Potato yield response to foliar application of phosphorus as affected by soil moisture and available soil phosphorus

Abstract

Foliar application of phosphorus (P) may be a supplementary treatment to sustain adequate P-status of potato (Solanum tuberosum L.). However, the prediction of the potential benefits of foliar P supply is difficult, since several factors, such as weather conditions and plant P-status influence the effects. We determined the impact of soil moisture and soil P-supply on the responsiveness to foliar P-application under controlled environmental conditions. Plant dry matter yields, P-accumulation and phosphorus use efficiency (PUE) with or without foliar application were determined at five soil P-levels in combination with two soil moisture levels. The results suggest that water status is of importance for the responsiveness to foliar P-application and may be related to diffusion of P through the leaf cells, which require a good water status. The PUE was significantly improved with irrigation while adding P to the soil decreased the PUE.

Keywords:

• Foliar fertilization
• irrigation
• phosphorus use efficiency
• potato

Introduction

Efficient use of phosphorus (P) fertilizers is essential, since it is a non-renewable resource and crucial for maintaining high productivity in arable agriculture. The fact that soil-P losses from farmland to surface waters is the major cause of eutrophication of surface water further emphasizes efficient use of P (Hart et al., 2004; Bergström et al., 2007). Potato is generally considered to be a crop with a high P demand due to its low phosphorus use efficiency (PUE), which is defined as the ability of plants to both acquire phosphorus from the soil, but also the ability to utilize phosphorus for the production of dry matter (Dechassa et al., 2003; Shenoy & Kalagudi, 2005). Consequently, P-fertilizer recommendations are higher for potatoes than for most other crops (Great Britain. Ministry of Agriculture and Food, 1994; Allison et al., 2001). In potato production, P-fertilizer is normally applied to the soil, but foliar application is sometimes recommended as a supplementary treatment. It has been suggested that foliar application could reduce the need for soil-applied phosphorus and thereby also the leaching potential (Mukherjee et al., 1966; Girma et al., 2007). However, Johnson and Vaidyanathan (1993) concluded that foliar-applied P can increase yields, but soil-applied phosphorus should not be reduced compared to recommended levels. They summarized 49 field trials and found yield increases > 10% in 33% of the experiments, while 23% of the experiments showed zero or negative response to foliar P-application. Allison et al. (2001) tested the effects of foliar P-application in six field experiments and found no significant response on either yield or tuber number, even though two of the experiments were conducted on soils with an Olsen-P index of 0 and included treatments that had received no soil-applied P-fertilizer. Similar results have also been reported from Kilpatrick (1993) and Prasad and Brereton (1970).We conclude that the response to foliar P-application of potato yield and tuber set is inconsistent.

The foliar fertilizer spray solution is absorbed through the cuticles of the epidermal cells which, to some extent, are permeable (Fageria et al., 2009), but may also be taken up through the stomata. Crucial for this diffusion is that a concentration gradient is present and that the intermolecular spaces of the cuticle and the interfibrillary spaces of the cell wall are saturated with water (Eichert et al., 1998). For these reasons it has been suggested that the plant water status may influence the uptake of nutrients through leaves (Alexander, 1986). It has been previously shown that, for example, barley plants which are well supplied with P acquire P less rapidly through leaves than plants with low P-status (Clarkson & Scattergood, 1982). This might be explained by a decreased diffusion gradient between the external leaf surface and the leaf apoplasm. Furthermore, the duration of the diffusion is important; thus, external factors such as temperature, sunlight and humidity which effect the evaporation rate from the foliar sprays are of importance (Marschner, 1995).

Only a few authors have studied the response of foliar P-applications in relation to other factors such as potato plant water status and P-status of the plant (Gooding & Davies, 1992). The aim of this study was to determine the influence of soil moisture and soil P supply on the potato response to foliar-applied P under controlled environmental conditions.

Material and methods

Plant material and substrate

This study was conducted in climate chambers from January to March 2007 using potato (Solanum tuberosum L., cv. Ditta). The plantlets were punched out from one sprouting eye of the seed potato which had been pre-cultivated for 6 weeks. The plantlets weighed between 2.8 and 3.2 g and were treated with Monceren FS 250® (Pencycuron) prior to planting to protect against black scurf. The plantlets were kept for 3 days in humid conditions to allow small roots and a protecting crust to develop. A total of 120 pots measuring 30 cm in diameter and 22 cm in depth were filled with a pre-mixed unfertilized low P (Olsen-P 0.5 mg/l) potting medium composed of 50% (v/v) pumice stone (0.5–3.0 mm) and 50% peat with humification level 3–4 (Olsen et al., 1954). One plantlet was planted 4 cm below soil surface in each pot; additional substrate was added as the plant emerged so that the final planting depth was 10 cm.

