Petar Yankov, Miglena Drumeva ©Showeet.com EFFECT OF SOIL TILLAGE SYSTEMS ON SOME PHYSICAL PROPERTIES OF THE SOIL Petar Yankov, Miglena Drumeva

INDEX Introduction Soil structure Bulk density Soil porosity Soil temperature Moisture content in soil Capillary rise of water

1 INTRODUCTION The essence of soil tillage consists in the mechanical impact of the working parts of the tillage tools and machines on the soil with the aim of creating the most favorable conditions for growth and development of the cultural plants. This is achieved by controlled modification of the physical structure on which the water, air, thermal and nutrition regimes of soil are dependent.

1. INTRODUCTION The contemporary agricultural production and the increasing demands toward it are the reason for the greater importance of the ways and systems for tillage of soil aimed at improving its physical properties (structure, bulk density, porosity and temperature), its water regime (moisture content and capillary rise), and its nutrition regime (content of organic matter, nitrogen, phosphorus, potassium) in order to obtain sustainable results, to preserve the soil fertility and the environment.

2 SOIL STRUCTURE Soil structure is a main factor of agronomic significance, on which the physical properties of soil are dependent, as well as the ratio and interaction of water and air in it. It is determined by the total mass and ratio of the soil units and aggregates, which have different sizes, porosity, mechanical resistance and water resistance (Fig. 1). Figure 1: Types of soil aggregates (Image from Victorian Resources on line at http://vro.dpi.vic.gov.au/dpi/vro/vrosite.nsf/pages/soilhealth_soil_structure)

2. SOIL STRUCTURE Soil tillage at appropriate soil moisture and especially the rational minimal tillage reduce the processes of destruction of the plough layer structure and to some extent contribute to the recovery of the structure of the subsoil. The structural units are also formed as a result from the alternated moisturizing and drying of soil, or its alternated freezing and defrosting. Under alternated freezing and defrosting, the effect is higher at moisture 70-80 % of the maximum field water capacity and in a moderately loose (porous) soils since more optimal-size soil units are formed in them.

early spring ploughing 2. SOIL STRUCTURE Therefore pre-winter soil tillage contributes to the formation of soil structure more valuable from an agronomic point of view (Photo 1 and Photo 2). Photo 1: Soil after autumn ploughing Photo 2: Soil after early spring ploughing

2. SOIL STRUCTURE The percent of water sustainable structural soil units characterizes not so much the variations in the decomposed soil units but highlights the rate of weakening of the forces which hold together the separate particles in the structural elements under the effect of the used soil tillage and their respective strengthening when soil is left undisturbed (Fig. 2).

2. SOIL STRUCTURE Figure 2: Water resistance of structural soil units under different types of soil tillage (%)

3 BULK DENSITY Apart from ploughing and minimal tilths, direct sowing of the agricultural crops is also being practiced in the recent decades. The determining factor for its use is bulk density. Bulk density is the weight of a unit volume of absolute dry soil in its undisturbed condition (Fig. 3). Figure 3: Bulk density (Image from Engineering Intro on line at http://engineeringintro.com/concrete/concrete-strength/bulk-density-of-aggregates-loose-and-compact-bulk-density/)

3. BULK DENSITY By decreasing the total and primarily the non-capillary porosity, soil compaction reduces the air permeability and the air retention capacity thus impairing the gas exchange between the atmospheric and the soil air (Fig. 4). Compaction affects also the thermal regime of soil. The compact soil, which is dry or moist up to a certain degree, has higher thermal conductivity than the loose soil. Figure 4: Water and air penetration in normal and compacted soil (Image from WM EarthCareTM on line at http://wmearthcare.com/everyday-soil-science-4-bulk-density-porosity/)

3. BULK DENSITY By altering the water-air and the thermal regimes of soil, compaction affects also the nutrition regime. The impaired gas exchange impedes respiration and root growth, suppresses the activity of the aerobic micro organisms and all oxidation processes in soil, and as a result decreases the efficient utilization of the nutrients (Photo 3). Photo 3: Restricted root depth in compacted soil (Image from WM EarthCareTM on line at http://wmearthcare.com/everyday-soil-science-4-bulk-density-porosity/)

Figure 5: Bulk density under different types of soil tillage (g/cm3) The long-term ploughing lead to a relative stabilization of bulk density in the cultivated soil layer (Fig. 5). The permanent intensive cultivation at the some depth through disking devices, although temporarily decreasing the bulk density of the cultivated layer lead with time to high compactness of the horizon under the cultivated layer. By chisel ploughing and no-tillage bulk density changed under the effect mainly of physical and climatic factors. Figure 5: Bulk density under different types of soil tillage (g/cm3)

4 SOIL POROSITY Soil porosity is the ratio between the total volume of pores in soil and the total volume of soil (Fig. 6). It results from the property of the soil solid particles and aggregates to arrange and link permanently in such a way that void spaces (pores) are formed between them. The narrowest pores are called capillary, and the wider ones are referred to as non-capillary. Figure 6: Soil porosity (Image from Digital Textbook Library on line at http://tankonyvtar.hu/en/tartalom/tamop425/0032_talajtan/ch06s06.html)

