Nutrient management in asparagus

During field preparation, the year before planting, take a 12-inch-deep soil sample to determine how much phosphorus, potassium and other nutrients to add. The soil test results will also reveal whether the soil pH must be amended with lime or sulfur in order to achieve the ideal pH range of 6.5-7.0.

Because amendments like lime and sulfur gradually alter the soil pH over several months, it is important that the soil test be done a year before planting to allow time to apply and incorporate the amendments and for pH conversion to occur.

Growers who have already been farming the field, have tested the soil recently, and know the soil pH to be in the proper range may be able to skip this step.

Use the results of the soil test to determine how much and what type of amendment to add to the soil. Whether using fertilizer, manure or compost, each amendment has a certain concentration of each nutrient.  In order to determine how much amendment to apply, know the following:

• Current P and K concentrations and organic matter (%OM) from the soil test.
• Recommended soil nutrient applications for asparagus in the tables below for new and established plantings.
• Nutrient concentrations in the amendment (actual or approximate).
  • With compost or manure, you also need to know the moisture content in order to know the available nutrient content (pounds of N/ton, P₂O₅/ton). 

Nitrogen (N)

Use the table below, along with your soil test report, to determine how much actual nitrogen (lb/acre) to apply. Then, determine how much amendment to apply based on the concentration of N in the amendment.

The soil test report gives a %OM reading but does not list an N reading. This is because nitrogen is very mobile and the concentration in a small area of soil can change rapidly. Therefore, %OM is used to determine N requirements.

According to Table 1, a new field with low organic matter such as 1.5% should receive 120 lb/acre of N over the course of the first season. Established plantings require less as the root systems become more established and more internal nutrient recycling occurs.

Phosphorus (P) and potassium (K)

Phosphorus levels of 30 ppm or over are adequate for asparagus and no further additions are necessary.  Table 2 shows phosphate (P₂O₅) recommendations for asparagus based on the test report. For example, a new field with 25 ppm P on the soil test should receive 100 lb/acre of P₂O₅ in the form of fertilizer or organic amendment. An established field with the same test report should receive 25 lb/acre.

Follow the same procedure for potassium using Table 3. For example, a field with a soil report reading of 101-150 ppm K should receive 100 lb/acre of potash (K₂O) before planting, or 25 lb/acre for an established field.

For asparagus, %OM less than 3.1% is considered low, 3.1- 4.5% is medium, and anything over 4.6% is high. Building up organic matter in soils is a long term process that can include cover crops, green manures, animal manures and compost addition.

While organic matter can be added in any of these forms prior to planting, it will not be a long term solution as it will break down over time. To maintain organic matter, additional organic amendments may be needed over time. Various types of organic amendments can be used in an asparagus bed, including composted manure.

Raw manure can be incorporated prior to planting, but adding raw manure directly to the crowns at planting is not advised. High-ammonium manure from poultry or swine, or one with high salt content, can burn the crowns if placed directly on top of them.

Manure with less ammonium and more organic matter, from dairy or composted manure, is less likely to cause damage. To reduce risk, incorporate manure into the soil prior to planting or incorporate it lightly to the backfilled soil rather than placing it directly onto the crowns.   

Additional recommendations for using manure in asparagus:

• Send the manure to be tested for N, P, K and salinity.
• Right rate is important. Make sure the manure rate matches up with asparagus nutrient recommendations, taking into account what nutrients are already in the soil. More manure is not always better.
• Incorporate the manure after broadcasting.
  • If using manure at or before planting, some soil separation between the roots and manure is good.
  • Rake in the manure before planting the roots, or backfill some soil onto the crowns before laying manure.
• In mature plantings, never apply manure in the spring prior to spear emergence. Topdress and incorporate it after harvest ends and before fern growth.
• If composted manure is used, the same recommendations apply.

Prior to planting, nutrients should be incorporated with tillage or cultivation to help ensure even distribution.

In the fall or spring prior to planting, broadcast and incorporate most of the recommended phosphorus and all of the potassium by tilling 8-12 inches deep with a chisel plow or rototiller. Compost and manure can be added at this time as well.

