**5. Results and discussion**

**4.2. Treatment calculations and planting**

40 Organic Fertilizers - From Basic Concepts to Applied Outcomes

lations were based on the following equation:

amendments were calculated based on the total N content.

used to estimate fresh and dry biomass yield per unit area.

Treatment rates for each of the six species were based on soil test recommendations. Sources of nutrients were ammonium nitrate (34% N), triple superphosphate (46% P), muriate of potash (60% K), poultry litter (54% N), and Megabloom (2% N), a fish protein fertilizer. Organic

Poultry litter, unlike commercial fertilizers, is quite variable and according to ref. [48] can vary up to 50% based on animal sources. The available values of litter nutrients using data from Fulhage and Pfost [47] were total N of 54 lb/ton, comprised of 48 lb/ton organic⋅N and 6 lb/ton NH4N, 59 lb/ton P2O5, and 38 lb/ton K2O. In addition, the amount of organic N available was based on days from collection to incorporation, which is 20% beyond 7 days. The calcu‐

> crop N - residual N available NH N + available organic N

Ten plants from each species were transplanted into three-row plots 1.2 m × 6 m at the recommended within- and between-row spacing for each species and drip irrigation applied. Fertilizer treatments for each species were based on soil test recommendations and were applied in single bands approximately 15–20 cm away from the plants. Six plants of each species from the middle row only were harvested. Physiological measurements were per‐ formed once per week, starting approximately one week after planting. These included stem diameter, plant height, and leaf area. Plant height and stem diameter were measured on each species starting at 2 cm above the soil stem interface to the terminals (for the former) and at the widest section for the latter, and recorded as cumulative growth over time. Leaf area was determined from leaf samples collected at each harvest every two weeks, using a LICOR-1800 leaf area meter (LI-COR, Lincoln, Nebraska, USA). All species were harvested periodically throughout the growing season (succulent stems of Amaranth and Celosia of approximately 15 cm length were harvested every two weeks) and once over at the end of the season. Fresh weights of harvested samples were recorded and subsamples collected for nutritional analysis. Samples were dried in ovens at 65°C for 72 h and the dry weights recorded. These data were

The sweetpotato study was conducted as a randomized complete block design with a 4 × 4 factorial treatment arrangement in three replications. The treatment factors were conventional NPK fertilizer, poultry litter, Megabloom (fish fertilizer; FSH), and an untreated check (O). The sweetpotato cultivars were J6/66, NCC-58, TU Purple, and Whatley-Loretan. Treatments were split-applied at the rate of 134–67–67 kg/ha NPK equivalent based on soil tests recommenda‐

Triplicate rhizosphere soil samples from each plot were taken at harvest, composited and analyzed for pH, organic carbon (SOC), and enzyme activity. pH was determined using 1:2.5 soil/water and SOC using the wet oxidation method [49]. Phosphomonoesterases activity was determined by the method of ref. [50]; β-glucosidase and *N*-acetyl-β-glucosaminidase activity

tions one and four weeks after planting as single bands 15 cm from the plants.

4

Organic amendments had no significant influence on fresh and dry biomass production, while species exerted a greater impact, and there were no significant interaction between organic amendments and species for any biomass variable (data not shown). Plants treated with Megabloom produced greater fresh fruit biomass (774, 572, 345 kg/ha, for Megabloom, NPK, and poultry litter, respectively), whereas NPK-treated plants produced greater dry biomass (302, 297, and 226 kg/ha, for NPK, Megabloom, and poultry litter, respectively). Although Amaranth, Celosia, and Okra produced greater total biomass, it was not statistically different from that of either Gboma or Longbean. Gboma plants had larger leaves than Amaranth and Celosia but similar to Longbean, Okra, and Eggplant. Amaranth and Okra plants were taller than the other species, with Amaranth having the greatest stem diameter but Okra having the highest total number of fruits than the other species (**Table 6**).


a Mean separation within columns followed by the same letter are not significant based on LSD, 5% level.

**Table 6.** Main effect of species on the total biomass yield, leaf area, plant height, and the stem diameter of vegetablesa .

