Quantification of soil nutrient balance and stock on smallholder farms at Agew Mariam watershed in northern Ethiopia

Abstract

Soil fertility has been declining in many parts of Ethiopia, moreover limits agricultural production, sustainability and food security. Nutrient balance is used to evaluate the state of soil fertility, rate of nutrient depletion, sustainability of land productivity, as well as to take the appropriate management decisions. This study was conducted to quantify soil nutrient balance and stocks on smallholder farms at Agew Mariam watershed, in northern Ethiopia in the 2020/21 season. The inflows-outflows of NPK into, and out of barley, tef, and wheat farms were determined through, field measurement, laboratory analysis and interviews. The nutrient balance in each crop was quantified by subtracting nutrient outputs from the inputs. The N partial balance of barley, tef and wheat was −66, −9.8, and −50.7 kg ha⁻¹ yr⁻¹fields, respectively. The P balance was also −5.9, 0.9, and −2.6 kg ha⁻¹ yr⁻¹ for barley, tef, and wheat fields, respectively. The K balance was −12.3, −3.2, and −5.4 kg ha⁻¹ yr⁻¹ in barley, tef, and wheat fields, respectively. The analysis revealed that N, P, and K had negative values except for P in tef. The stock of N was 1295, 1510, and 1240 in barley, tef, and wheat kg ha⁻¹fields, respectively. The P stock was 63, 18.7, and 27.5 kg ha⁻¹ in barley, tef, and wheat farms, respectively. Similarly, K stock was 1092.7, 1059.4, and 1090.6 kg ha⁻¹ in barley, tef, and wheat cropping systems, respectively. Reversing the imbalance between inflows and outflows via adding organic and inorganic fertilizer is essential for barley, tef, and wheat cropping systems in the study area.

1. Introduction

Soil fertility limits agricultural production in Ethiopia including the study area [1]. However, soil fertility level has been declining as a result of low fertilizers application, poor land management practice, and soil erosion [[2], [3], [4]]. Similarly, continual nutrient elimination via crop harvests with inadequate substitutes depletes the nutrients [5]. Soil nutrient depletion negatively affects agricultural productivity [6], sustainability [[7], [8], [9]], and food security [10,11]. Hence soil fertility management is a serious issue for farmers and researchers as soil properties vary spatially and temporally [12]. This is because crop production and productivity improvement mainly depend on soil nutrient management [13]. To reverse such nutrient management practices immediate and proper corrective measures should be done on time in place [14].

Soil nutrient balance is the summation difference between nutrient inflows and outflows within a particular framework over a certain period [[15], [16], [17]]. It is used to identify the present status of agricultural cultivated fields, soil health levels, and to take appropriate measures [18,19]. Likewise, it is an indicator of whether soil fertility is being maintained, improved, or degraded [17]. Similarly, it serves as an indicator of management practices, the sustainability of the farm, and the systems [20]. In Ethiopia, many studies on nutrient balance showed a negative balance [9,21]. This was due to the ineffective use of locally available nutrient resources, and the high cost of synthetic fertilizers [22,23]. Soil nutrient stock is the accumulation of plant nutrients in the soil that can be available to plants from 5 to 10 years [24]. Therefore, prudent nutrients management strategies for better crop yield and sustainability are indisputable.

In the study area, there is poor crop residue management, low organic and inorganic input addition, then low agricultural production and productivity [25]. Regarding to this, the productivity of barley, tef, and wheat were 2078, 550.5, and 1021 kg ha⁻¹ fields, respectively. That leads food insecurity and continuous refuge [26]. This study was initiated to quantify partial nitrogen, phosphorous, and potassium balance and stock of smallholder farms at Agew Mariam watershed in northern Ethiopia. Mainly to assess the sustainability of the farms, and to tackle the increasingly severe soil fertility decline. Based on the study result to identify most effective nutrient stock maintenance practice. The generated nutrient balance information is critical for decision makers to plan and implement integrated nutrient management strategies at a watershed level.

