YIELD, NUTRITIONAL QUALITY AND HEAVY METALS TOXICITY IN ROSELLE (Hibiscus sabdarrifa L.) AND JUTE MALLOW (Corchorus olitorius L.) CROPS AS AFFECTED BY TWO COMMERCIAL COMPOSTS AND CHICKEN MANURE

Abstract

Field and laboratory experiments were conducted to determine the effect of different compost formulations on yield, nutrient status, and possible heavy metal toxicity in roselle (Hibiscus sabdarrifa L.) and jute mallow (Corchorus olitorius L.). The vegetable crops were cultivated on an upland field (Latitude 09o 25ꞌꞌ N, Longitude 00 o 58 ꞌꞌ W) with an Altitude of 183 m above sea level consisting of the Kpalsawgu soil series, located at Changnaayili in the Northern Region of Ghana. The different compost formulations were DeCo (decentralized compost), ACARP (Accra Compost and Recycling Plant), CDLCM (composted deep litter chicken manure) and the control to which each of the two test-vegetable crops was assigned to as treatments. These treatments were then randomly distributed in each of four replications for the cultivation. Randomized complete block design (RCBD) was used for the experimental field where data was collected on growth parameters, chlorophyll content (using the SPAD meter), and leaf yield. Leaves of sample crops were harvested and shade-dried, sent to the laboratory for the nutrient status analysis. They were subsequently analyzed for the presence and concentration of heavy metals using the Atomic Absorption Spectrophotometer (Buck Scientific Model 210 VGP). Prior to this, the same test was done earlier on the resident soil series and on the different compost formulations. Residual levels of heavy metals (Cd, Pb, Zn, and Cu) found in the sample leaves were then compared with maximum residue levels. Completely randomized design was used for analysis of data obtained from the laboratory experiments. All data were analyzed using the Analysis of Variance (ANOVA) technique and the GENSTAT statistical program (9th edition). Least Significant Difference (LSD) at 5 % probability level was used to determine treatment differences. Heavy metals and some chemical constituents were naturally inherent in the soil of the Kpalsawgu soil series, but at safe levels; and as well contained in ACARP, DeCo and CDLCM composts but in varying concentrations/rates. Application of compost resulted in leafyield increase as compared to the control where CDLCM gave the highest leaf-yield. Dried leaves of roselle and jute mallow contained protein and minerals; they also contained Cu, Zn, Cd, and Pb but concentration levels of Pb and Cd were above recommended maximum residue levels reported by FAO and the European Commission and so considered a risk to the consumer (adults and children). It is recommended that soils be analyzed for the presence and concentration of heavy metals prior to cultivation of leafy vegetables (hyperaccumulators of heavy metals) so that the soil does not become a potential recipe for the bio-transfer of heavy metals to edible parts of the plant. Compost, and for that matter any soil amendment should be analyzed for the presence and concentration of heavy metals prior to application i.e. in situations where this requirement is not displayed on the compost package. ACARP, DeCo and CDLCM composts should be subjected to corrective reformulation to become soil amendments that are either free of heavy metals or at least to contain heavy metals in the range below or at recommended maximum residue levels.