# Effect of Phosphorus and Potassium Fertilizers Application on Soil Chemical Characteristics and Their Accumulation in Potato Plant Tissues

Authors: Habtam Setu
Journal: Applied and Environmental Soil Science
 (2022)

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## Abstract

Applications of nutrients are determined by the plant’s nutritional requirements and the soil’s available nutrients; however, the precise nutrient application is difficult. At Assosa, a field experiment was conducted to determine how soil chemical characteristics and accumulation of phosphorus and potassium in potato tissue responded to phosphorus and potassium fertilizers application. The treatments included four potassium levels (0, 100, 200, and 300 kg·K2O ha−1) and six phosphorus levels (0, 46, 92, 138, 184, and 230 kg·P2O5 ha−1). The experiment was set up as a factorial randomized complete block design (RCBD) and was repeated three times. According to the preplanting soil analysis, the cropping area’s soil has a medium CEC, low accessible phosphorus, and very low exchangeable potassium. The application of phosphorus had a highly significant influence on accessible phosphorus, exchangeable potassium, and CEC, the concentration of phosphorus in the leaf and tuber tissue of the potato plant. While potassium had a significant effect on exchangeable potassium, CEC, phosphorus concentration in potato plant tuber tissue, and potassium level in both the leaf and the tuber, the interaction effect of phosphorus and potassium on all of the above-mentioned soil chemical properties, as well as the amount of P and K in potato plant tissues, was observed. The critical potassium level in the leaf blade for optimum crop yield tissue content was determined to be 100 kg·K2O ha−1, while the critical level of phosphorus was determined to be 184 kg·P2O5 ha−1.


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## Body

## 1. Introduction

Potatoes (Solanum tuberosum L.) are steadily contributing to the food supply in developing countries, particularly in the highlands of subSaharan Africa [1]. Potato yields are heavily influenced by genetic background [2]. Because of biotic and abiotic factors that interfere with plant growth and tuber development, the actual yield is lower than the yield potential [3]. Aside from selecting the right cultivar, protecting the plants, and providing consistent water, proper nutrient management is also essential. Mineral nutrients help potatoes resist adverse conditions, increase yields, and ensure quality [4]. In general, because of their high yield and nutritional value, potatoes are among the crops that require the most mineral nutrients per kilogram of dry matter [4].In the potato industry, increasing potato yield primarily depends on soil fertility, which refers to the soil’s ability to provide the nutrients that the plants require. Major effects of declining soil fertility include low yield, loss of agrobiodiversity, low nutrient use efficiency in cropping systems, and soil loss [5]. The three most required plant nutrients, nitrogen, phosphorous, and potassium, are becoming scarcer in soils throughout Sub-Saharan Africa [6]. Fertilizers are used to boost the supply of plant nutrients in the soil, allowing potatoes to produce yields up to their full genetic potential under changing environmental conditions [7]. The amount of fertilizer nutrients required for optimum potato production is determined by the soil’s inherent capacity to supply adequate levels of nutrients to growing plants [8], the yield potential of the crop variety grown, the cost and availability of fertilizers, and the environmental conditions that prevail during the crop’s growth [9].Phosphorous is required in relatively high amounts for potato production [4]. However, poor phosphorus availability is a major impediment to potato production in tropical areas, particularly where it is the scarcest nutrient in soils [10], due to its high fixation and slow diffusion in most soil conditions [11]. Because it is not present in water-soluble form, it is usually insufficiently available to plants, causing the application of additional phosphate fertilizer [12]. Soil physicochemical characteristics have been shown to influence phosphorus solubility and adsorption in soils. These factors include the kind and quantity of soil minerals, soil pH, cation and anion effects, the extent of P saturation, response time and temperature, floods, and fertilizer management [13]. Furthermore, the accessibility of P from fertilizer may be influenced by soil reactivity, the degree of soil P deficit, the pace and manner of administration, crop demands, and soil variances [14].Potassium is the nutrient that potato crops consume most often. In addition to carbohydrate metabolism and enzyme activation, potassium regulates osmotic pressure and water use, supports nitrogen uptake and protein synthesis, and facilitates the translocation of assimilates in plants [15, 16]. Potassium, however, is primarily a free or absorbable bound cation, making it a fast-displacement cation in plants and cells [17]. This high mobility in the plant states the key functional properties of K as the main cation involved in charge neutralization and the most common inorganic osmotic active substance [18]. The roots absorb potassium as a K+ ion [19]. Apart from this, agricultural exports and potassium leaching lead to a decrease in soil potassium content [20]. Potassium deficiency can occur on a wide variety of soils, such as acidic sandy soils, waterlogged soils, and saline soils [21]. Along with low yields, potassium-deficient plants become prone to drought, excessive water, low and high temperatures, pests, diseases, and nematodes [22, 23]. Crops, such as potatoes, have high potassium demands, thus depleting soil potassium unless it is replenished externally [24]. In addition to the cropping system used, soil native potassium capacity as well as potassium rate, time, source, and method can also influence the effectiveness of potassium applications in potatoes [23]. It is difficult, however, to set universal critical ranges for soil potassium because of variable growing conditions, climate, a large number of available varieties, and great differences in yield. The soils in western Ethiopia are unable to provide all the necessary plant nutrients in sufficient quantities to support good crop growth, which is why fertilizers are one of the most effective methods of increasing nutrient uptake and improving yields in crop plants. Furthermore, the soils of the area are unhealthy due to nutrient mining in crop production and a lack of organic and inorganic replenishment. In general, it is less well documented how inorganic fertilizers affect the soil character and the concentration of nutrients in potato plant tissues in an area; instead, the emphasis is on yield and quality. Therefore, this study examines how inorganic phosphorus and potassium fertilizers affect soil chemical properties and potato tissue phosphorus and potassium concentration levels.


## 2. Materials and Methods

### 2.1. Description of the Experimental Site

The research was conducted at the Assosa Agricultural Research Center (AsARC), one of the Ethiopian Institute of Agricultural Research (EIAR) centers, which is located at 10°02′ N and 34°34′ E in western Ethiopia, about 665 km from Addis Ababa, the capital. The experimental site is approximately 1553 meters above sea level. The experiment was conducted during the main cropping season, and the area has an annual rainfall of 1100 mm on average. The rainy season lasts from May to October, with the most rain falling between June and August. It has a warm, humid climate, with mean maximum and minimum annual temperatures of 32°C and 17°C, respectively.