2. Materials and methods

2.1. Description of the study area

The study was conducted in the Waghimera Administrative zone, Amhara National Regional State, Ethiopia in the 2020/21 season. It is located from 38° 53′14″to 38° 56′15″ longitude and 12° 31′40″ to 12° 32′ 33″ latitude with an altitude of 2104 to 2361 m. a. s. l. It found 720 km from Addis Ababa, and 20 km south of Sekota town in Sayda kebele of Sekota district (Fig. 1). The watershed was delineated in 2016 by Sekota Dryland Agricultural Research Center as a model watershed for agricultural technology generation, adaptation, and dissemination. It has an area of 147 ha.

Fig. 1.

Location map of Agew Mariam watershed.

The study area has uni-modal rainfall pattern that extends from the early July to early September, the mean annual rainfall was 590 mm. While the mean annual minimum and maximum temperatures were 13 °C and 27 °C, respectively from 2000 to 2020 years (Agew Mariam and Kombolcha metrological station). The area belongs to dry semi-arid midland [25]. The soil types of Sekota district including the study watershed are: Nitisols, Vertisols, eutric Regosols, and eutric Cambisols. There are four land-use types; these are cultivated land (71.4%), bush land (19.7%), area closure (8.2%), and residence (0.7%) [27]. The farming system is characterized by subsistence mixed crop production and livestock husbandry. The major grown crops are Bread wheat (Triticum aestivum L.), sorghum (Sorghum bicolor L.), teff (Eragrostis tef (Zucc.) Trotter), barley (Hordeum vulgar L), and faba bean (Vicia faba L.). The area has a high potential for livestock production including cattle, apiculture, poultry, goat, sheep, and donkey.

2.2. Data collection and analysis methods

The representative 23 smallholder farms were selected by purposive random sampling technique considering slope, wealth, and fertility level, farmers’ field used as a replication. The field sizes were barley 25 m × 40 m (10⁴ m²), tef and wheat 30 m × 30 m (9 × 10² m²) each. The numbers of sampled farms were wheat 10, barley 3, and 10 tef selected based on their dominance coverage. The farms covered by barley in the watershed was less, then, the sample size was lower than tef and wheat. The inflows of N, P, and K through mineral and organic fertilizers were determined by interviewing the farmers. While the outflows through above ground biomass were measured by multiplying the amount of grain and straw by its nutrient contents [28]. The data of grain, and straw yield were collected directly from the entire whole field using hanging balance. Moreover, the N, P, and K nutrient contents of the crops were analyzed in the laboratory, based on their standard procedures.

One kg composite soil sample was collected by auguring from ten subsamples from each sampled farms at a depth of 0.2 m. The samples analyzed for N, P, and K contents, soil SOC, and soil separate particles. The collected composite soil samples were analyzed at Sekota Dryland Agricultural Research Center and Amhara Design and supervision works Enterprise soil laboratory (ADSWE). The soil was air-dried and sieved through 2 mm sieve for NPK and textural class analysis, but for SOC analysis it was sieved at 0.5 mm. Total nitrogen was determined by the Kjeldahl method [29]. Available potassium determined by Morgan's solution and K in the extract was measured by a flame photometer [30] and available phosphorus was determined following the Olsen method [31]. Soil bulk density is the weight of a dry soil per unit of soil volume and its values were calculated based on the core method. Drying undisturbed soil samples by oven dry at 105 °C for 24 h, then divided the dry soil by its volume [32]. Soil texture was analyzed through the hydrometer method [33,34]. Soil organic carbon was determined following the wet digestion method as described in Ref. [35].

Additionally, undisturbed soil samples were collected from the selected farms using core sampler for the analysis of bulk density. The inputs of N and P obtained from NPSZnB and urea fertilizers, the total P inflows in kg ha⁻¹ by multiplying the amount of P₂O₅ by 0.44. But, the total quantity of applied commercial urea fertilizer was changed into elemental nitrogen amount [23]. The data collected in field surveyed, laboratory analysis of soil and plant inputs, outputs, and stocks were analyzed by analysis of variance using SAS software version 9.1, and the mean separation was analyzed by using 0.05 least significance difference level (LSD). Additionally, the mean and standard deviation of the data estimated by Microsoft Excel spreadsheets.