On the day of planting, form the furrows and apply an additional 25-30 lb/acre of phosphate (P₂0₅) into the trench right before placing the crowns. If using compost, this rate of actual phosphate still applies. Read the concentration of phosphate on the fertilizer label or get a manure/compost analysis to determine how much product to apply.

If the phosphorus levels on the soil test report are rated “high,” omit the fall or spring broadcast application and only do the trench application.

A small amount of compost may also be added at planting. Since 2-3 inches of soil are pushed over the crowns to cover them immediately after planting, a portion of this cover can consist of compost.

After planting, apply approximately one-third to half of the total recommended rate for nitrogen. The remaining N should be side-dressed later in the season when the trenches are backfilled.  

In the first and second years after planting, soil amendments can be incorporated with shallow cultivation before spears emerge in the early spring, according to the soil test and tables above. A very wet spring may prohibit entering the field to fertilize prior to spear emergence, and delaying cultivation can damage the spears. As the soil warms up, add 30-40 lbs N/acre if needed based on the soil OM%.

Asparagus has a very fleshy root system that is capable of storing a lot of nutrients. The roots can store an estimated 150 lbs N/acre, 37 lbs P/acre and 170 lbs K/acre.

The plant can use these stored nutrients, in part, for the development of spears in the early spring. The actual amount of nutrients removed during a harvest of 2.5 ton/acre is 23 lbs N/acre, 3 lbs P/acre and 20 lbs. K/acre. Therefore, it may not be necessary to apply fertilizer every year. This decision should be made based on crop vigor, yield, and soil testing. Good record keeping is important.

Generally, it is not necessary to apply fertilizer for an established asparagus crop until after harvest. In fact, delaying fertilization until after harvest can reduce early weed growth. For sandy coarse-textured soils, however, 20-25 lbs N/acre in the spring may be beneficial for spear development.

Tables 1, 2 and 3 above present fertilizer recommendations for established plantings. This fertilizer should be top-dressed after harvest to encourage the ferns to grow.

The response of asparagus to applications of secondary macronutrients (Ca, Mg, S) and micronutrients (B, Cu, Fe, Zn, Mn, Mo, Cl, Ni) is not well documented in Minnesota. In general, asparagus does not respond strongly to micronutrients, so applying micronutrients should not be a focus of an asparagus nutrient management plan.

While many soils in Minnesota are high in calcium, most soils low in calcium (Ca) and magnesium (Mg) are acidic and should be limed with dolomitic lime prior to planting. Sulfur may be limiting on sandy soils with low organic matter. Suspected nutrient deficiencies should be confirmed with soil tests or tissue analysis.

Adequate soil moisture is necessary for fern development, spear development the following year, establishment of new crowns, and spear quality and yield. Asparagus plants use 0.10 to 0.20 inches of soil water per day during the period of fern growth.

Asparagus roots can penetrate up to 10 feet to get soil water if not restricted, but their greatest water uptake occurs from the top 6-24 inches of the soil. Maintaining adequate soil moisture in this zone, especially during the fern stage, should be the goal of an irrigation program.

Soil moisture during fern growth should not be allowed to deplete more than 50-60% of the soil's water holding capacity in the active rooting zone or go beyond a soil tension of 70 centibars before another irrigation. Asparagus plants do not generally show visual signs of wilting when moisture-stressed, so use extra care to ensure there is adequate soil moisture throughout the growing season.

However, not all asparagus fields need to be irrigated. In fact, most asparagus producers in Minnesota only water the crowns in the first year in order to promote establishment. Irrigation is most needed during drought, or in sandy and shallow soils in central Minnesota. In most years, growers feel that the soil moisture in their fields remains adequate during the season.

Irrigation is a balance. Infrequent, moderate soaking of the soil is preferred over frequent, light irrigation which can lead to foliar disease development. On the other hand, over-irrigation may cause some of the applied nitrogen to leach below the plant's root zone and possibly into the groundwater.