There were significant interactions between organic amendment and the different species for contents of vitamin C, betacarotene, total phhenolics, and total antioxidant capacity (**Table 7**). Effects of the interaction between Amaranth and fertilizer amendments on vitamin C showed the highest content among plants receiving NPK compared to the other two treatments. The betacarotene content was similar among plants receiving both Megabloom and poultry litter and substantially greater than plants receiving NPK (**Table 7**). The total phenolic content was higher with NPK whereas plants receiving Megabloom had higher 2,2-diphenyl-1-picrylhy‐ drazyl (DPPH) activity. Results of the interaction between Celosia and organic amendments on nutrient content show that NPK enhanced vitamin C content and, along with Megabloom, betacarotene content. There were greater total phenolics among plants receiving Megabloom followed by those receiving poultry litter, and DPPH activity was similar among plants receiving Megabloom or NPK. For Gboma, vitamin C content and DPPH activity were enhanced among plants treated with Megabloom, whereas NPK significantly increased the betacarotene content compared to those of the other treatments.


a ORAMD, organic amendments (poultry litter, Megabloom-fish protein-based).

b 2,2-Diphenyl-1-picrylhydrazyl (DPPH) % radical scavenging quenched.

c Significant at *P* = 0.01 (\*), *P* = 0.001 (\*\*), *P* = 0.0001 (\*\*\*).

**Table 7.** Effect of interaction between species and fertilizer amendments on nutrient content of vegetables.

Interaction between Longbean and fertilizer amendments on nutrient content was not determined for vitamin C and betacarotene due to sample size. The total phenolic content was similar among plants receiving megabloom and NPK, whereas DPPH activity was signifi‐ cantly greater among plants receiving Megabloom.

These results indicated that species exerted a stronger influence on yield than organic amend‐ ments. Longbean had a 44% greater fresh biomass than okra or eggplant and a 24% greater fresh fruit yield. However, okra produced 52% greater total fruit number than longbean or eggplant. Eggplant had greater leaf area and stem diameter than the other fruit-bearing species whereas okra plants were taller. Among the leafy greens, Amaranth and Celosia produced a 39% greater fresh and dry biomass yield than Gboma that had a 19% greater leaf area. Amaranth and Celosia were taller than Gboma and had thicker stems. Although organic amendments had no significant impact on biomass, there were trends toward a positive response by the plants. For example, plants receiving poultry litter produced 10% greater fresh and dry biomass. Similarly, plants receiving Megabloom had 23% greater fresh fruit biomass than those treated with NPK, whereas the total fruit number and leaf area and NPK-treated plants were 15% higher (Table 6).

enhanced among plants treated with Megabloom, whereas NPK significantly increased the

**Total phenolics (mg/100 g)**

**antioxidant capacityb (μmol AAE/g)**

betacarotene content compared to those of the other treatments.

**Betacarotene (mg/100g)**

Megabloom 156 91.6 467 30.6 NPK 188 77.9 542 25.3 Poultry 133 93.5 362 24.5 Significance \*\*\*c \*\*\* \*\*\* \*\*\*

Megabloom 163 81.1 400 30.7 NPK 197 87.9 420 31.8 Poultry 185 54.2 440 19.8 Significance \*\*\* \*\* \*\*\* \*\*\*

Megabloom 160 189.4 200 77.2 NPK 102 200.2 304 68.6 Poultry 102 157.8 453 57.2 Significance \*\*\* \*\*\* \*\*\* \*\*\*

Megabloom – – 556 47.1 NPK – – 592 37.1 Poultry – – 401 39.8 Significance – – \*\*\* \*\*\*

ORAMD, organic amendments (poultry litter, Megabloom-fish protein-based).

**Table 7.** Effect of interaction between species and fertilizer amendments on nutrient content of vegetables.

Interaction between Longbean and fertilizer amendments on nutrient content was not determined for vitamin C and betacarotene due to sample size. The total phenolic content was similar among plants receiving megabloom and NPK, whereas DPPH activity was signifi‐

These results indicated that species exerted a stronger influence on yield than organic amend‐ ments. Longbean had a 44% greater fresh biomass than okra or eggplant and a 24% greater

2,2-Diphenyl-1-picrylhydrazyl (DPPH) % radical scavenging quenched.

cantly greater among plants receiving Megabloom.

Significant at *P* = 0.01 (\*), *P* = 0.001 (\*\*), *P* = 0.0001 (\*\*\*).