### 2.2. Treatments and Experimental Design

The treatments comprised four levels of potassium (0, 100, 200, and 300 kg·K2O ha−1) and six levels of phosphorus (0, 46, 92, 138, 184, and 230 kg·P2O5 ha−1). The experiment was laid out as a randomized complete block design (RCBD) in a 4 × 6 factorial arrangement and replicated three times. Each plot received one of 24 treatment combinations, which were assigned at random.Phosphorus was provided by triple superphosphate (TSP, 46% P2O5), while potassium chloride (KCl, 60% K2O) was used as a source of potassium. The nitrogen source was urea (CO[NH2]2) (46% N). The granules of potassium and phosphorus fertilizers are applied below and around the seed tubers at planting. To avoid leaching as a result of high rainfall, potash was applied in two parts (half when the plant emerged and a half at midstage after planting), while phosphorus was applied all at once. Each plot received 138 kg·N·ha−1 of urea, evenly applied three times as recommended (1/4th at planting, 1/2 at midstage (about 40 days after planting), and 1/4th at tuber initiation (at the beginning of flowering)).


### 2.3. Soil Sampling and Analysis

Before planting and after harvesting, soil samples were taken from the experimental site. Composite soil samples were collected from the experimental field using an auger at a depth of 30 cm before planting. Further, for postharvest soil sampling, five soil samples were taken from each plot and composited per treatment. A total of 72 composite samples were made. Air-dried composite samples were ground and sieved for chemical analysis using a 2 mm sieve after being air-dried on paper trays. The soil was analyzed for total nitrogen, available phosphorous, exchangeable potassium, organic matter, soil pH, cation exchange capacity (CEC), and soil texture. The analysis was done at the AsARC soil laboratory.Soil organic matter determination was based on the oxidation of carbon with an acid potassium dichromate (K2Cr2O7−2) medium following Walkley and Black’s method as described by Estefan et al. [25]. A micro-Kjeldahl digestion and distillation method were used to determine the total soil nitrogen [25]. Available soil phosphorus was determined using the methods of Olsen method [26] and exchangeable potassium by extraction with 1 N ammonium acetate (method of Morgan) and determined by reading with a flame photometer [27]. A method described by Rhoades [28] was used to determine the cation exchange capacity of the solution. Soil pH was determined in 1 : 2.5 soils to water suspension using a glass electrode as described by McLean [29]. In addition, soil texture was determined using the hydrometer method of Ashworth et al. [30] as described by Rowell [31] using the USDA texture triangle.


### 2.4. Plant Tissue Sampling and Analysis

Mature, non-necrotic, and healthy leaves were randomly stripped off from plants in each plot, along with the petiole, immediately before blooming began (tuber initiation). From each plot, 60–80 leaves were collected as a sample. The leaves were dried in a forced draft oven at 65°C until they reached a consistent weight. In the case of tubers, tubers of all sizes were randomly selected from each plot before maturity (65 days after planting). They were finely cut (3 mm) with a knife. They were dried in a forced hot air circulation oven at 70°C until they reached a consistent weight. The samples were subsequently pulverized to a size less than 1 mm to determine the phosphorus and potassium levels of the leaf and tuber tissues. The plant’s dried and crushed leaves and tubers were ashed at a temperature of 480°C. The ashy plant material was treated with a nitric acid (HNO3) solution diluted in three volumes of distilled water. The P was assessed calorimetrically using the Vanadomolybdate method [32], and the potassium content was determined using a flame photometer to measure the potassium atoms produced at 766.5 nm from the same extract [33].


### 2.5. Statistical Analysis

The data were subjected to analysis of variance (ANOVA) using SAS [34] version 9.3’s generalized linear model (GLM), and interpretations were derived using the Gomez [35] technique. The least significant difference test was used to distinguish significant differences between treatment means at a 5% level of significance.


## 2.1. Description of the Experimental Site

The research was conducted at the Assosa Agricultural Research Center (AsARC), one of the Ethiopian Institute of Agricultural Research (EIAR) centers, which is located at 10°02′ N and 34°34′ E in western Ethiopia, about 665 km from Addis Ababa, the capital. The experimental site is approximately 1553 meters above sea level. The experiment was conducted during the main cropping season, and the area has an annual rainfall of 1100 mm on average. The rainy season lasts from May to October, with the most rain falling between June and August. It has a warm, humid climate, with mean maximum and minimum annual temperatures of 32°C and 17°C, respectively.


## 2.2. Treatments and Experimental Design

The treatments comprised four levels of potassium (0, 100, 200, and 300 kg·K2O ha−1) and six levels of phosphorus (0, 46, 92, 138, 184, and 230 kg·P2O5 ha−1). The experiment was laid out as a randomized complete block design (RCBD) in a 4 × 6 factorial arrangement and replicated three times. Each plot received one of 24 treatment combinations, which were assigned at random.Phosphorus was provided by triple superphosphate (TSP, 46% P2O5), while potassium chloride (KCl, 60% K2O) was used as a source of potassium. The nitrogen source was urea (CO[NH2]2) (46% N). The granules of potassium and phosphorus fertilizers are applied below and around the seed tubers at planting. To avoid leaching as a result of high rainfall, potash was applied in two parts (half when the plant emerged and a half at midstage after planting), while phosphorus was applied all at once. Each plot received 138 kg·N·ha−1 of urea, evenly applied three times as recommended (1/4th at planting, 1/2 at midstage (about 40 days after planting), and 1/4th at tuber initiation (at the beginning of flowering)).


