Chemical fertilizers. Samples of different chemical fertilizer mixtures. The main elements required by plant life are nitrogen (N), phosphorus (P) and potassium (K). Supplements based on compounds that contain these elements are referred to as NPK fertilizers. Different ratios, and additional trace elements, are used depending on the crop and soil. Clockwise from far left, the N:P:K ratios are: 15:5:20, with 2 parts magnesium (Mg) and 3 parts sulphur (S); 15: 30:15, with trace elements (commercial Miracle- Gro mixture); 12:10:27 with added sulphur, iron (Fe) and maganese (Mn); 1:1:1; and 16:8:13 with 2 parts Mg (tomato fertilizer).
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