Most soils cannot supply all essential plant nutrients in sufficient amounts to support good growth of crops, and hence, the application of fertilizer is one of the most effective means to increase nutrient uptake in crop plants and improve yields [1]. Agriculture and food production are dependent on a phosphorus (P) supply to produce adequate food, fibre and fuel for society [2]. However, P is often an important limiting factor for crop yields, so one of the ways to tackle these issues is improving phosphorus use efficiency (PUE).

According to Vita, Ethiopia has a high potential for potato production with 70% of the 13.5 million ha of arable land suitable for potato cultivation [3]. Furthermore, root crops are good sources of cash and foreign exchange for most smallholder farmers in Ethiopia [4]. However, potato is widely regarded as a secondary non-cereal crop in part because it has never reached the potential that it has in supporting food security. In addition, low soil fertility and crop nutrient imbalances are two major obstacles preventing Ethiopia’s farmers from realizing the high agricultural productivity [5].

When fertilizer prices represent a large portion of a producer’s costs, it is very important to maximize fertilizer use efficiency. Fertilizer use efficiency by most crops and farming systems is still very poor in most developing countries like Ethiopia. Therefore, improving agronomic efficiency provides both direct and indirect economic benefits and hence larger yield increases can be achieved for a given quantity of fertilizer applied. Integrated nutrient management (INM) by involving the combination of organic manure and fertilizers is an essential tool for balanced fertilization and sustainability of crop production on long term basis [1].

The Southern Nations, nationalities and Peoples (SNNP) region of Ethiopia contributed (22.93%) of the area under potatoes and (38.71%) production to the country’s total. Sidama Zone is one of the major potato producing zones in the SNNPR. The farmers in southern Ethiopia use traditional soil fertility improvement mechanisms such as intercropping and farmyard manure application rather than depending only on inorganic fertilizer [4]. However, organic manure alone is not sufficient (and often not available in large quantities) for the level of crop production, the farmer is aiming at. Hence, additional mineral fertilizers must be applied [6]. However, there is a lack of scientific studies to examine the effect of separate and combined applications of organic and inorganic fertilizers in improving nutrient uptake efficiency of potatoes in Sidama highlands. This study, therefore, undertaken in the Arbegona district with the objectives: and (i) to evaluate the effect of sole and combined application of FYM and inorganic fertilizers on soil phosphorous (P) balance (ii) to identify P levels in dry matter accumulation and use efficiency in response to different rate of the applied FYM and inorganic fertilizer.

Description of the study area

This study was carried out in Arbegona district of Sidama zone, Southern Ethiopia. The woreda (Arbegona) geographically lies between 6°35’18” to 6°56’37”N and 38°35’60” to 38°53’36”E (Figure 1). The experimental site is located at 74 km east of Hawassa town at geographic coordinate of 06°41’ N and 38°43’ E with elevation of 2,521 masl. The study area has a mean annual rainfall of 1400 mm and a mean annual reference evapotranspiration of 1123 mm.

Figure 1: Geographical location of the study area in Sidama zone of Southern Ethiopia.

According to FAO 1998 soil classification system, the study district has dominantly three soil types of chromic Luvisols, eutric Vertisols and humic Nitosols. However, the trial site is humic Nitosols.

Experimental design and field experimentation

Based on the recommended dose of nitrogen (N) for potato, considering only 50% N availability from FYM, the available N were calculated and treatment setup formulated to the applied full dose of FYM and the combine treatments of half FYM with half of inorganic fertilizers [7]. The field experiment laid out in a Randomized Complete Block Design (RCBD) with five treatments (nutrient management options) and one control (Table 1) replicated three times within a farm. Blended fertilizer formula identified for the woreda containing the most deficient nutrients (NPSZnB+K) selected based on the nutrient map produced [5].

Table 1: Treatments set up for potato productivity, nutrient uptake and use efficiency as influenced by soil amendments.

