effect of organic and fertiliser treatments on soil physical properties and erodibility

Soil & Tillage Research, 22 ( 1992 ) 199-209 199 Elsevier Science Publishers B.V., Amsterdam

Effect of organic and fertiliser treatments on soil physical properties and erodibility

E.I. Ekwue 1 Silsoe College, CranfieM Institute of Technology, Cranfield, UK

(Accepted 10 June 1991 )

ABSTRACT

Ekwue, E.I., 1992. Effect of organic and fertiliser treatments on soil physical properties and erodibility. Soil TillageRes., 22: 199-209.

The effect of different organic and fertiliser treatments on soil physical properties and erodibility was studied in the laboratory using soils from the organic manuring plots of Rothamsted Experimen- tal Station at Woburn, UK. Measurements of soil detachment, aggregate stability, particle size distribution, bulk density, porosity and infiltration rates were made to evaluate the role of organic treatments in determining physical properties and how these properties in turn affect soil erodibility. In comparison with farmyard manure, straw and fertilisers-only treatments, ley treatments improved aggregate stability and other measured soil properties, and this led to reduced soil detachment and improved infiltration rates.

INTRODUCTION

Despi te the awareness tha t organic m a t t e r reduces soil e rodib i l i ty (Luk, 1979; Ke tcheson , 1980) , there r ema ins a d o u b t abou t the re la t ive effects o f d i f fe ren t organic mater ia l s used as soil a m e n d m e n t s . T h e mater ia l s used in- c lude f a r m y a r d m a n u r e ( F Y M ) , peat , compos t , green m a n u r e and organic m a t t e r f r om grass leys. Some o f these t r e a tmen t s are m o r e effect ive than others in i m p r o v i n g soil aggregate stabil i ty to wa te r d i s rup t ion an d in reduc ing e rod ib i l i ty in d i f fe ren t soil types.

Wil l iams and Cooke ( 1961 ) and G r e e n l a n d ( 1 9 7 7 ) s tressed the super ior - i ty o f leys ove r o the r a m e n d m e n t s in i m p r o v i n g aggregate s tabi l i ty in some soils. F Y M i m p r o v e s soil physical p roper t i e s (T ia rks et al., 1974) and increases soil pe rmeab i l i t y (T i sda le and Nelson , 1956) , and thus m a y reduce r u n o f f in the field. Also, the large sized soil aggregates p r o d u c e d by F Y M are not easily t r anspo r t ab l e ( M a z u r a k et al., 1975 ) and m a y the re fo re reduce soil

~Present address: Department of Agricultural Engineering, University of Maiduguri, Maidu- guri, Nigeria.

© 1992 Elsevier Science Publishers B.V. All rights reserved 0167-1987/92/$05.00

200 E.I. EKWUE

transport. However, Mazurak et al. ( 1975 ) observed that FYM increased soil detachment by reducing soil cohesion and crust strength. Some organic materials that decompose slowly, such as straw and peat, contribute little to soil structure but may reduce soil erodibility by improving other soil physical properties which increase the permeability of soils (Troeh et al., 1980).

Previous research has concentrated on the effect of organic matter on soil aggregate stability, but not on soil erodibility itself. Few studies have compared the effects of different organic treatments on soil erodibility, and the relative effectiveness of these treatments when compared with mineral fertilisers. This paper seeks to rectify this by investigating the effects of organic and fertiliser treatments on soil physical properties linked to soil erodibility.

MATERIALS A N D M E T H O D S

Thirty two study soils were collected from the plots of the organic manuring experiment of the Rothamsted Experimental Station at Woburn, UK. The details of the previous treatments on the soils were documented by Mattingly (1974). The different organic contents in the plots resulted from the applications of various organic and fertiliser treatments at varying levels. It was therefore possible to study the effect of these treatments on soils of similar type.

The soils are developed on Lower Greensand beds and are of the Cotten- ham series (Mattingly, 1974). The soil is friable, weakly structured and of loamy sand texture. The soils of the area are classified as Xerumbrepts ac- cording to the USDA soil taxonomy. The organic manuring experiment con- sists o fa randomised block design involving four blocks, each with eight plots (30.5 m × 8.55 m) where the individual treatments are applied. The plots have received various treatments since 1965. The first phase of the experiment lasted until 1971 and involved continuous leys on two plots and arable cropping on the other six plots. All the treatment plots apart from the one with FYM received fertiliser treatments during this period. In addition, four of the arable p!ots received straw, peat, FYM and green manure while the other two had no organic treatment. The two arable plots with fertilisers-only treatment then served as a control against which to compare the results from the plots with organic treatments. Details of the initial treatments are given in Table 1.

