Nutrient management options for enhancing productivity and profitability of conservation agriculture under on-farm conditions in central highlands of Kenya

: Decline in soil fertility is one of the major constraints to sustainable crop production and profitability. To meet the increasing demand for the growing population the issue of low soil fertility needs to be addressed. An on-farm experiment was established to evaluate the effect of interaction between NPK fertilizers and minimum tillage on soil fertility, maize crop yield and on farm profit margins. The field trials were set up in a split-plot design organized in a randomized complete block design in 28 farmers’ fields. Application of all the three macronutrients i.e. NPK had the highest and significant (P < 0.0001) yields relative to treatments, where any of the three nutrients was omitted. Maize grain yield increased by over 150% with NPK, NP and NK application in all the three cropping seasons over the control. Grain losses with the omission of a single nutrient were highest with N (2.06, 2.40 & 2.42 t ha −1 ) followed by P (0.8, 0.59 & 0.43 t ha −1 ) in the short rain 2014 (SR2014), long rains 2015 (LR2015) and SR2015 use of conservation agriculture and all the three macro-nutrients (NPK) is the best bet for increasing, maize crop yield and associated return on investment.


Description of the study area
The study was conducted in Runyenjes Division of Embu County. Runyenjes Division lies in agro-ecological zones; Upper Midland zone (UM2) to Lower Midland zone (LM3) on the eastern slopes of Mt. Kenya at an altitude of 1500 m.a.s.l. [22]. The area receives a bimodal rainfall with long rains (LR) lasting from mid-March to May and short rains (SR) from late October to December, hence two cropping seasons per year. The annual rainfall ranges from 930 to 1395 mm per year with mean monthly temperatures of 20 ℃ [22]. The soils are predominantly humic-nitisols which are deep weathered and with moderate to high inherent fertility [22]. The farming systems in the study area are complex and intensively managed consisting of an integration of crops, trees and livestock. Maize

Farm selection and sampling
A household field survey was conducted in July 2014 to characterize the smallholder farms in the study area. The farms were categorized in terms of farm types and sizes, main soil types, the cropping system, farm management practices as well as the socio-economic factors influencing the farming systems. Soil samples were collected using an alderman soil auger at 3 random points of the demarcated fields at a depth of 0-20 cm and composited to one sample to establish the fertility status of the soils. Results from the household survey and soil data were used to guide the selection of the farms where trials were to be established. A total of 28 farms were selected and trials established and monitored for three seasons (SR2014, LR2015 and SR2015). The trial farms were selected on the basis that they were either flat or on a gentle slope, they represented the main cropping system, had uniform soil fertility and could accommodate the 5 treatments each measuring 5 m by 5 m. The rainfall data was collected using an automatic rain-gauge.

Trial design, treatments and crop management
The trial was laid out in a split-plot design and treatments arranged in a randomized complete block design in the 28 selected farms. The trial plot size was 5 m by 5 m. Out of the 28 farms 14 farms were under conservation agriculture while the other 14 were under conventional agriculture. Conservation agriculture system entailed, minimum tillage and retention of crop residues; while the conventional agriculture system entailed, manual ploughing and weeding and no residue retention. Maize (Zea mays L.), Duma 23 variety which is commonly grown in the area was the test crop. Two maize seed per hole were planted at a spacing of 0.75 × 0.25 m between and within rows, respectively. Thinning was done after germination leaving one seed per hole to maintain a population of 53,000 plants ha −1 in accordance with the regional plant population and density recommendation. A blend of straight fertilizers {urea as a source of N, triple superphosphate (TSP) as a source of P and muriate of potash (MOP) as a source of K} were applied at the rate of 120-60-60 kg ha −1 (N-P 2 O 5 -K 2 O), respectively (Table 1). Urea fertilizer was applied in 3 splits; 40 kg N ha −1 at planting, 40 kg N ha −1 at first top dressing (3 weeks after crop emergence) and 40 kg N ha −1 during second top dressing (5 weeks after crop emergence). Triple Super Phosphate (TSP) and Muriate of Potash (MOP) were applied at planting. Dry maize stover was used as a mulching material and applied after crop emergence at the rate of 5 t ha −1 under conservation agriculture treatments. Weeding was done twice using hoes in conventional agriculture treatments prior to first and second N top dressing. To control the preannual weeds and ensure that crops were established on clean fields, a mixture of selective Dual Gold 960EC  (pre-emergence) and non-selective Weedal 480 SL (post-emergence) were sprayed two days after planting in the conservation agriculture plots. Weeding under conservation agriculture treatments was done twice by spraying 2, 4 D-Amine herbicide 21 days after emergence (DAE). The Bulldock  0.05 GR insecticide was applied in all the treatments three weeks after the crop emergence to control maize stalk borer (Busseola fusca).

