Starter Fertilizer: Right Place, Right Rate

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.

Seed Safety Calculator from SDSU for Starter Fertilizer Article

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/).

Starter fertilizer demonstration example for starter fertilizer article

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.

Using banded fertilizer for corn production (University of Minnesota)

Guidelines for safe rates of fertilizer applied with the seed (Saskatchewan Agriculture, Natural Resources and Industry)

Corn response to phosphorus starter fertilizer in North Dakota (North Dakota State University)

Phosphorus fertilization of wheat significantly improved yield and crop vigor (North Dakota State University)

Wheat, barley and canola response to phosphate fertilizer (Alberta)

Starter Fertilizer Display: How low can YOU go?

When profits are squeezed, more farmers are asking about optimal starter fertilizer rates and how low starter fertilizer rates can be. These questions are the result of wanting to keep fertilizer costs down, to plant as many acres per day as possible, and to take advantage of more efficient, lower rates of banded phosphorus fertilizer compared to higher rates of broadcast phosphorus fertilizer.

To illustrate the role of starter fertilizer rates and seed placement, we put together displays showing the distance between fertilizer granules or droplets at various rates and row spacings. You can see several pictures with canola, corn, soybean, sugar beet, and wheat. We greatly thank John Heard with Manitoba Agriculture for helping with the displays.

The displays show the normal seed spacing for several crops with different dry or liquid fertilizer rates alongside the seed. These displays help visualize the distance between the seed and fertilizer at several rates. University research shows that to achieve the full starter effect, a fertilizer granule or droplet must be within 1.5-2.0 inches of each seed. If the fertilizer granule or droplet is more than 1.5-2.0 inches away from the seed, the starter effect is lost. Some people wonder about these displays, but you can prove it to yourself pretty easily. Just run the planter partially down on a hard surface at normal planting speed. You will see what you imagine as a constant stream of liquid fertilizer, ends up being individual droplets at normal speed, especially with narrow row spacings and lower fertilizer rates.

These displays help illustrate the minimum starter fertilizer rate to maintain fertilizer placement within 1.5-2.0 inches of each seed for the full starter effect. In addition to an adequate starter fertilizer rate, additional phosphorus and potassium should be applied to prevent nutrient mining, causing soil test levels to decline in years when minimum fertilizer rates are applied.

Split the Risk with In-season Nitrogen

For some farmers, applying fertilizer in the fall is a standard practice. You can often take advantage of lower fertilizer prices, reduce the spring workload, and guarantee that fertilizer is applied before planting. As you work on developing your crop nutrition plan, you may want to consider saving a portion of the nitrogen budget for in-season nitrogen topdress or sidedress application.

Some farmers always include topdressing or sidedressing nitrogen as part of their crop nutrition plan. These farmers have witnessed too many years with high in-season nitrogen losses, usually on sandy or clayey soils, through nitrate leaching or denitrification. Split-applied nitrogen is one way to reduce early season nitrogen loss, but do not delay too long before rapid crop nitrogen uptake begins.

Short-season crops, like small grains or canola, develop quickly. Your window for topdress nitrogen is short, so earlier is better than later. To maximize yield in small grains, apply all topdress nitrogen before jointing (5-leaf stage). Any nitrogen applied after jointing will mostly go to grain protein. In canola, apply nitrogen during the rosette stage, before the 6-leaf stage. For topdressing, the most effective nitrogen sources are broadcast NBPT-treated urea (46-0-0) or urea-ammonium nitrate (UAN, 28-0-0) applied through streamer bar (limits leaf burn). Like any surface-applied urea or UAN, ammonia volatilization is a concern. An effective urease inhibitor (e.g. Agrotain, generic NBPT) offers about 7 to 10 days of protection before rain can hopefully incorporate the urea or UAN into soil.

Long-season crops, like corn or sunflower, offer more time. Rapid nitrogen uptake in corn does not begin until after V6 growth stage. The Presidedress Soil Nitrate Test (PSNT), taken when corn is 6 to 12 inches tall, can help you decide the appropriate sidedress nitrogen rate. Topdress NBPT-treated urea is a quick and easy option when corn is small (before V6 growth stage). After corn reaches V10 growth stage, you should limit the topdress urea rate to less than 60 lb/acre (28 lb/acre nitrogen) to prevent whorl burn.

Sidedress nitrogen provides great flexibility in nitrogen sources and rates in row crops like corn, sugarbeet, or sunflower. Sidedress anhydrous ammonia can be safely injected between 30-inch rows. Anhydrous ammonia is not recommended in wet clay soils because the injection trenches do not seal well. Surface-dribbled or coulter-injected UAN can be applied on any soil texture. Surface-dribbled UAN is vulnerable to ammonia volatilization until you receive sufficient rain, so injecting UAN below the soil surface helps reduce ammonia loss. Injecting anhydrous ammonia or UAN below the soil surface also reduces contact with crop residue and potential nitrogen immobilization.

An effective in-season nitrogen program starts with planning. In years with substantial nitrogen loss, a planned in-season nitrogen application is usually more successful than a rescue application. If you are considering split-applied nitrogen for the first time, consider your options for nitrogen sources, application timing and workload, and application equipment. Split-applied nitrogen is another tool to reduce nitrogen loss risk and maximize yield potential.

Corn Stalk Nitrate Test

To help evaluate nitrogen management in corn, you may want to try the corn stalk nitrate test as a post-mortem tool. The corn stalk nitrate test is a late-season or end-of-season plant analysis on mature corn stalks. Iowa State University developed the corn stalk sampling protocol and interpretation. If corn did not have sufficient nitrogen, the corn stalk nitrate level will be low. If corn had excess nitrogen, the corn stalk nitrate level will be high.

The corn stalk nitrate test can be useful in cropping systems with manure or corn-after-alfalfa, where a significant portion of the crop nitrogen budget comes from nitrogen mineralization. It is also helpful in more humid climates, where the residual soil nitrate-nitrogen test is not utilized. For corn silage production, it is easy to collect corn stalk samples on the go during silage harvest, making it a quick and useful tool.

Since the corn stalk nitrate test is a post-mortem tool with the goal to provide information for future years, it is not recommended in years with abnormal precipitation. In drought years, potential crop productivity is reduced, so the plant nitrogen requirement is lower than normal. In high precipitation years, soil nitrogen losses will reduce the available nitrogen supply. As a result, the corn stalk nitrate level can be very high in drought years or very low in wet years. Such results say more about environmental conditions, not the adequacy of the nitrogen fertilizer program.

When to sample

  • Early: One-quarter milk line (R5 growth stage) on majority of corn kernels. Nitrate concentration may be high if collected early.
  • Optimum: One to three weeks after physiological maturity (black layer, R6 growth stage) on 80% of corn kernels.
  • Late: Up to harvest. Nitrate concentration may be low if rainfall has leached nitrate from plant material.

How to sample

  • Measure 6 inches from the ground, cut the next 8 inches of corn stalk (the 6-14 inch stalk section measured from plant base). Remove outside leaf sheath.
  • Collect 12 to 15 corn stalks.
  • Place corn stalks in ventilated plant tissue bag. Do not use plastic or zipped bag.
  • Do not collect diseased or damaged corn stalks.

Table 1. Corn Stalk Nitrate Test Interpretation

Nitrate-N (NO3-N), ppm Interpretation Comment
<250 Low Nitrogen supply was likely deficient and limited corn grain yield
250-2000 Sufficient
>2000 High Nitrogen supply exceeded plant requirement

for corn stalk nitrate test article