Potato Guidelines Page 5.

7.1 SOIL SELECTION AND THE AVAILABILITY OF SOIL WATER

Potatoes grow well on a variety of soil types, but not all soils give optimal yields of good quality. Several physical and

chemical factors must be considered when selecting soils for the cultivation of potatoes. Potatoes can be cultivated on soils of essentially all texture classes, but deep, well-drained soils of light to medium texture

are preferred. Soil texture (the ratio of clay to silt and sand) influences the rate of water infiltration and the water holding

capacity of a soil. In general, coarse textured and light loamy soils are best for potato cultivation. Although sandy soils are

usually ideal for mechanical tillage and harvesting, these soils have a limited water holding capacity and will only produce

good yields if the rainfall is adequate or if adequate irrigation water is available. The availability and quality of soil water is

an important factor in determining the yield and quality of the potatoes. Inadequate water during critical growth stages may

increase the occurrence of diseases (such as common scab) and tuber deficiencies (such as growth cracks, malformation

and secondary growth). Excessive water from rain and irrigation in sandy soils may also cause leaching of mineral

nutrients, especially nitrogen, beyond the root zone of a shallow rooted crop such as potatoes.

Fine textured soils (higher clay content) have lower infiltration rates than sandy soils. Infiltration rates are especially

important in the selection of type, rate and duration of irrigation. Fine to medium textured soils are able to hold more plant

available water than coarse textured soils, and are therefore more suitable for rain fed production. Such soils require less

regular irrigation than lighter soils. The disadvantage of heavier soils is that the formation of clods may complicate tillage

and may easily cause waterlogged conditions. Poorly drained soils are undesirable, since the accompanying poor aeration

may harm tuber development because of an oxygen deficiency. Apparently tubers are more sensitive than roots to an

oxygen deficiency in the soil. Especially in heavier soils, the growing tubers compact the soil around them, and this further

decrease oxygen uptake through the lenticels. The better aeration of light soils therefore constitutes one of the reasons

why tubers of a better quality may be expected from such soils.

The physical condition of the soil must also be considered when selecting a soil. A minimum soil depth of 600 mm is

required, with no compacted or limiting layers present. In comparison with crops such as maize and wheat, potatoes have

a very shallow and poorly developed root system, which will not grow through soil layers with a resistance greater than

1000 kPa. Compacted layers also limit the movement of water in a soil profile, and temporary water tables may even be formed. A

well developed, deep root system is essential for optimal utilisation of nutrients and to tolerate periods of drought. The

accumulation of rain water and a deeply developed root system is essential, especially in the case of rain fed production.

Compacted layers (plough layers) usually develop on sandy soils and must be alleviated by deep ripping. Mechanisation is

usually complicated and may even be impossible on very heavy soils, stony soils or on sloping fields. With stony soils,

which may otherwise have good physical properties, the stones may possibly be removed mechanically to facilitate tillage

and harvesting. Chemical analysis of the soil is essential before the potato crop is planted. Potatoes are adapted to a wide range of pH

conditions and may even tolerate quite acidic soils (pH 4). To ensure optimal production it may, however, be necessary to

adjust the pH, since the availability of nutrients and the activity of certain pathogens are affected by the soil pH. The

availability of nutrients is usually the factor that influences yield and quality the most.

Both a high concentration of dissolved salts and sodium in the soil may influence the physiology of potatoes. A high salt

content adversely affects the uptake of water from the soil solution by the roots. Such soils usually have a high pH, which

affects the availability of nutrients. High sodium content leads to a degradation of soil structure and the formation of surface

crusts, which limit the infiltration and movement of water. Potatoes are moderately salt sensitive and should preferably not

be produced on brackish soils or irrigated with water of a high salt content. Yield is hampered when the conductivity of the

saturated soil extract increases above 170 mSm-1. With a further increase in salt content, a 12% reduction in yield per 100

mSm-1 increase in conductivity may be expected. The history of the soil, particularly the crop rotation programme, should

also be taken into account in soil selection. 7.2 WATER REQUIREMENTS OF POTATOES

Introduction

Successful potato production depends on adequate water supply throughout the crop’s growing season. Water, which

comprises 90 to 95% of plant tissue and 70 to 85% of tubers, is one of the most important production factors limiting potato

production in South Africa. It plays an important role in several physiological processes and also serves as a source of

carbon, hydrogen and oxygen to the plant.

