Efficiency of P Fertigation for Drip-Irrigated PotatoGrown on Calcareous Sandy Soils

Mamdouh A. Eissa1

Received: 27 February 2018 /Accepted: 8 August 2018/# European Association for Potato Research 2018

AbstractMost potato growers apply phosphorus (P) before cultivation and there is little infor-mation available about P fertigation for drip-irrigated potato. A field experiment wasconducted during 2017 and 2018 to study the effect of P fertigation on P uptake andyield of potato (Solanum tuberosum L.) grown on a calcareous sandy soil. Threefrequencies of P fertigation (daily, every 2 days and every 3 days) and two forms ofP (urea phosphate (UP) and monopotassium phosphate (MPP)) were examined. UPreduced the soil pH by 7.2% and increased the P availability by 24% compared to MPP.Daily P fertigation increased P concentrations in the petioles of potato by 19–47%compared to the application every 3 days. Water and P were used more efficientlyunder daily P fertigation treatments and the use of acidic fertiliser (UP) was moreefficient in this respect. The injection of UP with irrigation water every day caused a22% increase in the marketable yield of potato compared to the injection of MPP every3 days. It can be recommended that for potato grown on sandy calcareous soils, UPmust be added daily to obtain high economic return and minimise the negativeenvironmental effects of unsustainably high P rates.

Keywords Drip irrigation . Efficiency . Fertigation frequency . Phosphorus use P uptake .

Water use efficiency

Introduction

Potato is the most important vegetable in Egypt due to the area planted and cropvalue. Annual production increased from 1.77 million tonnes in 2000 to 4.80million tonnes in 2013 (FAO 2013). According to the Ministry of Agriculture andLand Reclamation of Egypt (2014), the total amount of potatoes approved forexport had reached 697,710 tonnes in 2014 (18% of this amount goes to the

Potato Researchhttps://doi.org/10.1007/s11540-018-9399-7

* Mamdouh A. Eissamamdouhessa@gmail.com

1 Department of Soils and Water, Faculty of Agriculture, Assiut University, Assiut 71526, Egypt

European Union, 47% to the Russian market, and 33% to the Arabian countries).Most of agricultural expansion soils in arid and semi-arid regions have sandytexture with low fertility, low water holding capacity and poor physical, chemicaland biological properties (Kafkafia 1994; Kolahchi and Jalali 2013; Eissa andAhmed 2016). Availability of nutrients in calcareous soils is strongly affected byunfavourable soil properties (Eissa et al. 2016; Amin and Eissa 2017). Highcontent of calcium carbonate raises soil pH and this leads to (i) more ammoniavolatilisation (Liyanage et al. 2012), especially under arid conditions (Zhang2014); (ii) formation of less soluble Ca-phosphate (Al-Khateeb et al. 1986; Eissa2016); and (iii) precipitation of essential plant micronutrients (e.g. Fe, Mn, Zn andCu) in unavailable forms (Caselles et al. 2005; Amin and Eissa 2017).

Potato is sensitive to irrigation and yield is reduced by both over- and under-irrigation (Stark et al. 2004). Drip irrigation has been shown to increase water useefficiency (WUE), phosphorus use efficiency (PUE), quality and yield of potato grownin sandy soils (Iqbal et al. 2003; Badr et al. 2010, 2012; Xin-kai et al. 2012). A limitednumber of studies have shown that fertigation is an important practice for potatoproduction and it is possible to increase yield by well-scheduled programs throughoutthe growing period (Kashyap and Panda 2003; Darwish et al. 2006).