The straw, peat and FYM were applied and then ploughed into the top 25 cm of soil while the green manures were ploughed into the soil after cultivation. Starting in 1972 on Blocks I and III, and in 1973 on Blocks II and IV, all the plots were mechanically ploughed and potatoes, winter wheat, sugar beet, spring barley, potatoes and winter wheat were grown in rotation; they were given phosphorous, potassium and magnesium (P, K and Mg) fertilisers at appropriate rates, and the effects of eight levels of nitrogen were assessed. The

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aim was to evaluate the effect of organic matter accumulated during the first phase on crop yields. In 1979 on Blocks I and III, and in 1980 on Blocks II and IV, plots previously treated with peat and green manure in the first phase were sown to grass-clover ley. The other six plots grew sugar beet and spring barley to complete two cycles of the four-course rotation. The organic and fertiliser treatments used in the first phase were re-imposed on the six plots (Table 1 ). The past and present organic and fertilisers-only treatments in the plots are:

Fd/Fd--Fer t i l i ser equivalent to PKMg in FYM Fs/Fs--Fert i l iser equivalent to PKMg in straw Lc/Lc--Grass-c lover ley Lc/Ln--Grass-c lover ley with previous grass ley with nitrogen treatment St / St--Straw (barley) Lc/Pt - -Grass-c lover ley with previous peat treatment F YM/FYM- - Fa rm ya rd manure Lc /Gm--Grass -c lover ley with previous green manure treatment. The symbols represent treatments, t reatment 1979 to present/init ial treat-

ments, 1965-1971. Soil samples for the present study were collected from the top 15 cm of the

32 experimental plots. The particle size distribution of the soils is given in Table 2. The measurements made on the soil samples were of parameters linked with dispersibility of the soil and its hydrological behaviour which are in turn related to soil erodibility. Three replicate measurements were made for each soil parameter. Organic matter was assessed with the Walkley-Black

TABLE 2

Particle size distribution (%) of the test soils a sampled in May, 1985

Treatment Coarse sand Sand Fine sand Total sand Silt Clay symbols b (2-0 .6mm) (0.6-0.21mm) (0.21-0.06mm) (2.00-0.06mm) (63-2pm) ( < 2 p m )

Fd/Fd 1.4+0.2 31.5_+1.0 47.2_+2.3 80.1_+3.5 11.5+0.2 8.4_+0.2 Fs/Fs 1.8_+0.1 32.0+ 1.0 46.0 + _ 1.1 79.8+__2.1 12.2_+0.1 8.0_+0.4 Lc/Lc 1.8+-0.3 32.6_+ 1.7 43.9_+ 1.7 78.3_+2.7 12.6+-0.5 9.1 _+0.5 Lc/Ln 1.9_+0.3 34.1+_1.8 42.8-+1.4 78.8_+2.9 12.7+-0.4 8.5_+0.2 St/St 1.6_+0.1 31.3_+1.2 43.1_+1.9 76.0+-1.3 14.2+-0.7 9.8_+0.2 Lc/Pt 1.6_+0.1 34.3_+1.8 45.2_+1.2 81.1_+1.4 10.7+-0.6 8.2_+0.3 FYM/FYM 1 .6+_0 .2 34.9+2.6 45.3_+1.2 81.8_+2.1 10.2+__0.5 8.0+_0.4 Lc/Gm 1.8+_0.1 32.6+_0.8 45.1_+1.1 79.5+_0.5 12.2_+0.4 8.3_+0.2

aSoil classification is by the British Standards Institution system. Values are means of three repli- cates -+ standard deviation. ~Fd/Fd--Fertiliser equvalent to PKMg in FYM; Fs/Fs--Fertiliser equivalent to PKMg in straw; Lc/ Lc--Grass-clover ley; Lc/Ln--Grass-clover ley with previous grass ley with nitrogen treatment; St /St- - Straw (barley); Lc/Pt--Grass-clover ley with previous peat treatment; FYM / FYM--Farmyard manure; Lc/Gm--Grass-clover ley with previous green manure treatment.