## 2.3. Soil Sampling and Analysis

Before planting and after harvesting, soil samples were taken from the experimental site. Composite soil samples were collected from the experimental field using an auger at a depth of 30 cm before planting. Further, for postharvest soil sampling, five soil samples were taken from each plot and composited per treatment. A total of 72 composite samples were made. Air-dried composite samples were ground and sieved for chemical analysis using a 2 mm sieve after being air-dried on paper trays. The soil was analyzed for total nitrogen, available phosphorous, exchangeable potassium, organic matter, soil pH, cation exchange capacity (CEC), and soil texture. The analysis was done at the AsARC soil laboratory.Soil organic matter determination was based on the oxidation of carbon with an acid potassium dichromate (K2Cr2O7−2) medium following Walkley and Black’s method as described by Estefan et al. [25]. A micro-Kjeldahl digestion and distillation method were used to determine the total soil nitrogen [25]. Available soil phosphorus was determined using the methods of Olsen method [26] and exchangeable potassium by extraction with 1 N ammonium acetate (method of Morgan) and determined by reading with a flame photometer [27]. A method described by Rhoades [28] was used to determine the cation exchange capacity of the solution. Soil pH was determined in 1 : 2.5 soils to water suspension using a glass electrode as described by McLean [29]. In addition, soil texture was determined using the hydrometer method of Ashworth et al. [30] as described by Rowell [31] using the USDA texture triangle.


## 2.4. Plant Tissue Sampling and Analysis

Mature, non-necrotic, and healthy leaves were randomly stripped off from plants in each plot, along with the petiole, immediately before blooming began (tuber initiation). From each plot, 60–80 leaves were collected as a sample. The leaves were dried in a forced draft oven at 65°C until they reached a consistent weight. In the case of tubers, tubers of all sizes were randomly selected from each plot before maturity (65 days after planting). They were finely cut (3 mm) with a knife. They were dried in a forced hot air circulation oven at 70°C until they reached a consistent weight. The samples were subsequently pulverized to a size less than 1 mm to determine the phosphorus and potassium levels of the leaf and tuber tissues. The plant’s dried and crushed leaves and tubers were ashed at a temperature of 480°C. The ashy plant material was treated with a nitric acid (HNO3) solution diluted in three volumes of distilled water. The P was assessed calorimetrically using the Vanadomolybdate method [32], and the potassium content was determined using a flame photometer to measure the potassium atoms produced at 766.5 nm from the same extract [33].


## 2.5. Statistical Analysis

The data were subjected to analysis of variance (ANOVA) using SAS [34] version 9.3’s generalized linear model (GLM), and interpretations were derived using the Gomez [35] technique. The least significant difference test was used to distinguish significant differences between treatment means at a 5% level of significance.


## 3. Results

### 3.1. Preplanting Physicochemical Properties of the Soil

The results of a laboratory analysis of some physical and chemical parameters of the soil at the experimental site before planting are presented in Table1. The results suggest that the soil has a silt texture (49%) and is highly acidic in response, with a pH of 5.1. Furthermore, the experimental soil has a medium CEC (19.1 cmol·kg−1 soil), low total nitrogen (0.06%), accessible phosphorus (8.52 ppm), very low exchangeable potassium (0.12 cmol·kg−1 soil), and organic carbon (0.75%) (Table 1).Table 1
Soil’s physical and chemical properties of the experimental site before fertilization.
Soil depth (cm)Particle size (%)pH (H2O)OC (%)Available P (ppm)Total N (%)Exchangeable cations(cmol kg−1 soil)SandSiltClayK+Na+Ca++Mg++CEC0–301749345.10.758.520.060.1360.259.833.3719.1where OC = organic carbon; P = phosphorus; N = nitrogen; K = potassium; Na = sodium; Ca = calcium; Mg = magnesium; CEC = cation exchange capacity.


### 3.2. Soil Chemical Properties after Harvest

A study of soil fertility before and after cropping revealed that applying phosphorus and potassium fertilizers raised the CEC, accessible phosphorus, and potassium, indicating that the soil’s fertility state has improved (Tables1–3). As a result of the phosphorus application, soil chemical characteristics (CEC, available phosphorus, and exchangeable potassium) significantly improved. Similarly, except for available phosphorus, which was minor, exchangeable potassium and CEC changed dramatically with potassium fertilizer treatment (Table 3). CEC, accessible phosphorus, and exchangeable potassium levels were not affected by phosphorus and potassium additions to the soil (Tables 2 and 3).Table 2
Mean squares of soil fertility status after harvest as influenced by phosphorus and potassium applications.
Source of variationd.fAvailable phosphorus (ppm)Exchangeable potassium (cmol kg−1)CEC (cmol kg−1)Block25.19Ns0.0005Ns2.03NsP5259.95∗∗0.0013∗∗5.62∗∗K32.58Ns0.3228∗∗96.86∗∗P × K153.54Ns0.0003Ns1.18NsError462.330.00040.90where∗∗ = indicate significant difference at 1%; Ns = nonsignificant at 5% level of significance; d.f = degree of freedom; CEC = cation exchange capacity; P = phosphorus (P2O5); K = potassium (K2O).Table 3
Main effects of phosphorus and potassium fertilizers on available phosphorus, exchangeable potassium, and CEC status of the soil after harvest.
TreatmentsAvailable phosphorus (ppm)Exchangeable potassium (cmol kg−1)CEC (cmol kg−1)P2O5 (kg ha−1)06.52c0.23a20.71a466.60c0.21b20.45a927.43c0.21b19.18c13810.92b0.21b19.58bc18415.85a0.20b19.99ab23016.77a0.20b19.01cF test∗∗∗∗∗∗LSD (5%)1.250.020.7784K2O (kg ha−1)011.130.06d17.87c10010.660.15c17.88c20010.210.26b21.05b30010.730.36a22.48aF testNs∗∗∗∗LSD (5%)1.020.010.64CV (%)14.299.394.78Means of the same main effect followed by the same letter or with no superscript letter within a column are not significantly different at the 5% level of significance;∗∗ = significant at P<0.01 probability level; Ns = nonsignificant at P<0.05 probability level; CEC = cation exchange capacity; LSD = least significant difference; and CV = coefficient of variation.Increasing the phosphorus rate from nil to 46 and 92 kg·P2O5 ha−1 had no statistically significant effect on the soil’s available phosphorus concentration. However, increasing the phosphorus content from 0 to 138, 184, and 230 P2O5 ha−1 resulted in significant increases of 67, 143, and 157%, respectively (Table 3).Increased phosphorus application dramatically lowered available potassium and CEC. The exchangeable potassium was lowered by around 9% when the phosphorus rate was increased from zero to 46 kg·P2O5 ha−1. However, the decrease in exchangeable potassium among phosphorus-treated soils was not statistically significant. However, raising the phosphorus content from 0 to 92 kg·P2O5 ha−1 reduced the CEC by around 5.5%. However, plots that got 46 and 184 kg·P2O5 ha−1 were statistically equivalent to plots that received no phosphorus (Table 3).Unlike phosphorus, potassium increased the levels of exchangeable potassium and CEC. The exchangeable potassium increased by roughly 150% when the potassium level was increased from nil to 100 kg·K2O ha−1. Increasing the potassium level between 200 and 300 kg·K2O ha−1 raised it by around 333–500%, respectively. Similarly, increasing the potassium fertilization rate from nil to 200 and 300 kg·K2O ha−1 enhanced exchangeable potassium by around 18–26%, respectively. However, when compared with the control treatment, the application of 100 kg·K2O ha−1 did not affect the CEC (Table 3).