Water use efficiency (WUE) is a measure of a crop’s capacity to convert waterinto plant biomass. Phosphorus (P), in a balanced nutrient management program,can improve WUE and helps crops achieve optimal performance under limitedmoisture conditions (Eissa 2016). Phosphorus increases WUE through increasingthe vegetative growth and drought resistance, and reducing the rate of disease(Marschner 1995; Uchida 2000; Eissa 2016). The common method of P applicationis to broadcast or incorporate in the soil before sowing (Iqbal et al. 2003; Xin-kaiet al. 2012). However, most of P applied this way interacts rapidly with soilcompounds and the availability and uptake are reduced especially in calcareoussoils (Al-Khateeb et al. 1986). P fertigation seems the most appropriate approachfor fertiliser management for intensive sustainable agriculture, as it can increaseefficiency of fertilisers, increase yield, protect environment and sustain irrigatedagriculture (Silber et al. 2003; Eissa 2016). The application of 75–80% of recom-mended dose of NPK through drip fertigation resulted in higher tuber yield andquality and saved 40% of water requirement (Singh and Sharma 2002; Sasani et al.2006; Darwish et al. 2006). Most potato growers incorporate P fertiliser prior toplanting (Hopkins et al. 2010; Stark et al. 2004), so, little knowledge about Pfertigation exists. Different sources of P have been used in fertigation (e.g.monopotassium phosphate (MPP) and urea phosphate (UP)). MPP was used infertigation of drip-irrigated eggplant, cucumber and muskmelon (Herson et al.1997; Zipelevish et al. 2000). UP was used in fertigation of drip-irrigated tomato,eggplant, wheat and corn (Mikkelsen and Jarrell 1987; Papadopoulos and Ristimäki2000; Eissa et al. 2013).

Frequent applications of P fertigation may increase the P use efficiency (Eissa et al.2013). Approximately 85% of the potato roots concentrate in the upper 0.3-m soil layer(Opena and Porter 1999). Thus, P fertigation may have advantage for potato productionespecially under calcareous soils. The aim of the present research was to explore theeffect of P fertigation frequency and form of P fertiliser on the uptake of P and yield ofpotato grown on calcareous sandy soils.

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Materials and Methods

Field Experiments

The field experiments were conducted for two successive seasons during 2017 and2018 in sandy calcareous soils located at Beet Dawood village, Sohag, Egypt. The soilwas classified as Typic Torripsamments according to Soil Taxonomy (Soil Survey Staff2010). Selected physical and chemical properties are summarised in Table 1. Theexperimental site was irrigated using a drip irrigation system. The in-line GR dripperlaterals were installed 1.5 m apart, each irrigation jet contained a valve to controlthe fertilisation process. The emitters were spaced 0.30 m apart with a flow rate of1.8 L h−1. Potato cv. Diamant was planted on October 10, 2016, and October 15, 2017.The tubers were hand planted in hills (10–15-cm depth) spaced 0.30 m apart. Thetubers were arranged on both sides of the dripper line at a density of approximately44,000 plants ha−1. Nitrogen in the form of ammonium nitrate (33.5% N) and K in theform of potassium sulphate (50% K2O) were applied jointly, every 10 days via theirrigation system at rates to supply a total of 345 kg N ha−1 and 230 kg K2O ha−1,respectively. Harvesting was done when maturity was attained (after the yellowing ofpotato aboveground, some plants in the experiment’s surrounding belt were harvestedto determine the maturity of tubers). Harvesting was done by hand on February 5, 2017,and February 1, 2018, for the two seasons, respectively. Marketable yield accounts forall saleable tubers, after the removal of the small (< 3.5 cm), green, growth cracked,malformed or decayed tubers.

Experimental Design and Treatments

Two factors were examined in the study as follows: (1) P application frequency at threelevels with (i) every day (F1), (ii) every 2 days (F2), and (iii) every 3 days (F3) and (2)P fertilisers in two forms, namely (i) monopotassium phosphate (MPP) and (ii) urea

Table 1 Some physical and chemical soil properties (0–30 cm)

