Fertilizers are nutrient additives applied to the soil periodically to main­tain optimum crop productivity. The need for fertilization may result from a deficiency of one or more mineral elements in the soil, their pres­ence in a form unavailable to the plant, or the leaching of elements into the soil to a depth below the root zone.

Chemical Fertilizers

Since nitrogen, phosphorus, and potassium are the soil elements used in greatest quantity by the green plant, a fertilizer that provides all three elements is termed a complete fertilizer. The actual percentage by weight of each of the three primary elements in a fertilizer determines its analysis; for example, 100 pounds of 10-6-4 analysis fertilizer con­tains 10 pounds of nitrogen (N), 6 pounds of phosphoric acid (P2O5), and 4 pounds of potash (K2O). The analysis figures are always expressed in the same order and represent the same nutrients. Note that phospho­rus and potassium are not present in elemental form, but as chemical compounds. Thus the amount of actual element provided for the plant is less than the analysis implies (Figure 3-7).

With simple arithmetic, fertilizers can be compared on the basis of their nutrient ratio. The ratio is a reduction of the analysis to the low­est common denominator. For example, a 5-10-10 analysis has a ratio of 1-2-2. (Each of the numbers has been reduced through dividing by a common factor of 5.) A fertilizer analysis of 10-20-20 also has a ratio of 1-2-2. Therefore, a 5-10-10 fertilizer supplies the three major nutrients in the same proportion as a 10-20-20 fertilizer, but twice as much of the actual product must be applied to obtain the same amount of nutrients (Table 3-3).

When less than 30 percent of a complete fertilizer’s weight repre­sents available nutrients, it is termed a low-analysis fertilizer. When the amount of available nutrients is 30 percent or more, the product is a high-analysis fertilizer. The remaining material in a fertilizer is filler, either organic or synthetic. The filler may provide some additional essential elements, and may even be important as a source of micronu-


figure 3-7. Interpreting fertilizer analysis figures: the nutrients are always shown in the same order. (Delmar/ Cengage Learning)

TABLE 3-3.

pr Nutrient Ratio

50 Pounds of 5-10-10 Fertilizer

50 Pounds of 10-20-20 Fertilizer



21/2 pounds of N (nitrogen)

5 pounds of N

5 pounds of P2O5 (phosphoric acid)

10 pounds of P2O5

5 pounds of K2O (potash)

10 Pounds of K2O

trients, but essentially the filler is a carrier for the available macronu­trients. It allows them to be applied evenly to the soil and crop. It also adds weight and bulk, both undesirable features. Table 3-4 compares high-analysis and low-analysis fertilizers.

The more common chemical fertilizers include: anhydrous ammo­nia, ammonium nitrate, urea, sodium nitrate, and ammonium sulfate as carriers of nitrogen; superphosphates, ammoniated phosphates, and round rock phosphate as carriers of phosphorus; and potassium nitrate, potassium chloride, potassium sulfate, and potassium-magnesium sul­fate as carriers of potassium.