### 3.3. Phosphorus and Potassium Levels in Plant Tissues

The main effect of phosphorus significantlyP<0.01 influenced the concentration of phosphorus in the potato plant’s leaf as well as in the tuber tissue. The main effect of potassium significantly P<0.05 influenced the concentration of phosphorus in potato plant tuber tissues. However, the main effect of potassium did not affect the concentration of phosphorus in potato plant leaf tissue. The two factors did not interact to affect the concentration of phosphorus in the potato plant’s leaf and tuber tissue (Table 4).Table 4
Nutrient accumulation in potato leaves and tubers mean squares as influenced by phosphorus, potassium application rate, and their interaction.
Source of variationd.fPhosphorus content (ppm)Potassium content (cmol kg−1)LeafTuberLeafTuberBlock20.0013∗0.0002Ns0.071Ns0.112∗P50.1257∗∗0.1285∗∗0.047Ns0.202∗∗K30.0001Ns0.0009∗12.097∗∗18.272∗∗P × K150.0003Ns0.0004Ns0.041Ns0.013NsError460.00030.00020.0290.035where∗∗, ∗ = indicate a significant difference at 1% and 5% level of significance, respectively; Ns = nonsignificance at 5% level of significance; d.f = degree of freedom; P = phosphorus (P2O5); and K = potassium (K2O).Increasing the rate of phosphorus linearly resulted in significantly higher nutritional concentrations in both leaf and tuber tissues. Thus, raising the fertilizer rate from 0 to 46, 92, 138, 184, and 230 kg·P2O5 ha−1 significantly enhanced the nutrient concentration in leaf tissue by approximately 100, 350, 450, 550, and 650%, respectively. Similarly, phosphorus concentrations in tuber tissue increased by 117, 267, 300, 383, and 467% in the sequence mentioned (Table 5).Table 5
Phosphorus and potassium concentration levels of dried potato leaves and tuber tissues.
TreatmentsPhosphorus content (ppm)Potassium content (cmol kg−1)LeafTuberLeafTuberP2O5 (kg ha−1)00.04f0.06f2.802.95b460.08e0.13e2.723.19a920.18d0.22d2.863.19a1380.22c0.24c2.813.25a1840.26b0.29b2.713.33a2300.30a0.34a2.713.27aF test∗∗∗∗Ns∗LSD (5%)0.010.0130.140.15K2O (kg ha−1)00.180.22a1.79c2.14d1000.180.22a2.40b2.61c2000.180.21ab3.40a3.68b3000.180.21b3.49a4.35aF testNs∗∗∗∗∗LSD (5%)0.0120.010.11360.1247CV (%)9.906.976.125.82Means of the same main effect followed by the same letter or with no superscript letter within a column are not significantly different at 5% level of significance;∗∗ = significance at P<0.01 probability level; ∗ = significance at P<0.05 probability level; Ns = nonsignificant at P<0.05 probability level; LSD = least significant difference; and CV = coefficient of variation.The application of phosphorus did not affect the potassium levels in the plant’s leaf tissue. The two nutrients did not interact to affect potassium concentrations in potato crop leaf and tuber tissues. The application of phosphorus, however, significantly affected the potassium content of the tuber tissue (Table4). Increasing the phosphorus rate from nil to 46 kg·P2O5 ha−1 improved the potassium concentration in crop tuber tissue by around 8%. This could imply that phosphorus and potassium have a synergistic role in the plant’s metabolic processes. However, raising the phosphorus amount did not increase the nutritional concentration in tuber tissue (Table 5).The use of potassium fertilizer did not influence the phosphorus concentration in leaf tissue. However, it had a substantialP<0.05 effect on phosphorus content in tuber tissue and a highly significant P<0.01 influence on potassium concentration in both leaf and tuber tissues (Table 5). Increasing the potassium rate from 0 to 100, 200, and 300 kg·P2O5 ha−1 significantly raised potassium concentrations in leaf tissue by approximately 34, 90, and 95%, respectively. Similarly, potassium concentrations in tuber tissue increased by 22, 72, and 103%, respectively, in the sequence mentioned (Table 1). The highest tissue potassium content, however, was measured at 100 kg·K2O ha−1 (i.e., 4.5%) (Table 5). The results of the concentrations of the two nutrients observed in this experiment also revealed that the optimum yield obtained may be 10% or less than the maximum since other factors may have reduced the plant’s optimum uptake and utilization of the nutrients.The findings of this experiment revealed that the concentrations of phosphorus at the optimum level of fertilizer input (184 kg·P2O5 ha−1) were 0.26 ppm, which was nearly equivalent to the critical level of the nutrient in the leaf blade for optimum crop yield. Similarly, a sufficient concentration (4%) of potassium in the plant’s leaf tissue was documented at the potassium fertilizer level that resulted in optimum or near-optimum marketable and total tuber yields of 200 kg·K2O ha−1. At both nutrient levels, these findings may explain increased marketable and total tuber yields, as well as a large and medium section of tubers.