Sand (%) 91 ± 2.2

Silt (%) 7.0 ± 0.5

Clay (%) 2.0 ± 0.05

Texture Sandy

Field capacity (w/w%) 14 ± 0.8

Wilting point (w/w%) 10 ± 0.40

CaCO3 (%) 17 ± 0.80

pH (1:1 suspension) 8.10 ± 0.07

EC (1:1 extract) dS m−1 1.20 ± 0.02

Total nitrogen (mg kg−1) 100 ± 4.5

Organic matter (g kg−1) 5.0 ± 0.05

Available Olsen P (mg kg−1) 3.50 ± 0.20

Available−K (mg kg−1) 120 ± 5.5

Each value represents a mean of three replicates

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phosphate (UP) with (iii) control treatment without P application (C). The experimentaldesign was a split plot design with four replications. The frequency of P fertigationtreatments was imposed onto the main plot, whilst the form of P fertiliser was placed inthe subplot. The dripper’s lines (one for each replicate) were placed on the midline ofnorth–south oriented soil beds at 1.5 m apart and the line length was 20 m (plot area is1.5 × 20 = 30 m2). Phosphorous fertilisers were added at the same rate (150 kg P2O5 ha−1)but over three different times. All the fertilisation treatments were adjusted to provide thesame amount of nitrogen (N), phosphorus (P) and potassium (K) (the details of Ptreatments are shown in Table 2). The amount of fertilisation for each P treatment neededwas calculated and dissolved directly in the water to prepare the concentrated fertilisersolution that was delivered with irrigation water into the irrigation system. Most P istaken up at early growth stages (Jones et al. 2011); therefore, the fertigation of P started aweek after planting and terminated 2 months after planting. The rest of the nitrogen (N)and potassium (K) requirements were applied as ammonium nitrate and potassiumsulphate. The fertigation of N and K started 2 weeks after planting and was repeated at10-day intervals until plants reached maturity (approx. 3 months after planting).Fertilisation rates of nitrogen, phosphorus and potassium were according to the recom-mendations of the Ministry of Agriculture and Land Reclamation (Egypt). The experi-ment contained a control treatment (without P) to calculate the P use efficiency.

The application rate of P was 150 kg P ha−1 applied as 60, 30 or 20 equal doses, i.e.2.5, 5 and 7.5 kg P for every day, 2- and 3-day treatment, respectively. The water wasadded every day according to the calculated ETwhich is explained under the section onirrigation water requirements. The water was applied daily for all treatments at rate of30–40 m3 ha−1 depending on the growth stage. The P fertilisers were dissolved in 200 Lof water and then injected into the irrigation system.

Irrigation Water Requirements

The daily reference evapotranspiration (ETo) was estimated using Penman–Monteith’smodified equation (Allen et al. 1998). The actual evapotranspiration (ETc) was calcu-lated according to the equation (ETc = ETo × Kc). The crop coefficient (Kc) for potatogrowth stages (initial, developmental, middle and tuber maturity stages) was 0.50, 0.65,1.15 and 0.75, respectively (Allen et al. 1998). The estimated ETo was 378 and 279 mmand the ETc was 296 and 238 mm in 2017 and 2018, respectively. The applied irrigationwater during the whole growth season was 3830 and 3088 m3 ha−1 in the first andsecond season, respectively (the application efficiency for drip irrigation was Ea = 85%and the leaching fraction was considered as 10% of water requirement). The potato

Table 2 Amount of different fertilisers for each phosphorus treatment

Treatments Fertilisers type and amount

C 126 kg of urea (46-0-0) + 196 kg of potassium sulphate (0-0-50 + 18 S)

UP 341 kg of urea phosphate (17-44-0) + 196 kg of potassium sulphate (0-0-50 + 18 S)

MPP 288 kg of monopotassium phosphate (0-52-34) + 146 kg of ammonium sulphate(21-0-0 + 24 S) + 60 kg of urea (46-0-0)

C, UP and MPP refer to control, urea phosphate and monopotassium phosphate, respectively

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plants in all the experimental treatments were irrigated daily by the same amount ofwater. Table 3 summarises the meteorological data during the period of the twogrowing seasons.