2.3. Plant sample analysis

The plants harvested manually on their maturity dates were preferred by the farmers. Crop biomasses were weighed using a hanging balance. Similarly, the grain yield of barley, tef, and wheat was collected and measured. Representative straw samples were taken into oven-dry at 65 °C for 72 h to avoid moisture contents. Then both straw and grain were taken for laboratory analysis [31]. Plant tissue (grain and straw) was air-dried and grinded to pass through a 0.15 mm mesh [36,37]. The concentrations of the total nitrogen in the plant were determined by micro-Kjeldahl digestion, distillation, and titration method [29]. Phosphorous and potassium concentration were measured by spectrophotometer and flame photometry respectively and determined with the procedure described by Ref. [38].

2.4. Soil nutrient balance analysis

The partial N, P, and K balance was quantified by using [15,16] model. Partial soil NPK balance = inputs of NPK through (organic and inorganic fertilizers) – outputs of NPK (via grain and strain). Soil N, P, and K stocks was estimated based on the [39] model. The stock of N, P, and K (kg ha⁻¹) = bulk density (kg m⁻³) x soil sampling depth (m) x respective concentration of soil N, P and K content (kg kg⁻¹) x area 1ha (10⁴ m²). Inputs data like N, P and K nutrient contents of the soil, bulk density, and sampling depth were collected in the field survey and laboratory analysis.

3. Results and discussion

3.1. Inflow of nutrients

Cereals barley, tef, and wheat were the major grown crops in the Agew Mariam watershed. The result of this study revealed that the addition of nutrients into the farms through mineral and organic fertilizers sources was very low. Only a few farmers (30.4%) use inorganic fertilizers for the production of tef, barley, and wheat which was ratified by this research through interviews. The farmers applied only synthetic fertilizers in the form of NPSZnB and urea for the production of field crops. The average used nutrient for the barley was 7.7 kg N ha⁻¹yr⁻¹ but no addition of P fertilizer sources (Table 1). For tef 5.1 and 2.6 kg ha⁻¹ yr⁻¹ N and P were used, respectively, in wheat 3.1 and 2.7 kg ha⁻¹ yr⁻¹ N, and P added, respectively. Since proper nutrient management is very critical to increase crop production and sustain soil productivity [40]. Although there was no K fertilizer addition has been done for each study crop farms. The added N and P amounts could not meet the crops' optimum requirement of nutrients for better production. The recommended amount of nitrogen and phosphorous for tef and wheat to the area were 92 and 10 kg ha⁻¹ N and P, respectively [41,42]. However, the crops in the study area had no responses to K fertilizer application on crop yields, since the soil had optimum K amount [43]. The result aggresses with [44] who reported K had no yield advantage on wheat, tef and maize yield in northwest Ethiopia. But, this statement disagrees with the findings of [45,46] who reported K fertilizer had responses on wheat yield in Chencha, Hagere Selam and Tsegede in Southern and northern Ethiopia.

Table 1.

Nutrient inflows into the farmlands of the study watershed (kg ha⁻¹ yr⁻¹).

Crop field	IN1			IN2
	N	P	K	N	P	K
Barley	7.7 ± 13.3	0	0	0	0	0
Tef	5.1 ± 6	2.6 ± 1.5	0	0	0	0
Wheat	3.1 ± 5	2.7 ± 0.5	0	0	0	0

Where IN1 refers to inputs from mineral fertilizer, IN2 stands for inputs from organic fertilizer.