## 3.1. Preplanting Physicochemical Properties of the Soil

The results of a laboratory analysis of some physical and chemical parameters of the soil at the experimental site before planting are presented in Table1. The results suggest that the soil has a silt texture (49%) and is highly acidic in response, with a pH of 5.1. Furthermore, the experimental soil has a medium CEC (19.1 cmol·kg−1 soil), low total nitrogen (0.06%), accessible phosphorus (8.52 ppm), very low exchangeable potassium (0.12 cmol·kg−1 soil), and organic carbon (0.75%) (Table 1).Table 1
Soil’s physical and chemical properties of the experimental site before fertilization.
Soil depth (cm)Particle size (%)pH (H2O)OC (%)Available P (ppm)Total N (%)Exchangeable cations(cmol kg−1 soil)SandSiltClayK+Na+Ca++Mg++CEC0–301749345.10.758.520.060.1360.259.833.3719.1where OC = organic carbon; P = phosphorus; N = nitrogen; K = potassium; Na = sodium; Ca = calcium; Mg = magnesium; CEC = cation exchange capacity.


## 3.2. Soil Chemical Properties after Harvest

A study of soil fertility before and after cropping revealed that applying phosphorus and potassium fertilizers raised the CEC, accessible phosphorus, and potassium, indicating that the soil’s fertility state has improved (Tables1–3). As a result of the phosphorus application, soil chemical characteristics (CEC, available phosphorus, and exchangeable potassium) significantly improved. Similarly, except for available phosphorus, which was minor, exchangeable potassium and CEC changed dramatically with potassium fertilizer treatment (Table 3). CEC, accessible phosphorus, and exchangeable potassium levels were not affected by phosphorus and potassium additions to the soil (Tables 2 and 3).Table 2
Mean squares of soil fertility status after harvest as influenced by phosphorus and potassium applications.
Source of variationd.fAvailable phosphorus (ppm)Exchangeable potassium (cmol kg−1)CEC (cmol kg−1)Block25.19Ns0.0005Ns2.03NsP5259.95∗∗0.0013∗∗5.62∗∗K32.58Ns0.3228∗∗96.86∗∗P × K153.54Ns0.0003Ns1.18NsError462.330.00040.90where∗∗ = indicate significant difference at 1%; Ns = nonsignificant at 5% level of significance; d.f = degree of freedom; CEC = cation exchange capacity; P = phosphorus (P2O5); K = potassium (K2O).Table 3
Main effects of phosphorus and potassium fertilizers on available phosphorus, exchangeable potassium, and CEC status of the soil after harvest.
TreatmentsAvailable phosphorus (ppm)Exchangeable potassium (cmol kg−1)CEC (cmol kg−1)P2O5 (kg ha−1)06.52c0.23a20.71a466.60c0.21b20.45a927.43c0.21b19.18c13810.92b0.21b19.58bc18415.85a0.20b19.99ab23016.77a0.20b19.01cF test∗∗∗∗∗∗LSD (5%)1.250.020.7784K2O (kg ha−1)011.130.06d17.87c10010.660.15c17.88c20010.210.26b21.05b30010.730.36a22.48aF testNs∗∗∗∗LSD (5%)1.020.010.64CV (%)14.299.394.78Means of the same main effect followed by the same letter or with no superscript letter within a column are not significantly different at the 5% level of significance;∗∗ = significant at P<0.01 probability level; Ns = nonsignificant at P<0.05 probability level; CEC = cation exchange capacity; LSD = least significant difference; and CV = coefficient of variation.Increasing the phosphorus rate from nil to 46 and 92 kg·P2O5 ha−1 had no statistically significant effect on the soil’s available phosphorus concentration. However, increasing the phosphorus content from 0 to 138, 184, and 230 P2O5 ha−1 resulted in significant increases of 67, 143, and 157%, respectively (Table 3).Increased phosphorus application dramatically lowered available potassium and CEC. The exchangeable potassium was lowered by around 9% when the phosphorus rate was increased from zero to 46 kg·P2O5 ha−1. However, the decrease in exchangeable potassium among phosphorus-treated soils was not statistically significant. However, raising the phosphorus content from 0 to 92 kg·P2O5 ha−1 reduced the CEC by around 5.5%. However, plots that got 46 and 184 kg·P2O5 ha−1 were statistically equivalent to plots that received no phosphorus (Table 3).Unlike phosphorus, potassium increased the levels of exchangeable potassium and CEC. The exchangeable potassium increased by roughly 150% when the potassium level was increased from nil to 100 kg·K2O ha−1. Increasing the potassium level between 200 and 300 kg·K2O ha−1 raised it by around 333–500%, respectively. Similarly, increasing the potassium fertilization rate from nil to 200 and 300 kg·K2O ha−1 enhanced exchangeable potassium by around 18–26%, respectively. However, when compared with the control treatment, the application of 100 kg·K2O ha−1 did not affect the CEC (Table 3).