Calculation of Water and Phosphorus Use Efficiency

Water use efficiency (WUE) was calculated using the equation (WUE = TY/ETc),where TY equals total tuber yield, ETc equals seasonal actual evapotranspiration(mm). Irrigation water use efficiency (IWUE) was estimated using the formula(IWUE = TY/IW), where IW equals seasonal crop water applied (mm). Phosphorususe efficiency (PUE) was estimated using the following equation: (PUE = (Yt − Yo)/P),where Yt equals total tuber yield under treatment, Yo equals total tuber yield undercontrol and P equals applied phosphorus (kg P2O5 ha−1). Total tuber yield in allequation is in units of kg ha−1.

Soil and Plant Analysis

A composite soil sample was collected before cultivation from the top 30 cm of soil,air-dried, crushed and sieved to pass through a 2-mm sieve. Physical and chemicalproperties of the soil were determined according to Burt (2004). The soil pH wasmeasured in 1:1 soil to water suspension using a digital pH meter. Available soil P wasextracted by 0.5 M sodium bicarbonate solution at pH 8.5 according to Olsen et al.(1954). The electrical conductivity (EC) was estimated using the salt bridge method(Rhoades 1982).

After 70 days from planting, composite soil and plant samples were collected.A plant sample consisted of 100 petioles/plot of the fourth leaf from the top ofthe plant. These samples were cleaned, washed with tap and distilled water, airdried, then dried in an oven at 70 °C until constant weight, ground and stored forchemical analysis. A soil sample (0–30 cm) from each treatment was collected todetermine available P and soil pH. Plant samples were digested in H2SO4 and

Table 3 Average monthly maximum (Tmax) and minimum (Tmin) temperature, relative humidity (RH), windspeed (WS) and reference evapotranspiration (ETo) during 2017 and 2018 growing seasons

Month Tmax Tmin RH(%)

ETo(mm)

WS(km h−1)

October, 2016 28.0 18.0 55 2.4 5.2

November, 2016 26.8 13.3 58 2.1 3.4

December, 2016 20.7 6.7 59 2.3 2.7

January, 2017 17.2 5.3 57 2.1 2.4

October, 2017 27.6 17.8 27 1.2 2.8

November, 2017 26.5 15.7 30 1.1 1.7

December, 2017 20.4 7.6 60 2.3 2.8

January, 2018 17.5 5.0 58 2.4 2.4

Rainfall was 0 for the two growing seasons

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H2O2 as described by Parkinson and Allen (1975) and analysed for P byspectrophotometer.

Statistical Analysis of Data

To test the significance of differences between the treatments, two-way analysis ofvariance (ANOVA) was used followed by Duncan’s multiple range test, both usingthe SPSS statistical program.

Results

Effect of Urea Phosphate and Monopotassium Phosphate on Soil pH and AvailableP

The data in Fig. 1 illustrate the effect of urea phosphate and monopotassium phosphateon the soil pH. The P form has a significant effect on the soil pH (P = 0.05). The highestsignificant values of soil pH were recorded in the soil fertilised with MPP or control(C), whilst the lowest values were recorded in the soil fertilised with UP. The injectionof UP in the irrigation water reduced the soil pH by about 7.2% compared to MPP. UPreduced the soil pH from 8.1 (the initial value of soil pH, Table 1) to 7.43. Figure 2illustrates the effect of P form on the availability of soil P. The P form has a significant(P = 0.003) effect on the availability of soil P. The highest significant value of P wasrecorded in the soil fertilised with UP, whilst the lowest value was recorded in thecontrol treatment. UP increased the availability of P by 24% compared to MPP. UP wassuperior in reducing the soil pH and increasing P availability compared to MPP.

