Mulches: Types, Effects and Factor | Conservation | Soil Management

In this article we will discuss about:- 1. Types of Mulches 2. Effect of Mulch Cover 3. Mulch Factor 4. Vertical Mulching.

Types of Mulches:

The mulches may be of following types:

(a) The natural mulches are borned by nature itself, no man’s effort is required.

(b) Synthetic Mulches:

Includes organic and inorganic liquids that are sprayed on the soil surface to form a thin film for protecting the soil against various atmospheric happenings, taking place over the top soil surface.

The different synthetic mulching materials are as under:

i. Rasins

ii. Asphalt emulsions

iii. Latex and cut back asphalt

iv. Canvas

v. Plastic and paper products

vi. Polythene and polyvinyle chloride (PVC)

vii. Bitumen emulsions etc.

The plastic mulches are very useful for nurseries in semi-arid and arid regions. Now days, plastic mulches are available in different thicknesses and colours. Cost is also very less. Due to this reason, these mulches can be used at large scale. In respect of soil erosion/soil loss control the plastic mulch could be very promising. By spreading on erosion susceptible soils, the soil loss can be controlled, absolutely. Also, it can be used for moisture conservation and water harvesting, efficiently.

In farm ponds the longevity of stored water can be increased by lining the pond surface using plastic sheets. Apart from above, the plastic mulches are also used with drip system in different crops (horticultural) for moisture conservation and weed control points of view.

The plastic mulching is not suitable for the taller vegetations. In nurseries, the dark coloured plastic mulches make the soil surface very warm during the day hours, because black body absorbs greater heat from the sunlight. This type of characteristics results to lower diurnal and higher nocturnal temperature.

(c) Petroleum Mulch:

The petroleum mulches are easier to apply and also less expensive. These mulches are available in the form of emulsions of asphalt in water, can be sprayed on the soil surface at ambient tem­perature to form a thin film in continuous form, that clings the soil, but not penetrate deeply. A thin film of petroleum substance, made so is termed as mulch film.

The mulch film increases the rate of seed germination and also plays a significant role for vigorous growth of seedlings. The surface film is also suitable for erosion control. Also, this mulch is stable against erosion, sufficiently porous to allow water in the soil, but insoluble in water and resistant enough to the weather forces causing it to last as long as necessary for vegetations to become established.

(d) Conventional Mulch:

In few cases the conventional mulches such as hay or straw are more effective than the petroleum mulches. These mulches not only conserve the moisture and reduce the fluctuation of soil tem­perature, but also protect the soil from rain drop impact and hold the excess surface water in contact with the soil, and thus to increase the infiltration rate, and reduce the runoff and soil erosion.

In addition, during day hours these mulches also absorb as much insulation as bare soil does, but little energy is conducted downward, resulting the surface of the mulch hot and the soil remains cool. On the other hand, during night hours the mulch gets cool down and permits the soil to remain warm. The paper mulches are counted under conventional mulch, are reported to provide a remarkable result.

Paper mulches increase the soil temperature, especially of the surface soil layers. There are several evidences to show that, the paper mulching proves better performance in improvement of soil condition, besides promoting the earthworm activities. However, the toxic contents of chemicals are leached out of the paper, and develops bad effects on soil health should be avoided. The treated papers such as bitumanised have toxic effect on the plants.

(e) Inorganic Mulches:

i. Soil Mulch:

It is an important mulch for the arid and semi-arid regions. Its application should be avoided during summer and rainy seasons, especially. The soil mulch is effective to reduce the evaporation, particularly where the soil is saturated to a lower depth below the top surface or the moisture content is in-excess of field capacity, but not in contact with water-table.

Sometimes, the soil mulches are not effective under ordinary conditions, because large amount of moisture evaporation establishes a protective dry layer of the soil, which if worked, causes excessive soil moisture loss.

In soil mulching, a loose and dry soil layer of 5 to 8 cm thick is established on the soil surface. For this purpose, the land surface is ploughed for the depth as specified above, and left for some-time to get dry; after that by planking operation the tilled soils are planned. In this way a layer of dry soil in loose condition is prepared over the soil surface, which acts as soil mulch.

