Field experiment and treatments.

The present experiment was conducted twice in different areas of the same field during 2004–2005 and 2005–2006 growing seasons at the Onion Research Institute's experimental farm, Changnyeong district, Korea (long. 35°55′ N, lat. 128°47′ E). The experiment site has been under continuous cultivation by the double cropping system of rice followed by onion. Top soil texture was silt loam with an organic matter (OM) content of 16.5 g·kg⁻¹, pH 6.9; and residual NO₃-N, available P, and exchangeable K were 5.3 mg·kg⁻¹, 107 mg·kg⁻¹, and 0.50 cmol_c·kg⁻¹, respectively, before planting onion. Onion cv. Superball (an F₁ hybrid cultivar for fall transplanting) was sown onto bare soil on 8 Sept. 2004 and 9 Sept. 2005 and transplanted into beds mulched with a sheet of transparent polyethylene on 6 Nov. 2004 and 8 Nov. 2005 with a spacing of 15 cm in row and 20 cm between rows with seven rows. The bed size was 12 × 1.4 m, accommodating 560 plants per plot. Harvesting was conducted after 80% of the tops had fallen down on 9 June 2005 and 13 June 2006.

The experiment consisted of three sets of treatments: 1) constant P and K with variable N rate; 2) constant N and K with variable P; and 3) constant N and P with variable K. The constant N, P, and K rates were 240 kg·ha⁻¹ N, 34 kg·ha⁻¹ P, and 128 kg·ha⁻¹ K, which were recommended fertilization rates for intermediate-day onion in Korea. The N rates were 0, 120, 240, and 360 kg·ha⁻¹ N applied as urea. The P rates were 0, 18, 35, and 52 kg·ha⁻¹ P as fused phosphate fertilizer. The K rates were 0, 67, 133, and 200 kg·ha⁻¹ K as potassium sulfate. Every treatment also received decomposed commercial pig compost at 8.0 t·ha⁻¹. Compost, phosphate fertilizer (P), one-third of the urea (N), and 40% of potassium sulfate (K) were applied pre-plant with the rest split into two equal applications in-season in February [107 d after transplanting (DAT)] and March (137 DAT). Irrigation, weeding, and other cultural practices were completed as necessary. The experiment designs each year were a randomized complete block with three replicates.

Properties of rice straw and compost used.

The rice straw was cut in pieces of ≈5 to 10 cm long as soon as the rice grain was harvested, spread, and incorporated. Straw was spread at a rate of 4.5 t·ha⁻¹, which is normal production for the area. Straw composition is presented in Table 1. Nutrient application locally from straw was 29 kg·ha⁻¹ N, 8 kg·ha⁻¹ P, 107 kg·ha⁻¹ K, and 3720 kg·ha⁻¹ OM. The produced commercial pig compost was composted of pig manure and sawdust, which contained 16.6 g·kg⁻¹ N, 6.6 g·kg⁻¹ P, 14.3 g·kg⁻¹ K, 82.9% OM, and a carbon:N ratio of 30:0. The resulting total nutrient amounts of pig manure applied to soil per hectare were 63 kg N, 25 kg P, 55 kg K, and 3157 kg OM. Consequently, the total from both organic sources was 92 kg·ha⁻¹ N, 33 kg·ha⁻¹ P, 162 kg·ha⁻¹ K, and 6,897 kg·ha⁻¹ OM.

Table 1.Nutrient contents and rates of rice straw and decomposed pig manure applied to onion.

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Plant growth and plant and soil chemical analysis.

Onion growth was measured on 7 and 27 Feb. (92 and 112 DAT), 12 Mar. (125 DAT), 7 and 21 Apr. (151 and 165 DAT), 9 and 23 May (183 and 199 DAT), and 11 June (216 DAT) with 10 plants per replicate. The soil samples were collected for chemical analysis from the surface layer (0 to 15 cm) at the same time that plant growth was measured and before basal fertilization.

Onion plants were separated into the bulb and green leaves followed by measuring fresh leaf weight (g/plant) and bulb weight (g/plant) on the individual plot's basis. The samples at harvest were dried to constant weight at 70 °C. The dried samples were ground, weighed, and dissolved in H₂SO₄ and H₂O₂. Total N was measured by the Kjeldahl method. P was measured colorimetrically with an ammonium–vanadate–molybdate method and K content was determined by an atomic absorption spectrophotometer. Total soluble solids (TSS) were estimated with a portable digital refractometer.

Air-dried soil samples were analyzed for pH, NO₃-N, available P, and exchangeable K. Nitrate N content was determined by reflectometry (RQ plus, Merck, Germany). The Lancaster method [RDA, 2000; 5 g soil was extracted with 20 mL of 0.33 M CH₃CHOOH, 0.15 M lactic acid, 0.03 M NH₄F, 0.05 M (NH₄)₂SO₄ and 0.2 M NaOH at pH 4.25] was used in determining available P. An atomic absorption spectrophotometer was used to measure exchangeable K. Soil pH was measured by an electrometer.