ORGANIC AND FERTILIZER TREATMENT EFFECTS ON SOIL 203

method; mechanical analysis was performed using the pipette method; bulk density, porosity and water content at saturation were assessed using undisturbed core samples (54 m m in diameter and 20 m m deep) following the methods described by Brady (1984). The proportions of water stable aggregates (WSA) were assessed using the procedure of Low (1954) with the application of a sand correction.

Details of the measurements of soil detachment were given by Ekwue (1990). Soil samples of less than 5 m m in diameter were put in splash cups which were 73 m m in diameter and 50 m m deep. A 20 min simulated rainfall was applied at an intensity of 82 m m h-1 using a rotating disc simulator (Morin et al., 1967). The soil detachment reported here is the dry weight of soil thrown out from the splash cups during rainfall.

Comparative infiltration rates were assessed both before and after rainfall using 100-mm-diameter cylinder cores with a depth of 110 mm. Sample prep- aration and the design storm were the same as those in the soil detachment test described above. To measure infiltration rates, a filter paper was placed on the soil surface and empty cylinders of the dimensions used in the soil infiltration test were placed on the top of the soil surface cores and sealed with electricians' tape. Water intake rates were determined by keeping a constant 8-10 cm head of water on the samples and periodically measuring the depth of water that had moved through the soil surface. Final infiltration rates are reported.

A criticism of this experimental investigation may be the use of disturbed samples for soil detachment and infiltration tests. Disturbed samples, however, simulate the tilled layer of the soil (Moldenhauer, 1965) which repre- sents a highly erodible material on these loamy sandy soils. Woodburn and Kozachyn (1956) worked with both disturbed and undisturbed samples and observed that although disturbed samples had higher soil detachment than did undisturbed ones, the relative detachment rankings of the samples re- mained the same. Rose ( 1962 ) made a similar observation in a similar study on infiltration. It is therefore very likely that the relative rankings of the soil detachment and infiltration parameters for the treatments examined in this study may not be significantly altered by their structural condition.

RESULTS AND DISCUSSION

The treatments are listed in order of increasing organic matter content. The results quoted are means of four blocks. Analysis of variance was carried out using the randomised complete block design method, and the Duncan's mul-

2 0 4 E.I. EKWUE

tiple range test (DMRT) was used to test for differences between mean results of individual treatments.

Organic matter contents

Table 3 shows the values of organic matter content. The FYM and L c / G m plots contained the highest mean organic matter contents. The mean values of all the organic treatments were significantly higher at the 1% level than the fertiliser-only treatments. Generally, all the soils of the experiment had low organic matter content (less than 2%), and this is common in arable soils with high sand content.

Soil physical properties

The results of measurements of some properties that depict the physical condition of the soil are presented in Table 4. The FYM plots had the lowest bulk density, and the highest porosity and water content at saturation. This was probably because of the coarse nature of the organic material (Williams and Cooke, 1961 ) which physically prevents close packing of soils, thus al- lowing channels which enhance root movement . The high water retention of the plots is attributable to the high water holding capacity of FYM (Hafez, 1974). The ley and straw plots had lower mean bulk densities and higher porosities, and had water retention values higher than those of the control soils with fertilisers-only treatment.

The differences in mean values were small. This may reflect the narrow range of organic matter content of the soils. However, the trend in the mean values showed that FYM treatment produced the best physical conditions in

T A B L E 3

O r g a n i c m a t t e r , i n f i l t r a t i o n ra t e s , a g g r e g a t e s t a b i l i t y a n d soi l d e t a c h m e n t

T r e a t m e n t s y m b o l s O r g a n i c m a t t e r F i n a l i n f i l t r a t i o n

( % ) r a t e s ( m m / h r )

- R a i n + R a i n

W a t e r s t a b l e Soi l d e t a c h m e n t a g g r e g a t e s > 0 . 5 ( k g m - 2 )

m m ( % )

F d / F d 1 .02 ¢ 6 0 cd 44 u~ 0 . 8 8 c 2 . 2 6 e F s / F s 1 .02 c 5 4 d 30 c 0 .91 ¢ 2 . 2 0 de

L c / L c 1 .28 b 106 a 72 a 1.67 b 2 . 0 7 bc L c / L n 1 .37 ab 93 ~ 6 9 a 1 .84 b 2 . 0 4 abe S t / S t 1.38 ab 50 d 34 c 0 . 9 7 c 2 .15 Cd

L c / P t ! . 4 2 a 142 a 75 ~ 2 . 4 7 a 1 .97 ab F Y M / F Y M 1.44 a 98 b 6 3 ab 1.11 c 2 .12 Cd

L c / G m 1.44 a 112 a 63 ab 2 . 2 2 a 1 .94 ~

T h e d i f f e r e n c e s b e t w e e n v a l u e s in a c o l u m n f o l l o w e d b y d i f f e r e n t s u p e r s c r i p t s a r e s i g n i f i c a n t a t t h e 5% level .


