INFLUENCE OF NUTRIENT SOURCE ON THE ELEMANTAL COMPOSITION OF IRRIGATED GARLIC

The effect of five nutrient (fertilizer) sources on garlic (Allium sativum L.) grown under irrigation in Bauchi state was in vestigated. Irrigation was scheduled at 25% management allowable deficit, giving an irrigation interval of 4 days and gross application depth of 56 mm of water per application. The nutrient sources were NPK fertilizer (15:15:15), wood ash, poultry manure, and fermented cow dung slurry, while the control was zero fertilization. The composition of crop samples grown under application of the various fertilizers was assessed in the laboratory following recommended analytical procedures. Pungency level was taken as a measure of the bulb sulphur content. Results of the study indicate that NPK has the effect of significantly increasing (p ≤ 0.01) the bulb moisture content and crude protein level of irrigated garlic, while causing significant reduction (p ≤ 0.01) in the pungency of the crop relative to the control. The organic fertilizers (wood ash, poultry manure, and fermented slurry) yielded crops having relatively higher pungency with wood ash giving the highest pungency strength. The mineral contents of the crops grown with organic fertilization were equally higher than those from the control and those grown with NPK. Furthermore, the crops produced with organic fertilizers had relatively lower moisture content. Results of this study demonstrate that organic fertilizers would give better quality garlic with higher pungency than the widely used NPK fertilizer and no fertilizer application.


INTRODUCTION
Garlic (Allium sativum L.) is a universally popular crop with varied functions to humankind. Garlic produces unique flavours savored by almost all of the world's culture (Havey, 1999). The bulb is characterized by antiseptic, diuretic, expectorating, anti-scurvy and antirheumatic properties (FAO, 2006). The multiple uses of garlic today, translates into increasing demand for the crop for domestic consumption and as production input in pharmaceutical and cosmetic industries as well.
Garlic is a cold weather perennial crop with high nutrient and water requirement (Anon, 2006). The crop is grown both under rainfed and under irrigation. Garlic is approximately 40% dry weight with the major complex carbohydrate being fructan. Its characteristic flavour results from the formation of organosulfur compounds when the precursor S-alk (en)yl cysteine sulphoxide is converted by allinase to thiosulfinate (Jones et al., (2004).
The relative pungency of the Alliums results from both genetic and environmental components (Ketter andRandle, 1998., Havey, 1999). Today, efforts to obtain higher yields of garlic have resulted in the application of various types of fertilizers. The different types of fertilizers have dissimilar concentrations of plant nutrients and therefore affect the soil environment differently. This would ultimately bear varying consequences on the pungency of garlic and other Alliums. Garlic is important because of the culinary value of its flavour and rich reserve of essential compounds like Diallyl disulfide, flavonoids and selenium amongst others (Ketter and Randle, 1998). As mankind strives to obtain higher yields of garlic through heavy application of fertilizers, such procedures must equally preserve the quality of the crop. A recent study by Cantwell et al., (2006) in California USA showed inconclusive results on the influence of different fertilizer treatment on the pungency of garlic. In their work, the pungency of garlic was assessed using three indicators viz: quantity of pyruvate, concentration of thiosulfinate and level of allin enzyme in the crop. Results indicated a reduction in quantity of pyruvate with increase in N fertilizer, whereas, the thiosulfinate level increased at higher doses of N-fertilization. However, when subjected to P and K fertilizer treatments, both pyruvate and thiosulfinate levels increased with increasing quantity of P at constant K. The trend in allin concentration was even more uncertain (Cantwell et al., 2006). This research therefore, further investigated the influence of different types of fertilizer on the pungency of garlic, particularly when grown under semi-arid tropical climate. The results serve as guide in fertilizer selection for high value garlic production.

Field Experiment
Garlic was field grown under basin irrigation at the irrigation research plot of Abubakar Tafawa  Irrigation was scheduled at 25% Management Allowable Deficit (Cantwell et al. 2006) giving an irrigation interval of 4 days and a gross application depth of 56 mm of water. The application time was computed (Hart et al., 1980) to be 105 seconds, from Eq 1: E Qu L n F Ta 600 = where: Ta = application time (minutes) Fn = net water requirement (mm) Q = unit inflow rate, m 2 /s The crops were grown on basins of dimensions 3m x3m, hence L was taken as 3m. A small petrol engine centrifugal pump with discharge of 4 l/s was used for delivering water to the field. This gave a unit inflow rate of 1.66667x10 -3 m 3 /s. The experimental treatment constituted five types of fertilizers commonly used for garlic production in Africa. The treatments consisted of wood ash (16.5t/ha), poultry manure (7.0t/ha), NPK 15:15:15 (0.3t/ha), fermented slurry (7.0t/ha) and control (no fertilizer). These treatments were arranged in a randomized complete block design with four replications. All other production practices were the same for all the treatments. The crops were harvested when the lower third of the leaves turned brown (Anon. 2006), and cured for two weeks before taking samples for laboratory determination of the constituent elements.