Farm yard manure that was used for the experiment had 28.75% moisture content on average correction factors. Nutrients in FYM other than NPK not considered and 100 kg blended has (17.3 N₃, 4.7 P₂O₅, 7.41 S₂, 23 Zn0, 3 B within 100 kg) and 100 kg K from (KCl).

The experiments carried out under rain-fed conditions in the season of summer to autumn (Bega to Belg season) of 2015/16. Irish potato (Solanum tuberosum L.) variety of Belete used as a test crop with spacing of 75 and 30 cm between rows and plants, respectively. The total plot size was 3.75 m wide and 3 m long having 1 m space between the blocks. The FYM and inorganic fertilizer banded in an open row on the ridges of the potatoes at 10 cm from the seeds covered by 2 cm layer of soil. The recommended rates of P, and K, S, Zn and B applied at planting, whereas N applied in split: half at planting and the remaining half at about 35 days after planting. FYM, Urea, DAP, KCl and blended (NPSZnB+K) fertilizers were used as sources of the nutrients and applied as banded. First earthing-up or hilling and weeding followed immediately after urea application. The second weeding and hilling done 8 weeks after planting and the plants harvested at maturity.

Data collection and analysis

Soil and plant samples collection and preparation: Before the start of the experiment, twenty surface soil sub-samples (0-20 cm) were randomly collected and composited for laboratory analyses of texture and selected chemical soil properties. After harvest, six surface soil subsamples collected from each plot (three per ridge and three per row) and a composite made for each treatment.

Tubers dry weights was determined after drying in oven at 65°C to a constant weight. Five-hundred-gram fresh weight used for determination of dry matter content. Uptake of N use efficiency and concentration was done separately for shoots (stems and leaves), roots plus stolen. The nutrient accumulation and partitioning were calculated by multiplying nutrient concentration with the dry matter of the respective plant parts and the uptake of N in economic and by-product of potato plant parts were estimated following the procedure outlined [8,9].

Soil and plant analyses: Determination of particle-size distribution was done by using hydrometer procedure as outlined by Sahlemedhin and Taye. The soil pH was potentio-metrically measured using pH meter in supernatant suspension of a 1: 2.5 soil to water mixture.

Total nitrogen analyzed by wet-oxidation procedure of the Kjeldahal methods. Available P determined by Bray II method; Available K, determined by Morgan’s extracting solution. Soil organic carbon (OC) was determined by the Walkley and Black method the procedure as outlined by Sahlemedhin and Taye. Plant samples subjected to the analysis of total N concentration using the above listed methods for soil N analysis procedure. The oven dried (65°C) plant samples which passed through 200 mesh sieves were used [10].

Calculation of plant nutrient uptakes and efficiencies

The nutrients use efficiencies were calculated by the formulas [2,9,10]

• Agronomic efficiency (AE): AE=(Gf-Gu)/Na (eq. 1)

where Gf and Gu are tubers yield from the fertilized and unfertilized plot (kg), respectively and Na is the quantity of nutrient applied (NPK) (kg).

• Physiological efficiency (PE): PE=(Gf-Gu)/(Ntf-Ntu) (eq. 2)

where Gf and Gu as described above, and Ntf and Ntu are the nutrient (NPK) (kg) accumulation by (total shoot and tuber) in the fertilized and unfertilized plot, respectively.

• Apparent recovery (AR): AR=(Nf-Nu) × 100%/Na (eq. 3)

where Nf and Nu are the nutrient accumulation by the total biological yield (shoot and tuber) in the fertilized and unfertilized plot (kg).

• Partial Phosphorous Balances: The assessment of P balances was considering P added from fertilizer as inputs and P taken up by total crop materials (economic and biological yield) as out from the soil system [11,12].

Statistical analysis: Data collected on yields uptake of N and use efficiency in economic and by-product of potato plant parts subjected to analysis of variance (ANOVA). SAS software (version 9.0) was used and the treatments comparison were made with LSD (P ≤ 0.05) level of significant.

