Cool and wet soil conditions can limit uptake of immobile nutrients early in the growing season. Banding fertilizer with the corn planter is a popular method of supplying nutrients to corn and banding can increase uptake early in the growing season. As planter size has increased more corn farmers have turned to liquid fertilizers banded directly on the seed because of the simplicity of equipment required and ease of handling liquid fertilizers. Banding fertilizer directly on the seed, which is also known as “in-furrow” or “pop-up” placement can be an effective way to supply small amounts of
macro- or micro-nutrients to a crop.
Placement of fertilizer directly on the corn seed does pose a risk as fertilizer materials can damage plant tissue thereby reducing plant growth and seed germination. The most noticeable effect of starter
fertilizer is the increase in plant growth early in the growing season. This effect can be seen even if broadcast fertilizer is applied (Figure 1). Small amounts of fertilizer can significantly increase plant early growth. In the example in Figure 1, a 2.5 gallon per acre rate of 10-34-0 increased early plant growth by an average of 15% while there was only a small additional increase (1-2%) from either 5
or 7.5 gallons per acre. High rates of liquid fertilizers may not be needed to increase early plant growth.
Some nutrients in seed placed fertilizers have a greater effect on early growth responses than others. Seed placed phosphorus (P) increases early plant mass more than nitrogen (N) and potassium [(K)
Figure 2]. Application of P will generally increase early plant growth even in the presence of high soil test P. Seed placed N, K, or S may appear to increase plant mass early in the season but differences are a result of low N, K, or S concentration in the soil solution which is limiting growth. Choosing a P seed placed fertilizer source that can economically supply at least 10 lbs P2O5 per acre is important if increased plant mass is a goal for application of starter fertilizer.
Damage Potential of Seed-Placed Fertilizers
There are two factors to consider when determining the potential damage a fertilizer source may cause. First, all fertilizers contain salts. In low concentration, salts will typically have little impact on the growth of a crop. All salts by nature are formed by ionic bonds which give the materials a slightly polar charge. Water is a polar molecule and will be attracted to any positively charged molecules in the soil. If the charge is in a high enough concentration, such as a fertilizer band, ions can hold water so strong and will also draw water out of tissues resulting in damage to growing tissue and
reduced seed germination.
The salt index of a fertilizer is a measure of the salt concentration of a fertilizer in the soil solution. Salt index is an important parameter to consider when comparing fertilizer products suitability for seedplacement.
Salt index is a unit less measure that is expressed as the ratio of the increase in osmotic pressure a fertilizer source induces to the same weight of sodium nitrate (NaNO3). Sodium nitrate has a relative
osmotic value of 100. Sodium nitrate was used as a baseline because it was a common fertilizer source when the concept of salt index was developed. All salt index values are expressed as per unit nutrients (Table 1). For most fertilizer sources one unit of nutrients equals 20 lbs. For acids, 100 lbs is used for calculating nutrient units.
The salt index of a fertilizer source is not directly measured but rather is calculated based on the salt index values of the components of a fertilizer source (Table 1). Table 2 summarizes the calculation of 7-21-7. To calculate the sa lt index of 7-21-7 in Table 2:
1. List all fertilizer materials and weight for each component [per ton of material (columns 1 and 2)] in the fertilizer blend.
2. Determine the nutrient units in columns 3-5 by multiplying the total lbs of material by the concentration of N, P, K, or S.
3. List the salt index per plant nutrient unit from Table 1 into column 6.
4. Determine the salt index from each component by multiplying the sum of the nutrient units from columns 3-5 by the per unit salt index value in column 6.
5. Add the salt index values from each component together to determine the salt index of the material.
Salt index values of common fertilizer materials are given in Table 3. Fertilizer sources with a salt index of 20 or less are typically best suited for in-furrow placement. Most fertilizer sources
with salt index values less than 20 were manufactured from potassium phosphate instead of potassium chloride. Salt index values greater than 20 are not suggested for in-furrow placement. It is important to note that the salt index is not a direct measurement of the amount of fertilizer that can be safely applied in the seed row.