Preparation of the Garlic samples
Laboratory analyses were conducted on four randomly selected samples taken from each treatment and replicate. The selected bulbs were first peeled to remove the outer scales and then reduced to their respective cloves. The cloves from each category were coarsely chopped with a sharp knife and shuffled in separate petri dishes. Samples were taken from each lot for pungency and constituent determination

Theory of Pungency
The principal compounds of intact garlic, which serve as precursors of the characteristic flavour of garlic, are the alkyl and alkenylcysteine sulphoxides (Anon. 2006). The major substrate for the pungency reaction of garlic is (+)-S-allyl-L-Cysteine Sulphoxide (allin) (Anon. 2006). When the walls of garlic cloves are broken, the vacuolar enzymes allinase rapidly lyses the cytosolic alk(en)yl cysteine sulphoxides to form sulfenic acids. These immediately condense to form the alkyl alkanethiosulphinates, which is the principal flavour compound of garlic (Anon. 2006). Pyruvic acid and ammonia are equally produced as by-products of the reaction. The elements and compounds associated with this chemical reaction constitute the framework for pungency determination of the Alliums. Four pungency markers of the Alliums are: i. Enzymatically developed pyruvate (EPY), a by-product of the allinase enzyme activity. ii.
Thiosulfinate concentration, the principal product produced by allinase activity iii.
concentration of allin, the substance for allinase activity iv.
The quantity of bulb or total sulphur. (Watson, 2005;Cantwell et al., 2006) Sulphur is a constituent of thiosulfinate, one of the major pungency compounds of the Alliums. Use of bulb sulphur as a measure of pungency has been used over the years (Resemann and Carle, 2003;Babatunde et al., 2003;Jones et al., 2004;Watson, 2005) because it has direct bearing with the pungency compound (thiosulfinate). Pungency was therefore assessed in this study using total or bulb sulphur content. The sulphur content was determined through dry ashing following the recommended procedure (A.O.A. C., 1984).

Determination of Composition and Sulphur of the Experimental Crop.
The composition of the garlic bulbs was determined based on the relative abundance of major plant constituents viz: Moisture content (MC), Crude Protein (CP), Calcium (Ca 2+ ), Iron (Fe 2+ ), Copper Cu 2+ ) and Sulphur (S). Moisture content was Influence of Nutrient Source on the Elemantal Composition of Irrigated Garlic determined using the gravimetric method while CP, Ca 2+ , Fe 2+ , and Cu 2+ were determined still through dry ashing according to recommended procedures of the A.O.A. C., (1984). Bulb sulphur was determined from wet digested sample on CE 1020, UV Spectrophotometer Instrument (CECIL ) Instrument, Cambridge, England

Data analysis
Analysis of variance and correlation were done using the MINITAB (Version 11) computer software for the major tissue constituents of the bulb. Direct and combined contributions of some constituent to the other were obtained using a computer simulated pathway analysis newly developed by Agada and Babatunde (2007) Table 1 shows the physico-chemical properties of the experimental soil. The crops fertilized with NPK had the highest protein level averaging 82.3 g/kg, while the control crops, grown without fertilizer had an average protein content of 52.9 g/kg (Table 2). NPK gave the highest crude protein level because it has a relatively high concentration of Nitrogen which is a major constituent of amino acids, the basic constituent of proteins (Wong et al. 2001).The recorded range of protein content of 52.9-82.3 g/kg compared well with the average value of 66.7 g/kg earlier reported by Anderson (2006).

