Role of planting depth and mulching on growth and yield components of autumn potato crop sown at different dates

Field experiments were carried out to study the role of various planting depths i.e. 10, 15 and 20 cm along with mulching on growth and yield attributes of autumn potato crop sown at different dates (October01, 15 and 29) at Horticulture Research Farm, The University of Agriculture Peshawar, during 2014-15 and 2015-16. The experiment was laid out in Randomized Complete Block Design (RCBD) with split plot arrangement. Sowing dates were kept in main plot, while planting depth and mulching were in sub plot. The two years average results revealed that sowing dates, planting depth, mulch treatments and their interactions have significant effects on potato growth and yield parameters. Potato crop sown earlier on October 01 showed maximum plant height (50.4 cm), number of leaves plant (33) and marketable tuber yield (16.2 t ha). Regarding planting depths, tuber planted at a depth of 15 cm produced maximum emergence (86 %), plant height (45.5 cm), number of leaves plant (32) and marketable tuber yield (13.8 t ha) as compared to tubers planted at a depth of 10 and 20 cm. Mulching resulted in maximum emergence (83 %), plant height (42.6 cm), number of leaves plant (30) and marketable tuber yield (13.6 t ha) as compared to no mulching. The interaction effect of sowing dates, planting depth and mulching significantly affected all the growth and yield parameters. Therefore, it can be concluded from the results that early sowing of potato crop on October 01 at a depth of 15 cm with organic mulching resulted in better growth and yield of autumn potato crop under the climatic condition of Peshawar, Khyber Pakhtunkhwa, Pakistan.


Introduction
Potato (Solanum tuberosum L.) a member of the nightshade family (Solanaceae) is a main world food crop produced and consumed worldwide.It is ranked fourth place among the most important food crops, after wheat, rice and maize [1].The potato crop is sensitive to heat, cold, and drought; consequently, climate change is projected to have a large, and in many cases, negative impact on yields in many potato growing regions.Heat stress is a serious threat for potato production.Elevated temperatures accelerate leaf senescence and double respiration rates for each 10 0 C increase, resulting in a negative carbon balance on hot days [2].Tuber initiation is restricted in most genotypes, when night temperatures exceed17 0 C [3] and the optimal temperature for tuber bulking ranges between 14 0 C and 22 0 C. Higher temperatures decrease tuber growth [4] and can cause misshapen or chain tubers, high sugar levels, low starch contents, and glycoalkaloid accumulation [5].Under hot and dry conditions, high vapor pressure deficit increases transpiration and insufficient water supply causes drought stress, resulting in yield losses.Soil mulching with organic material is one method of soil water conservation and also helps in maintaining a constant soil temperature within the crops root system [6]; [7].Mulch is very important for plants, because it maintain proper moisture by reducing daily temperature fluctuations, and also reduce soil warming in summer months, thus making better soil condition for plant growth and development [8].Temperature stress that occurs at high soil temperature at uncovered soil (32-34 °C), may be minimized by the use of the mulch [9].A variety of types of mulch leads to an increase in soil moisture content as a result of decreased evaporation from the soil surface compared to that of un-mulched soil [10].The effects of planting depth on potato growth and development have been investigated by many scientists [11,12].Rapid sprout emergence from deeper planted seed tubers or seed pieces have been reported.It has been observed that soil moisture is the limiting factor for sprout emergence at shallow planting depths [11].Conversely [12], indicated shallow planting hastened plant emergence.[13] found soils at 5.1 cm, on average, had warmer soil temperatures following planting than those at 10.2 cm.Warmer soil temperatures had a positive influence on sprout growth when moisture level was adequate throughout the profile.The relationship between tuber greening, planting depth and tuber position within the hill have been studied [12].These studies report that as mother tubers or seed pieces are planted closer to the soil surface, daughter tubers typically grow closer to the soil surface.Tubers closer to the soil surface are more likely to become green, sunburn [14], or infected with phytophthora infestans as the season progresses [15].Therefore, this study was conducted to find out the role of different sowing dates, planting depth and mulching treatments on growth and yield of autumn potato crop.

Materials and methods
The experiment entitled "Role of planting depth and mulching on yield and quality components of autumn potato crop sown on different dates" was carried out at Horticulture Research Farm, The University of Agriculture Peshawar during the autumn growing season of the years 2014-15 and repeated in 2015-16.Prior to the sowing of potato crop in each year, five soil samples from different sites of the experimental block, were taken with the help of an auger at 0-15 cm depth.Different soil physicochemical characteristics were analyzed.The land was well prepared through twice plough two weeks before the planting during both the years.The ridges were prepared manually 70 cm apart.Sound and healthy seed tubers harvested from the last autumn crops were used.Tubers of the available variety Kuroda packed in bags.More or less uniform whole seed tubes of about 20 -30g in weight were used for planting.Seed potatoes were planted in each unit plot maintaining a spacing of 70cm × 25cm.Both organic and in-organic fertilizers were applied to the experimental field.Well-rotten Farm Yard Manure (FYM) was applied @ 25 t ha -1 just after the soil was ploughed with tiller 15 days before planting.While chemical fertilizers were applied @ 120:100:80 NPK kg ha -1 in the form of Urea, Di ammonium phosphate (DAP), and Potassium Sulphate.

