (per hectare) Soil depth

0-50 mm 50-100 mm 100~200 mm 200-300 mm

GoMo GoM1 GoM2 GIMo GIMI GIM2 G2Mo G2M1 G2M2

60 kg P 60 kg P + 16 t mulch 60 kg P + 32 t mulch 8 t gypsum 8 t gypsum + 16 t mulch 8 t gypsum + 32 t mulch 20 t gypsum 20 t gypsum + 16 t mulch 20 t gypsum -- 32 t mulch

36 26 13 3 39 35 16 8 47 44 16 5 43 29 15 4 43 29 9 3 42 31 15 6 64 44 16 5 61 42 14 7 78 43 17 8

* Refer to tab. 2 for explanation of symbols *" All plots initially received 20 kg P as superphosphate.

Thereafter, only the Go plots received further applications totalling 60 kg/ha P.

Tab. 4: Soil phosphorus levels (Bray-2 extractable P to 300 mm depth)Jbllowing 4 crop cycles and ameliorant applications.

5 Conclusions

1. The ameliorating effect o f both phosphogypsum and plant residue mulches, applied regularly at planting, has been demonstrated under field conditions.

2. A difficulty encountered is the variability of surface soil conditions, resulting in considerable variation in infiltration rate measurements on

.

the same or replicate plots. It was nevertheless concluded that the field measurement of final infiltration rate (FIR) resulted in much higher FIR values than those obtained under laboratory rainfall simulators.

Initially it was not known to what extent the favourable effects of gypsum and mulches on crust formation would persist over a growing season

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Surface Treatments, Crusting and Inl~ltration 251

(under no-tillage or minimum tillage opera t ions ) . O u r d a t a s h o w tha t at

least ove r shor t c r o p p i n g cycles (4-5

mon ths ) , these a m e l i o r a n t s re ta in a

ve ry l a rge p r o p o r t i o n o f the i r in i t ia l

a m e l i o r a t i n g effects.

4. M i c r o s c o p i c e x a m i n a t i o n o f su r face

crusts r evea l ed tha t u n d e r m o i s t

c o n d i t i o n s m i c r o b i a l hyphae , s l imes

and r ema ins m a y p lay an i m p o r t a n t

role, e i the r benef ic ia l o r d e t r i m e n -

tal. Th i s o b s e r v a t i o n w a r r a n t s fur-

the r inves t iga t ion .

5. C h e m i c a l ana lyses c o n f i r m e d the

benef ic ia l effect o f g y p s u m on soil

so lu t i on e lec t ro ly te c o n c e n t r a t i o n .

T h e c o n t r i b u t i o n o f p h o s p h o g y p -

s u m to the soil P rese rvo i r is sig-

n i f ican t a n d m a y o b v i a t e the need

for P fer t i l izer in m a n y ins tances .

A c k n o w l e d g e m e n t s

The authors whish to acknowledge the assistance and supervision of staff of the Makatini experimental farm. Mr. Graeme Croft supervised laboratory chemica l analyses . T h e c o - o p e r a t i o n a n d

he lp o f Prof. J. C o e t z e e a n d s taf f o f

the U n i v e r s i t y o f P re to r i a ' s E l e c t r o n m i -

c r o s c o p e U n i t is a lso g ra te fu l ly a c k n o w l -

edged.

R e f e r e n c e s

AGASSI, M., MORIN, J. & SHAINBERG, I. (1985): Effect of raindrop impact energy and water salinity on infiltration rates of sodic soils. Soil Sci. Soc. Am. J. 49, 186-190.

BEN-HUR, M., SHAINBERG, I., BAKKER, D. & KEREN, R. (1985): Effect of soil texture and CaCO 3 content on water infiltration in crusted soil as related to water salinity. Irrig. Sci. 6, 281-294.

FAO (1965): Soil erosion by water. Some mea- sures for its control on cultivated lands. Food

and Agricultural Organization of the United Nations, Rome, 284 pp.

HADAS, A. & STIBBE, E. (1977): Soil crusting and emergence of wheat seedlings. Agron. J. 69, 547-550.

HUDSON, N. (1971): Soil Conservation, Bats- ford, London.

LEVY, G.J. (1988): The effects of clay mineralogy and exchangeable cations on some of the hydraulic properties of soils. DSc(Agric) Thesis, University of Pretoria, Pretoria.

MORIN, J. & BENYAMINI, Y, (1977): Rainfall infiltration into bare soils. Water Resour. Res. 13, 813-817.

LEVY, G.J., BERLINER, P.R., DU PLESSIS, H.M. & VAN DER WATT, ILv.H. (1988): Mi- crotopographical characteristics of artificially formed crusts. Soil Sci. Soc. Am. J. 52, 784-791.

MORIN, J., GOLDBERG, S. & SEGINER, I. (1967): A rainfall simulator with a rotating disk. Trans. ASAE 10, 74-79.

SEGINER, I. (1965): Tangential velocity of sprinkler drops. Trans. ASAE 8, 90-93.

SHAINBERG, L (1985): The effect of exchangeable sodium and electrolyte concentration on crust formation. Advances in Soil Sci. 1, 101- 122.

Address of authors: H.v.H. van der Watt A.S. Claassens Department of Soil Science and Plant Nutrition University of Pretoria Pretoria, South Africa

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effect of surface treatments on soil crusting and infiltration

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