RESULTS AND DISCUSSION
The bulb sulphur content, which is taken as a direct measure of the degree of pungency of the crop (Jones et al., 2004;Watson, 2005), equally varied significantly (p ≤ 0.01) among the samples, from as low as 4.0 g/kg with NPK to 15.2 g/kg with wood ash. On this basis, wood ash is an appropriate nutrient source for growing high pungency garlic. If however, the pungency were to be moderated to a low level, then NPK or fermented slurry would be more appropriate. Anderson (2006) had recorded sulphur content of 0.5 g/kg to 3.7 g/kg for garlic grown in Europe. In a related research, Resemann and Carle (2003) documented bulb sulphur content of onion (Allium cepa L.) in the range of 4.55-6.77 g/kg   Table 3, however, show that moisture content has a negative relationship with Ash content and Bulb sulphur content. Using a fertilizer like NPK that leads to high moisture accumulation in garlic would result in the reduction in both the ash and sulphur contents of the bulbs. This can also be attributed to the fact that, the K component of NPK has the ability to bind to elements with non-bonding pairs of electrons such as sulphur (Personal communication with a Professor of Analytical Chemistry). Table 3 again revealed that to attain high protein levels in irrigated garlic, the moisture content would equally increase. However, the Cu, Ca, Fe, and ash levels would drop. Fe is important in crop physiology because it is used in the biochemical reactions that form chlorophyll, and is a part of one of the enzymes that is responsible for the reduction of Nitrate-N to ammoniacal-N (Hochmuth et al., 2006). Calcium, on the other hand is a component of Calcium pectate, a constituent of cell wall (Hochmuth et al., 2006). High moisture content has adverse effects on storability of agricultural crops. Babatunde et a.l (2004) also reported high post harvest loss from onion bulb grown with NPK fertilizer. Thus apart from the high crude protein level offered by NPK, the fertilizer would generally not be a very good nutrient source for garlic cultivation. Table 3 further showed that usage of organic nutrient sources that increased the sulphur content of garlic, equally increased Cu, Ca, Fe, and ash contents. This further reduce the bulb moisture content, which is desirable in preserving garlic over long terms. High sulphur content is particularly cherished in garlic because it provides a base for formation of alkyl alkenethiosulphinates (R 1 -SS(O)-R 2 ), sulfenic acid (R-SOH) and allicin, which are the major pungency compounds of garlic (Watson, 2005;Anderson, 2006;Cantwell et al., 2006). Allicin is a natural antibiotic (about one-fiftieth as powerful as penicillin and one-tenth as powerful as tetracycline) that could kill many kinds of bacteria (Anderson, 2006). Sulphur is also the base constituent of the fatsoluble Diallyl disulfide and Diallyl trisulfide both of which have been shown to have anti-cancer and antitumor activities (Anderson, 2006). Sulphur, rather than Nitrogen is responsible for the flavour and medicinal attributes of garlic. Nevertheless, Jones et al. (2006) had reported that sulphur metabolism is intimately related to Nitrogen metabolism through a production of the amino acid cysteine. Yet, same researchers further observed that Nitrogen does not apparently affect the level of inorganic sulphur in garlic bulbs. Wood Ash and poultry manure enable attainment of increased sulphur and Nitrogen contents in garlic. These fertilizers are therefore recommended for production of garlic having good flavor and medicinal values. Table 4 showed that Ash and crude protein contents strongly contributed to the sulphur level of irrigated garlic. The implication is that slight variations in these two constituents would easily alter the sulphur level of garlic. Since it has been demonstrated that Crude protein level has an inverse relationship with sulphur content (Table 3), Table 4 further illustrated that increasing the crude protein level especially by increasing NPK level, would cause an associated reduction of the Sculpture content and hence pungency of garlic. This observation can serve as an alternative means of reducing pungency, as may be desired by some who prefer low pungent garlic. Apart from this, cultural practices like application of wood ash and poultry manure for garlic production is encouraged because they produce high quality garlic vis-à-vis its use as spice and as a medicinal plant.

CONCLUSION
This research has shown that the different nutrient sources commonly used for irrigated garlic production within Nigeria, significantly affect the composition and pungency of the crop. NPK produces garlic with high moisture content and crude protein level, but the mineral composition of the bulbs would be low. High moisture content militates against prolonged storage of the crops, while the low mineral composition reduces the medicinal value of the garlic bulbs. Organic fertilizers like wood ash, poultry manure, and fermented slurry produce garlic having low moisture content, high pungency and higher mineral composition. NPK was not found to be a very suitable fertilizer for use in garlic production. Rather, organic fertilizers, particularly wood ash was recommended for production of good quality garlic.