In world food production, the potato is an important crop along with wheat and barley. In 2001, the world production of potato was estimated at about 307 million metric tons (Fennir, 2002). Increasing population and limited arable land have caused researchers to focus on the increased mechanization of potato production (Rembeza, 1993; Bentini, 1992), including soil preparation, planting, protection, harvesting and post harvest operations (Spiess, 1994; Gupta et al., 1994; Ridder et al., 1993). In fact any improvement in mechanization can affect the quality and quantity of potato production (Bentini, 1992).
One of the most important goals in tillage is maintaining a high degree of aggregation in a soil, because then roots can develop and penetrate the soil better and the maximum amount of water can be stored for plant needs. In addition, aggregated soil surface particles resist breakdown under rainfall better, preventing sealing over and allowing maximum water intake, reducing erosion due to runoff and reducing the breakdown of aggregates into fine particles which are transported by water. Also aggregated soil particles provide improved resistance to compaction by wheels or tracks of tractors and field machines and to the action of tillage machines (Ghazavi, 2004).
Tillage implements are significant part of the cost involved in an agricultural system. The researchers have concentrated on energy and draft requirement aspects because it is an important factor in decreasing cost and increasing work rate (Amran et al., 1999; Kasisira and du Plassis, 2005; Rohit and Raheman, 2005; Sayin et al., 2004).
There is no standard tillage system used in the preparation of potato fields, but it is obvious that as few operations as possible would be better to save time and energy and to reduce excessive soil compaction. The wide range of soil types on which potatoes are grown and the variety of tillage implements available for soil preparation are two important factors in tillage management. The correct amount of crop residue must be incorporated into the soil to allow the planter to work without trouble. The amount of tillage required to do this will depend upon the soil type and the ability of the planter to operate properly in crop residues. Tillage should produce enough loose soil to allow the planter shoe to penetrate to the desired depth and to provide the hilling tools with enough loose soil to construct a proper hill over the seed.
In certain soil types, the use of an inappropriate tillage tool, or cultivation
at an unsuitable soil moisture content, will produce excessively strong soil
aggregates that will remain intact throughout the growing season and into harvest.
These soil clods affect tuber form and quality and also are difficult to separate
from the potatoes on the mechanical harvester, thus reducing harvester efficiency
and increasing harvesting cost. Hard dry clods that come into contact with the
tubers on the harvester and other handling systems can cause black spot bruising.
Further bruising can be created by the clod elimination mechanisms or methods
used on the harvesters.
The most common tillage procedure is to plow down the previous years crop residue with a moldboard plow in the fall or spring. Then a disk is used in the spring to break large soil clods. This is normally followed by a third operation with a spring tooth harrow that levels or smooths the soil and can incorporate fertilizer (Skorupinska and Wasilewska, 1993; Unger, 1994).
Spiess (1994) and Edwards et al. (2000) reported that mulch treatments in soil preparation can reduce erosion and compaction, improve soil conditions for crop water and nutrient uptake and potentially increase yield. Agebede (2006) found effects of four tillage methods on soil properties and yam yield. An investigation is necessary to compare the performance of a moldboard plow with other tools that can leave some plant residue on the soil surface. Therefore, some experiments were performed in this study to compare the performance of a combination plow with that of a moldboard plow and to study their energy requirements, soil behavior and potato yield (Lutter et al., 1991; Doerkes, 1992; Ghazavi, 1997).
MATERIALS AND METHODS
Two series of experiments were carried out during 8 years. The first was conducted
on the Nafferton Farm at the University of Newcastle Upon Tyne, UK. The soil
was a sandy clay loam and its physical properties were measured before and after
the tillage treatments, which were conducted immediately after the harvest of
a barley crop in August 1995. The average soil moisture content at the time
of treatments and measurements was 19.4%. The soil physical property and energy
measurement tests and instruments are shown in Table 1. All
tillage draft measurements were carried out using a Zetor model 7211, 56 kW
tractor equipped with a three point linkage dynamometer developed at the Koln
Fachhochschule, Germany. Data acquisition was achieved through a notebook computer
with an Amplicon 226 16 channel data acquisition card (Ghazavi, 1997).
||Soil physical property and energy measurement tests and instruments
The Microsoft Windows based software system used was from Signal Centre Inc.
Eight channels were used to collect the required data from six force transducers,
one speed meter and a position sensor attached to the tractor linkage. The complete
real-time PC laboratory was mounted within the tractor cabin with three transducers
placed exterior to the cab between the implement and the tractor. The transducers
measured the vertical and horizontal forces used to calculate tillage energy
data during field operations. To measure 3-point linkage angles, a linkage position
sensor was mounted at the end of the lift-arm shaft. A small generator was used
as a signal for ground speed measurement, mounting on the axle of a fifth tire
connected to the frame by a universal joint at the front of the tractor. A three-point
hitch dynamometer that was designed by Lutter et al. (1991) was used
to measure implement energy following the method of Doerkes (1992). This dynamometer
comprised two bottom-link transducers and one on the top link and left the geometry
of the tractor linkage unchanged.