Experimental setup and growth conditions

The experiment consisted of 20 treatments: foliar or no foliar P-application in combination with moist and dry soil conditions and five soil P levels. These soil phosphorus levels were: 0 (P0), 72 (P1), 150 (P2), 222 (P3), and 300 (P4) mg P/pot (Figure 1) which corresponds to approximately 0–50 kg P ha⁻¹. The experiment had six replicates. All pots were randomly distributed in three climate chambers which all were adjusted and controlled to have the same climate conditions. The plantlets were grown for 10 weeks under 18 h daylight at 20°C/18°C (day/night). The humidity was continuously adjusted to 65%, and the fluorescent light intensity was 320 µmol m⁻² s⁻¹.

Figure 1.  Experimental overview.

Foliar P-application

The foliar P treatments included two spraying events. The first one was carried out 18 days after emergence (d.a.e), just before tuber initiation, and the second application 10 days later. Pots with windows were used to determine the right time for application with respect to tuber initiation. A dose of 0.0109 g P/plant (corresponding to 2.7 kg P/ha) was applied with monopotassium phosphate (KH₂PO₄) in the first spraying event and 0.008 g P/plant (2.0 kg P/ha) in the second event. All foliar applications were sprayed by hand, using a professional Hardi sprayer, until runoff almost occurred. The pots were covered with fabric to prevent spraying liquid from reaching the soil. In order to allow sufficient time for the diffusion uptake to occur, the humidity was brought up to 90% for 4 hours after the application.

Moist treatment

Two soil moisture levels were defined, dry and moist, starting one week after emergence. For the ‘dry’ treatment, the soil was kept between 0.25 and 0.35 g water cm⁻³ and for the ‘moist’ treatments, between 0.35 and 0.45 g water cm⁻³. Each pot was watered individually by weight and allowed to reach its lower limit before distilled water was applied until the upper limit was reached. The pots were placed on trays to prevent water and nutrients to drain through the pot.

Nutrient supply

For the different P-levels, KH₂PO₄ were mixed into nutrient solutions corresponding to each P-level. All solutions contained the same amount of all other essential nutrients. Since P was supplied to the different nutrient solutions levels as KH₂PO₄, KCl was added to each treatment to ensure that all plants received the same amount of K. The total amount of nutrients added to each pot were 1.765 g N, 1.853 g K, 88 mg S, 274 mg Ca, 265 mg Mg, 95 mg Fe, 124 mg Mn, 4 mg Zn, 3.5 mg Cu, 0.25 mg Mo, and 7.06 mg B. The nutrient solutions were divided into eight doses and applied in equal amounts weekly starting at planting.

Data collection and statistical analysis

The following data were collected from each pot: (1) fresh weight (FW) of foliage and tubers, (2) dry weight (DW) of foliage and tubers, (3) total P taken up by the tubers and the foliage, (4) P concentration in the recently matured petiole, analyzed 22 d.a.e.

Phosphorus use efficiency was defined as (g DW. g⁻¹ P),

where DW is the total dry weight and Ps is the total amount of applied P, including foliar-applied P.

The foliage and tubers from each plant was oven dried separately, at 70°C, until no weight loss could be recorded. The samples were ground, combusted in nitric acid by means of microwave technique and analyzed by ICP-AES analysis for total P concentration.

All data and parameters were subjected to analysis of variance using the GLM procedures of IBM SPSS statistics 20.0 for Windows. The main effects and two-way interactions of soil moisture, foliar treatment, and soil application level were included in the model. The three-way effects were excluded from the model since no significant interaction occurred in any of the parameters. Plants within the zero soil P-application level were excluded from the data-set since the plants wilted half way through the experiment. When statistically significant differences were found, further differences between treatments were evaluated using the LSD test. For all analyses, the significance level of P<0.05 was used.

Results

Soil moisture

Soil moisture had a significant (P<0.05) effect of all investigated traits except the P concentration in foliage (Table I). Plants growing in the moist soil conditions had significantly higher P concentrations in the tubers, total P accumulation, tuber FW, total DW, and PUE (Tables II and III, Figures 2 and 3). The moist treatment also increased the number of tubers per plant with 11% from 5.1 to 5.7 (data not shown).