The amount of added nutrients to the study area was low even as compared with other part of Ethiopia [[47], [48], [49]]. This might be related to the poor dissemination of mineral fertilizers in the study area. Most of the farmers in the study area could not afford the money to purchase and use mineral fertilizers on their farms. The reasons were high poverty levels, lack of reliable credit services, and the ever-increasing cost of mineral fertilizer affect farmers’ fertilizer usage. According to Refs. [22,50], the above-mentioned problems enforce the farmers to use low inputs. In Ethiopia, smallholder farms get only 30–40% fertilizer [51]. As a result, cereal yields and fertilizer use are low in Ethiopia [52]. Additionally, unreliable, and erratic rainfall is another factor since in dry areas these fertilizers negatively affect crop production [53]. The result of this study was in line with the findings of [9,54] who reported that poor farmers purchase lower amounts of chemical fertilizers compared with the rich.

According to the results of the interview, farmers did not apply organic fertilizers (farmyard manure and compost) to their barley, tef, and wheat farms. Since the number of animals per household is very low in number for the production of excess farmyard manure. The smaller amount of farmyard manure produced per household is mostly used around the homesteads plots and as fuelwood. So that, there were no input flows of N, P, and K from organic sources to the major cereal field crops. The availability of organic sources' of fertilizers depends on livestock number and family labor size for transporting to the farmlands [22]. However, currently in the study area as well as in the rest of the country, farmyard manure is used as a source of energy [55].

3.2. Outflow of nutrients

The outflow of nitrogen, phosphorous, and potassium via harvested crop yield in barley farms were 40.1, 3.8, and 3.1 kg ha⁻¹ yr⁻¹ fields, respectively (Table 2). Simultaneously, the loss of N, P, and K in tef fields were 6.2, 0.6, and 0.6 kg ha⁻¹ yr⁻¹, respectively. The outflows in wheat fields were 22.5, 2.4, and 1.3 kg ha⁻¹ yr⁻¹ N, P, and K, respectively. The magnitude differs among crop types due to their production potential, soil type, agronomic practice, and nutrient uptake [[56], [57], [58], [59]]. The outflows of N, P, and K by crop residue removal in barley were 33.6, 2.2, and 9.2, in tef 8.7, 1.2, and 2.6, and wheat 31.3, 2.9, and 4.1 kg ha⁻¹ yr⁻¹, respectively.

Table 2.

The amount of nutrient outflows from major farmlands (kg ha⁻¹ yr⁻¹).

Crop	OUT1			OUT2
	N	P	K	N	P	K
Barley	40.1 ± 30	3.8 ± 1.8	3.1 ± 2.1	33.6 ± 22	2.2 ± 0.3	9.2 ± 6.2
Tef	6.2 ± 2.1	0.6 ± 0.2	0.6 ± 0.3	8.7 ± 2.7	1.2 ± 0.2	2.6 ± 1.1
Wheat	22.5 ± 7.7	2.4 ± 0.7	1.3 ± 0.5	31.3 ± 11	2.9 ± 0.9	4.1 ± 1.8

Where OUT1 refers to outputs via grain yield, OUT2 represents outputs by crop residue removal.

The outflows of N through (grain + straw) in barley, tef, and wheat fields were 73.7, 14.9, and 53.8 kg N ha⁻¹ yr⁻¹, respectively (Table 3). Similarly, the outflows of phosphorous via grain and straw yields in barley, tef, and wheat fields were 5.9, 1.7, and 5.3 kg ha⁻¹ yr⁻¹, respectively. The output of P from tef fields was the lowest one compared with barley and wheat; this might be related to its smaller above-ground biomass yield. The current research finding is in line with the findings of [7,9], but it contrasts with [47,49,60]. The outflows of K via (grain + straw) in barley, tef, and wheat farms were 12.3, 3.2, and 5.4 kg ha⁻¹ yr⁻¹, respectively. The loss of K through straw residue removal is greater than grain yield since straws had a high K content [59,61,62]. Whereas, the straw of cereal crops had lower N and P contents than grain [47,59]. Moreover, the overall loss of K was low compared with other studies. This might be due to low nutrient uptake by the crops as the N and P were not supplied sufficiently that resulting in low crop yield. The current study revealed the loss of K was lower than other findings [48,60].

Table 3.

The amount of nutrient inputs and outputs from major farmlands (kg ha⁻¹ yr⁻¹).