Effect of soil amendments on chemical properties

The soil of the study area was loamy textured (Table 2). Loam textured soils are usually considered to be fertile and can hold sufficient available water indicated that such types of soils are often found in mountainous regions and are suitable to grow potatoes. The soil reaction rated as very strongly acidic (Table 2). Analysis of the experimental soil before planting revealed that it was low in OC, medium in total N, low in available P and medium in K contents in accordance with the rating of the same author for tropical soils (Table 3). Similarly, soil pH and available P of the study area in line that grouped OC contents ranging between 2.00 and 2.99 as very high [5,13,14]. On the other hand, after harvest, available P was significantly (P ≤ 0.05) higher at application of sole FYM, sole blended fertilizer and their half-combined usage over that of the control and the other amended plots (Table 2). This reveals that applying FYM is important at least partially address the problem of P fixation in the strongly acidic soils of the study area and the consequent low crop productivity due to P shortage.

Table 2: Selected soil chemical properties before planting and FYM.

Table 3: Partial P balances in soil after harvest as influenced by soil amendments at Arbegona area of Sidama zone, Southern Ethiopia.

Partial soil phosphorous balances: Plots treated with RNP and FYM alone had net negative available P balances, while positive available P balances were obtained from all treatments including the control (Table 2). The positive P balance in the control might be attributed both available nutrients replenished in more quantities from the soil exchangeable site as compared to the uptake. Similar findings in net available P balances was reported by [15]. Farm yard manure is a good source of nitrogen, phosphorus and potassium (NPK) and other macro- and micro-nutrients. In long term application of FYM, the nutrient supplying power of the soil is improved, so that treatment with FYM may be superior to an equivalent application of mineral fertilizer. In many soils, the rate of removal of plant nutrients by crops, leaching and denitrification is well more than nutrient release by weathering and mineralization. Potato is a high nutrient feeder. A negative nutrient balance thus resulted unless nutrients supplied in the form of fertilizers or manures to make up the difference [16]. The observed net negative P balances might have been imposed by low P availabilities coupled with high uptake. Loss of phosphorous might have also occurred in plots [7,17].

Effect of soil amendments on P concentration and uptake in different parts of potato

Significant (P ≤ 0.05) differences in P uptake by potato plant parts were observed due to the use of different amendments (Table 4). The lowest P uptakes in the shoot and tubers were exhibited by the control while significantly higher P uptakes were measured all plants received different treatments. The highest shoot and tuber P uptakes were recorded in plants treated with blended fertilizer alone and half FYM combined with either half RNP or half blended fertilizer over the control and other amendments (Table 5). Similar findings were reported and the differences in uptakes were attributed to the supply of P from the amendments and/or the varietal difference [18-20]. The quantities of P taken up by shoot and tubers were more than double (100%) with applications of sole blended fertilizer, half FYM combined with half RNP and integrated half FYM and blended fertilizer over the control. The uptake of P might have been pronounced by availability of K and other micronutrients (S, Zn and B) from the blended fertilizer. Significant (P ≤ 0.05) and lowest P concentration in the shoots and roots of potato were recorded in control plant followed by the plants treated with recommended rate of NP while the remaining treatments exhibited significantly higher concentrations than control and RNP treatment (Table 4). The highest P concentrations in shoots and roots were obtained from plants treated with sole blended fertilizer and half FYM in conjunction with either half RNP or half blended fertilizer and sole FYM equally recorded the highest P concentrations in shoots.

Table 4: Effect of soil amendments on P concentration and uptake in potato plant materials as influenced by soil amendments at Arbegona area of Sidama zone, Southern Ethiopia.

Table 5: Effect of soil amendments on P use efficiency of potato as influenced by soil amendments at Arbegona area of Sidama zone, Southern Ethiopia.

However, the highest P concentration in tubers was recorded with sole application of blended fertilizer and half FYM and blended fertilizer. The increments in P uptake and concentrations in the plant parts might be attributed to synergistic effects of K and other micronutrients from blended fertilizer. Similar findings were reported by Neshev and Manolov and who observed that the application of K in the form of K₂SO₄ led to increase of P uptake by potatoes [21].