The soil mulch prepared so, obstructs the capillary loss of water from the lower layers due to following reasons:

i. Lack of contact with moist soil layer lying below to the open atmosphere.

ii. Increase in non-capillary pore spaces.

iii. Resistance to wetting.

These effects are more apparent under isothermal conditions in regards to soil and air temperature. Soil mulching has beneficial effect on soil aeration, also.

The formation of crust on top soil surface is also counted as soil mulching. It results in clogging of the soil pores, which effectively seals the lower horizons from contact with the atmosphere, and also prevents the diffusion of O₂ and CO₂.

The soil mulching becomes more effective, when it is composed of crumbs and clods of proper size not liable to get break down by the impact of subsequent rains. In sandy areas, the soil mulch should be prepared immediately after rains and renewed after subsequent rains.

The general function of mulch is to raise the soil temperature during winter season and to lower the same during summer season, by checking heat conduction through the mulched layer, and thus to maintain the soil temperature at a uniform level. In many cases, the soil mulching also plays an important role to diminish the weeds growth from the soil.

ii. Stone Mulching:

It involves the spreading of stone pieces on the ground surface to conserve the moisture and also to reduce the wind erosion. It is a very old practice, followed in arid zones. Soil under the stone pieces tends to be in moist condition, but the soil temperature becomes slightly higher. The soils lying below the stones, harbour small animals and cause high nitrification.

The stone mulching is also used for trapping the dews, particu­larly in that locations where significant dew fall takes place. Central Arid Zone Research Institute, Jodhpur has reported the use of rubble mulch in combination of small fragments of stones and bricks, provides better result on moisture conservation, as compared to the synthetic mulching and mulching made by the straw materials.

(f) Organic Mulches:

The tree branches, twigs, leaves, leaf litter, grasses, weeds etc. are used as organic mulch to cover the soil surface. The organic mulches are found superior than the artificial mulches in respect of moisture conservation, reduction in evaporation and runoff. By this mulch, the control of evaporation is more effective, particularly when rainfall takes place at frequent intervals, but not found so effective when the rains are few and scattered.

In other words, the infrequent rains and small showers may not be saved at all, but for the intense rains, which result a wet surface for several days with excess surface water for deep percolation, these mulches have better efficiency.

Furthermore, the light mulches are ineffective for controlling the evaporation, because moisture conserving efficiency of mulches is inversely related to their capacity to absorb water or to extract from the soil by capillary action. Resistant mulches do not decay shortly, but last for a long time, as result they are more effective for conserving the soil moisture.

Benefits of Organic Mulches:

Various advantages are listed as under:

1. Very effective in reducing the soil erosion, because they promote interception loss and infiltration of rain water.

2. They obstruct the impact of rain drops over the ground surface, and thus dissipate the erosive power of rainfall.

3. Very effective in preserving and improving the soil structure by eliminating the crusting of soil surface and sealing of pores by runoff.

4. Organic mulches also enhance the dew fall by insulating the soil thermally and electrically from the atmosphere.

5. The organic mulches under high air temperature keep the soil temperature below the maximum air temperature. This effect is penetrated into the soil up to the depth ranging from 15 to 20 cm or so from the top soil surface.

6. These mulches drastically enhance termites population in the top soils. The termites eat the mulches and leave a very fine tilth, behind, causing increase in percolation and infiltration rate. The small termite channels are formal in the soil. This happening is responsible to reduce the runoff and soil loss, significantly.

Effect of Mulch Cover:

A mulch cover prevents surface sealing by preventing the direct impact of raindrops on the soil surface and by encouraging biological activities, which leads to the development of macropores in the soil. These effects have precidental importance on reduction of runoff and soil erosion, both. The effectiveness of mulch for reducing the soil erosion has been studied by several researchers. Lal (1977) reported the effect of mulch on soil loss and runoff in uncropped land at four level of slopes, i.e. 1, 5, 10 and 15%, shown in Table 11.6.

He also developed a general relationship between soil loss, mulch rate and runoff, given as –

Soil Loss = C” M^–k … (11.1)

In which, C” and k are the constant; the value of k lies between 0.06 and 0.30 for the soil of Ibadan (Nigeria) and M is the mulching rate. This equation shows that the runoff and soil loss decrease exponentially with increase in rate of application of mulch. Similarly, Woodburn (1942) also reported that, on bare silt-loam soil with 7° slope, the annual soil loss was obtained as 2.46 kg/m², but only 0.11 kg/m² was noticed on the same land when covered with straw mulch at the rate of 0.5 kg/m².