An actively growing potato plant requires large amounts of water and may replace its water content up to four times per day

under optimal conditions and average transpiration rates. More than 95% of the water taken up by the roots of the plants is

therefore lost by transpiration, and only a small fraction actually contributes to growth. Water stress, whether from too

much or too little water, may have serious adverse effect on potato plants and tuber progeny. Too much water can lead to

water logged conditions and the leaching of nutrients. Water stress from too little water is usually the most serious problem

with potato production. The potato plant is sensitive to water stress, even of short duration. This sensitivity may probably

be attributed to the plant’s shallow, poorly developed root system and the harmful effects of moderate water deficiencies on

physiological processes. Water stress is usually reflected in slower growth rate, the development of a smaller leaf canopy, early senescence and

eventually lower yields. Several tuber disorders may also occur as a result of too much, too little or fluctuating levels of

ground water. The occurrence of such disorders is often increased by the combined effect of water and another

environmental factor, usually high temperature. In most soils, stress starts to develop when 35 to 40% of the plant

available water has been depleted from the root zone. This corresponds to ground water potentials of about –30 to –55

kPa.

The rate of evapotranspiration (water usage) depends on a number of factors:

● the availability of ground water – transpiration decreases when there is a shortage.

● growth stage of the plant – a fully developed crop canopy will transpire more water than one which covers only part of

the soil surface. ● net radiation – high light intensity and long days increase evapotranspiration.

● humidity of the air – transpiration is higher at low humidities. ● temperature influences evaporation and indirectly humidity as well.

● wind speed – water consumption on a windy day may be as high as double that on a windless day. The water requirements of potato plants during different growth stages will now be discussed briefly. The influence of a

shortage or excess of water during each growth stage is summarised in Table 7.1. Table 7.1: The effect of shortage or excess water at different growth stages on potatoes

Water requirements during different stages of the growing season Period before planting

The soil must contain about 70 to 80% of plant-available water so that large applications to fill up the soil profile just after

planting, will not be necessary. However, as a guideline the soil water content must be at such a level that the soil may still

be tilled and planted with ease. Period between planting and emergence

During this period the soil around the seed potatoes must be kept moist, to ensure that adequate water is available for

sprouting and root growth. Care must be taken, however, that the soil is not too wet, since the seed pieces may rot.

Irrigation must be conducted with care, and only light applications (7 to 10 mm at a time) must be applied when it becomes

essential to irrigate. If the temperature is high, it is important to cool the soil with light irrigations.

Emergence to tuber initiation (vegetative growth)

During this stage the plants are small and the foliage only covers a small fraction of the soil surface. The transpiration rate

is low and the plants require little water. Root depth is still shallow and irrigations must be limited to 15 to 20 mm at a time.

Too much water can lead to the development of an undesirably shallow root system. A depletion of 20 to 25% of plant-

available water (PAW) from the root zone may be allowed.

Period of tuber initiation

Adequate water supply is very important, since water stress may lead to the initiation of less tubers. Since tuber yield is

determined by both the number and the size of the tubers, the yield will be hampered by water stress. The small tubers are

also most sensitive to common scab infection at this stage. Adequate soil water around the tubers may protect them

against infection and development of common scab lesions. At this stage a maximum depletion of 20% PAW must be

maintained. Period of tuber growth (tuber bulking)

After tuber initiation the tuber growth stage starts, during which a lot of water is required. Water stress during this period

has the most detrimental influence on the final yield, in that the size of the tubers is limited. The large influence of water

stress in this phase (on yield) may be ascribed to the fact that this is the longest growth stage in the life cycle of the plant.

In addition wetting and drying out of the soil profile as a result of too long irrigation intervals may give rise to the

development of several tuber disorders, such as secondary growth, hollow heart, growth cracks and malformation. It is

harmful for the potato if the soil is too wet, as it may lead to waterlogged conditions and attack by diseases. Quality

(relative density and chip colour) may also be harmed. A maximum depletion of 20% PAW must be maintained during this

stage. Maturation Water requirements decline as the plants begin to senence, and a 40 to 50% depletion of PAW must be allowed to promote

skin set. A high soil water content (less than 35% depletion) must be avoided, since it can lead to the development of

enlarged lenticels and quality defects. Relative tuber density may especially be harmed, as the tubers may take up too

much water. Too dry a soil may in turn hamper harvesting and result in mechanical damage to the tubers.

7.3 IRRIGATION SCHEDULING

In South Africa the management of soil water is probably one of the factors which has the greatest influence on yield and

the quality of potatoes. Yield is influenced negatively in that either the number of tubers or their size may be adversely

affected by water stress. Several tuber disorders, such as malformation, growth cracks, brown spot, hollow heart and

secondary growth, may be ascribed directly to the amount and distribution of water during the growing season.

