Effects of Neem Leaf Extract on the Soil Properties, Growth, Yield, and Inorganic Nitrogen Contents of Lettuce

Abstract

While lettuce offers essential human nutrients, it also contains anti-nutrients, particularly nitrate (NO3−). The use of neem leaf extract as a natural nitrification inhibitor has proven itself promising to remediate lettuce tissue NO3− content. This study evaluated the effects of neem leaf extract on soil properties, soil nitrification, lettuce growth, yield, and NO3− content. Five nitrification inhibitor treatments were evaluated: (i) no inhibitor (control), (ii) nitrapyrin, and three rates of neem leaf extract based on the dry weight of the raw material: (iii) 1 g kg−1 soil (Neem1), (iv) 2 g kg−1 soil (Neem2), and (v) 4 g kg−1 soil (Neem4). Neem leaf extract generally increased soil concentrations: P (47.6–55.8 mg kg−1), K (45.8–62.7 mg kg−1), Ca (129–164 mg kg−1), and Mg (29.0–35.7 mg kg−1) compared with the control (50.6 mg P kg−1, 35.3 mg K kg−1, 123 mg Ca kg−1, and 24.8 mg Mg kg−1). Neem leaf extracts significantly increased soil NH4+–N concentrations (13.9–30.2 mg kg−1) and nitrification inhibition (12.5–70.5%), but significantly decreased soil NO3−–N concentrations (6.4–13.2 mg kg−1) and net nitrification rates (0.08–0.23 mg N kg−1 day−1) relative to the control (6.6 mg NH4+–N kg−1, 14.7 mg NO3−–N kg−1, 0.26 mg N kg−1 day−1, and 0% nitrification inhibition). The neem leaf extracts significantly decreased shoot fresh weight (13.5–43.1 g plant−1), shoot dry weight (0.84–3.91 g plant−1), and root dry weight (0.14–0.27 g plant−1) compared with the control (52.3 g shoot fresh weight plant−1, 5.36 g shoot dry weight plant−1, and 0.35 g root dry weight plant−1). The significant decreases in the lettuce biomass in the neem extract treatments paralleled the significant decreases in the shoot’s tissue NO3−–N contents and significant increases in tissue NH4+–N content and soil Al concentrations.

NPK Fertilizer Requirements for Jatropha Zeyheri Indigenous Tea under Microplot Conditions

“…Surprisingly, comparable results were obtained when different doses of NPK fertilizer were used on Cichorium intybus [8]. Consubstantially, increasing neem leaf extracts had density dependent growth patterns on lettuce [9]. Generally, biological entities react to extraneous and internal factors in a DDG patterns which are characterized by three growth phases; stimulation, neutral and inhibition in support of the theory suggested by Liu et al [10] In the present study, at lower rates of NPK fertilizer high level of growth was achieved.…”

Harnessing Neem (Azadirachta indica A. Juss): A Sustainable Approach to Natural Farming

“…The neem active ingredients, particularly azadirachtin and nimbolide were found to inhibit nitrifying microorganisms.The hindering effect of the neem leaf extract on nitrification led to ammonium toxicity.The neem extract did not elevate the growth and yields of lettuce but imposed deleterious effects on the lettuce. Nitrogen deficiency accompanied by ammonium and aluminium toxicities drove the detrimental effects of the neem leaf extract on lettuce [26]. There are issues with storage of plant extracts like neem as sunlight degrades their effectiveness as pesticides [27][28].…”