The mulching should be done to cover 70 to 75% of the soil surface. The straw, with application rate of 0.5 kg/m² is sufficient to achieve this. A less covering of soil surface does not adequately protect the soil. An estimate of the required application rate of mulch to control the erosion can be made for pre-selected set of conditions using the Manning’s equation of flow velocity.

Johnson and Moldenhauer (1982) derived following relationship for maize straw used as mulch material:

In which, n_m is the value of Manning’s roughness coefficient for mulch and M is the mulching rate (kg/m²). The above equations cannot be used for wheat straw, soyabean residues or other mulch materials.

Mulch Factor:

It is defined as the ratio of soil loss occurred from a mulched field to the soil loss from the field without mulch. It is related to the percentage residues or mulch cover by the following expression developed by Laffen and Colvin (1981) –

MF = e ^–a.RC … (11.4)

In which, MF is the mulch factor; a is the constant ranges from 0.03 to 0.07, but 0.05 is commonly used as an average value and RC is the percentage residue or mulch cover.

There have been developed several types of sub-surface tillers by using them the formation of mulch can also be done. The straight blade, wide sweep, narrow sweep, rotary hoe, rotary sub-soiler, disc harrow are the main implements. For selecting a suitable sub-surface tiller for mulch preparation, it is important to consider its size, clearance, weed killing capacity and sturdiness of construction.

If machine is too wide then difficulties may be encountered on irregular grounds. Also, a wide machine of more than one sections performs the func­tion in better way on contour, than a machine of similar width equipped on a single rigid frame. Looking this fact in view, it is essential to consider all related features of the implement for having better mulching work.

Chapman (1948) reported that, the canopy intercepts the rainfall and retains a portion of it, except small rain events. Most of the intercepted rainfall either become stem flow or get reform into drops; some of which may be larger than the original raindrops. Elwell and Stocking (1976) have derived a curvilinear relationship between soil loss and percentage canopy cover, given as under –

SL = A . e^B . FC … (11.7)

Where,

SL = soil loss

A = constant

e = exponent

B = coefficient, depends on the soil type and slope

FC = fraction canopy cover

Khan et.al (1988) studied the effect of mulch cover and canopy on the soil loss through laboratory studies on the soil “Parr silt loam”, which contains 15% sand, 61% silt, 21% clay, 1.4% carbon and 2.4% organic matter. The rainfall was performed through rainfall simulator from the height of 3.5 m. They also derived a relationship for computing the mulch cover, which is written as –

MC = 1 – e^{-0.67 MR}

The average soil loss at different canopy heights, canopy over and straw mulching rate, reported by Khan et.al (1988) is shown in Table 11.7.

Gregory (1982) also derived a relationship to predict the fraction of soil covered by crop residues, which is given as under –

Where,

MC = fraction of soil covered

A_m = area covered by a single layer straw divided by its mass. A typical value of A_m for wheat straw mulch is taken as 0.5 ha/t

MR = mulch rate (t/ha)

Vertical Mulching:

Normally, the mulches prepared through deep chiseling and sub-soiling are not so effective to reduce the runoff and soil erosion, because the tillage marks do not stay open for continuous rapid infiltration.

Spain and McCune (1956) introduced special type of mulching practice referred as “vertical mulching” to reduce the runoff by enhancing the infiltration for longer duration. It is carried out of inserting the stuffed plant residues, vertically into sub-soiler marks to keep the slots open and remain effective for longer duration.

The surface water moves downward into the sub-soil along the stuff inserted into the tillage mark, and also from the ground surface. In this operation, there develops a network of open slots on the ground surface, depending on the quantity of crop residues, used. A precaution should also be maintained that, the slots should not be covered by the soil during tillage operations.

The field experiences on this aspect revealed that, the vertical mulching has an appreciable effect on runoff control, when it is done on contour. The concept of vertical mulching was developed in Indiana, where this technology has been appeared to be very effective for removing the excess water from the ground surface. This practice has also been started to use as drainage tool for surface water removal from the low-lying areas; and is found better response from this.