Soil laboratory analysis
The soil samples from the field were taken to the laboratory, where they were air-dried and ground to pass through a 2 mm sieve. The soil pH in water was measured in a ratio of 1:2 soil to water using a pH meter [23]. Total nitrogen was analyzed through the Kjeldahl method [23]. Soil organic C was determined using the Walkley-Black method [24]. Soil extractable P was determined using Mehlich 1 method [25,26]. Soil K, Ca, Mg and Zn were analyzed using standard methods [27]. Table 2 shows the averages for the initial soil chemical and physical characteristics for the 28 farms.

Harvesting and yield determination
Maize grain yield and stover yield were harvested at physiological maturity from a 3 m by 2 m net plot. The cobs were manually separated from the stover. Cobs were then manually threshed, moisture content determined and then adjusted to 12.5% and presented in t ha −1 . Maize stover was cut at ground level and total above-ground fresh weight determined. The dry weight of the stover was determined after drying a sample of known fresh weight to a constant dry weight and expressed in t ha −1 .

Economic data
Data on costs of farm inputs (seeds, TSP, Urea, MOP, and herbicides) was collected through a survey of input prices from agro-input stockists in the study area. The time taken for the field operations (land preparation, planting, fertilizer application, thinning, weeding, pest control and harvesting) was taken using stopwatches and calculated as the work rate per hour. The average time taken was calculated and converted into monetary value at the rate of 0.25 USD per 8 hour working day. Maize stover was accounted for as an additional benefit and was valued at the market value of 19.61 USD ton −1 at harvest time ( Table 3).
The partial budget procedures were used for cost-benefit analysis [28]. Net benefits were calculated by subtracting total variable costs from gross benefits for each treatment from the sale of maize grain and stover yields. Benefit to cost ratio was calculated as the ratio of net benefits to total variable cost [28].

Data analyses
The maize yield, soil properties and economic data was subjected to analysis of variance (ANOVA) using SAS 9.3 software [29]. Post-ANOVA analysis (polynomial contrasts) was conducted to examine the potential contribution of individual and combined nutrients on maize grain yields. For both ANOVA and post-ANOVA the treatment means were separated using Fisher's least significance difference (LSD) at 5% level of significance. Paired t-test was done on soil properties to test whether the mean value changes on soil nutrient values at initiation and termination of the experiment was significant at 5% level of significance.

