THE EFFECT OF SURFACE ROUGHNESS ON THE SOIL LOSS

Field measurements and lab analysis were conducted to determine the effect of soil surface roughness ( oriented and random) resulted from discing and chiseling on the soil loss by wind erosion. Some soil ridges parameters were measured in the field and aggregation (clods and non-erodible aggregates) were determined in the lab. The soil loss of the two operations was determined as a function of these variables. The results showed that the large proportions of random roughness(large clods) and clear oriented roughness (soil ridges) in the tilled layer after soil discing have a greatest effect in reducing the amount of soil loss and potential erosivity of soil by wind action in comparison with soil chiseling .This is attributed to the fact that the soil clods easily crushed into fine aggregates after soil chiseling but remained unchanged in soil discing .The statistical analysis coming in agreement with the results of the lab and field investigation INTRODUCTION The objective of primary and secondary tillage for wind erosion control is to produce rough and cloddy surfaces which have a greatest effect in reducing erosion by sheltering and trapping the blown soil particles and decreasing the wind velocity near the rough surface ( Hofman and Franzen, 1997). In other word, for any given soil conditions the amount of soil which will be blown depends on the two factors; the prevailing wind velocity and the roughness of soil surface (Romken et. al. 2001). An empirical-logarithmic formula suggested by Kirkby and Morgan(1987) for the relationships showed that; Z = 0.81 (Log 5.5 ds ) ( 1) Z = The height above erodynamic surface at which the wind velocity =zero ds = Grain or aggregate diameter (mm) The value of Z was varied with the logarithemic of grain or aggregate diameter, therefore the Z value over rough surface was greater than that of smooth surface. For this reason the height of soil surface during the measuring processes should be corrected due to changes in height of reading it made(Gomez ,2005). Many researchers have reported that the disc and chisel are the most common tools used to roughen the soil and bring clods to surface for wind erosion control and reducing soil loss (Saleh, 1997, Hofman and Franzen, 1997) , but other workers mentioned that tillage in general can gradually reduces the soil surface roughness by breaking the clods and aggregates that resist wind erosion( USDA,2008). Surface roughness is very important factor when measured the total soil loss from a given field. Woodruf and Siddaway (1965) have developed an empirical equation for estimation the erodibility of soils. The wind erosion equation (WEE) developed by these workers estimate a potential erosion in ton/acre as a function of five independent variables.This equation is ; Mesoptamia J. of Agri. (ISSN 815-316X) Vol. (39) No.(2) 2011 51 Received 3/1/2010 accepted 21/6/2010 Mesoptamia J. of Agri. (ISSN 815-316X) Vol. (39) No.(2) 2011 52 A= f ( I , K , C ,L V )(2) A = Soil loss t/acre I = Soil erodibility factor K = Soil roughness factor C = Climatic factor L = Unshelterd length factor V = Vegetation cover factor f = function Among the factors which help to protect soil from wind erosion , soil roughness factor(K-factor). Garcia et. al. (2008) have reported that K-factor estimates the fractional reduction of erosion caused by ridges and non-erodible aggregates. Therefore K has been divided into; soil ridge roughnesss sub-factor ( Krd) and soil random roughness sub-factor ( Krr),then; K = Krd + Krr (3) The objective of the study reported here ,was to provide an important informations on the interaction effect of random roughness (aggregation) and oriented roughenss (soil ridges) resulted from soil discing and chiseling (which are widely used operations in agricultural dryland farms at northern Iraq) on the soil loss by wind action, and to compare the profitibility of the two operations under this condition in order to indicate areas where further researches are needed. MATERIALS AND METHODS The study was conducted on the selected farm located at Kaber AlAbid area (about 20 Km south Mosul city). Table (1) showing some physical, chemical characteristics and classification of studied farm's soil. Composite surface soil samples ( 0-25 cm in depth ) was taken from tilled layer after discing and chiseling .This depth selected because many field observations indicated marked soil structural differences occurred approximately within this depth. Table( 1 ): Selected characteristics of studied farm's soil Clay Silt Sand Texture pH EC OM CaCO3 Classification gm / kg dS/m gm / kg 372 367 261 Silty Clay 7.7 0 .44 9.8 288 Entisol The soil samples were airdried and gently crushed to pass through a 4 sieves. Dry sieving for four fractions (2, 1, 0.50, and 0.25 mm) were done according to the procedure mentioned by Kemper and Rosenau (1986). Samples were sieved two times. The accumulated percentage of soil aggregates passing through each sieve was substitute in the following equation to calculate the Arithmetic Mean Size (AMS) for each soil treatment as reported by Emond ( 1971); 1400 (Y1 + 3/2 Y2 +3Y4 + 8Y8) (4). AMS = 400 Mesoptamia J. of Agri. (ISSN 815-316X) Vol. (39) No.(2) 2011 53 Where; Y1= soil aggregates passing through 0.25mm sieve. Y2, Y4 and Y8 = the accumulated percentage of aggregates passing through 0.50 , 1.0, and 2.0 mm sieves respectively The standard dry sieving procedure repeated by resieving the non erodible aggregates (NEA) expressed as percent larger than 0.84 mm in diameter .The obtaining values were plotted on the table ( 2 ) to obtaining the soil erodibility factor( I ) of wind erosion equation (WEE). Table ( 2 ): Soil erodibility factor( I ) in Mg / ha. as a function of % NEA.