## 3.3. Phosphorus and Potassium Levels in Plant Tissues

The main effect of phosphorus significantlyP<0.01 influenced the concentration of phosphorus in the potato plant’s leaf as well as in the tuber tissue. The main effect of potassium significantly P<0.05 influenced the concentration of phosphorus in potato plant tuber tissues. However, the main effect of potassium did not affect the concentration of phosphorus in potato plant leaf tissue. The two factors did not interact to affect the concentration of phosphorus in the potato plant’s leaf and tuber tissue (Table 4).Table 4
Nutrient accumulation in potato leaves and tubers mean squares as influenced by phosphorus, potassium application rate, and their interaction.
Source of variationd.fPhosphorus content (ppm)Potassium content (cmol kg−1)LeafTuberLeafTuberBlock20.0013∗0.0002Ns0.071Ns0.112∗P50.1257∗∗0.1285∗∗0.047Ns0.202∗∗K30.0001Ns0.0009∗12.097∗∗18.272∗∗P × K150.0003Ns0.0004Ns0.041Ns0.013NsError460.00030.00020.0290.035where∗∗, ∗ = indicate a significant difference at 1% and 5% level of significance, respectively; Ns = nonsignificance at 5% level of significance; d.f = degree of freedom; P = phosphorus (P2O5); and K = potassium (K2O).Increasing the rate of phosphorus linearly resulted in significantly higher nutritional concentrations in both leaf and tuber tissues. Thus, raising the fertilizer rate from 0 to 46, 92, 138, 184, and 230 kg·P2O5 ha−1 significantly enhanced the nutrient concentration in leaf tissue by approximately 100, 350, 450, 550, and 650%, respectively. Similarly, phosphorus concentrations in tuber tissue increased by 117, 267, 300, 383, and 467% in the sequence mentioned (Table 5).Table 5
Phosphorus and potassium concentration levels of dried potato leaves and tuber tissues.
TreatmentsPhosphorus content (ppm)Potassium content (cmol kg−1)LeafTuberLeafTuberP2O5 (kg ha−1)00.04f0.06f2.802.95b460.08e0.13e2.723.19a920.18d0.22d2.863.19a1380.22c0.24c2.813.25a1840.26b0.29b2.713.33a2300.30a0.34a2.713.27aF test∗∗∗∗Ns∗LSD (5%)0.010.0130.140.15K2O (kg ha−1)00.180.22a1.79c2.14d1000.180.22a2.40b2.61c2000.180.21ab3.40a3.68b3000.180.21b3.49a4.35aF testNs∗∗∗∗∗LSD (5%)0.0120.010.11360.1247CV (%)9.906.976.125.82Means of the same main effect followed by the same letter or with no superscript letter within a column are not significantly different at 5% level of significance;∗∗ = significance at P<0.01 probability level; ∗ = significance at P<0.05 probability level; Ns = nonsignificant at P<0.05 probability level; LSD = least significant difference; and CV = coefficient of variation.The application of phosphorus did not affect the potassium levels in the plant’s leaf tissue. The two nutrients did not interact to affect potassium concentrations in potato crop leaf and tuber tissues. The application of phosphorus, however, significantly affected the potassium content of the tuber tissue (Table4). Increasing the phosphorus rate from nil to 46 kg·P2O5 ha−1 improved the potassium concentration in crop tuber tissue by around 8%. This could imply that phosphorus and potassium have a synergistic role in the plant’s metabolic processes. However, raising the phosphorus amount did not increase the nutritional concentration in tuber tissue (Table 5).The use of potassium fertilizer did not influence the phosphorus concentration in leaf tissue. However, it had a substantialP<0.05 effect on phosphorus content in tuber tissue and a highly significant P<0.01 influence on potassium concentration in both leaf and tuber tissues (Table 5). Increasing the potassium rate from 0 to 100, 200, and 300 kg·P2O5 ha−1 significantly raised potassium concentrations in leaf tissue by approximately 34, 90, and 95%, respectively. Similarly, potassium concentrations in tuber tissue increased by 22, 72, and 103%, respectively, in the sequence mentioned (Table 1). The highest tissue potassium content, however, was measured at 100 kg·K2O ha−1 (i.e., 4.5%) (Table 5). The results of the concentrations of the two nutrients observed in this experiment also revealed that the optimum yield obtained may be 10% or less than the maximum since other factors may have reduced the plant’s optimum uptake and utilization of the nutrients.The findings of this experiment revealed that the concentrations of phosphorus at the optimum level of fertilizer input (184 kg·P2O5 ha−1) were 0.26 ppm, which was nearly equivalent to the critical level of the nutrient in the leaf blade for optimum crop yield. Similarly, a sufficient concentration (4%) of potassium in the plant’s leaf tissue was documented at the potassium fertilizer level that resulted in optimum or near-optimum marketable and total tuber yields of 200 kg·K2O ha−1. At both nutrient levels, these findings may explain increased marketable and total tuber yields, as well as a large and medium section of tubers.


## 4. Discussion

### 4.1. Preplanting Physicochemical Properties of the Soil

The productivity of every crop is determined by its genetic potential, cultural practices such as soil fertility, and climatic conditions. Crop productivity and quality are mostly determined by balanced nutrition under certain agro-climatic conditions. Most soils in tropical and subtropical areas are deficient in nutrients due to nutrient depletion, soil erosion, and adverse soil characteristics such as acidity, salinity, and others that limit the efficiency of plant nutrient uptake by plants. As a result, productivity is low and food insecurity exists. Furthermore, poor nutrition, which causes a nutrient imbalance in plants, is a major contributor to low food production.The soil has a low level of soil-accessible phosphorus (8.52 ppm), according to Mengel and Kirkby [21]. The soil’s low phosphorus content is most likely due to the soil’s strong phosphorus-fixing capacity [36]. While the low potassium concentration may be related to the composition of the clay (kaolinite), which has a low potassium ion retention capacity and thus excessive cation leaching due to heavy precipitation in the studied area [21]. As such, the preplanting physicochemical soil characteristics imply that the experimental soil has some limitations in terms of crop performance. These data indicated that the soils required the external application of fertilizers following crop recommendations.


### 4.2. Soil Chemical Chemical Properties after Harvest

Higher phosphorus fertilizer applications enhanced phosphorus availability in the soil, despite the possibility of significant nutrient fixation. This also suggests that applying the fertilizer saturates the soil’s adsorption sites, allowing some of the nonlabile P to replace the P in the soil solution and make P available to plants. The lack of significant changes in available soil P in response to adding 46 and 92 kg·P2O5 ha−1 fertilizer implies that such fertilizer amounts were still insufficient to bring available P to the level required by plants to grow and develop normally and productively. According to Horneck et al. [37], the amounts of valuable soil P achieved beyond the application of 138 kg·P2O5 ha−1 was sufficient for potato crop growth and development using the Olsen technique.As the result indicated, the decrease in exchangeable potassium and CEC could be linked to the fact that the extent and type of charge on soil colloids impact a soil’s ability to retain important plant nutrients in the face of water moving through the soil profile. Due to the high precipitation in the study area as well as clay (kaolinite), which has poor potassium ion retention capacity, cation leaching increased as the negative charges in the soil increased, as Mengel and Kirkby [21] stated.On the other hand, due to the country’s lack of potassium use, excess nitrogen and phosphorus fertilizers may aggravate the situation, and continuous use would further deplete native soil potassium reserves and this coincides with Akhtar et al. [38]. Aside from depleting soil potassium, it will also reduce crop yield. As a mobile element, potassium does not remain in the soil for long after it is released. Furthermore, potassium in soil solutions is either fixed within the clay lattice or exchanged with the NH4+1 ion within the exchange complexes. Accordingly, potassium fertilizers applied at higher rates have a greater chance of fixing than fertilizers applied at lower rates. This is in accordance with Schneider et al. [39] and Taiwo et al. [40].


