Evaluation of Mineral Contents of Lettuce and Carrot Harvested from Soil Treated with Organic and Inorganic Fertilizers

The dichotomy of striking a balance between sustainable food crop production for the skyrocketing human population and ensuring agricultural practices that mitigate environmental degradation has prompted much research into sustainable crop production methods. The application of amendments has become an integral part of arable soil management in restoring declining soil fertility for sustainable and high-quality crop production. This study was conducted on lettuce and carrot cultivated on soil treated with three different amendments: cow dung, sewage sludge, and nitrogen–phosphorus–potassium (NPK) mineral fertilizer. The vegetables were harvested at maturity at 60 and 110 days for lettuce and carrot, respectively, dried in a hot air oven, crushed, and then digested to obtain an aliquot sample. The level of macronutrients was quantified from the aliquots using inductively coupled plasma optical emission spectrometry (ICP-OES), Avio 550 Max, PerkinElmer, USA. It was observed that both soil treatment and types of vegetables significantly impacted the level of mineral contents in the vegetables. The highest values of 58.00 ± 8.36 mg/kg and 72.97 ± 12.53 mg/kg were recorded for Na and P in carrots from soil treated with sewage sludge, respectively. The highest values of 247.97 ± 17.07 mg/kg and 104.72 ± 4.12 mg/kg were recorded for Ca and Mg in lettuce from sewage sludge-treated soil, respectively. Similarly, the highest value of 546.75 ± 76.44 mg/kg for K was also recorded in lettuce, but from cow dung-treated soil. The overall pattern of mineral accumulation by vegetables shows that carrots accumulate more Na than lettuce, and lettuce accumulates more Mg, Ca, and K than carrots, while there was no significant difference in the level of P in both lettuce and carrots. The findings reveal that lettuce and carrots from soil treated with organic manure cow dung and sewage sludge accumulated higher mineral contents of most of the investigated minerals. It was also observed that lettuce accumulated higher contents of most of the minerals investigated. This study therefore concludes that organic manures are better alternatives to mineral fertilizers for vegetable production, which supports the effort to strike a balance between sustainable and ecofriendly agriculture.