All about the soil

Your soil is the storage vehicle for the nutrients for your onion crop. Replenishing the soil with these essential nutrients is critical to having success. With all the rain that has fallen across most of the country this spring, your soil may have lost lots of these nutrients due to leeching. Specifically, nitrogen is the most common nutrient that is easily leeched from the soil by excessive rainfall. In this article, we will dive into the importance of nitrogen in your onion crop, and how to maintain the right balance.

Ensuring your onions have enough nitrogen

The easiest way to determine if the onion plant has enough nitrogen available is to look at the leaves. Plants that are lacking nitrogen have a pale green to yellow appearance. If the tops are stunted and not growing, this also indicates a lack of this essential nutrient. Applying adequate nitrogen to the plant not only greens the foliage but also increases the length of the leaves. This enhances efficient uptake and utilization of other nutrients especially phosphorous and potassium, as well as increasing the number of leaves. Remember that each leaf translates in to another ring of onion so the more leaves you have, the larger the onion bulb. When bulbing takes place, it is a transfer of carbohydrates from the leaves to the bulbs; therefore, larger tops will equal larger bulbs.

Applying Ammonium Sulfate (21-0-0) or a similar source of nitrogen at a rate of 1 cup per 10 feet will be adequate to generate a good response in your onion crop. If you use another source of nitrogen such as Urea (40-0-0), adjust the application based on the first number of the analysis. Each application will give your crop approximately 40 actual lbs of nitrogen. After your plants recover, continue applying nitrogen every two to three weeks at a rate of 1 cup per 20 feet to maintain adequate nitrogen levels.

The amount of nitrogen already in your soil depends on what was planted in the soil previously. Grain or corn will provide approximately 50 lbs of Nitrogen if it was planted previously in the same location that you are now planting onions. Most row crops such as beans, peas, and potatoes will not provide any nitrogen the next year. Getting a soil sample will provide you with your starting point.

Total nitrogen needed is in the range of 150-160 pounds of nitrogen so in this case it is presumed that your soil already has around 28-58 units of nitrogen to begin with.

Calculating the nitrogen

Calculation of N application rate. (lb N per acre)

N requirement:       160

Soil test N:              -44

Previous crop:       +25

Irrigation water:    -10

N rate to apply:      131 lbs N per acre

If you’re fertilizing less than an acre, see how to calculate your rate on a square foot basis.

In this case on a per acre basis, approximately 625 pounds of Ammonium Sulfate is required as a general rule of thumb. This will produce units of actual Nitrogen (625 X 21% nitrogen content of Ammonium Sulfate).

What to do after bulbing begins

learn more about furrow irrigating your onion beds

Once the onions begin to swell at the bulb, you will need to stop any additional applications of nitrogen. Applying nitrogen late in the growing season will produce what is called “thick necks”. This will reduce the shelf life of the onions in storage. Onions with thin necks store better than onions with thick diameters.

Have more questions about nutrient needs in your onions? Contact Customer Service at (830) 876-2430 or email We look forward to another season of providing the highest quality onion plants for you!