TABLE4

Some physical properties of the study soils

Treatment symbols Bulk density Porosity ( g c m -3) (%)

Moisture content at saturation (%)

F d / F d 1.71Cd 34.0 b 19.8 b F s / F s 1.73 cd 34.0 b 19.8 b Lc/Lc 1.69 ~d 36.0 a 21.6 ab Lc /Ln 1.67 a~ 37.0 a 21.9 a St /St 1.68 a~ 36.0 a 21.4 ab Lc /P t 1.66 ab 37.0 a 22.2 a F Y M / F Y M 1.65 a 37.0 a 22.9 a L c / G m 1.69 bed 36.0 a 21.6 ab

The differences between values in a column followed by different superscripts are significant at the 5% level.

the soil of all the organic treatments. FYM was followed by the ley and straw treatments, while the plots that received only fertilisers produced the poorest soil conditions for crop growth and water infiltration.

Water stable aggregates

All the study soils had low stability to water disruption; that is, they contained water stable aggregates (WSA) greater than 0.5 m m in diameter with a range of mean values of 0.88 to 2.47 (Table 3). These soils, which have a high proportion of fine sand (Table 2), are usually unstable (Low, 1955 ).

The ley treatments produced the highest aggregate stability in the soil. The order of ley effectiveness was L c / P t > L c / G m > L c / L n > Lc/Lc. The values from these ley treatments were significantly higher either at the 1% or 5% levels, than those from the other treatments. FYM plots had lower values of stability despite their relatively high organic matter content. The mean stability was less than half of that in the Lc /P t and L c / G m plots. There were no significant differences between the stability values of the FYM, straw and fertiliser treatments.

These results indicate that some organic materials were more effective than others in stabilising soil aggregates. The highest stability to the soil that was imparted by the ley treatments agrees with the findings of Williams and Cooke ( 1961 ) and Greenland ( 1977 ). Allison ( 1968 ) attributed the superiority of leys to their provision of ideal conditions for both aggregate formation and stabilisation. Williams and Cooke ( 1961 ) attributed their effectiveness on sandy soils to the ability of fine roots of grass to tie soil particles together. The results from the present study have further shown that the effect of leys in stabilising soil aggregates was increased when they were preceded by other treatments such as peat, green manure and nitrogen, in that order. These

206 E.l. EKWUE

treatments may have provided a good environment in which the leys then responded favourably.

The results also showed that FYM was more effective in increasing the organic matter content of the soil than in increasing soil stability. Williams and Cooke ( 1961 ) observed that organic matter in the form of FYM increases soil aggregate stability by sticking soil particles together. They added that, since sands are less cohesive than clay, the FYM treatment is effective in clay but not in sandy soils such as those used in the present study.

The straw treatment did not increase soil stability as much as the leys did, despite its plots having a higher mean organic matter content than those of two of the ley treatments. This suggests that straw may not be a good stabiliser of sandy soils. This may be because straw has a high carbon:nitrogen ratio (Foth, 1978 ) which means that it decomposes slowly in the soil. Cook ( 1967 ) suggested that since FYM is composed mainly of straw, both products may be expected to act in the same way.

The fertilisers-only treatments produced soils with the lowest stability to water disruption. This may be because of the inability of fertilisers alone to increase the organic content of the soil as much as do the organic treatments. Fertilisers are therefore less effective in soil stabilisation than organic treatments.

Soil detachment

The mean range of soil detachment varies from 2.26 kg m - 2 in the F d / F d treatment to 1.94 kg m -2 in the L c / G m treatment. The treatments that produced the highest mean stabilities also had the least detachment. This is because stable aggregates resist detachment by utilising part of the rainfall en- ergy (which would otherwise cause soil detachment ) to disperse the aggregates before appreciable detachment can occur (Mazurak and Mosher, 1970 ). There was a significant negative correlation coefficient of -0 .701 between soil detachment and WSA for all the 32 study soils (Ekwue, 1987).

