Fertilizer Application With Small-grain Seed at Planting

Applying fertilizer with the seed at planting is one successful soil management practice that has long been recognized as a means to improve crop yields. Grain drills with fertilizer attachments enable the farmer to apply a small amount of fertilizer with the seed and plant in one operation.

A common practice in the northern Great Plains dryland small-grain production area is to apply 50 pounds per acre of 11-52-0 (N+P205+K20) fertilizer with the seed at planting. This practice is called applying ‘starter fertilizer’, which in small grains improves early root and plant growth in low- to medium-P-testing soils.

The starter fertilizer also has the potential to enhance plant development early in the spring, when soil temperatures are normally low and soil nutrient availability to a young root system is lower.

High small grain yields possible with advances in soil management, improved cultivars and reduced soil organic matter have increased the need for fertilizer, particularly N. Most fields require more fertilizer than can be placed with the seed.

Application of higher rates of fertilizer with the seed at planting often causes severe seed germination damage, resulting in poor seedling emergence, poor stands with a subsequent yield loss. Additional fertilizer (NPK) required for maximum economic yield greater than the limits put forth in this publication should be applied in another operation, either broadcast, or banded away from the seed.

Although the broadcast and band application of fertilizers have been successful soil fertilizer management practices, producers once again are interested in applying higher rates of fertilizer with the seed, especially N. This interest has been driven by a desire to [1] reduce the number of field operations, [2] emphasize conservation tillage to improve soil water management, [3] reduce erosion by maintaining adequate residue on the soil surface and [4] use high-residue seeding equipment such as hoe drills and especially air seeders that utilize a higher portion of the seedbed than the traditional double-disc drill.

The question continually being asked is: How much N can be applied safely with small-grain seed at planting? Many factors influence the actual rate of fertilizer to apply, and these factors may vary with specific environmental conditions and locations.

Some site-specific factors that need to be considered when determining the rate of N to apply with the seed include row spacing, seed furrow opener type, seedbed utilized (width of seed spread), soil texture, soil pH, soil water, precipitation, fertilizer placement, fertilizer form, fertilizer material and crop.

Row spacing generally is controlled by the type of seeding equipment. Some normal row spacings found on various seeding equipment include 6, 7, 7.5, 10, 12, 14, 16, 20, 22 and 30 inches. Row spacing usually is set by the equipment manufacturer specific for the particular crops to be planted. Wider row spacings often are achieved with narrow row spacing equipment by plugging some of the seeding units that meter the seed to the openers.

The rate per acre of N that can be applied safely with the seed will decrease as the row spacing increases because at a given rate per acre, the fertilizer is more concentrated and thus more fertilizer is in direct contact with the seed. Some seeding equipment may be designed with a variety of seed openers.

The seed opener refers to the method used to place the seed into the soil. The main categories of seed openers include single-disc, double-disc, offset double-disc, disc-shoe, hoe and sweep or wide shovel, which progressively disturb more soil at the time of seeding. The more soil disturbance, the higher the rate of fertilizer that can be applied safely with the seed because more soil mixing occurs and less fertilizer remains in direct contact with the seed.

The seed opener used on a particular piece of seeding equipment may dictate the row spacing allowed because closely spaced rows on the front gang of openers may be covered by the rear gang with more soil disturbance. Thus the seedbed utilized (SU) becomes one factor controlling the amount of fertilizer that can be applied safely with the seed at planting.

The amount or percent of seedbed that is utilized is determined by the row spacing, the particular type of opener that controls the soil disturbance and the actual width or spread of the seed within the area of soil disturbance.

SU can be calculated by the following formula:

% seedbed utilized (SU) = Seed spread (in.) over Row spacing (in.) x 100

The typical double-disc opener disturbs about 1 inch of soil, and the seed is placed or spread in the same 1 inch. The SU for a double-disc opener with 6-inch row spacing would be approximately 17%. Hoe openers, depending on size, width and depth of seeding, will disturb 2 to 5 inches of soil and normally spread the seed 2 to 3 inches. A hoe opener with 10-inch row spacing that spreads the seed 3 inches would have an SU value of 30%.

The wide shovel or sweep opener, normally associated with air seeder units, will disturb soil as wide as the sweep, and the seed spread will depend on the type of spreader attached to the rear of the sweep on the tillage shank. A chisel plow with 12-inch spaced shanks and 12-inch sweeps that spreads the seed over a 6-inch band would have an SU value of 50%.

The rate of fertilizer that can be applied with the seed will vary based on the calculated SU value. The higher the value of SU, the higher the rate of fertilizer that can be applied with the seed at planting. The maximum rate of N fertilizer that can be applied with the seed at planting based on selected row spacings and SU can be found in Table 1 for the double-disc, hoe and wide shovel air seeder with various widths (inches) of seedbed utilized.

Table 1. Maximum nitrogen fertilizer rates with small-grain seed at planting based on planter spacing, planter type and seedbed utilization.

Soil texture refers to the percentage of sand, silt or clay in the soil. Coarse-texture soils – loamy sand or sandy loam – contain a high proportion of sand. Fine-texture soils – clay loam or clay – contain a high proportion of clay. Medium-texture soils – loam and silt loam – contain a higher proportion of silt.

Soil texture influences the amount of fertilizer that can be applied with the seed at planting in two ways. Texture determines (1) the amount of water retained by the soil and (2) the cation exchange capacity (CEC), or the ability of the soil to adsorb the damaging ammonia ions (NH3) released by nitrogen fertilizers.