Cropland	IN1 + IN2			OUT1 + OUT2
Nutrients	N	P	K	N	P	K
Barley	7.7	0	0	73.7	5.9	12.3
Tef	5.1	2.6	0	14.9	1.7	3.2
Wheat	3.1	2.7	0	53.8	5.3	5.4

3.3. Partial soil nutrient balance

The partial N, P, and K balance is the summation difference between input, and output flows. The partial N balance of barley, tef, and wheat were −66.1, −9.9, and −50.7 kg ha⁻¹ yr⁻¹, respectively (Table 4.). Comparatively, tef's partial nutrient balance was better than barley and wheat, as a result of the outflows through grain yield, and straw removal was low. This may be due to low above-ground biomass yield of tef. As a whole, the result revealed that N input into the croplands' was highly lower than outflow out of the soil system. Hence, the balance was negative. The result implies that farms have negative partial nutrient balance their sustainability will be at risk, unless reversing the trend of the balance [20]. Then if situation continue as it is, it will be impossible to cultivate crop at all and increase crop production.

Table 4.

Partial soil nutrient balance of major crop types.

Crop field	Partial soil nutrient balance (kg ha−1 yr−1)
	N	P	K
Barley	−66b	−5.9	−12.3
Tef	−9.9a	0.9	−3.2
Wheat	−50.7b	−2.6	−5.4
LSD (0.05)	−23.5	NS	NS
CV	29.6	24.9	27

Phosphorous partial balance of barley, tef, and wheat were −5.9, 0.9, and −2.6 kg ha⁻¹ yr⁻¹, respectively. Tef farms had positive balance, even though like N the inflows were very low, because of low outputs by crop grain yield, and residue removals (Table 2). Whereas, barley and wheat had negative balances, as a result, the outflows did not counterbalance by the inflows. Phosphorous was the second most important essential plant nutrient, but farmers’ could not add sufficient organic and inorganic P fertilizer sources. Consequently, the yield of crops was low which lead to low agricultural income and household food insecurity problems in the study area. This study finding was in line with [54] who reported 6.3 in Alaje, and 10.6 kg P ha⁻¹ yr⁻¹ balance in Raya-Azebo, Ethiopia, it might be due to the areas nearly similar physical and socio economic characteristics. Moreover, the present study result was in line with the finding of [47] who reported 11 and -1 kg P ha⁻¹ yr⁻¹ balance for tef, and wheat fields, respectively. On the contrary, our finding differed from the findings of [48] who reported 6 kg ha⁻¹ yr⁻¹ for Amhara National Regional State. Moreover, according to Ref. [18] report a barley-enset farming system had positive P while N and K revealed slightly negative balances. Generally, phosphorus is an important agricultural input in the world, but it is limited by known phosphate reserves and geological time scales [63], hence, it requires a proper management strategy.

Potassium nutrient fluxes (partial balance) of barley, tef, and wheat fields were −12.3, −3.2, and −5.4 kg ha⁻¹ yr⁻¹, respectively. The result revealed that K exported from the farms was more than the imported into the farms. As a result, the loss of K by harvested crop yield and straw removal might not cause severe K depletion in the major cereal crops' farmlands. However, it needs the application of K fertilizer sources. Our finding is in agreement with [48] who reported −2 kg K ha⁻¹ yr⁻¹ is the balance for Amhara National Regional State. But it contradicts with the national value of 7 as well as a cereal-pulse system of −87, −11, and −23 kg ha⁻¹ yr⁻¹ for barley, tef, and wheat fields, respectively. Similarly, it was contradicted with the findings of [64] who reported K balance for the poor, medium, and rich were −53.98, −54.46, and −56.17 kg ha⁻¹ yr⁻¹, respectively.