Rainfall trend over the experimental period
The highest rainfall was recorded during the SR2015 season (October-December 2015) while the least was recorded in the SR2014 season (October-December 2014) (Figure 1). A 12 days' drought towards the end of the SR2014 season was experienced in December. Rains were uniformly distributed during the SR2015 season. Table 4 presents, the effect of tillage and applied nutrients on maize grain yield. Tillage effects on yield were not significant (P < 0.05). Cumulative yields for the three seasons, ranged from 12.9 to 15.1 t ha −1 for tillage practices with N included, between 7.7 and 8.2 t ha −1 for tillage practices with N omitted and between 4.6 and 5 t ha −1 for the control. All the fertilizer treatments, yielded significantly higher yields relative to the control, irrespective of the omitted nutrient (P < 0.01). The NPK treatments increased maize grain yields by 197, 237 and 196% over the control during the SR2014, LR2015 and SR2015 seasons, respectively (Table 4). A trend, of highest yields in nitrogen by tillage interactions was evident (P < 0.001). Omitting N resulted in cumulative yield penalties of more than 4 t ha −1 over the three seasons, irrespective of the tillage type. The yields were influenced by the amount of rainfall received across the seasons.   The highest individual macronutrient response to maize grain yield was N followed by P and K, respectively (Table 5). Orthogonal contrast showed omission of N (NPK vs PK) had the highest significant (P ≤ 0.0001) losses on maize grain yields of 2.06, 2.40 and 2.42 t ha −1 during the SR2014, LR2015 and SR2015 seasons, respectively. Omission of P (NPK vs NK) contributed a significant influence on crop yields losses with 0.8, 0.59 and 0.43 t ha −1 during the SR2014, LR2015 and SR2015 seasons, respectively (Table 5).

Soil nutrients changes
There was no observable tillage effect on total soil N during the study period. Total nitrogen decreased significantly by 13, 17, 14 and 9% in NPK, NP, NK and PK fertilizer inputs, respectively over the study period ( Table 6). The soil P level increased significantly in the conservation agriculture treatments but not in the conventional agriculture treatments (P ≤ 0.05). The nutrient management practices had no effect on the soil P level. Extractable K was significantly (P ≤ 0.05) higher under conventional agriculture than conservation agriculture at the start of the experiments but not after the three cropping seasons. A significant positive change (P ≤ 0.05) of extractable K was observed in PK treatments after the three cropping seasons (Table 6).

Cost benefit analysis
Conservation agriculture had significantly lower total variable costs compared to the conventional agriculture. The net benefits and benefit-to-cost ratio were significantly higher under N inclusion treatments (NPK, NP, and NK) compared to N omission treatments (PK and control) ( Table 7). On average, N inclusion treatment generated over 40% higher net benefits, compared to N omission treatments irrespective of the tillage practice in each of the two seasons. The benefit-cost ratio (BCR) was significantly (P ≤ 0.05) higher under conservation agriculture than conventional agriculture.

AIMS Agriculture and Food
Volume 5, Issue 4, 666-680.  During the LR2015 and SR2015 seasons, application of inorganic fertilizer led to a significant (P ≤ 0.0001) increase of net benefits of 282 and 266% with NPK, 278 and 239% with NP, 258 and 120% with NK & 228 and 126% with PK over control, respectively. Benefit to cost ratio was increased by 26 and 24% with NP application, 25 and 23% with NK application and 18 and 12% with NPK application during the LR2015 and SR2015 seasons, respectively. Results showed that investing in NPK, NP and NK fertilizers, could yield 3-fold higher net return above the control under the two tillage systems.

