A common product used in managing soil fertility is 19-19-19, a blend that supplies the three primary macronutrients necessary for robust plant growth. Determining the precise amount of this fertilizer to apply per acre is highly variable, depending on numerous factors specific to the field and the hay crop itself. Achieving the correct application rate is paramount for maximizing yield and quality while managing input costs efficiently.
Decoding the 19-19-19 Formula
The three numbers represent the NPK ratio: Nitrogen (N), Phosphorus (P), and Potassium (K). The 19-19-19 blend indicates that 19% of the product’s weight is elemental nitrogen, 19% is phosphate, and 19% is potash. This equal distribution makes it a highly balanced fertilizer, providing an even supply of the three major nutrients.
The second and third numbers represent the chemical forms required by law for labeling, not elemental phosphorus and potassium directly. The middle number refers to available phosphate (\(\text{P}_2\text{O}_5\)), and the last number represents soluble potash (\(\text{K}_2\text{O}\)).
Nutritional Demands of Hay Production
Hay crops have significant demands for all three primary macronutrients to sustain multiple harvests throughout a season. Nitrogen is the nutrient most responsible for vegetative growth, directly promoting tiller development and increasing the crude protein content of the harvested forage. Its application is designed to boost the plant’s leaf area and overall tonnage.
Phosphorus is central to energy transfer, supporting root development and photosynthesis. Adequate phosphorus levels ensure the necessary foundation for vigorous regrowth after cutting. Potassium plays a specific role in water regulation, helping the hay crop manage drought stress and improving stand longevity and disease resistance.
Pure grass hay requires substantial nitrogen supplementation since grasses cannot fix nitrogen from the atmosphere. Conversely, fields containing a high percentage of legumes, such as alfalfa or clover, can fix their own nitrogen. Maintaining a mix of 25% or more legumes in a stand essentially makes the crop self-sufficient for nitrogen, requiring little to no applied nitrogen fertilizer.
Calculating the Standard Application Rate
Determining a standard application rate involves establishing a baseline nutrient requirement, usually for nitrogen, as it is the most limiting nutrient for grass hay. A common recommendation is to apply approximately 40 to 50 pounds of actual nitrogen per acre for each cutting. This split application strategy, applied after each harvest, is more efficient for the grass than a single large dose.
To calculate the amount of 19-19-19 product needed, the desired pounds of actual nutrient are divided by the nutrient’s percentage in the blend. Using a target of 50 pounds of actual nitrogen per acre (19% N), the calculation is 50 pounds divided by 0.19. This yields a result of approximately 263 pounds of 19-19-19 product per acre per cutting.
Applying 263 pounds of the 19-19-19 product supplies 50 pounds of actual nitrogen, 50 pounds of \(\text{P}_2\text{O}_5\), and 50 pounds of \(\text{K}_2\text{O}\) per acre. This standard calculation assumes the field’s needs for phosphate and potash are roughly balanced with the nitrogen requirement. If the soil has a deficiency or excess of phosphorus or potassium, this balanced fertilizer blend may not be the most appropriate choice.
This standard rate provides sufficient nutrition for moderate yield goals in fields with average soil nutrient levels. The application rate is applied after each cutting to promote rapid regrowth and sustain the stand’s productivity throughout the season. For a field cut three times, the total annual application of 19-19-19 would be approximately 789 pounds per acre.
Variables Requiring Rate Adjustments
The calculated standard rate of 263 pounds per acre requires adjustment based on several field-specific variables. A current soil test identifies existing nutrient deficiencies or excesses in the soil. If the soil test indicates high phosphorus or potassium levels, a balanced 19-19-19 is an inefficient choice, and a different blend should be used.
Target yield goals directly influence the required application rate, as higher yields remove greater amounts of nutrients from the field at harvest. Producers targeting higher tonnage will need a higher annual nutrient input. Furthermore, the timing and frequency of harvest dictate the application strategy, as a single spring application requires a different, usually larger initial rate than the split applications applied after each cutting.
A history of applying manure to the field also changes the required synthetic fertilizer rate. Manure is a rich source of nitrogen, phosphorus, and potassium, which must be accounted for to prevent over-application and nutrient runoff. Ignoring this organic input leads to unnecessary expense and potential nutrient imbalance, meaning the final rate is determined by integrating the baseline calculation with specific field conditions.