Figure 2.  Tuber fresh weight as affected by foliar-P-application at two soil moisture regimes. The columns represent mean values for soil-applied P. Columns with different letters are significantly (P<0.05) different with the LSD test.

Figure 3.  Phosphorus use efficiency (PUE) as affected by foliar P-application and soil moisture. The columns represent mean values for soil-applied P. Columns with different letters are significantly (P<0.05) different with the LSD test.

Table I. Analysis of variance for investigated traits.

Source	DF	P concentration tubers	P concentration foliage	Total P accumul-ation	Tuber FW	Total DW	Tuber number	PUE (Tw/Ps)
Moisture	1	0.000	0.155	0.000	0.000	0.000	0.033	0.000
Foliar	1	0.001	0.000	0.000	0.135	0.065	0.877	0.000
Soil-applied P	3	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Foliar×Moisture	1	0.153	0.245	0.238	0.006	0.009	1.00	0.026
Foliar×Soil-applied P	3	0.078	0.003	0.406	0.073	0.248	0.204	0.276
Moisture×Soil-applied P	3	0.000	0.072	0.003	0.371	0.122	0.236	0.055
Moisture includes dry and moist irrigation regime, Foliar includes treatment with and without foliar application of P and ‘Soil applied P’ includes 72, 150, 222 and 300 mg P per pot. Tw, Total dry weight; Ps, Soil-applied P; PUE, phosphorus use efficiency; DW, dry weight; FW, fresh weight; DF, degrees of freedom. P values, bold numbers indicates significant (P<0.05) effects.

Table II. Mean values for total dry weight (DW) and total P accumulation (g/pot) as affected by moisture level and foliar P-application.

		Soil-applied P (mg/pot)
		72		150		222		300
		Total DW	Total P accumulation	Total DW	Total P accumulation	Total DW	Total P accumulation	Total DW	Total P accumulation
Moist soil	No foliar P	38.9	0.038	74.5	0.086	82.8	0.132	85.4	0.180
	Foliar P	46.6	0.043	80.3	0.097	88.7	0.148	89.3	0.196
Dry soil	No foliar P	37.2	0.034	67.6	0.075	78.6	0.120	84.3	0.150
	Foliar P	41.2	0.036	63.0	0.075	80.4	0.129	78.8	0.162
	LSD	5.3	0.005	6.9	0.010	7.7	0.017	9.8	0.021
LSD, least significant difference (P<0.05).

Table III. Mean values for P concentration in tubers and foliage (mg P/g DW) as affected by moisture level and foliar P-application.

		Soil-applied P (mg/pot)
		72		150		222		300
		P concentration tubers	P concentration foliage	P concentration tubers	P concentration foliage	P concentration tubers	P concentration foliage	P concentration tubers	P concentration foliage
Moist soil	No foliar P	0.93	0.84	1.23	0.97	1.71	1.34	2.26	1.76
	Foliar P	0.95	0.86	1.23	1.18	1.78	1.40	2.32	1.90
Dry soil	No foliar P	0.97	0.77	1.15	1.00	1.58	1.39	1.87	1.50
	Foliar P	0.96	0.85	1.21	1.15	1.67	1.41	2.13	1.89
	LSD	ns	0.066	0.069	0.087	0.156	0.178	0.163	0.172
LSD, least significant difference (P<0.05).

Foliar-P

Foliar application of P significantly increased the P concentration in both tubers and foliage (Tables I and II). The increased concentration also contributed to a higher total P accumulation (Tables I and II). Even though foliar application increased the P accumulation it did not affect tuber FW, total DW, or the number of tubers (Table I). Since foliar application in general failed to increase the total DW, the PUE (g DW. g⁻¹ P) decreased with foliar application (Table I and Figure 3).

Soil-applied P

The total P accumulation and the P concentration in both tubers and foliage significantly increased with increasing amounts of soil-applied P (Tables I and III). Soil application of P also increased the tuber FW, total DW, and tuber number, but only up to 222 mg P/pot. Increasing the P supply to more than 222 mg P/pot caused no further significant effects. The P concentration in the most recently matured petiole 22 d.a.e. increased linearly (R ²=0.70) with increasing P treatment (data not shown). The critical petiole P concentration for cv. Ditta was found to be approximately 3 mg P/g DW measured 22 d.a.e. This concentration was reached when 222 mg P/pot was applied.