Data analysis.

All data were averaged from results of two successive growing seasons and analyzed with SAS Enterprise Guide 4.2 (SAS Institute Inc., Cary, NC). Fisher's protected least significant difference was calculated at P ≤ 0.05.

Plant growth and bulb yield.

Crop growth was very slow until March, but thereafter increased for all fertilization treatments (Table 2). Bulb weight was increased with N rates higher than 0 kg·ha⁻¹ on 23 May and 11 June. Foliage weight was increased with N fertilizer on 21 Apr. and 23 May, but not on 12 Mar. or 11 June. In the case of P fertilizer, 18 kg·ha⁻¹ P resulted in a significant increase for foliage weight on 23 May and for bulb weight on 12 Mar. and 21 Apr. By the time of harvest, there was no effect of P fertilizer. K levels did not affect foliage and bulb weight throughout the growing season except for bulb weight on 11 June.

Table 2.Changes in the average onion bulb and foliage weight of onion plants as affected by N, P, and K fertilizer levels.

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Onion bulb TSS at harvest decreased with N application and was higher with 240 kg·ha⁻¹ N than 120 or 360 kg·ha⁻¹ N (Table 3). On the other hand, TSS was unaffected by P or K levels. Percent stand reduction was substantial at 360 kg·ha⁻¹ N with a 7.7% reduction. It is likely to have been caused by the initial high salt concentration. Onion marketable bulb yield was lowest at the 0 N rate.

Table 3.Onion soluble solid, stand reduction, and yield as affected N, P, and K levels.

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Chemical fertilizers N, P, and K on onions effectively enhance growth and yield up to a threshold level and are detrimental over this threshold (Amin et al., 2007; Greenwood et al., 1992; Jilani et al., 2004). The thresholds vary depending on the soil conditions, fertility, and productivity as well as the local climate and varieties. Amin et al. (2007) reported that fertilizer application significantly increased bulb yield with N fertilizer over the no fertilizer treatment, but there were no differences between different N levels. This result was similar to a study in the subtropical hilly region of Bangladesh (Mozumder et al., 2007). In other studies, fertilizing N rates from 0 to 200 kg·ha⁻¹ N showed that 120 kg·ha⁻¹ was best (Jilani et al., 2004), which was also supported by the findings of Ghaffoor et al. (2003). The stand reduction rate at very high N levels (360 kg·ha⁻¹) was similar to findings of Mogren et al. (2008) who showed that onion seedling emergence was sensitive to soil N concentrations.

In some studies, P fertilizer applied to onions provided a slightly positive effect, resulting in an increase in bulb yield as compared with no P fertilization (Amin et al., 2007; Laughlin, 1989), whereas in others, there were no differences (Boyhan et al., 2007). In the cases of K fertilization levels, some researchers (Aisha and Taalab, 2008; El-Bassiony, 2006) reported that the highest bulb yield and quality were observed with increased potassium sulfate level in the clay or clay loamy soil of Egypt. However, most results showed that the increased K levels did not affect bulb yield (Amin et al., 2007; Boyhan et al., 2007).

In the present study, neither P nor K fertilizers influenced bulb yield significantly, perhaps as a result of the high level of nutrients in the organic materials applied to the experimental field, which were especially high in P in the pig manure compost and K in the rice straw (Table 1). The input of organic materials such as animal manure, compost, or crop residues can reduce chemical fertilizer doses and contribute to a build-up of soil nutrients (Prasad et al., 1999; Yaduvanshi, 2002). Gill and Meelu (1982) demonstrated that 12 t·ha⁻¹ of farm yard manure could substitute for 40 kg N as inorganic fertilizer in rice and gave residual effects equivalent to 30 kg·ha⁻¹ N and 13.1 kg·ha⁻¹ P in the following wheat crop. Shah and Ahmad (2006) showed that wheat fertilized with the integrated use of urea and farm yard manure (FYM) yielded better than the use of urea or FYM alone, although the level of applied N was the same. Prasad et al. (1999) have suggested that soil incorporation of rice or wheat residue is an ecofriendly practice without any adverse effects on crop yield and incorporated residue gradually improves soil fertility. Nguu (1987) has also observed that there were no significant differences in maize yield between 60 and 120 kg·ha⁻¹ N or 30 and 60 kg·ha⁻¹ P when crop residue of 4 t·ha⁻¹ was used as mulch and soil was not tilled.

Soil chemical properties and nutrient uptake.