With rain fed production the producer is largely dependent on nature, but irrigation farmers can manage this input to a large

extent. The factors involved in efficient soil water management include the method, amount and timing of irrigation. The

different methods of irrigation used in the production of potatoes are discussed elsewhere in this chapter. The judicious

management of water application, by which frequency and quantity of irrigation is purposefully managed, is known as

irrigation scheduling. The prevailing weather conditions, soil type and the growth stage of the plant determine irrigation

frequency and quantity. The growth stage determines the rooting depth and foliage coverage and as a result also the

amount of water that may be depleted during each growth stage (see section on water requirements). The amount of water

available for uptake by plants is determined by the soil properties.

Table 7.2 Approximate soil-waterparameters for soil of different texture classes Example 7.1 Available ground water and allowable depletion

Plant water stress commences when the soil water content drops below a critical value, expressed as a percentage of plant

available water (PAW). For a specific soil, PAW is defined as the difference between field capacity (the soil water content

after a saturated soil has drained for about 48 hours) and the permanent wilting point (the stage at which plants cannot

withdraw any more water and thus wilt and die off). Other concepts, such as the drained upper limit and the lower limit

(“DUL” and “LL”) are also sometimes used. Yield and quality are, however, already harmed before the permanent wilting

point is reached. Accordingly, there are critical amounts of water that may be depleted before plants are stressed and

production is harmed. This is known as the allowable depletion (of PAW). The amount of available soil water is

determined mainly by the texture class of the soil (see section on soil selection) and the effective root depth of the potatoes

in that soil. The PAW of soils initially increases with increasing clay content, but later decreases again (Figure 7.1). The

plant available water of soils may be determined by the farmer himself (field capacity and wilting point), or approximate

values for specific texture class soils may be used. Table 7.2 serves as an example of this. As soon as the PAW of a soil is

known, the allowable depletion for a certain growth stage can be calculated. Example 7.1 illustrates how the allowable

depletion for a particular soil and growth stage must be calculated. When the soil water is supplemented, it is important that irrigation quantities will never exceed the amount required to refill

the soil profile to field capacity, unless it is intended to leach salts from the root zone.

Scheduling practices

Several methods are available to assist the producer in his decision as to when, and how much, to irrigate. With most

methods it is attempted to measure or predict one or more components of the soil-plant-atmosphere continuum. For

example, gravimetric sampling may be used to determine soil water content, and the irrigation quantity is then calculated

according to the allowable depletion. This method is labour intensive and tedious, however, and is seldom used on a

routine basis. Otherwise, instruments such as a neutron moisture meter (Figure 7.2) may be used. Several new

techniques which facilitate data collection, are also available lately (e.g. Time Domain Reflectometry – TDR), since the soil

water content is automatically registered. Although most of these apparatus are very accurate, their costs unfortunately are

often very high.

Several methods can also be used to determine the matrix potential of the soil. This gives an indication of how difficult it is

for plants to take up water from the soil, and thus indirectly of the amount of water in the soil. In other words, it measures

the suction force which the roots must exercise to take up water. This method only indicates when to irrigate, and not the

amount required. Tensiometers (Figure 7.3) and several types of resistance blocks may be used for these measurements.

As standard for potatoes it is recommended that the soil water potential at 300 mm depth of sandy soils should not drop to

lower (drier) than –30 to –40 kPa and not lower than –40 to –55 kPa for loamy soils. The user will learn from experience

what irrigation quantity is required at a specific reading to raise the soil water content to field capacity again. Placing one

tensiometer below the root zone (e.g. at a depth of 600 mm) can be very useful. If the reading of this tensiometer has not

returned to zero after an irrigation, the application was too little. If the tensiometer reading remains at zero between

irrigations, it is a sign of over irrigation.

The method of pan evaporation and crop factors attempt to address both the atmospheric and the plant components.

Evaporation from an American class A pan gives an indication of the atmospheric evaporative demand (Eo). The crop

factor (f) is the ratio of actual evapotranspiration of the plant (Et) to pan evaporation, i.e.

f =

Et/ Eo

Crop factors are dependent on the plant growth stage and are specific for the locality for which they have been determined.

It is very important to note that crop factors are only guidelines for a specific area and might not give a good indication of

water usage for other localities and under abnormal conditions, such as very hot or cool seasons. It is also important to

confirm the actual soil water status gravimetrically or with tensiometers from time to time. In the absence of other methods

this method may serve as a good point of departure, to be refined for specific conditions. Example 7.2 shows the crop

factors for a medium-length cultivar during a spring season for Roodeplaat. Computer simulation models are the latest, and probably the best, additions to scheduling methods and have been adopted world-wide during the past decade. Plant and soil inputs are combined with weather data to simulate daily water usage.