Experimental design and layout
The experiment was laid out in Randomized Complete Block Design (RCBD) with split plot arrangement having four replications.Date of sowing was kept in main plots, while planting depth and mulch treatments were allotted to sub plots.The size of each main plot was 10.92 m 2 , while that of each sub-plot (experimental unit) was 1.30 x 1.40 m 2 .There were 2 rows of potato plants in each sub-plot, each row 1.30 m long and spaced 0.70 m apart.Treatments details were as under (Table 1).

Results and discussion
Emergence (%) Data concerning emergence (%) of tubers in response to different sowing dates (SD), planting depth (PD) and mulching (M) is shown in Table 2. Emergence (%) was significantly affected by planting depths, mulching and year as source of variation, while there was no significant effect of sowing dates on emergence (%).The interactions were not significant except SD x M and PD x M interactions (Figure 1 & 2).The emergence (86 %) was highest; where tubers were sown at 15 cm depth, while lowest (72 %) at 20 cm depth.Mulching significantly increased emergence (83 %) as compared to (75 %) in control.The mean data showed that emergence in 2015 (80 %) was higher than that in 2014 (77 %).The interaction effect of SD x M showed that mulching have more emergence than no mulching irrespective of sowing date as compared to no mulching.In case of no mulching emergence was depressed, when the crop was sown on October 15 (Figure 1).
The PD x M interaction as presented in (Figure 2) indicated that mulching was effective, when sowing depth was increased from 10-15 cm, while emergence linearly decreased with increased in depth of seed sowing.Mulching is recommended when tubers are planted at depth of 10-15 cm only.
Planting depth and mulching has an effect on emergence.Tuber planted at a depth of 15 cm resulted in maximum emergence (%).The increase in emergence (%) at 15 cm depth could be due to optimum temperature, soil moisture at sowing time.Planting depth also has a significant effect on plant height.As the planting depth increased, the stem length and so plant height decreased.The decrease in stem length with increase in planting depth could be, because at shallow planting the plants emerged fast and had more growth period.Also at lower planting depth, the difference in the exposed stem could have contributed positively to stem length, while the buried stem for higher planting depth contributed negatively to stem length.This could also be attributed to light availability as it has been reported that increase of steam number will lead to increase of bush height due to light availability and its effect on increase of length number of node.[34], also reported decrease in plant height of potato with increase in planting depth.Straw mulch acts as a source of organic matter.The increase in plant height [35] with mulching might be due to the decomposition of organic matter, with the subsequent releases of available nutrients to the plants from the soil, resulted in increased growth of the plants.Application of mulch also significantly produced maximum plant height as compared to non-mulch [36].Also, [37] found that plants cultivated on mulched soil were higher than plants from the control plot; however, treatments consisting of mulch and row covers produced significantly higher plants than treatments without row covers.A higher soil and air temperature under covers provides better conditions for plants immediately after planting and allows them to produce a higher mass of aboveground parts [38].Significant variation was observed in number of leaves plant -1 with different sowing dates, planting depth and mulching (Table 4).The interaction effects of all the treatments, sowing dates were not significant except that of PD x M. The maximum number of leaves plant -1 (33) was recorded with October 01 sowing, while minimum (25) were noted with 29 October sowing.Regarding planting depth highest number of leaves (32) were counted in plants at 15 cm depth, followed by ( 28) at 10 cm depth, while deep planting at 20 cm produced least number of leaves plant -1 (25).Maximum number of leaves (30) was counted in plants applied with mulch, while without mulch plants produced less ( 27) number of leaves.The year effect was found significant.Number of leaves plant -1 was higher (29) during 2015, as compared to (28) in 2014.The PD x M interaction showed that significantly more number of leaves were produced at 15 cm depth both in mulch and without mulch treatment.However more leaves were recorded for mulching along with 15 cm planting depth (Figure 4).Number of leaves was significantly decreased with late sowing, which might be due to chilling temperature during the growing period that caused in reduction in overall growth.The total number of leaves plant -1 may be increases greatly by an increase in temperature, because of the combined effect of an increase in number of basal and apical lateral branches and an increase in the number of leaves on these branches [39].There was increase in leaf number of sweet potato with increasing temperature from 20 to 28 °C.The increased in number of leaves at medium planting depth of 15 cm could be attributed to the depth being the optimum for plant development.
The roots not being too close to the surface where the soil dry up quickly or too deep had chances of poor aeration due to too much water and soil compaction.The roots will grow in the most favorable conditions to support above ground growth.Marketable tuber yield was significantly affected by sowing dates, planting depth and mulching (Table 5).The interaction SD x PD and PD x M were significant.Early sowing in first week of October resulted in maximum marketable tuber yield (16.2 t ha -1 ), which was followed by (14.1 t ha -1 ) in October 15 sown plants, whereas late sown plants on October 29 produced less marketable tuber yield (8.9 t ha -1 ).Data regarding planting depth showed that higher marketable tuber yield (13.8 t ha -1 ) was obtained from plants sown at a depth of 15 cm, while lowest marketable yield (12.4 t ha -1 ) was recorded in plants planted at a depth of 20 cm.Marketable yield was increased from (12.6 t ha -1 ) in control to (13.6 t ha -1 ) with mulching.
The year used as a source of variation also significantly affected marketable tuber yield t ha -1 .The maximum marketable tuber yield (13.3 t ha -1 ) was produced during the year 2015 as compared to (12.9 t ha -1 ) in 2014.The  5) exhibited the SD x PD interaction, which showed that marketable tuber yield t ha -1 consistently decreased with delay in sowing from start of October to end of October at all planting depths.However sowing late in October at 20 cm depth resulted in maximum decreased in marketable tuber yield t ha -1 with delay in sowing from October 01 to October 29 at 10 to 15 cm depth.The interaction between PD x M revealed that sowing at a depth of 15 cm with mulching produced more marketable tuber yield t ha -1 (Figure 6).Marketable yield increased with early sowing as compared to late sowing.Since earlier sowing produced higher percentage of larger and medium sized tubers, thus contributed to the marketable yield.As the sowing delayed, the yield was reduced.Similar results have also been reported by [42].Other studies also confirm that potato tuber yield increased with early planting [43].Data collected also support the results regarding tuber yield as the vegetative growth was significantly improved by planting the crop earlier and harvesting later, which increased the number of large and medium sized tubers and decreased the small sized tubers.Similarly planting depth of 15 cm produced maximum number of stems plant -1 , number of leaves plant -1 , number of medium and large size tubers plant -1 , which contributed to increase in marketable yield.[44] reported that the increase in yield of mulched plot was probably associated with the conservation of moisture and improved microclimate both beneath and above the soil surface and great weed control.Mulch provides many benefits to crop production through soil and water conservation, enhanced soil biological activity and improved chemical and physical properties of the soil [45], effectively controlled the weed growth and increased soil temperature resulting in faster emergence, early canopy development and higher tuber yield [46].The higher yield in the second year 2015 was probably due to slightly more favorable conditions like temperature, photoperiod, soil fertility, moisture and rainfall for potato development.The high temperature during the growing season had a negative effect on yield in 2014.Water has been recognized as an important manageable input influencing potato tuber growth, development, quality and yield [47]. Water stress not only reduces total tuber yield, but also changes tuber size distribution, and tuber quality traits [48].

