Starter fertilizer placed with or near the seed is essential for vigorous early season growth in grass crops such as corn and wheat. We plant these crops early because we know vigorous early season growth is important to high yields. Early seeding also means cold soils. Starter fertilizer is an insurance policy to get the crop off to a fast start despite cold soil conditions.
Each year we receive many questions about starter fertilizer placement and rates. These questions are the result of growers wanting to plant as many acres per day as possible, take advantage of more efficient banded P fertilizer, and of course lower fertilizer costs.
The two most common questions we get are “What is highest rate of starter fertilizer I can apply with the seed?” and “What is the lowest rate of starter fertilizer I can apply with the seed” and still get a starter effect? South Dakota State University (SDSU) made a downloadable spreadsheet that calculates the maximum seed-safe fertilizer rate (Figure 1). The spreadsheet will ask for the crop choice, fertilizer product, seed opener width, row spacing, tolerable stand loss, soil texture, and soil water content. The spreadsheet calculations are based on greenhouse and field studies from SDSU.
Figure 1. Fertilizer Seed Decision Aid from South Dakota State University. You can download the spreadsheet here.
University research shows, that to achieve the full starter effect, a fertilizer granule or droplet must be within 1.5 to 2.0 inches of each seed. If the fertilizer granule or droplet is more than 1.5 to 2.0 inches away from the seed, the starter effect is lost. To show the effect fertilizer rate has on the distance between each seed and fertilizer particle/droplet, we at AGVISE created visual displays. Below is an example of wheat seeded in 7” rows, with 30 lb/a P2O5 (57 lb MAP) banded and corn seeded in 30” rows with 30 lb/a P2O5 (7.5 gallons 10-34-0) banded (Figure 2). To see more displays with several crops, fertilizer rates, and row spacing, go to the link shown below (many thanks to John Heard with Manitoba Agriculture for helping with these displays). These displays help visualize why lower starter fertilizer rates just won’t cut it for the full starter effect. Remember, starter fertilizer rates must be high enough to keep fertilizer particles/drops within 1.5 to 2.0 inches of each seed (link to all of the figures on several crops: https://www.agvise.com/starter-fertilizer-display-how-low-can-you-go/).
Figure 2. Two examples from our starter fertilizer displays series. Click here to see more crops and rates.
In addition to starter fertilizer, additional P and K fertilizer is needed to prevent nutrient mining. Nutrient mining, or applying less total fertilizer than the crop removal rate, causes P & K soil test levels to decline over time. Many broadleaf crops are sensitive to seed placed fertilizer so only low rates of starter fertilizer can be used. In contrast, most grasses can tolerate much higher rates of P fertilizer with the seed (Table 1). If you want your starter P fertilizer rate to keep up with crop removal across your rotation, you will need to apply higher rates to crops like wheat and corn. The safe rate of P fertilizer with wheat seed is much higher than crop removal. This allows you to do some catch up on P for years when you grew soybeans or canola and could only put a low rate of starter P with the seed. If you cannot keep up with P removal in your rotation with normal starter fertilizer rates, you will need to apply additional P in a mid-row band or broadcast application at some point in the rotation.
Table 1. Seed-safe fertilizer rates may not meet crop removal. The seed-safe limit is based on 1-inch disk or knife opener and 7.5-inch row spacing for air-seeded crops and 30-inch row spacing for corn. Phosphorus (P) balance: Seed-safe limit (lb/acre P2O5) minus crop P removal (lb/acre P2O5). A negative P balance indicates the seed-safe limit does not meet crop removal, which may decrease soil test P.
Starter fertilizer is an important part of any crop nutrition plan. Having the right resources will help in making the best decisions for you or your growers. Below are additional links to information on seed-placed fertilizer.