The second factor to consider is the potential for ammonia (NH30) to form in the band near the seed. Ammonia can damage plant tissue similar to salts. Nitrogen contained in liquid fertilizers can be in any of three forms, ammonium (NH4+), nitrate (NO3-), or urea. Of these three, ammonium and nitrate can contribute to the salt content of the fertilizer but do not have the same damage considerations as ammonia. The exception to this rule is ammonia on high pH soils. Soil pH controls the relative percentage of ammonia to ammonium in the soil. The percentage of ammonium is greater as soil
pH increases. When ammonium is applied to a high pH soil a portion can be converted to ammonia. Urea can result in the greatest accumulation of ammonia in the band as ammonia is formed as an intermediary in the conversion of urea to ammonium.
Dry urea is not suggested as an in-furrowfertilizer source. Many common liquid fertilizer sources contain some urea as part of the total N in the fertilizer (Table 4). Research in Minnesota has indicated fertilizer sources that contain materials that will liberate NH30 in the seed furrow can be more damaging than fertilizers that contain high amounts of salts. In order to mitigate the potential for stand loss, materials such as 28% or 32% UAN solutions are better suited for band application on the soil surface or banded away from the seed row with at least one inch of soil between the seed row and
fertilizer band (for example, a starter placement).
The thiosulfate ion in liquid starter fertilizers requires special consideration as it can be highly damaging to plant tissue. Two common liquid fertilizer sources, ammonium- and potassium thiosulfate, contain the thiosulfate ion. Ammonium thiosulfate can liberate free ammonia in the seed furrow. Potassium thiosulfate is safer but should not be applied at high rates in the seed furrow. If sulfur is required in a starter fertilizer blend, alternative sources should be used as placement of thiosulfate directly with the corn seed can be very risky. Alternative placement methods such as a surface band or starter band at least one inch away from the seed row should be considered when using a fertilizer source containing thiosulfate.
The damage potential of fertilizers can easily be measured by evaluating the number of plants emerged following fertilizer application and comparing it to a control or untreated area. Plant emergence does not give the full picture of damage potential as reduction in plant growth may occur before the fertilizer product decreases the number of plants per acre. For example, the final number of emerged plants following an in-furrow application of ammonium thiosulfate (ATS) was not noticeable until rates exceeded 5 gallons per acre (Figure 3). The mass of plants above ground steadily decreased even
with the lowest rates of ATS applied. Over application of most fertilizer sources, especially in-furrow application, will eventually lead to reductions in plant mass and plant population. Keeping application rates of all fertilizer sources low makes the most sense to reduce the risk of damage and give the best change for an economic return.
Various placement options have been developed to reduce the damage potential of liquid starters placed in-furrow. One of these methods is a dual band of fertilizer placed to both sides of the seed row slightly above the seed on the side of the furrow (dual band above). Data from a sandy soil indicates that this placement was no better or worse than traditional placement with the seed or a single band placed above the seed row, when applying a high rate of multiple fertilizer sources (Table 5). The 10-34-0 fertilizer source contained the most total N at the low and high rates and resulted in the greatest reduction in stand. What is important to note is there were no differences among the placement methods. Maintaining at least two inches of soil between the band and the seed row is sufficient to reduce the risk of starter damage. Most placement methods within a half of an inch from the seed row should be viewed similarly to placement directly on the seed.
Effect of Soil Type On Seed Safe Fertilizer Rates
Soil chemical properties can have a significant impact on the amount of fertilizer that can be safely applied with the corn seed. The most important property of a soil is the cation exchange capacity (CEC). The CEC is a measure of the amount of positive ions a soil will hold. All salts contain positive and negative ions. Soils with a greater CEC can reduce the risk of salt damage from high rates of fertilizer. Soils with high CEC in Minnesota typically have greater concentration of organic matter which affects the water holding capacity of the soil. When liquid fertilizers are placed in the seed row of a soil with a greater water holding capacity, when at field capacity, the risk of seedling damage is decreased. As soils dry, the risk for damage from fertilizer salts or free ammonia liberated greatly increase.
