Soil testing is the primary tool for assessing phosphorus (P) and potassium (K) nutritional needs of asparagus before planting and for established plantings of asparagus. Nitrogen (N) fertilization follows general guidelines based on the age of asparagus and is discussed below. Excessive applications of N and P can be detrimental to the environment through runoff into surface waters. Nitrates can also leach into groundwater. Careful management of these nutrients can minimize these environmental problems.
Preplant fertilization of P and K should be based on soil tests. For the most accurate estimation of soil nutrient availability, collect and analyze soil. Although asparagus roots can extend 3 to 4 feet deep, collecting soil from the top 12 inches is sufficient to obtain accurate results. Collect a composite sample of a minimum of 12 soil cores from each field; if zones of different soil texture exist within the same field, take separate samples to represent each major soil type. The following table suggests appropriate soil levels for phosphorus and potassium:
| Soil test interpretation | Phosphorus (bicarbonate extraction) in ppm |
Potassium (ammonium acetate extraction) in ppm |
|---|---|---|
| Low (Crop response likely) |
<10 | <100 |
| Medium (Crop response possible) |
10–15 | 100–150 |
| High (Crop response unlikely) |
>15 | >150 |
| Key: < = less than; > = greater than; ppm = parts per million | ||
For phosphorus and potassium, a low soil-test value suggests the need to fertilize; at medium soil levels, yield response to fertilizer application is possible; at high soil levels, yield response is less likely. Phosphorus is generally applied preplant as a banded or broadcast application prior to direct seeding or planting transplants or crowns; potassium can be applied preplant, sidedressed during fern stage, or fertigated (injected into drip irrigation water) during the growing season or when planting crowns.
When preparing a field for an asparagus planting, it is recommended to add no more than 100 pounds of phosphate (P2O5) per acre when soil levels are greater than 20 ppm, and 200 pounds or more for soil levels of 10 ppm or less. For fields with soil K-test values of less than 100 ppm, up to 200 lb potassium (K2O) per acre can be applied.
Early Establishment
The use of soil tests for nitrogen management in asparagus has not been researched. A moderate nitrogen application (100 lb nitrogen/acre) is recommended to build up the crown tissue for the first year of fern growth. However, the first 2 to 3 years after establishment, higher rates of up to 200 lb nitrogen per acre are warranted in order to build up nitrogen levels in the crowns of the plant.
Established Asparagus
Once an asparagus planting is established and producing, the annual phosphorous and potassium removal is modest; maintenance applications in the range of 50 lb P2O5 per acre and 50 to 75 lb K2O per acre should be sufficient to maintain soil fertility. Fertilization timing varies widely, but phosphorous and potassium are generally applied before the harvest period.
Maintenance levels of nitrogen in the range of 100 to 150 lb per acre should be sufficient to maximize yield. Application timing varies, but nitrogen applications are commonly split between the preharvest and postharvest period. Nitrogen application is most efficient during active fern growth.
In established asparagus, nutrients are typically applied as a sidedressing but can be applied through fertigation if drip irrigation is used.