**ORAMDa Vitamin C**

Amaranth

Celosia

Gboma

Longbean

a

b

c

**(mg/100g)**

42 Organic Fertilizers - From Basic Concepts to Applied Outcomes

Nutrients in organic fertilizers are released through mineralization by soil microorganisms [53]. Depending on soil conditions such as pH and moisture content, mineralization rates can be impacted. It is probable that the lack of response to organic amendments in this study could be due in part to slow mineralization rates resulting in fewer nutrients available for plant uptake [53]. In fact, Whitmore [54] reported that 40% of the total N from composted chicken litter was available in the first year and the remainder at the rate of 6–12% per year thereafter because of the slow mineralization rates, and researchers have recommended applying 50% more organic fertilizer 14–20 days earlier than normal to compensate for slow mineralization rates.

Nutrient content of the vegetables varied with species. NPK enhanced vitamin C and total phenolics in Amaranth but not betacarotene or DPPH activity. These results are inconsistent with those of Wheeler et al. [55] and Muso and Ogaddiyo (in kale and hibiscus) [56] who reported lower vitamin C with increased nitrogen fertilization. The increase in vitamin C could be due in part to a decrease in protein production and an increase in carbohydrate production [54]. High vitamin C in the leaves may make plants more tolerant of stress since reducing vitamin C increases susceptibility to stresses [57].

All three amendments enhanced betacarotene content. Megabloom and poultry litter amend‐ ments produced similar levels in Amaranth similar to Megabloom and NPK in Celosia and Gboma. This increase is probably due to increased chlorophyll from nitrogen and or lightabsorbing pigments including carotenoids that are critical in photosystems I and II of the photosynthesis process [54]. Indeed, research has shown that light enhances the biosynthesis of phenolics in the chloroplasts of the cells and thus tends to accumulate in high amounts in the vacuoles or deposits in secondary cell walls as lignin [58].

Betacarotene content of Amaranth, Celosia, and Gboma increased with time for all species up to 51 days after transplanting except for Amaranth and Gboma plants receiving NPK. The betacarotene content of Amaranth plants receiving NPK appeared to decline with time whereas Celosia plants receiving NPK increases substantially with time (data not shown).

Sweetpotato results showed an interaction between the fertilizer amendments and cultivar for rhizosphere pH that varied depending on cultivar and cultivar response varied with pH (data not shown). The pH was lowest in rhizospheres of Whatley/Loretan and NCC-58 receiving Megabloom, and generally, pH ranged from 6.1 to 6.8. Thus, fertilizer amendments lowered rhizosphere pH values with TU Purple plots receiving PL and Whatley/Loretan and NCC-58 plots receiving Megabloom, having the lowest values, respectively. SOC was similar among amendments but was highest for TU Purple and J6/66 and ranged from 0.63 for Whatley/ Loretan to 1.07 for J6/66 (data not shown). Storage root yield was similar regardless of the amendment applied ranging from a low of 12.0, 10, 21.1 t/ha for control and plants receiving NPK, respectively (**Table 8**).


a ACP, acid phosphatase; ALKP, alkaline phosphatase; βGLU, β-glucosidase; βNAG, β-glucosaminidase. \*, \*\*, \*\*\*significant at 0.05, 0.01, 0.001 levels of probability.

**Table 8.** Main effect of fertilizer amendments on storage root yield soil enzyme activitya .

Therefore, the addition of organic amendments increased both soil enzyme and microbial activity, which is consistent with the findings of others [59–61]. NPK-treated plots had higher enzyme activity compared to the controls and, the organic amendments as a nutrient source did not adversely affect enzyme activity relative to NPK treated plots.

In general, the addition of fertilizer and organic amendments had a significant impact on bacteria at every taxonomical level while TU Purple and Whatley/Loretan impacted the *Gemmatimonadetes* at every taxonomical level (**Table 9**).


**Table 9.** Effect of fertilizer, organic amendments, and cultivars on class bacterial composition of sweetpotato

rhizosphere.

The results indicated that *Proteobacteria* was the most dominant phylum and class identified, of which three of its classes (*alphaproteobacteria, betaproteobacteria*, and *gammaproteobacteria*), as well as the class *actinobacteria*, were the most prevalent in the class groups. This observation is consistent with the literature that proteobacteria are ubiquitous in the soil ecosystem. TU Purple and Whatley/Lortan significantly impacted *Gemmatimonadetes* at every taxonomical level suggesting that these cultivars produce exudates that may attract these bacteria. Bacteria belonging to phylum *Gemmatimonadetes* are frequently detected in a variety of environments and are noted as one of the nine most commonly found phyla in 16S rRNA genelibraries from soil [62, 63]. Further, *emmatimondetes* play a role in P cycling by improving P removal in wastewater and could play a similar role in soil.
