Introduction

• Water is the key input in crop production and plants take nutrients from the soil in the medium through water.
• It also helps maintaining the plant temperature and enables the plants to face the varied climatic conditions smoothly.
• The irrigation schedule should be such that neither plant faces any excessive water stress nor there is excess water causing leaching of nutrients.

An efficient irrigation management involves 3 considerations

1. When to apply ?............(Time of application)
2. How to apply ?............(method of application)
3. How much to apply ?....(depth of water per irrigation)

• The time interval between irrigation depends upon the
• Crop : ability of the crop to with stand the extent of moisture stress without suffering any adverse effect on growth and yield.
• Growing season : Climate & weather conditions i.e. evaporativity demand of the season.
• Root system : Ability the plants root system to explore the root zone for moisture uptake and maintain the water uptake rate commensurate with the E.T. needs.
• Succulency : Need for maintaining high turgor pressure with in the plant cells method of application varies with the ridge and furrow bed planting.
• Method of crop establishment : Flat bed or ridge or flat-bed and furrow planted.
• Row to row spacing : Whether furrow irrigation possible not.
• Method of irrigation application : Border, furrow, gated pipes sprinklers, or drip system.
• Kind of crop (root crop/grain crop) : Determines frequency and depth of irrigation.

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Critical Stages for Irrigation

• In Potato, following stages are considered critical from point of view of maintenance of adequate moisture in the root profile; as even small shortages at any of these stages could result in proportionally very high loss in yield.
• Maintaining adequate moisture in the rhizosphere (top 100 cm or so), therefore, is imperative to success. The critical stages are :

Germination

• for assured, rapid, uniform emergence of seedlings and good uniform crop stand at the start; and synchronous maturity at the end of crop season.

Stolonization

• Stolon formation initiation 30-35 days after the seeding of the crop.
• This stage coincides with earthling up of the crop which is done to provide soft soil medium for development and growth of stolons (:the stolon is an under-ground stem, on the tip of which is borne a single tuber).

Tuberization

• When the stolons, formed under-ground start to swell at the tips.
• This follows 10-15 days after stolonization initiation.
• Any shortage at this phase reduces the chances of stolons to be converted into tuber-bearing stolons, or tubers if formed may fail to progress in their size gain and remain as chats.
• Early tuber development : Early tuber development phase is very critical, as the tuber development proceeds in order of precedence of their initiation & the tubers if once hindered in growth, would cease to grow & plants energy is then wasted in initiating new tubers or promoting of secondary tubers.
• This may give rise to final produce of variable size of tubers, with large proportion of chats & fewer ware grade tubers; which results both from uneven growth and uneven (non-synchronous) maturity of tubers at harvest.

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Depth of irrigation largely depends upon

• The depth of root system
• Frequency of irrigation
• Method of irrigation
• Depth of soil and nature of soil - clayey, sandy etc.

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Specific Irrigation Management Schedule for Potato

• Fore, high yield realization of Potato, apply irrigation immediately after planting which leads to proper germination of the crop in general.
• The Potato crop in Punjab should be irrigated at 7-10 days interval.
• This schedule does not allow the crop to suffer from any water stress, this corresponds to an irrigation at ASMD of 20-30 per cent or an irrigation at soil moisture tension of 0.3 bar, measured at 15-20 cm depth or an irrigation at a depth of net CPE of 15 mm to 20 mm.
• The last irrigation should be with held 10-15 days before harvest to allow the tubers to harden their skin before harvesting/digging of potato.
• More moisture at digging time leads more soil to stick to their surface.
• The harvest of tubers of proper moisture moderate moisture auto matically leads the roots shed away from the tuber's surface.
• More-over the efficiency of potato digger's improved a lot when it is operated in mildly moist soil conditions.
• Too-less moisture at the digging, leads to more energy requirement and formation of large clods and to separate Potatoes from these clods needs more time and labour.
• Therefore, favorable moisture content in the soil is considered critical for getting clean and large produce at minimal costs.
• After harvesting let the Potato lie in then layer in well aerated dark stores/racks for 10-15 days, so as to allow the skin to harden further & let the potatoes mature enough to with-stand the rigours of transportation & handling without getting bruised.

Note

1. The ASMD means the "Available Soil Moisture Depletion Regime; meaning the range over which the moisture in the root profit is allowed to fluctuate between the Field Capacity and to a given point within the range of available soil moisture (RAM).
2. The range of available moisture (RAM) is defined as the moisture retained in the soil between the Field Capacity or FC (the upper most limit of available water for plant growth) and the permanent wilting point or PWP (the lower most limit of available water at which plants can sustain their growth).
3. The IW/CPE means an irrigation regime scheduled on the concept of cumulative Pan Evaporation i.e. climatological concept. The CPE means the net cumulative pan evaporation measured from USDA Class I Open Pan Evaporometer, located in a meteorological observatory, nearby as minus the Effective Rain Fall (ERF) received over the irrigation cycle. The IW stands fro quantity of water for applied per irrigation.