### 4.3. Phosphorus and Potassium Concentration Levels in Plant Tissues

The potato plants absorbed more nutrients as their availability in the soil solution increased. This signifies that the plants absorbed more potassium when the nutrient’s exchangeable content increased due to fertilizer treatment. The level of exchangeable K in the unfertilized soil inhibited plant growth, as demonstrated by soil test findings before planting. Similarly, the increase in phosphorus concentration in tuber tissue in response to increasing the potassium application rate may be related to the synergistic roles that the two nutrients play in plant metabolism. Potassium concentrations in the crop’s leaf blade and tuber tissue, however, increased significantly. In the present study, the concentrations of phosphorus (2.5%–6.0%) and potassium (3.5%–5.0%) were compared in potato leaves, based on Walworth & Muniz [41]. These are moderate ranges applicable to determining the P and K concentrations of leaves on potato plants grown in Ethiopia, according to Dechassa [42]. A reference range of tuber P and K concentrations was also used in this study, as reported by Walworth & Muniz [41].Untreated experimental soil had an exchangeable potassium content of only 0.117 cmol·kg−1 soil. This potassium level in the soil is exceedingly low for high crop output, and it should be at least 0.3 cmol·kg−1 soil for adequate crop growth and yield, as stated by Mengel and Kirkby [21], Reis and Monnerat [43], and Roy et al. [44]. As a result, the significantly increased potassium concentrations in both the crop’s leaf blade and tuber tissue may be attributed to the plant’s high uptake of the nutrient as a result of increased availability of the nutrient in the soil in response to the application of the fertilizer containing it. This may have enabled the plant to develop better and generate more tubers. The highest tissue potassium content, however, was measured at 100 kg·K2O ha−1 (i.e., 4.5%). The results of the concentrations of the two nutrients observed in this experiment also revealed that the optimum yield obtained may be 10% or less than the maximum since other factors may have reduced the plant’s optimum uptake and utilization of the nutrients. The critical concentration of a nutrient in plant tissues is the concentration of the nutrient in a particular plant part, usually, a mature leaf sampled at a given growth stage (usually tuber initiation for potato), below which plant growth and yield are suppressed by 5–10% according to Prado and Caione [45] or the tissue nutrient concentration resulting in maximum yield [46]. Critical phosphorus and potassium concentrations in potato leaf blade dry matter for optimum yield are typically 0.3 and 4.0%, respectively, as stated by Walworth and Muniz [41]. While according to Reis and Monnerat [43], a potassium concentration between 1.8 and 2.2% should be present in tuber tissue for optimal yield and quality. Nevertheless, the higher potassium content in tuber tissue in this study may be attributed not only to the potassium application but also to the plant’s age, which was 65 DAP, since tissue nutrient concentrations change with age. Similar results were found by Jackson and Haddock [47] as potassium levels in tubers decline with aging. Beside this, the reported tissue nutrient contents differ due to differences in tissue sampling techniques, tissue types, and potato varieties used in the study, as Walworth and Muniz [41] indicated.On the contrary as Hawkins [48] noted that tuber phosphorus levels decreased throughout the entire growing season, starting at 0.57% early on and declining to 0.16% after harvest. The early season phosphorus concentration levels range between 0.57 and 0.22%, the midseason (70–74 DAP) values range between 0.29 and 0.21%, the late season (91–95 DAP) values range between 0.18 and 0.16%, and the harvest (112–152 DAP) tuber K levels range between 0.16 and 0.14% [47, 48]. Hernandez et al. [49]and Barben et al. [50] indicated that such phosphorus deficiency reduces potato tubers’ size and yield, slows plant growth, and affects the quality of the tubers by causing a decrease in starch production and causing necrotic spots on the tubers [51].


## 4.1. Preplanting Physicochemical Properties of the Soil

The productivity of every crop is determined by its genetic potential, cultural practices such as soil fertility, and climatic conditions. Crop productivity and quality are mostly determined by balanced nutrition under certain agro-climatic conditions. Most soils in tropical and subtropical areas are deficient in nutrients due to nutrient depletion, soil erosion, and adverse soil characteristics such as acidity, salinity, and others that limit the efficiency of plant nutrient uptake by plants. As a result, productivity is low and food insecurity exists. Furthermore, poor nutrition, which causes a nutrient imbalance in plants, is a major contributor to low food production.The soil has a low level of soil-accessible phosphorus (8.52 ppm), according to Mengel and Kirkby [21]. The soil’s low phosphorus content is most likely due to the soil’s strong phosphorus-fixing capacity [36]. While the low potassium concentration may be related to the composition of the clay (kaolinite), which has a low potassium ion retention capacity and thus excessive cation leaching due to heavy precipitation in the studied area [21]. As such, the preplanting physicochemical soil characteristics imply that the experimental soil has some limitations in terms of crop performance. These data indicated that the soils required the external application of fertilizers following crop recommendations.


