Hay production involves harvesting forage crops, such as grasses or legumes, for animal feed. Maintaining consistent yield and quality requires replacing the nutrients removed from the field with each cutting. The amount of fertilizer needed per acre is not a fixed number; it depends on the specific crop, the soil’s existing nutrient content, and the desired yield goal. Fertilization’s primary role is to replenish these nutrients to ensure the long-term health and productivity of the hayfield.
Establishing the Baseline: Why Soil Testing is Essential
Determining the precise fertilizer application rate begins with a comprehensive soil test. This test provides a snapshot of the soil’s existing fertility levels, including the \(\text{pH}\), and the available amounts of phosphorus (P) and potassium (K). Soil \(\text{pH}\) is important because it governs the availability of all other nutrients; for instance, alfalfa prefers a near-neutral \(\text{pH}\) (6.5 to 7.0), while grass hay tolerates a slightly lower range (6.0 to 6.5).
The soil test results for phosphorus and potassium are translated into fertilizer recommendations using a “build and maintain” philosophy. If levels for \(\text{P}_2\text{O}_5\) and \(\text{K}_2\text{O}\) fall below the optimal range, the recommendation includes a “build” component to gradually raise nutrient levels over several years. Once the soil reaches the optimal range, the recommendation shifts to a “maintain” component, applying fertilizer only to replace nutrients removed by the harvested hay. This approach ensures fertilizer is applied only where needed, preventing waste and promoting soil health.
Crop-Specific Nutrient Removal Rates
While soil testing guides the application of \(\text{P}\) and \(\text{K}\), the nitrogen (N) requirement is determined by the hay crop type and the yield goal. Nitrogen is highly mobile and not effectively measured by standard soil tests. Hay crops fall into two categories: grass hay, which requires substantial nitrogen, and legume hay, which fixes its own nitrogen from the atmosphere. Legumes like alfalfa or clover, if they make up 25% or more of the stand, can supply enough nitrogen for themselves and companion grasses, often negating the need for added nitrogen fertilizer.
For grass hay, a common recommendation is to apply 40 to 50 pounds of actual nitrogen per acre for every ton of expected hay yield. For example, a grass field expected to yield 3 tons of hay per acre over a season would require 120 to 150 pounds of nitrogen per acre. This application boosts tonnage and increases the protein content of the harvested forage.
Hay crops remove significant amounts of phosphorus and potassium with each ton of harvested forage, which must be replaced to prevent soil depletion. On average, a ton of harvested hay removes about 12 to 15 pounds of phosphate (\(\text{P}_2\text{O}_5\)) and 48 to 55 pounds of potash (\(\text{K}_2\text{O}\)). A 4-ton-per-acre harvest would remove roughly 48 to 60 pounds of \(\text{P}_2\text{O}_5\) and 192 to 220 pounds of \(\text{K}_2\text{O}\) annually.
Calculating and Timing Fertilizer Applications
The final step is converting the recommended pounds of pure nutrient (\(\text{N}\), \(\text{P}_2\text{O}_5\), \(\text{K}_2\text{O}\)) into the actual pounds of commercial fertilizer product. Commercial fertilizers are labeled with three numbers, known as the grade, which represent the percentage by weight of \(\text{N}\), \(\text{P}_2\text{O}_5\), and \(\text{K}_2\text{O}\). To determine the total amount of product needed, the required pounds of nutrient are divided by the nutrient’s percentage in the fertilizer, then multiplied by 100.
For instance, if the soil test calls for 50 pounds of \(\text{K}_2\text{O}\) per acre and the farmer uses potash (0-0-60), the calculation is 50 pounds of \(\text{K}_2\text{O}\) divided by 0.60 (60%), equaling approximately 83 pounds of the 0-0-60 product per acre. A similar calculation is performed when using a blended product like 13-13-13, ensuring the target nutrient is met without over-applying the others. When multiple nutrients are needed, the blend should be chosen carefully to match the required ratio of \(\text{N}\), \(\text{P}_2\text{O}_5\), and \(\text{K}_2\text{O}\).
The timing of nitrogen application is important for maximizing uptake and yield in grass hay. Since \(\text{N}\) is highly mobile, it is most efficient to use split applications, applying the total seasonal amount in smaller doses. The first application should occur during early spring green-up, with subsequent applications immediately following each hay cutting. This strategy ensures the grass receives the nitrogen when actively regrowing, increasing nitrogen use efficiency and minimizing the risk of nutrient loss or runoff.