Maximum seed safe rates for common fertilizer sources on loam and sandy soils are given in Table 6. The “High” application rate reported in the table represents the maximum rate that could be applied given adequate soil moisture at planting. The “low” rate reported is for situations where moisture is limiting. The rates given in Table 6 were found to result in similar emergence and growth compared to when no starter was applied. The fertilizer rate that produced the greatest plant growth was typically similar to the “low” value. The rates given in Table 6 DO NOT guarantee damage
WILL NOT occur within a given year. Fertilizer placement with the seed always presents some risk for damage for all fertilizer sources and rates. The maximum application rate for coarse textured (sandy) soils should be 50% or less than that of soils with loam textures.
An accepted theory for application of in- furrow starter fertilizers has existed for many years. It stated, no more than 10 lbs per acre of N + K2O applied in the furrow for corn. The data in Table 6 supports the notion of no more than 10 lbs per acre of N + K2O applied directly on the corn seed. For sandy soils no more than 4 lbs of N + K2O should be applied. The general guideline does not apply for products such as ammonium thiosulfate or products where potassium phosphate is the source of K (low salt sources). Only nitrogen needs to be considered for low salt sources. However,
high rates of N in low salt sources can still result in damage from nitrogen (particularly urea) and are seldom economical.
Impacts on Corn Grain Yield and Moisture of the Harvested Grain
The influence of liquid in-furrow fertilizer on corn grain yield needs to be considered separately from impacts on early plant growth. Seed placed fertilizer will almost always have a greater impact on early plant growth than on grain yield. As the corn plant develops, height differences that occurred early in the growing season will disappear. Enhanced early growth can impact corn silking date. Three years of Minnesota research indicated that corn silking date could be advanced by 1 to 2 days across a range of planting dates and hybrid relative maturities.
Enhanced maturity can impact grain moisture at maturity. Differences in grain moisture at harvest when starter is used will most likely occur when harvesting fields early where grain moisture is in the range of 20-30%. In dry to average years, in-furrow application can decrease grain moisture from 0 to 1 percentage points. In exceptionally cool and wet years this decrease can be as much as 0 to 2 percentage points.
Increased corn grain yield at the end of the season depends on many factors. In a few cases where soils test high for most nutrients, a grain yield response may be caused by the advancement of growth also termed the “starter effect”. The potential for the starter effect to increase grain yield in high testing soils is low. An increase in grain yield from the “starter effect” in Minnesota is estimated to occur about 10% of the time compared to a grain moisture reduction which may occur up to 25% of the time. Grain yield increases are mainly caused by one or more of the nutrients applied in the band being deficient in soil for a significant part of the growing season. This nutrient deficiency may be caused by a lack of an available nutrient in the soil or the nutrient being temporarily unavailable because of environmental factors such as cold, wet soils. The prophylactic addition of micronutrients in seed placed fertilizers has not been shown to increase the probability of a grain yield response. Zinc may be the only micronutrient of benefit in a seed placed mix. A fully chelated source of zinc should be used in order to avoid Zn forming complexes with orthophosphate in seed placed fertilizer mix and precipitating out of solution.
Fallow syndrome is caused by a reduction of the activity of vesicular arbuscular mycorrhiza (VAM) by growing a non-host crop prior to growin g corn. A common non-host crop grown in Minnesota is sugar beet or canola. In soil, VAM colonize the roots of corn and are important for the uptake of P and zinc (Zn). Fallow syndrome can occur in soils with high soil test P or where broadcast fertilizer is applied and can result in significant loss in grain yield. Application of 20-30 lbs of P2O5 with 1-2 quarts per acre of fully chelated Zn can be effective at reducing the risk for fallow syndrome in corn following sugar beet. The rate of P needed to reduce the risk of fallow syndrome may exceed the previously suggested rates. If applying fertilizer in the seed furrow to correct fallow syndrome, be certain that the rate of the fertilizer source selected will not result in significant stand loss. It is important to note that growing corn following sugar beet does not guarantee that fallow syndrome will occur.