The Signal Centre software is specifically designed for data acquisition applications. It allows mathematical functions and file handling operations to be carried out on the data as it is being collected through the Amplicon 226 data acquisition card. Further facilities included on-screen information regarding the data during collection. Its flexible design allowed it to be used for a large number of diverse applications.
In the first series of experiments, tillage tests were conducted using a fully randomized strip design with four replications of the tillage treatments. The plots in each experiment were 100 m in length and 4 m wide. The tillage instruments were tested, an improved disk plow (combination of disk and chisel tine), a conventional three furrow moldboard plow. The combinatory plow was designed by Yule and Roddy (1994) and it was completed by Ghazavi (1997) with some modifications.
A second series of experiments was conducted on a clay soil in the Khatoon Abad Farm at the Islamic Azad University of Khorasgan in Isfahan, Iran. The implements tested were the high speed disk plow and a conventional three furrow moldboard plow. The plots were the same size as that in the first series of experiments mentioned above. A randomized complete block design was used to compare the primary tillage machines behavior on potato growing. A manually fed planter was used to plant the tubers at 6 cm below the surface of the soil in the last week of March 2000 and 2001. Four workers fed the seeds manually onto a rotating disk before the seeds were dropped in the soil. After planting and as the plants began to grow, the same cultivation was performed each year using the same implements. This was a mechanical press that piled soil on top of the seeds to produce a hill structure. Nearly flat coverage was provided by the hill to protect the growing tubers from becoming sunburned. The irrigation system and other conditions were similar for all plots.
At harvest, a tractor semi-mounted direct potato harvester produced by the Sabzdasht Company in Isfahan, Iran was used. The harvester dug the potatoes from the rows and immediately passed them to the separator within the machine. A day before mechanical harvesting, some potato samples were taken by hand and carried to the laboratory for different tests and measurements on tuber yields.
RESULTS AND DISCUSSION
In Fig. 1-5, samples of the results of
penetrometer cone index, dry bulk density and soil moisture content are illustrated
in the plow layer, which was considered to be of 25 cm depth. In terms of the
soil cone index, the moldboard plow left the soil in the weakest condition,
while the improved disk plow achieved better consolidation at Nafferton Farm.
The same results were seen in some experiments at Khatoonabad Farm. Cone index
measurements were consistent with soil dry bulk density results, while the measurement
of soil moisture content showed that the conventional moldboard plow caused
losses in moisture content, probably due to soil inversion.
||Soil cone index in the plow layer before and after tillage
||Soil cone index in the plow layer before and after tillage
Cone-index and dry density increased with depth of plowing. Comparison of dry
density and cone index versus depth before and after cultivation resulted higher
soil resistance in uncultivated plots, then in cultivated field by improved
disk plow and moldboard plow, respectively. Resultly, the improved disc plow
output is closer to uncultivated field condition than the moldboard plow output.
Possibly due to amount of operation occurred by each implement in the field
or clod breakdown (Owende and Ward, 1990; Gemtos et al., 1999; Raper
et al., 2000).
In the series of experiments reported in this section, conditions were dry
by United Kingdom standards with an average moisture content in the cultivation
layer of 19 g/100 g of dry mass. The energy measurement results of the different
tillage implements are shown in Table 2.
||Soil dry density in the ploe layer before and after tillage
||Soil dry density in the plow layer before and after tillage
||Soil moisture content in the plow layer before and after tillage
|| Plot means, average and mean square of draft (KN), tractive
power (KW), peak draft, peak TP and speed for two kinds of Plows
|a: ID = Improved Disk Plow and MP = Moldboard Plow; *, **
= Significant at p<0.05 and 0.01, respectively
In the tractive effort measurements, the minimum coefficient of variation
was seen with the moldboard plow. When examining draft, the moldboard plow results
had a greater coefficient of variation than those of the high speed disk plow.
Yet when examining values for tractive power, the moldboard plow had a reduced
level of variation in relation to the improved disk plow. The data can be explained
by the response of the tractor to sudden increases in draft. With the moldboard
plow, because of the higher loading, increases in draft were met with a drop
in tractor speed, whereas with the improved disk plow the tractor had enough
reserve power and speed to be less responsive to sudden change in draft, where
the coefficient of variation was higher for the moldboard plow results than
for the improved disk plow. Also, the mean, minimum and maximum values of draft
were lower for the improved disk plow than for the moldboard plow in all experiments.