4. SOIL POROSITY Long-term investigations carried out in a stationary field trial have shown that the disturbance of the natural position and integrity of the soil layer by the soil tillage, and its undisturbed status without cultivation for shorter or longer periods of time cause changes in soil porosity as well (Fig. 7). Figure 7: Total soil porosity under different types of soil tillage (%)

5 SOIL TEMPERATURE The temperature can vary greatly with increasing and decreasing solar radiation throughout the day. Depth from the surface differences in soil cover such as mulch, stone content and grassing can cause a wide variation in the thermal conductivity of a soil. The thermal properties of a soil will indicate the levels of soil water content since water is a better thermal conductor than air. The thermal conductivity of the soil increases as the water level increases in a soil.

5. SOIL TEMPERATURE Replacement of ploughing with a tillage without turning of the plough layer, with minimal and with no-tillage led to decrease of soil temperature in the surface layer both in cold and warm days during the year. Most significant variations were observed in the 0-10 cm layer, which, on its part, determined the conditions for accumulation and transference of heat in soil. Under conditions of negative mean daily air temperatures, soil temperature decreased down the depth profile with the reduced number and depth of the soil tillage operations (Fig. 8). At high positive mean daily air temperatures, the temperature amplitudes between the surface layer and the lower soil horizons at minimal tillage and the areas with minimal or no-tillage were lower in comparison to the ploughed areas (Fig. 9).

5. SOIL TEMPERATURE Figure 8: Soil temperature after some soil tillage systems at negative average daily air temperatures (oC)

5. SOIL TEMPERATURE Figure 9: Soil temperature after some tillage systems at high average daily air temperatures (oC)

6 MOISTURE CONTENT IN SOIL One of the main functions of soil is to store moisture and supply it to plants between rainfalls or irrigations. If the water content becomes too low, plants become stressed. The plant available moisture storage capacity of a soil provides a buffer which determines a plant’s capacity to withstand dry spells (Digital Textbook Library on line at http://tankonyvtar.hu/en/tartalom/tamop425/0032_talajtan/ch07.html) (Fig. 10). Figure 10: Composition of an unsaturated soil sample (Image from Pioneer Agronomy Library on line at http://pioneer.com/home/site/us/agronomy/library/soil-water-mgmt/#available)

6. MOISTURE CONTENT IN SOIL The results from a long-term stationary field trial showed that under conditions of long drought, prior to sowing of wheat, the content of productive moisture in soil at depth 0-30 cm was highest under constant chisel ploughing (Fig. 11). The deep loosing of soil facilitated water filtration, the water retention capacity of soils increased and a significant reserve was formed to be used during periods of drought. The productive moisture was lowest in the soils subjected to more intensive cultivation. The higher amount of moisture in the soils with minimal or no-tillage was maintained in the 80-100 cm horizon, as well.

6. MOISTURE CONTENT IN SOIL A – ploughing; B – disking; C – no-tillage; D – chisel plough Figure 11: Productive soil moisture during the individual growth stages of wheat under different types of soil tillage (mm)

6. MOISTURE CONTENT IN SOIL Under much higher rainfalls during the same period, the amount of productive moisture in the 0-30 cm layer was highest after annual ploughing. It was followed by constant chisel ploughing. In the areas with disking and no-tillage, the soil moisture content was equal. Down the soil profile, this distribution of productive moisture under the separate tillage systems remained the same, the moisture reserves in the 80 – 100 cm layer decreasing in comparison to the above horizon, which was best expressed under constant no-tillage. The greater moisture content in the soils with intensive tillage during years with rainfalls exceeding the mean norm was due to their higher water permeability and water retention capacity.

7 CAPPILARY RISE OF WATER Capillary water is of primary importance for the growth, development and productivity of the plants. It is retained and moves under the effect of meniscus forces (Photo 4). The capillary phenomena in soil are determined by the available system of capillary pores and depend to a large extend on the porosity of soil. Photo 4: Capillary rise of water (Image from Massachusetts Institute of Technology on line at http://web.mit.edu/nnf/education/wettability/gravity.html)

7. CAPPILARY RISE OF WATER The capillary rise of water in the soils with minimal and no-tillage was higher in comparison to the soils subjected to intensive tillage (Fig 12). The independent application of ploughing reduced the capillary rise of water in the slightly leached chernozem soils. Highest weighting coefficients with regard to capillary rise of water were determined in the 10-20 and 20-30 cm layers. In these layers after minimal and no-tillage the system of capillary pores and the network of inter-aggregate cavities formed by the roots of the plants remained intact in soil and hence the capillary rise of water was higher. After ploughing the large-sized structural soil units along the entire profile of the cultivated layer increased the cavities of non-capillary origin.

7. CAPPILARY RISE OF WATER Figure 12: Capillary rise of water under different types of main soil tillage (cm)

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