• In potato where irrigation is to be applied in furrows, the quantity of water per irrigation, i.e. IW approximates 50 mm. Therefore an W/CPE regime of 0.25 would mean
  • IW/CPE = 0.25
  • CPE = IW/0.2 = 50 mm/0.25 = 20 mm
• i.e. scheduling of an irrigation at net Cumulative Pan Evaporation of 20 mm ; net of Effective Rainfall, received our each irrigation cycle.
• Effective Rainfall is defined as the net rainfall which goes to bring back the given root profile depth (say about 150 cm in case of potato in Punjab) back to the field capacity.
• Any quantity higher than the net dificit (say PET x factor correction) = Reference ET.
• Crop ET = Reference ET x KC (crop coefficiant), is takes as equivalent to ERF ; and is substracted from CPE ; whenever a shower is received over an irrigation cycle (or interval between two irrigations).

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Effect of Water Stress

• Severe water stress (irrigation of 40-50 mm CPE) at tuber initiation alone reduced tuber seed (2.5-5.0 cm tubers) yield on an average by 3.24 t/ha; and number of seed tubers by 8.3 tubers/m².
• Severe stress at tuber enlargement depressed 'ware' grade tuber field (more than 5.0 cm) rather than seed ; and at later the tuber enlargement, the effect was only on ware grade tubers.
• The effect of mild stress (irrigations at 25-33 mm CPE) on yield was small as compared to severe stress.
• The crop received frequent irrigations based on 16 mm CPE (total CPE minus rainfall) did not show any water stress on crop.
• Water management practices vary from region to region. Water requirement of Potato ranges from 400-600 mm depending upon the length of growing season. i. e. whether it is a short duration early season crop of 110-120 days (as is commonly raised in Punjab) or it is a long season crop of long duration variety like Kufri Sindhuri which extends over 150-170 days, from September to March; and covers much of its life cycle during the latter rising temperature phase of February-March; besides having to encounter high temperatures during the early growth phase during September, October & November. Normally, the early season crop grown in Punjab has it growth Cycle of 110-120 days; coinciding much its latter growth (tuber development) with falling temperature regime of low evaporativity and its seasonal water needs are moderate.
• Potato requires well drained soil and good aerated root environment for healthy development of large size tubers. Soil moisture depletion, infact, should never be allowed to fall below 30-40% of available soil moisture of the root zone in top 100 cm soil.
• Water deficit during stolonization, tuber initiation and early tuber formation adversely affect the yield of Potato; particularly that of the ware grade tubers as well as the quality the produce-both physical and chemical.
• In plains of Punjab where at the time of sowing the temperature is moderate, lower rootzone is well charged after the end of rainy season; and much of growth and almost all of the entire tuberization phase coincide with cool weather, of moderate low evaporative demand (<2 mm day). Potato is given light irrigations at rather frequent intervals to sustain un-interrupted tuber-development at an accelerated rate and to produce smooth skinned, large, uniform, round tubers to get high proposition of war grade produce & reduce the extent of chats to the minimum.
• Potato crop is very responsive to scientific irrigation management and gives good response to scientifically scheduled irrigations. Highest yield of quality grade ware tubers was obtained when irrigation of potato were scheduled at soil moisture depletion of 0.3 atmosphere (bars) through teniometer based readings; tensiometers fixed at 15-20 cm depth in sandy loam soils of Ludhiana; involving an application of 7-10 irrigations since after 2-3 common irrigation given for crop establishment during the first 2-3 weeks after seeding the crop.
• While applying irrigation, care is taken that in no case the water within the furrows should reach more than two-thirds of the height of the ridge/over-flow over it; as it may lead to encrustation of ridges; cause an aerobic conditions; and prove injurious to crop, both during the germination and tuber development.
• Length of the ridges & furrows may depend upon soil type, soil moisture intake rate, slope and field gradient and source of water. It may vary from 10m (sandy soils, or well irrigated situations) to 200 m or more on heavier type of soil with low intake rates & having high water discharge source of tube-well/tank system as irrigation source. More the precision in the leveling of the field, more could be the length. The main objective is to supply uniform rate of water supply to crop through-out the length and breadth of the field (furrow-lengths).
• In medium heavy (clayey) soils which become hard on during, delay in irrigation could result in production of non-uniform, disfigured and less tuber formation maintenance of optimum moisture in these soils is this the only way out for harvesting good yields of high quality tubers.
• On the other hand, the sandy soils, considered most ideal for potato cultivation, frequent irrigations because of their low water holding capacity and sudden development of highly moisture stressed conditions on depletion of moisture even by a small fraction. This makes the potato rather more 'exacting' in its water management.

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