Discussion
Significantly higher yields in NPK treatments as compared to controls over the study period indicate the importance of balanced nutrition on crop performance. The N, P and K are the major limiting nutrients in the area and their application is essential to obtain optimal yields. Several authors have reported more than double the yields with application of NPK over the control [16,[30][31][32]. In our present study, omission of N (PK and control) affected yields more severely than omission of other nutrients. Overall P was the second most important nutrient affecting the maize yield. This is in agreement with the expected trend, since overall nitrogen is the most limiting nutrient in crop production, followed by P. Most fields tend to have N deficiencies due to high losses and uptake of nitrogen by the growing crops. P deficiencies are also evident in the area due to high P fixation in these acidic soils. The average pH of these soils was 5.87 with the pH across the farms varying between 4.84 and 6.60.
In this study omission of K did not significantly influence the yields. Although K is also a macro nutrient, in most regions its supply from the soil is adequate. However, continued crop cultivation with application of only N and P supplying fertilizers, like; DAP, NPK 23:23:0, UREA and CAN by over 80% of the farmers [33] has led to continued harvesting of K by growing crops resulting often in low supply of K [30]. The results agree with [34] who reported maize grain yields decrease of 84% due to no fertilization, 77% due to N omission, 78% due to P omission and 26% due to K omission. Besides, [35] reported grain yields decrease under maize wheat rotation that followed the order NPK > NP > NK > N > Control. This trend was also reported [36] in a study that evaluated the effects of inorganic fertilizer application on grain yield, nutrient use efficiency and economic returns of maize in western Kenya.
The significant increase in extractable P on the conservation agriculture relative to conventional agriculture was probably due to addition of P through decomposition of mulching material under conservation agriculture system. Crop residues are an important source of P in the farms. Higher P levels under conservation agriculture than conventional agriculture has been reported [37,38] due to limited mixing of soil with fertilizer P [39] in the minimum tillage systems therefore reducing the surface contact for P fixation.
The higher K increase under conservation agriculture could either be as a result of additional input of K in soils through decomposition of crop residues applied as mulch. Previous studies have reported accumulation of most of the K that is taken up by plants in the stover [40]. In addition, high K under conservation agriculture could be attributed to minimum tillage which has been found to enhance K levels in the soil because of reduced losses through leaching as a result of minimal soil disturbance. For instance, [41] reported 1.65 and 1.43 times higher K in 0-5 cm and 5-20 cm soil layer in no-till when compared to conventional agriculture system, respectively. The significantly higher K increase in the NP and PK treatments could be associated with the positive synergistic interaction of N and K on nutrient uptake [42,43]. The K uptake in this case was reduced in the absence of either N or K. [44] reported that the uptake of K is strongly influenced by other elements such as N explaining the higher amounts of K in the PK treatment due to low K uptake.
Compared with conservation agriculture, higher total variable cost (TVC) was recorded under conventional agriculture in both LR2015 and SR2015 seasons. Use of herbicides under conservation agriculture could have contributed to the reduced TVC owing to the high labor cost of manual digging and weeding in the conventional agriculture [10,16,45]. The mulching under conservation agriculture could be another added advantage for reduced cost of production as it has been found to reduce weeding labour cost as well as weed density [46]. [16] reported that maize-bean rotation was KE 22,718 cheaper under no-till with crop residue retention than under conventional agriculture with no crop residue retention in Embu and Kirinyaga Counties.
Higher net benefits were recorded under conservation agriculture than conventional agriculture in both LR2015 and SR2015 seasons. This could be associated to the lower production costs under conservation agriculture than conventional agriculture. Similarly, [47] reported higher maize net returns under conservation agriculture (permanent beds) compared to conventional agriculture. Higher net benefits as a result of fertilizer application could be attributed to higher yields recorded in both seasons over the control.
The BCR was significantly higher under conservation agriculture than conventional agriculture while NPK, NP and NK treatments had signficantly higher BCR than PK over the two seasons. This could be attributed to the lower cost of production under conservation agriculture. This concurs to [48,49] who stated that N and P should be the basis of optimizing fertilizer use for maximum crop yield and profitability. The omission of N (PK) led to a lower BCR compared to control and this could be as a result of high cost of P and K fertilizers (high TVC), and relatively low maize grain and stover yields (low net benefits). The general low BCR due to the omission of N observed in this research corroborates well with those reported by other studies [35].

Conclusions
There were beneficial effects of applying a combination of all the three macronutrients (NPK) relative to applying any of three nutrients singly or omitting any of the three nutrients from the combination under both conservation agriculture and conventional agriculture. Rainfall variability in amount and distribution greatly affected maize yields across the seasons. Grain loses were higher with the omission of N and P affirming the importance of N and P in crop production. Treatments with N offered the most profitable options while conservation agriculture was more economical compared to conventional agriculture. There is therefore need to continue promoting the use of NPK fertilizers and conservation agriculture among the farmers for enhanced crop productivity and profitability.