The model makes use of weather data to predict plant growth and development, from which the water usage and soil water

balance are calculated. Some models permit the user to make updates during the season in respect of observations, such

as canopy cover and soil water content.

In South Africa the use of computer models is still limited, but will probably gain acceptance faster in future. Some

scheduling models currently available include PUTU, BEWAB and SWB. At this stage scheduling services are provided

mostly by private institutions. Producers can, however, also operate models themselves.

7.4 TYPES OF IRRIGATION

In modern irrigation, water is applied with a consideration of other important inputs, such as energy, labour, fertilisation and

pest control, in order to optimally increase production. The choice of a suitable irrigation system is complicated by factors

such as the availability of capital, the availability of water, labour inputs, land size, availability of energy, system

maintenance, topography and water quality, to name but a few. In principle there are three methods of water supply to

potatoes: ● Flood irrigation ● Overhead irrigation and

● Drip irrigation Flood irrigation Flood irrigation, or surface irrigation, is probably one of the oldest methods of irrigation. It comprises the supply of water to

the soil surface by means of furrows or channels. Water is spread over the land by the force of gravity (mainly where a

slope occurs). The advantage of flood irrigation is its relative cheapness and the very low energy costs. A further

advantage is that it may be effectively utilised on a small scale. However, flood irrigation has a number of pontential

disadvantages: ● Uneven water application

● It is labour intensive

● Erosion problems (with steep slopes) ● Cannot be used on coarse textured soils such as sand (owing to high infiltration rates) ● High evaporation losses ● Wastage of water by wetting too deeply

● Not suitable for large lands Flood irrigation of potatoes is done on a relatively small scale (in South Africa), and has been replaced largely by

overhead irrigation.

Overhead irrigation

The most common method of irrigation in potato production is with overhead sprinklers, better known as sprinkle irrigation,

which is based on the simulation of rain. General types of overhead irrigation used are:

Transferable systems (moving the pipes by hand, side roll, boom) ● Moving systems (centre pivot, lateral system and moving gun)

● Permanent or fixed systems Sprinkle irrigation, other than flood irrigation, is designed to spread the water uniformly. Run-off and the formation of

puddles should be avoided by ensuring that the application rate does not exceed the rate of soil infiltration. This may be

done by the suitable adjustment of the sprinkler and nozzle sizes, pumping pressure and moving speed of the system.

Wetting patterns for flood and sprinkle irrigation are shown in Figure 7.4. Sprinklers are spaced such as to obtain uniform

wetting.

The uniformity of application and efficiency of a well-designed overhead sprinkler system may be as high as 85%. Under

unfavourable climatic conditions (low humidity, windy and hot days) the efficiency of such a system may, however, drop to

as low as 40%, due to losses. Irrigation is sometimes applied after sunset, to enhance efficiency. The advantage of overhead irrigation is that light irrigations may be repeated regularly, especially on light soils. Cooling of

the soil on hot days is a further advantage. Fertilisers and herbicides may also be applied by means of an overhead

irrigation system. The mobile systems are not as labour intensive as hand-movable systems. The main disadvantages of sprinkle irrigation systems are their high capital investment, high energy costs (with high-

pressure systems) and relatively high maintenance costs. Drip irrigation

Drip irrigation is the slow (drop by drop) localised application of water on top of or beneath the soil surface. Water is

supplied to plants by means of a set of polyethylene pipes with drippers fixed therein or thereto. The working pressure

usually ranges from 70 to 300 kPa. The effective water pressure is considerably reduced by friction when the water flows

through the narrow passages of the drippers. Water drops are then released at atmospheric pressure. The dripper lines

are left in the field for the entire growing season. As with any other irrigation system, the dripper delivery should not exceed

the infiltration rate of the soil. Dripper delivery rates usually vary between about 0.8 and 10 litres per hour.

The ideal drip irrigation system is usually computerised, with electrically controlled valves to deliver the correct volumes of

water. However, valves may also be hand-controlled. From the position of the dripper the water moves laterally as well as

downward into the soil because of gravitation and matric pontential gradients in the soil. With drip irrigation, only parts of

the soil surface are wetted. Strip wetting is obtained by utilising the correct dripper discharge rate and spacing. It is

important to do a prior dripper calibration, in order to determine the wetting patterns of various drippers on a specific soil.

Thereafter the desired dripper delivery and spacing can be determined. Spacing between drippers in the line must be

about 80% of the wetted diameter, i.e. the wetting patterns in the row must overlap by about 20%. However, it is difficult to

determine this visually from the wetting pattern on the soil surface, as the wetted diameter just under the soil surface is

greater than on the surface.