The changes in soil pH and NO₃-N content, along with available P and exchangeable K⁺ content, were affected by different N, P, and K levels (Fig. 1). There was an approximately decrease from 6.93 to 5.97 to 6.67 in soil pH for all N treatments until 125 DAT and thereafter there was a slow increase to 6.30 to 7.10 in pH. The soil pH lowered with the rate of N fertilization, in particular during early growth. On the other hand, P and K levels did not influence soil pH. Work by Barak et al. (1997) has shown that acid-forming N fertilizer applications over 30 years caused reductions in soil pH, accumulation of exchangeable acidity, decreases in exchangeable calcium and magnesium, and reduction of cation exchange capacity associated with the N fertilization rates. Hence, higher N fertilization rates eventually led to lower N fertilizer efficiency (Barak et al., 1997). In particular, soil pH is an important factor in the productivity of high-value crops such as onions, which ideally should be in the range of 6.0 to 6.5. When the soil pH of 5.20 was adjusted to 6.23 after lime application of 5.0 t·ha⁻¹, onion yields increased from 20.9 to 37.3 t·ha⁻¹ (Stevens et al., 2003).

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Increasing N levels resulted in a definite rise of NO₃-N content in the soil (Fig. 1). Although NO₃-N content reached a maximum at 151 or 183 DAT, the content at harvest returned to close to the original level before the fertilizers were applied. During the growing season, the soil NO₃-N in 240 and 360 kg·ha⁻¹ N treatments ranged from 36.6 to 113.7 and 49.9 to 148.6 mg·kg⁻¹, which were at least twice as high as that in 120 kg·ha⁻¹ N level. Nevertheless, no difference in onion bulb yields was observed between these treatments, which might be the result of the lowered soil pH caused by higher N levels. Similar patterns of soil NO₃-N content can be seen by the findings of Mogren et al. (2008). They showed that the differences in organic fertilizer application caused remarkable variations in soil NO₃-N content but no significant influence on onion yields. Greenwood et al. (1992) indicated that in view of the sensitivity of onions to salinity, the severity of the depressive effect of fertilizer N on early growth depended on the concentration of nitrate in the rooting zone. Lee and Chung (2006) observed that lower NO₃-N content in the soil, which had compost applied, as compared with chemical fertilizer, led to increased weights of lettuce and reduced nitrate accumulation in leaf tissue.

In the case of P and K, available P and exchangeable K in the soil were enhanced according to increasing P and K fertilizer rates (Fig. 1) and differences persisted to harvest. The onion bulb yield, however, was not significantly affected by higher P and K availability. Organic amendments provided all the P and K needed for maximum yield in this study. Applied P and K were unnecessary.

N concentrations and uptake in onion bulbs increased with added N fertilizer, whereas P and K uptakes in bulbs were lowest at the 0 N rate (Table 4). However, fertilizer use efficiency of N was 36.0% in the 120 kg·ha⁻¹ treatment followed by 240 kg·ha⁻¹ at 28.0% and 360 kg·ha⁻¹ at 20.6%. On the other hand, increased P levels resulted in enhanced N, P, and K concentrations and uptake into onion leaf tissue but reduced K concentrations and uptake into the onion bulb. There was not a clear effect of higher P levels on the P conzcentration or total uptake into the onion bulb and P fertilizer use efficiency as well. K concentration and uptake in onion leaf tissue were increased with higher K levels. The K level of 133 kg·ha⁻¹ resulted in the highest K uptake in onion bulb, but K fertilizer use efficiency was the highest at 67 kg·ha⁻¹ level at 23.0% followed by 133 kg·ha⁻¹ at 14.0% and 200 kg·ha⁻¹ at 8.7%. Our results indicated that enhanced N and K uptake in onion bulbs was affected by higher N and K levels, respectively. This did not increase the N fertilizer use efficiency consistently and in turn did not increase onion bulb yield.

Table 4.Effect of N, P, and K treatments on the nutrient concentration and total uptake in onion bulb and leaf tissue as well as fertilizer use efficiency at the end of growing season.

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Onion root systems basically consist of superficial roots that are rarely branched and lack root hairs, requiring a much larger nutrient supply to compensate for this limited root system, but onion bulb yield responded little to fertilizer rates (Halvorson et al., 2008; Shock et al., 2004). The nutrient uptake of onions could be variable depending on fertilizer types, rates, application methods (Lee et al., 2009), and soil type (Mitsios and Rowell, 1987).

In conclusion, the annual application of compost and rice straw into the soil provided sufficient P and K to grow onions without additional P and K fertilizer while requiring a N fertilizer level of at most 120 kg·ha⁻¹. The reduced chemical fertilizer required resulting from the compost and rice straw incorporation may be a better solution in terms of the environment and resource use as well as economic benefit than the annual application of chemical fertilizer.