The highest draft and tractive power was required by the moldboard plow. A significant difference was seen between the draft and draft power measurements of the improved disk plow and the moldboard plow. These results are largely consistent with those of studies carried out by Yule and Roddy (1994) and Gol et al. (2007).
The peak measurements can be a good indication of the maximum energy requirement for the purposes of tractor selection or implement design. Farm managers need to know tractive power and total power required in order to select suitable and economical tractors for their implements. Design engineers can use the draft force to calculate the resistance of the machine parts and also they can select different materials with which to manufacture the machinery. A comparison of peak tractive power measurements resulted a result similar to those mentioned above. A highly significant difference was seen between the implements with respect to the peak draft.
||Frequency distribution of the improved disk plow draft forces
||Frequency distribution of the moldboard plow draft forces
Frequency distributions show the profiles of the forces encountered by each
machine (Fig. 6, 7). The range of 0 to 26
kN was divided into 13 force classes of 2 kN range each and the percentage of
samples in each range is shown. The upper limit of each of these force classes
is indicated as the x-axis. The graphs give an insight into the composition
of each mean value for draft. The skewness values were 1.4 and 1.6 and the kurtosis
quantities were 0.9 and 1.5 for the modified disk plow and moldboard plow, respectively.
||Profile Length Ratio (RZ), surface relief area and MWD of
||Soil plow layer sieve size classification after cultivation
Aggregate size distribution revealed broadly consistent results between machines.
Experiment 2 provided better conditions for clod breakdown, with a higher percentage
of aggregates below 6.3 mm (class 10) for the 2 implements. The correspondence
between sieve class number was categorized as numbers (1-10) and size in mm
Figure 8 shows the results from experiments 1 and 2 were
primarily influenced by differences in moisture content between the two experiments.
Clod breakdown appeared to be broadly comparable between implement treatments
in both case, however the very dry conditions in experiment 1 led to increased
clod strength and poorer breakdown. The mean weight diameter is slightly higher
in experiment 1, but again increased soil moisture and easier soil breakdown
conditions meant that this difference was reduced in experiment 2 significantly
due to T test result.. Profile Length Ratio RZ (total surface area per 1 square
meter area) comparison was not significant, where as T test showed difference
between them significantly (Table 3) (Ghazavi, 1997).
The costs of operating each machine were calculated based on the following data (Ghazavi, 1997):
||Fifty ha per annum work, interest on capital 10%, straight
line depreciation, 10 year life.
||Machine costs: 3 furrow plow 5000US$, 3 furrow disk plow 4400$.
||Work Rates: Moldboard plow 0.56 ha h‾1, improved disk
plow 0.66 ha h‾1.
||The hire cost of the 56 kW tractor was $23 Ah‾1.
Potato production rate for the moldboard plow and the improved
disk plow in two different years (Khatoonabad Farm)
||An improved disk plough picture (combinatory)
It was observed that the main advantage of using the moldboard plow was its
more complete soil inversion which allowed good burying of trash. The improved
disk plow achieved good mixing with a relatively clear soil surface and it had
the advantage of being cheaper to operate with a faster work rate than the standard
moldboard plow. Although the degree of soil inversion was less than the moldboard
plow and more plant residue was-left on the soil surface, the improved disk
plow was able to reduce moisture loss in late summer and autumn conditions (Fig.
Soil physical changes before and after cultivation and also, energy requirement are the most important factors to evaluate a tillage tool. There is a relationship between soil physical parameters such as soil type, moisture content, dry density, cone-index, mean weight diameter, RZ etc., energy requirement and yields of products. (Desbioles et al., 1997, 1999; Ghuman and Sur, 2001; Rohit and Raheman, 2005; Natsis et al., 1999).
The use of Improved Disk Plow was an effective method to reduce the force;
incorporate soil, moisture and residue better for improving the soil fertility;
achieve suitable pulverization and soil mixing and leaving some residue on the
surface to reduce soil erosion and water running; better dynamics of soil engaging
due to speed operation comparison result. However, it is exempt of creating
hard pan layer and has the advantage of being cheaper to operate with a faster
work rate than the standard moldboard plow especially in arid and semi arid
area like Iran (Yule and Roddy, 1994; Gol et al., 2007). Over all, the
new plow as an alternative implement gave good performance and would appear
to offer a financial advantage to farmers, making it worthy of further development
and testing. Also, it can play a main role in potato mechanization.
Further studies should be done to investigate the effect of new tillage management on potato quality and quantity.
The author acknowledges the financial support from the Shahre-Kord University and Islamic Azad University (Khorasgan branch) of Iran that made this research work and study possible.