According to Mengel and Kirkby the nutrient content of plant tissue reflects soil availability [16]. The low P uptake and concentrations in plant materials of the control might therefore be attributed to low P availability in the experimental soil; as was also confirmed by soil analysis before planting (Table 2). Additionally, phosphorus availability to plants is determined by the chemical characteristics of the soil and the P fertilizer source [7,22]. Thus, the use of blended fertilizer alone and in combination with FYM could be considered as best source for P availability.

Phosphorous efficiency as influenced by soil amendments

Agronomic Efficiency (AE) of P: Agronomic efficiency of P was significantly (P ≤ 0.05) lower at application of RNP as compared with all other amendments. The highest values were obtained when half FYM was used in conjunction with either half RNP or half blended fertilizer over RNP. Similar findings on AE of P were reported when P supplied from organic and inorganic sources and the authors stated that AE of P decreased when P fertilization increased [9,20,23].

Physiological Efficiency (PE) of P

Sole application of blended fertilizer resulted in significantly (P ≤ 0.05) lower phosphorous PE than that of other amendments except the combined application of half FYM with half blended fertilizer. Fertilization with sole FYM increased PE of P by 29% against the plot treated with blended fertilizer. Phosphorous PE in the present study was in agreements with the reports of P level and variety experiments in different cropping system [9,20]. These authors showed that at high P uptake the PE of crops decreases and the PE of P would depend on varieties. Nutrient efficiency of plants depends on: soil factor of production potential, chemical species of the fertilizer used and plant factors of interaction with environmental factor and root microbe [20,22,24].

Apparent Recovery (AR) of P

Lowest apparent recovery of phosphorous at (P ≤ 0.05) was attained in plots amended with RNP alone. Additionally, plots amended with sole FYM application showed significantly lower ARE of P as compared to those with combined applications of half FYM with half NP, combined half FYM and blended and sole blended fertilization. Phosphorous ARE was improved four times and more than threefold, respectively at application of combined half FYM and blended fertilizer over RNP. Phosphorous ARE is in line with the findings indicate that the level of nutrient fertilization affects the nutrient availability in soil, and at high contents of soil nutrients and their availability more nutrients might be taken up by plants [9,20,23]. It has been reported that variation in ARE is common due to the dissimilarities of cultivars [20,22,23]. These authors have also discussed that depending upon the nutrient absorption power of the crops and their utilization at the biochemical levels, crops may vary in the recovery of the applied nutrients.

Summary and Conclusion

Combined application of FYM with inorganic fertilizer better means to maintain soil P balance as the same time resulted significantly the highest tuber yield. Potato plants grown in soils amended with sole blended fertilizer, half FYM combined with RNP and half FYM and blended fertilizers improved dry tubers yield and total P uptakes more than twofold (100%) as compared to the control. In the amendments, on average agronomic efficiency of 138 kg dry tubers kg^-1 was attained for P supply. In the same way, physiological efficiency of 238 kg dry tubers kg^-1 was attained for P uptake. Furthermore, apparent recovery of 59% across treatments was obtained from P supply. The recoveries of and K was above 100%, indicating that the nutrients were depleted from the soil reserve, and the importance of K management. Generally, soil amendments either in sole or combined inorganic and organic fertilizers could be considered to improve soil fertility status and potato yield in the study area. In the mixed crop-livestock farming system of the Sidama highlands, where livestock is an important component of their livelihood, cattle manure is abundantly available. Hence, they should be encouraged to utilize widely FYM to amend the inherently poor and very acidic soils. Further researches need to be undertaken to replicate the study in a wider spatial and temporal aspects.

I would like to forward my deepest thanks to Southern Agricultural Research Institute (SARI) for its’ sponsorship and also to the Livestock and Irrigation Value chains for Ethiopian Smallholders (LIVES) project of the International Livestock Research Institute (ILRI) for financially supporting the field research.