## 4.2. Soil Chemical Chemical Properties after Harvest

Higher phosphorus fertilizer applications enhanced phosphorus availability in the soil, despite the possibility of significant nutrient fixation. This also suggests that applying the fertilizer saturates the soil’s adsorption sites, allowing some of the nonlabile P to replace the P in the soil solution and make P available to plants. The lack of significant changes in available soil P in response to adding 46 and 92 kg·P2O5 ha−1 fertilizer implies that such fertilizer amounts were still insufficient to bring available P to the level required by plants to grow and develop normally and productively. According to Horneck et al. [37], the amounts of valuable soil P achieved beyond the application of 138 kg·P2O5 ha−1 was sufficient for potato crop growth and development using the Olsen technique.As the result indicated, the decrease in exchangeable potassium and CEC could be linked to the fact that the extent and type of charge on soil colloids impact a soil’s ability to retain important plant nutrients in the face of water moving through the soil profile. Due to the high precipitation in the study area as well as clay (kaolinite), which has poor potassium ion retention capacity, cation leaching increased as the negative charges in the soil increased, as Mengel and Kirkby [21] stated.On the other hand, due to the country’s lack of potassium use, excess nitrogen and phosphorus fertilizers may aggravate the situation, and continuous use would further deplete native soil potassium reserves and this coincides with Akhtar et al. [38]. Aside from depleting soil potassium, it will also reduce crop yield. As a mobile element, potassium does not remain in the soil for long after it is released. Furthermore, potassium in soil solutions is either fixed within the clay lattice or exchanged with the NH4+1 ion within the exchange complexes. Accordingly, potassium fertilizers applied at higher rates have a greater chance of fixing than fertilizers applied at lower rates. This is in accordance with Schneider et al. [39] and Taiwo et al. [40].


## 4.3. Phosphorus and Potassium Concentration Levels in Plant Tissues

The potato plants absorbed more nutrients as their availability in the soil solution increased. This signifies that the plants absorbed more potassium when the nutrient’s exchangeable content increased due to fertilizer treatment. The level of exchangeable K in the unfertilized soil inhibited plant growth, as demonstrated by soil test findings before planting. Similarly, the increase in phosphorus concentration in tuber tissue in response to increasing the potassium application rate may be related to the synergistic roles that the two nutrients play in plant metabolism. Potassium concentrations in the crop’s leaf blade and tuber tissue, however, increased significantly. In the present study, the concentrations of phosphorus (2.5%–6.0%) and potassium (3.5%–5.0%) were compared in potato leaves, based on Walworth & Muniz [41]. These are moderate ranges applicable to determining the P and K concentrations of leaves on potato plants grown in Ethiopia, according to Dechassa [42]. A reference range of tuber P and K concentrations was also used in this study, as reported by Walworth & Muniz [41].Untreated experimental soil had an exchangeable potassium content of only 0.117 cmol·kg−1 soil. This potassium level in the soil is exceedingly low for high crop output, and it should be at least 0.3 cmol·kg−1 soil for adequate crop growth and yield, as stated by Mengel and Kirkby [21], Reis and Monnerat [43], and Roy et al. [44]. As a result, the significantly increased potassium concentrations in both the crop’s leaf blade and tuber tissue may be attributed to the plant’s high uptake of the nutrient as a result of increased availability of the nutrient in the soil in response to the application of the fertilizer containing it. This may have enabled the plant to develop better and generate more tubers. The highest tissue potassium content, however, was measured at 100 kg·K2O ha−1 (i.e., 4.5%). The results of the concentrations of the two nutrients observed in this experiment also revealed that the optimum yield obtained may be 10% or less than the maximum since other factors may have reduced the plant’s optimum uptake and utilization of the nutrients. The critical concentration of a nutrient in plant tissues is the concentration of the nutrient in a particular plant part, usually, a mature leaf sampled at a given growth stage (usually tuber initiation for potato), below which plant growth and yield are suppressed by 5–10% according to Prado and Caione [45] or the tissue nutrient concentration resulting in maximum yield [46]. Critical phosphorus and potassium concentrations in potato leaf blade dry matter for optimum yield are typically 0.3 and 4.0%, respectively, as stated by Walworth and Muniz [41]. While according to Reis and Monnerat [43], a potassium concentration between 1.8 and 2.2% should be present in tuber tissue for optimal yield and quality. Nevertheless, the higher potassium content in tuber tissue in this study may be attributed not only to the potassium application but also to the plant’s age, which was 65 DAP, since tissue nutrient concentrations change with age. Similar results were found by Jackson and Haddock [47] as potassium levels in tubers decline with aging. Beside this, the reported tissue nutrient contents differ due to differences in tissue sampling techniques, tissue types, and potato varieties used in the study, as Walworth and Muniz [41] indicated.On the contrary as Hawkins [48] noted that tuber phosphorus levels decreased throughout the entire growing season, starting at 0.57% early on and declining to 0.16% after harvest. The early season phosphorus concentration levels range between 0.57 and 0.22%, the midseason (70–74 DAP) values range between 0.29 and 0.21%, the late season (91–95 DAP) values range between 0.18 and 0.16%, and the harvest (112–152 DAP) tuber K levels range between 0.16 and 0.14% [47, 48]. Hernandez et al. [49]and Barben et al. [50] indicated that such phosphorus deficiency reduces potato tubers’ size and yield, slows plant growth, and affects the quality of the tubers by causing a decrease in starch production and causing necrotic spots on the tubers [51].


## 5. Conclusion

The results indicated that phosphorus application had highly significant effects on available phosphorus, exchangeable potassium, and CEC, whereas potassium did the same except for available phosphorus. Phosphorus fertilizer application enhanced soil accessible phosphorus while decreasing CEC and exchangeable potassium. Unlike phosphorus, potassium increased exchangeable potassium and CEC levels. However, the interaction effect of potassium and phosphorus on all of the above-mentioned soil chemical characteristics was insignificant.The effect of phosphorus profoundly changed the concentration of phosphorus in the leaf and tuber tissue of the potato plant, as well as the potassium content of the tuber. While potassium had a significant influence on the concentration of phosphorus in potato plant tuber tissue and the potassium level in both leaf and tuber, the two nutrients did not affect the concentrations of phosphorus and potassium in potato plant leaf and tuber tissue. The findings of this experiment revealed that the concentration of phosphorus at 184 kg·P2O5 ha−1 was 0.26%, which was nearly equivalent to the critical level of the nutrient in the leaf blade for optimum crop yield. Similarly, a sufficient concentration (4%) of potassium in the plant’s leaf tissue was documented at the potassium fertilizer level of 100 kg·K2O ha−1.



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*Source: 5342170-2022-12-15.xml*