Fig. 1 Soil pH as affected by the injection of urea phosphate (UP), monopotassium phosphate (MPP) andcontrol (C). C, UP and MPP refer to control, urea phosphate and monopotassium phosphate, respectively.Mean values (±SD, n = 12) are averaged across P fertigation frequency treatments. Means denoted by the sameletter indicate no significant difference according to Duncan’s test at P < 0.05

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Effect of Fertigation Frequency and P Form on P Concentrations in the Petiolesof Potato

Composite samples of potato petioles were collected 70 days after planting to evaluatethe effect of different treatments on P concentrations and the data are presented inTable 4. The form of P and the P fertigation frequency significantly affected theconcentrations of P. The injection of UP through irrigation water increased the Pconcentrations by 85%, whilst the injection of MPP increased the P concentrationsby about 43% compared to the control treatment. The daily injection of P (F1)

Fig. 2 Available soil P (mg kg−1) as affected by the injection of urea phosphate (UP), monopotassiumphosphate (MPP) and control (C). C, UP and MPP refer to control, urea phosphate and monopotassiumphosphate, respectively. Mean values (±SD, n = 12) are averaged across P fertigation frequency treatments.Means denoted by the same letter indicate no significant difference according to Duncan’s test at P < 0.05

Table 4 Concentrations of P in potato petioles as affected by P fertigation treatments

P form 2017 2018

C 2.4 ± 0.17 c 2.5 ± 0.14 c

MPP 3.7 ± 0.15 b 3.3 ± 0.13 b

UP 4.4 ± 0.20 a 4.7 ± 0.18 a

Pr˃F 0.004 0.0001

P fertigation frequency

F1 3.8 ± 0.20 a 4.4 ± 0.18 a

F2 3.5 ± 0.20 b 3.1 ± 0.17 b

F3 3.2 ± 0.15 c 3.0 ± 0.15 b

Pr˃F 0.002 0.002

C, UP and MPP refer to control, urea phosphate and monopotassium phosphate, respectively. Means (±SD,n = 12) denoted by the same letter indicate no significant difference according to Duncan’s test at P < 0.05

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increased the P concentrations by 19% in the first season and by 47% in the secondseason compared to F3.

Effect of Fertigation Frequency and P Form on Potato Yield

The data in Table 5 show the main effects of fertigation frequency and P form on thetotal (TY) and marketable yield (MY) of potato. The main effects of fertigationfrequency and P form were significant (P < 0.05) in the two growing seasons. Ureaphosphate (UP) and daily P fertigation (F1) treatments gave the highest significantvalues of total and marketable yield. UP increased the marketable and total yield ofpotato by 69 and 64% in first season and by 80 and 70% in the second season comparedto the control (C). The increases in the case of MPP were 56 and 54% in the first seasonand 63 and 58% in the second season. The daily P fertigation (F1) increased themarketable and total yield of potato by about 10 and 12% in comparison with the otherfertigation treatments.

Water and Phosphorus Use Efficiency

The main effect of P form on phosphorus use efficiency (PUE) was significant(P < 0.001) in both growing seasons as shown in Table 6. PUE in case of UP washigher by about 20 and 22% than that of MPP in the first and second season,respectively. PUE was significantly affected (P < 0.05) by the P fertigation frequency.PUE of potato fertilised every day (F1) was higher by about 28 and 68% than thatfertilised every 3 days (F3) for the first and second season, respectively. PUE wasaffected significantly (P < 0.001) by the interaction of fertigation frequency and P form.The highest significant values of PUE (80 and 93) were recorded in the daily UPfertigation treatment, whilst the lowest values (53 and 45) were recorded in the case ofMPP applied every 3 days. In general, the phosphorus was used more efficiently under

Table 5 Total yield (TY) and marketable yield (MY) of potato (t/ha) as affected by P fertigation treatments