3.4. Nutrient stock

Total N, P, and K stock for the upper 0.2 m depth of the watershed values were low as illustrated in Table 5. The stock of N for barley, tef, and wheat farms were 1295 ± 481.1, 1510 ± 600, and 1240 ± 181 kg ha⁻¹, respectively. Phosphorous stock on barley, tef, and wheat farms were 63 ± 81, 18.7 ± 4.3, and 27.5 ± 11 kg ha⁻¹, respectively. Furthermore, the stock of K on barley, tef, and wheat farms was 1092.7 ± 122, 1059.4 ± 169.4, and 1090.6 ± 168.5 kg ha⁻¹, respectively. The result revealed that the stocks were varied among croplands. The differences were related to bulk densities and nutrient content variation [65]. The stock of N, P, and K had no direct relationship with the current available amount, since it will be available to the plants gradually in the coming 5–10 years [24]. The objective of nutrient stock improvement was not to maximize their concentration on soil but to maintain the required optimum amount for sustaining agricultural production. In the tropics and sub-tropics food production usually relies on available soil nutrient stocks [66]. The current study result showed that inappropriate soil fertility as well as, land management activities was not effective in maintaining soil nutrient stocks [22,67]. Removal of crop residue for animal feed, low addition of compost, farmyard manure, and mineral fertilizers cause land degradation [68]. Soil fertility management practices should be modified continuously in space and time since it is not static [69].

Table 5.

The nutrient stock within farmlands (0.2 m soil depth).

Crop land	Soil nutrient stock (kg ha−1)
	N	P	K
Barley	1510	18.79	1059.4
Tef	1240.8	27.5	1090.6
Wheat	1295.3	63	1092.7
LSD (0.05)	NS	NS	NS
CV	28.37	32.5	14.9

Combined application of organic and inorganic fertilizers for a long time improved soil nutrient contents of total N, P, K, Ca, and Mg in the upper 0.1 m depth [70]. This study result value was lower than [71] who reported 5510, 1200, and 30800 kg N, P, and K ha⁻¹ stock, respectively in cereal-pulse based systems in Ethiopia. Similarly, lower than [72] who reported N stock value was 2890 kg ha⁻¹ for rain feed and 3180 kg ha⁻¹ for irrigation at 0.15 m depth. This might be related to the poor soil fertility management practices of our study area [25]. Low inputs addition, severe land degradation, and lack of crop residue retention reduce soil nutrient stock [71]. The stocks of N, P, and K had no statistically significant difference (P ≤ 0.05) among the farmlands (Table 3). The stock of N in barley, tef and wheat farms were similar, simultaneously, P stock was the same in all barley, tef and wheat farms. Like N and P stocks, K stocks also statistically the same in all farmlands of barley, tef and wheat (Table 5). However, N, P and K nutrient stocks were have there was farmlands barley, tef, and wheat had a significant effect on nutrient stocks. The stock of N > K > P in all field sites. The results revealed that the amount of N, P, and K throughout the watershed were similar, but there was an amount difference between the N, P, and K amount.

4. Conclusion and recommendations

This study result revealed that soil fertility management practice via using locally available nutrient sources, organic and inorganic fertilizers were underprivileged. Thus inflow addition of N, and P were below the recommended level. The study showed a negative nutrient balance with low nutrient stocks, because of inadequate input additions. The result implies that the sustainability of the farms' production system being at risk, hence the inputs and outputs of soil nutrient should be counterbalanced. To improve the agricultural production capacity of smallholder farms soil fertility should be corrected by adding optimum organic, inorganic fertilizers, and crop residue retention. Similarly, more extension services on the addition of organic and synthetic fertilizers shall be in place. Further studies on integrated soil fertility management activities should be practiced to recover the agricultural productivity of the farms.

Consent for publication

Not applicable.

Author contribution statement

Tilahun Esubalew: Conceived and designed the experiment; Performed the experiments; Analyzed and interpreted the data; Wrote the paper.

Tadele Amare: Performed the experiment; Analyzed and interpreted the data; Wrote the paper.

Eyayu Molla: Analyzed and interpreted the data; Wrote the paper.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement

Data will be made available on request.

Declaration of interest’s statement

The authors do not disclose any conflict of interest.

Additional information

No additional information is available for this paper.