Figure 3 .
Figure 3. Plant height (cm) of potato plant as affected by different sowing dates and planting depth Number of leaves plant -1Significant variation was observed in number of leaves plant -1 with different sowing dates, planting depth and mulching (Table4).The

Figure 4 .
Figure 4. Number of leaves plant -1 of potato as affected by planting depth and mulching Marketable tuber yield (t ha -1 )

Figure 5 .Figure 6 .
Figure 5. Marketable yield (t ha -1 ) of potato as affected by sowing dates and planting depth

Table 2 . Emergence % as affected by sowing dates, planting depth and mulching
[22]rate of sprout growth increased as soil temperature increased from 10 to 20 0 C. Soil temperature below 10 0 C delay and reduce sprout growth[22].
[21] temperature, water supply and soil fertility regulate the sprouting behavior of tubers[16].As shallow planting results in increase surface temperature and moisture stress, which affect tuber emergence.Also in case of deep planting failure of emergence can occurs as potato sprouts had to come across a long distance of the ground to emerge.The results are in agreement with those of [17], who reported that each potato cultivar has its own accumulated heat units for eye and sprout development and if there is enough moisture available maximum emergence will occur.Lack of available water will influence the water uptake by the seed tuber[18], which is needed for physiological processes and act as a medium for biochemical reactions in seed tuber[19], such as the transportation of soluble sugar into the growing bud.[20]reported that percentage of plant emergence was affected by planting depth; seed tuber placed deeper resulted in lowering the plant emergence.Also planting seed tubers too shallow, can lead to tuber greening and moisture stress due to soil surface heat and evaporation[21].

Table 3 . Plant height (cm) as affected by sowing dates, planting depth and mulching
Means not followed by the same letters are significantly different.*, ** show probability level of 5 and 1 %, correspondingly.ns means non-significant

Table 5 . Marketable yield t ha -1 as affected by sowing dates, planting depth and mulching Years Sowing Date (SD) 2014 2015
Means not followed by the same letters are significantly different.*, ** show probability level of 5 and 1 %, correspondingly.ns means non-significant