P form 2017 2018

MY TY MY TY

C 15.4 ± 1.0 c 17.1 ± 1.1 c 15.4 ± 1.1 c 17.1 ± 1.0 c

MPP 24.0 ± 1.2 b 26.3 ± 1.2 b 24.0 ± 1.4 b 26.3 ± 1.4 b

UP 26.0 ± 1.3 a 28.0 ± 1.4 a 26.0 ± 1.2 a 28.0 ± 1.5 a

Pr˃F 0.000 0.004 0.006 0.0001

P fertigation frequency

F1 23.4 ± 1.2 a 25.7 ± 1.8 a 22.1 ± 1.4 a 25.7 ± 1.7 a

F2 20.9 ± 1.1 b 22.8 ± 1.5 b 20.9 ± 1.8 b 23.1 ± 1.2 b

F3 21.1 ± 1.0 c 23.0 ± 1.0 b 20.3 ± 1.5 b 22.4 ± 1.4 c

Pr˃F 0.005 0.008 0.0006 0.0009

C, UP and MPP refer to control, urea phosphate and monopotassium phosphate, respectively. Marketable yieldrefers to tubers > 3.5 cm. Means (±SD, n = 12) denoted by the same letter indicate no significant differenceaccording to Duncan’s test at P < 0.05

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daily P fertigation treatment, and the use of acidic fertiliser (UP) was more efficientthan the use of MPP.

The data for water use efficiency (WUE) and irrigation water use efficiency (IWUE)are presented in Table 6 and varied widely under the fertigation frequency and P form.WUE and IWUE were significantly affected by the P fertigation frequency and P forms(P < 0.01) as well as by the interaction of the two factors (P < 0.001) in the two growingseasons. Water was used more efficiently in the daily fertigation frequency treatment.The use of UP significantly increased water use efficiency by about 7% in comparisonto MPP. The daily P fertigation (F1) increased the WUE by about 13% in comparison toF3. The highest values of WUE and IWUE were recorded for UP applied daily.

Discussion

Fertiliser availability and utilisation by crops are modified by the properties of fertiliserand soil and by micro site reactions which occur near the fertiliser granules, droplets orbands. Soil pH is often the most important chemical property affecting nutrientsorption, precipitation, solubility and availability (Basta et al. 2005). Slightly acid soilpH (6.8) is the optimum value for maximum P availability. Calcareous soils have highsoil pH values and hence P availability is low in such soils. The current study showedthat the injection of acid P form (UP) caused a considerable reduction in the soil pHassociated with an increased P availability and uptake. Acid P form applied tocalcareous soils may increase the availability of P by (i) reducing the soil pH therebyretaining soil conditions for better P availability (Mikkelsen and Jarrell 1987; Eissaet al. 2013) and (ii) with each application of acid form of P fertiliser, the newly addedacid dissolves some of the previously formed calcium phosphate, so that the released Pwill be available to the plant (Mortvedt and Kelsoe 1988). Phosphorus in calcareous

Table 6 Phosphorus use efficiency (PUE), water use efficiency (WUE) and irrigation use efficiency (IWUE)in the 2 years, as affected by P fertigation treatments

Fertigation frequency P forms 2017 2018

PUE WUE IWUE PUE WUE IWUE

F1 C – 61 ± 2.1 d 47 ± 3.0 d – 71 ± 1.8 d 55 ± 3.4 e

MPP 73 ± 2.2 b 98 ± 2.3 a 76 ± 2.4 a 80 ± 2.0 c 122 ± 3.1 ab 94 ± 2.0 b

UP 80 ± 2.5 a 101 ± 2.0 a 78 ± 1.5 a 93 ± 2.0 a 130 ± 3.0 a 101 ± 2.2 a

F2 C – 55 ± 1.2 e 43 ± 2.7 d – 64 ± 2.0 d 50 ± 1.5 e

MPP 58 ± 2.0 d 84 ± 1.5 c 65 ± 2.1 c 71 ± 2.0 d 109 ± 3.5 bc 84 ± 2.1 c

UP 71 ± 1.5 b 91 ± 2.1 b 70 ± 1.5 b 85 ± 2.0 b 118 ± 3.4 b 91 ± 2.4 b

F3 C – 57 ± 2.0 e 44 ± 3.4 d – 73 ± 2.0 d 56 ± 1.0 e

MPP 53 ± 1.1 e 84 ± 2.3 c 65 ± 2.0 c 45 ± 2.0 f 101 ± 3.0 c 78 ± 2.3 d

UP 67 ± 1.2 c 91 ± 1.5 b 70 ± 1.6 b 58 ± 2.0 e 109 ± 4.0 bc 84 ± 2.6 c

PUE values are in kg yield kg−1 P2O5; WUE, and IWUE values are in kg yield mm−1 of water. C, UP andMPP refer to control, urea phosphate and monopotassium phosphate, respectively. Mean values (±SD, n = 4)denoted by the same letter indicate no significant difference according to Duncan’s test at P < 0.05