The PUE decreased with increasing amounts of soil-applied P. Plants which received 72 mg P/pot had a PUE of 508 while the 150, 222, and 300 mg P/ pot treatment had 450, 358, and 274 g DW. g⁻¹ P, respectively, (data not shown).

Two-way interactions

There was a significant interaction between foliar-P-application and soil moisture in terms of PUE, tuber FW, and total plant DW (Table I). Plants within the moist soil regime had higher tuber FW and total DW yields, when treated with foliar application while the plants in the dry soil regime were non responsive to foliar P (Table II and Figure 2). The positive effects of foliar application on total DW within the moist soil regime resulted in reduced negative effects of foliar application on the PUE (Figure 3).

The effects of high soil moisture on total DW were also larger among the plants that were subjected to foliar spraying compared to the non-sprayed plants (Figure 2). Thus, applying P to the leaves enhanced the positive effects of irrigation.

The effect of foliar application on P concentration in the foliage was significantly different depending on the amount of P supplied to the soil (Table I). The effects of foliar P was surprisingly higher in plants which received intermediate (150 mg P/pot) and high (300 mg/pot) compared to plants which received 72 and 222 mg P/pot (Table III).

There was a significant interaction between soil moisture×applied amount of P in terms of P concentration in the tubers and total P accumulation (Table I). The positive effect of the moist soil regime on these traits increased with increasing amounts of P applied to the soil (Tables II and III).

Discussion

The results from this study show that the water status of the potato plant is of importance for the responsiveness to foliar-applied fertilizers. Plants within the moist treatment showed a greater yield response as a result of foliar P-application compared to plants grown under dry soil conditions, even though no visual water stress symptoms were observed. Therefore irrigation should be scheduled before applying phosphorus to the leaves. Yuncai et al. (2008) found similar results in maize where foliar application of P gave higher yields under well-watered conditions, but not under drought conditions. A possible explanation for these results is that the plants in the moist treatment may have had more turgid cells, which resulted in more open micropores, in which a higher diffusion rate of P could occur. It has been previously shown that a low internal P concentration could enhance the P uptake through leaves. This was explained by an increased diffusion gradient between the external leaf surface and the leaf apoplasm (Clarkson & Scattergood, 1982). However, this was not observed in our experiment, where foliar application increased yield similarly within the different soil application levels. Therefore, high internal P concentration in the potato plant may not affect the P uptake through leaves negatively, as previously suggested.

It is apparent that the growth of both tuber FW and total DW increased with increasing soil P supply up to 222 mg P/pot. Increasing the P supply beyond that did not increase further the yield. The optimal petiole P concentration for cv. Ditta measured 22 d.a.e. was just below 0.3% P of DW (data not shown), which is similar to earlier studies (Walworth & Muniz, 1993).

Phosphorus use efficiency describes the ability of the plant to produce biomass in relation to a given P supply. PUE is dependent on two factors, the uptake efficiency (ability to take up P from the soil) and the utilization efficiency (ability to produce biomass from P taken up) (Blair, 1993). In our experiment the PUE was higher in the moist soil regime compared to the dry because of higher total DW. The increased DW might have been caused by other factors related to high soil moisture or by the improved P accumulation. In general high soil moisture increase the P accumulation of soil P because the path length over which P has to diffuse to the root decreases with increasing soil water content. A lower concentration of P in the soil solution is therefore needed to maintain diffusive supply (Allison et al., 2001). Harris (1994) reviewed the effect of irrigation on the availability of P and concluded that increased volumetric soil water content will improve the availability of P to the potato crop.

Foliar application of phosphorus resulted in a decreased PUE. This may be explained by the fact that P is taken up less effectively through leaves compared to the roots; although, foliar application provides a more rapid utilization (Fageria et al., 2009). However, the significant interaction between foliar application and soil moisture shows that the PUE of foliar treated plant can be increased if the plants are well supplied with water.

Increasing P–application to the soil decreased PUE. This result is similar to findings by Balemi and Schenk (2009), who investigated the utilization efficiency in four different varieties with different efficiencies at two different soil P levels. The range in efficiency, approximately 250–500 mg d.m. mg⁻¹ P, where similar to our data. When comparing the results it seems that cv. Ditta used in this trial is an intermediate P use efficient cultivar.

Acknowledgements

The financial support of AnalyCen Nordic AB, Bara Mineraler, Greppa Näringen, Lantbrukarnas riksförbund (LRF), Partnerskap Alnarp and Stora Tollby Trädgård AB through a matching investment initiative is gratefully acknowledged.