The ley plots had the lowest detachment values followed by the FYM and straw plots. The plots with fertilisers-only treatments had the highest mean detachment. These results emphasise that both the total organic matter content of soil and the type of organic materials introduced into the soil are im- portant in reducing soil erodibility.

Infiltration rates

The measurements of final infiltration rates with and without rainfall show that the ley treatments which have the highest mean stability of soil also have the highest mean infiltration rates. The FYM plots also had high infiltration because of good physical properties, such as porosity, that FYM imparted to


the soil (Table 4). The fertilisers-only and straw treatments had the lowest values since neither structural stability nor other physical properties were improved.

The mean values of final infiltration rates without rainfall ranged from 50 mm h-~ in the straw treatment to 142 mm h - 1 in the Lc/Pt treatment. These values were reduced when infiltration was measured after the application of simulated rainfall. The mean rates then ranged from 30 mm h - 1 in the Fs/Fs treatment to 75 mm h-~ in the Lc/Pt treatment. The reductions in infiltration values were caused by the breakdown of soil aggregates and the compac- tion of the surface which led to surface sealing. The reductions were significant at the 1% level.

The straw treatment gave marginally better results for the parameters al- ready discussed than did the fertilisers-only treatments. However, the fertilisers-only treatments had higher infiltration rates than the straw treatments. This may well be a result of the lower mean sand content of the straw plots (Table 2 ).

S U M M A R Y A N D C O N C L U S I O N S

The effects of some organic and fertiliser treatments on properties linked to soil erodibility were studied. Soil erodibility depends mainly on soil properties linked to soil dispersion, detachment and also infiltration rates (Smith and Wischmeier, 1962 ).

The results from this study showed that the plots with ley treatments had the highest mean stability, lowest detachment and highest infiltration rates of all the treatments tested. This suggests that ley treatment may be the most effective in reducing detachment as well as runoff generation. This would be expected to lead to low soil transport. Leys were found to work better when preceded by other organic treatments such as peat and green manure.

FYM may not improve soil stability and reduce detachment in sandy soils as much as do the leys, but the high infiltration rates it induces may ensure low runoff and soil transport. The high infiltration rate is caused by reduced bulk density and increased porosity of the soil. Straw and fertilisers-only treatments produced the lowest aggregate stability, highest detachment and lowest infiltration rates, indicating that they are ineffective in reducing soil erodibility.

The straw treatment was marginally better than the fertilisers-only treatments. Cooke ( 1967 ) suggested that straw can be best used by mixing it with the faeces and urine of farm animals to produce FYM since it has little man- urial value on its own. The findings from this study support this assertion as far as reducing soil erodibility is concerned.

These results show that in unstable soils like the loamy sandy ones tested the use of organic manures produced more favourable results than the fertil-

208 E.I. EKWUE

isers-only treatments. Differences in the values of measured parameters were detected despite the low organic matter status of all the study soils. Significant differences were also found between the various organic treatments tested for some parameters. This study has therefore demonstrated the need to consider not only the amount of organic matter but also the type of organic materials present in soils when properties related to aggregate stability and erodibility by rainfall are being compared.

The author acknowledges that other substances such as roots and fungal hyphae may have contributed to soil stability, as reported by Tisdall and Oades (1982). It is possible that some of the organic treatments may have acted partly by encouraging the growth of these substances in the soil. Future work should examine the role of these other stabilising agents on the soils used in this study. Furthermore, in this experiment, all organic treatments were incorporated at low depths (the top 25 mm of soil) and this method is appropriate for encouraging decomposition and formation of humus in temperate regions. In tropical regions, however, it may be advisable to deeply incorpo- rate organic materials so as to reduce decomposition rates in order to obtain useful intermediary humus-like products in the soil rather than merely the final decomposition products of carbon and water. In addition to the type of incorporated organic materials, therefore, the depth of incorporation may be expected to influence how organic materials affect soil aggregate stability and erodibility.

ACKNOWLEDGEMENTS

The author is very grateful to Professor R.P.C. Morgan for his invaluable suggestions and reading of the manuscript. He is also grateful to Rothamsted Experimental Station for access to their field plots and to A.J. Johnston for his advice.

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