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soils moves very little from the point of application and reacts quickly with soil; thus,its availability and uptake by plants will be reduced. Rubiez et al. (1991) and Eissa et al.(2013) reported that the injection of UP through drip irrigation reduced the pH ofcalcareous soil by about 0.5 unit around the drip emitters and they also recorded aconsiderable P mobility in the soil within 20 cm from the emitters. Thus, more P useefficiency is expected especially with shallow rooting system of potato (Stark et al.2004). Opena and Porter (1999) reported that about 85% of potato root is concentratedin the upper 30 cm soil layer. The mobility of UP in calcareous soils under dripirrigation was found to be greater than MPP by Eissa et al. (2013).

The amount of plant material produced at the end of the growth season is a result ofmany interacting processes in soil-crop systems. Water and availability of nutrients arethe most important factors in plant production especially in newly reclaimed soils,where low water holding capacity and less soil quality predominate. The presentresearch illustrated that a high frequency of P fertigation (daily application) increasedpotato yield. The suggested mechanisms for that increase are the following: (i) contin-uous replacement of P in the depletion zone at the vicinity of root surfaces and (ii)reducing the interaction time between P and soil allowed more P availability anduptake. High frequency of P fertigation induced an increase in yield of lettuce (Silberet al. 2003), corn (Eissa 2016) and cucumber (Liang et al. 2014). For potato, carefulfertilisation and water management are required to ensure regular growth, high drymatter content and marketable tubers (Darwish et al. 2006). The current study revealedthat water and P were used more efficiently under daily P fertigation treatment, and theuse of acidic fertiliser (UP) was more efficient in this respect than the use of MPP. Theapplication of UP to drip-irrigated corn and wheat grown in calcareous sandy soilsincreased the uptake of P by about 10% compared to MPP (Eissa et al. 2013). Frequentapplications of UP increased the availability and uptake of P resulting in higher tuberyield, thereby water and P were used more efficiently. High frequency of P fertigationincreased PUE and WUE, as found by other researchers (Ben-Gal and Dudley 2003;Silber et al. 2003; Iqbal et al. 2003). In some previous studies, the yield of wheat, cornand potato was not affected by the form of P fertiliser (Hopkins et al. 2010; Eissa 2016).Hopkins et al. (2010) found that the application of MPP to drip-irrigated potato onlyincreased the tuber specific gravity compared to ammonium polyphosphate. The resultsof the current study revealed that UP has significant effects on yield, PUE and WUEand this may be due to the effect of UP on soil pH and availability of P (Eissa 2016).

Conclusions

Potato production in calcareous sandy soils needs efficient irrigation and fertilisationmanagement. Drip irrigation allows adding fertilisers into irrigation water, which mayincrease plant nutrients uptake and crop yield. Phosphorus is a necessary element for allgreen plants and therefore should have special additional attention in this respect,because its availability is low and movement in soil restricted by several reactions. Inthis study, fertigation with two P fertilisers and three different application frequencieswas investigated in potato cultivated in sandy soil with an initially high pH (8.1). Theresults indicate some differences between these treatments. The pH of the soil in theroot zone was reduced from the initial value, and the use of acidic fertiliser (urea

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phosphate) was most efficient in this respect. The current study demonstrated that ifphosphorus was applied daily in irrigation water, to supply nutrient at rates that matchplant requirements, we would be able to increase yield without increasing rate ofphosphorus fertiliser. The daily P application caused a 10% increase in the marketableyield which is equal to approximately 2 tonnes of potato. The cost of fertiliserapplication through the drip irrigation system is very low. Thus, it is recommendedto apply P daily, especially in sandy calcareous soils.

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