Dripper delivery is probably one of the factors most neglected in the design of drip irrigation systems. The depth of wetting

is influenced by dripper discharge rate. The higher the discharge the deeper the wetting per unit time, irrespective of the

type of soil. Soil does play a role in the lateral movement of the water. The lower the application rate (discharge) the lower

the downward movement and the greater the lateral movement of the water in the soil. The dripper spacing in the row will

thus be larger for drippers of a low delivery rate than with higher delivery rates. This is explained by the fact that the

combined effect of gravitation and downward conduction of the water is larger than the lateral hydraulic conductivity of the

soil (Figure 7.5). An adverse effect of this is that drippers of a high delivery rate are inclined to wet too deep, thereby

leaching nutrients from the root zone. In this way the efficiency of drip irrigation (may be as high as 95%) is considerably

lowered by water wastage. If drippers with a high delivery rate (>2 liter/h) are used, it is advisable to split the amount into

equal portions. For an irrigation of 4 hours the irrigation time will, for example, be split into two irrigations of 2 hours each,

with an interval of about 6 to 12 hours between irrigations. The total irrigation time may even be divided into more smaller

irrigations for sandy soils. For drip irrigation the wetted parts of the soil (root zone) are constantly kept “wet”. However, the soil is seldomly saturated,

and therefore remains well aerated. This is a great advantage over sprinkle and flood irrigation, because soil is never

allowed to dry out. A disadvantage is the high irrigation frequency required, and should difficulties be experienced with the

system and irrigation is interrupted, it is difficult to correct the large water deficiency thereafter. If irrigation is interrupted for

periods even as short as two days, the potato plants may experience stress. A further disadvantage of drip irrigation is that

rain water cannot be utilised as well as in the case of overhead irrigation, as the soil is maintained close to field capacity

and the rainwater then drains out of the root zone. It is therefore essential that a drip irrigation system be designed

correctly and managed well.

Water use efficiency is excellent with drip irrigation, and increases in yield of as much as 40% has been recorded.

Fertilisation may be applied (according to the needs of the plant) through the drip system (fertigation). It has been claimed

that the utilisation of nutrients is better with fertigation compared to conventional fertilisation and irrigation.

The success of drip irrigation is determined mainly by the irrigation water quality and the efficient filtration thereof.

Thorough filtration of water lowers the risk of blockages. It is essential, however, that drippers are inspected regularly for

blockages. The capital investment costs of drip irrigation are relatively high in comparison with those of sprinkle irrigation.

The lower energy costs and savings in water with drip irrigation may make the long term running costs (working costs)

much lower in relation to those of other systems. Drip irrigation may be successfully applied for potato production provided the design, layout and management thereof is

good. Figure 7.1

The plant available water in soils of different texture classes

Figure 7.2

A neutron moisture meter for the determination of soil water content.

Figure 7.3

Tensiometers for the determination of soil potential with potatoes. Figure 7.4

Infiltration patterns for two types of irrigation.

Figure 7.5

Wetting patterns with two different dripper deliveries.

Example 7.1.

Potatoes are produced on a sandy loam soil. During tuber initiation, the roots are 300 mm (0.3m) deep. Lets assume that

the soil to depth of 0.15 m contains 100 mm m-1 PAW while the next 0.15 mm contains 120 mm m-1 PAW. The total

amount of water available at this stage for the plant can be calculated as follow:

Depth of soil layer (in meter) X PAW per meter = mm PAW in the soil layer.

Thus: 0.15m x 100mm = 15 mm water 0.15m x 120mm = 18 mm water Total (0-0.3 m) = 33 mm water The total amount of water in the root zone of the potato plants is thus 33 mm. The allowable depletion can now be

calculated by multiplying the percentage permitted depletion for each growth phase with the PAW. In this case 25% of the

PAW is withdrawn:

33 mm x 25% = 8.25 mm. Thus, during the tuber initiation phase, we will irrigate each time when 8 mm of water has been

withdrawn for the soil profile. When the roots grow deeper, we will have to include the deeper soil layers in the calculation.

We must also take into consideration, the efficiency of the irrigation system. For example, if a pivot is 80% efficient, the real

amount of irrigation will be adjusted to: 8.25 ÷ 0.8 = 10.3 mm

Example 7.2 Crop factors for a medium growth cultivar (for example Up-to-Date) during a spring planting at Roodeplaat.

Lets assume it is 4 weeks after plant. The crop factor is 0.5. If the pan evaporation during the previous day was 8 mm, then

the evpotranspiration is calculated as follow:

= E0 x ƒ

= 8 mm x 0.5

= 4 mm

J. M. Steyn and H.F. du Plessis