Coarse-texture soils have low water retention and low CEC, so seed germination damage will be greater on these soils with the same fertilizer rate than fine-texture soils that have high water retention and high CEC. Table 2 gives a range of the amount of nitrogen fertilizer that can be applied successfully with the seed at planting based on soil texture and the SU for the double-disc, hoe and air seeder-type openers.

Table 2. Maximum nitrogen fertilizer rates with small-grain seed at planting based on soil texture and seeded utilization.

Soil water influences the amount of damage caused by fertilizer applied with the seed in two ways. First, the free ammonia released by N fertilizer materials has a high affinity for water. The water molecules in the soil essentially combine with the NH3, reducing the damaging effect on the seed. Dry soils, as a result of texture or climatic conditions, contain little water, and the excess NH3 (not adsorbed by the soil exchange sites) moves easily through the cell walls into the seed, actually seeking out the water in the seed embryo to cause damage.

Second, fertilizer placed in direct contact with the seed can have a salt effect (burning). In dry soils, water in the seed embryo can move outward to dehydrate the seed, or the soil water can be absorbed by the fertilizer material, then adequate water is not available for the seed to germinate. Rates of fertilizer placed with the seed at planting must be reduced under dry soil conditions.

Another important factor in relation to soil water is whether precipitation is received soon after planting. Because this cannot be predicted, the recommended amount of N applied with the seed should be on the conservative side, especially with urea as the fertilizer source.

The distance the fertilizer is placed from the seed can have a tremendous effect on the rate of fertilizer placed with the seed at planting. Fertilizer placed in a narrow band in direct contact with the seed will have the greatest potential for damage. Damage decreases as the distance from the seed is increased.

This partially is related to SU because greater soil-seed-fertilizer mixing action occurs as the seed and fertilizer is spread out. This can be observed by the diagrams in Figure 1, which show a comparison between narrow placement and three types of different seed and fertilizer distribution patterns.

Figure 1. Four distribution patterns of fertilizer placed with the seed at planting.

The area of fertilizer release will have an effect on placement and mixing of fertilizer in the soil. If the fertilizer is in the same flow pattern as the seed, little mixing occurs unless a spread pattern is employed. However, if the fertilizer is released separately from the seed, to the side, below or behind the seed, greater soil mixing will occur, reducing the potential for fertilizer damage. The type of fertilizer material (granular, liquid or gas) also can affect the desired distance fertilizer is placed from the seed.

Nitrogen fertilizer is manufactured as dry granular materials (ammonium nitrate or urea being the most common forms), liquid materials (aqua ammonia or nitrogen solutions) and gas (anhydrous ammonia). Granular materials remain and react close to the area where placed. If the granular material is mixed with the soil adequately, the germination damage is minimized. Liquid and especially gaseous materials will move farther in the soil from the point of placement.

N movement in the soil is controlled by texture, which determines the voids or pore space where water or air can move. Liquid and gaseous materials can be placed away from the seed and still move to the seed to cause damage. The extent of the damage is controlled by the reaction of the fertilizer with the soil and water, which are controlled by texture. Greater movement and germination damage are caused on coarse-textured soils. Germination damage also is controlled by the fertilizer form.

Fertilizers can damage the seed in two ways. The first, and most serious, way that fertilizers damage the seed is by specific toxicity. For most N fertilizers, ammonia (NH3) toxicity is the largest factor that causes seed damage.

The second way is by salt damage. All commercial fertilizers dissolve in water and make the soil solution saltier near the point of application. Fertilizers such as potassium chloride (potash) and ammonium nitrate can injure the seed by the salt effect. Most phosphate fertilizers have a minimal salt effect.

Fertilizer form controls the amount of nitrogen that is released into the soil as NH3. The NH3 causes severe injury when close to the seed. The greatest potential for germination damage occurs with anhydrous ammonia because it is released into the soil as NH3.

Urea fertilizer has the next highest potential to lower seed germination. Although urea is not an ammonium fertilizer when applied in the granular form, it quickly hydrolyzes to ammonium carbonate in the presence of the urease enzyme commonly found in soil. The ammonium carbonate is unstable and quickly decomposes to release NH3.

Liquid nitrogen materials are nonpressure solutions that contain mixtures of water, ammonium nitrate, urea and ammonia. Liquids contain no free NH3 and generally have lower potential for germination damage, but this depends on the proportion of each fertilizer form in the solution and its respective reaction in the soil.

At high application rates of fertilizer with the seed, the salt injury or “burn” can contribute to germination damage. Fertilizers increase the salt content in the soil solution, which influences the osmotic pressure, which, in effect, causes water movement from a lower to a higher salt concentration. Water moves out of the seed to the fertilizer pellet, actually drying out the seed and causing “burn,” which lowers germination.

A salt index per unit of plant nutrients is given to various fertilizer sources. Nitrogen and potassium fertilizers have a higher salt index than phosphorus fertilizers. Higher analysis fertilizers generally have a lower salt index because less material is applied for the same amount of nutrients. Anhydrous ammonia, ammonium nitrate and urea have a salt index of 0.6, 3.0 and 1.6, respectively.

As higher rates of fertilizer are applied with the seed at planting, the salt effect, in addition to the NH3 effect, becomes a more important factor in determining the amount of germination damage.

The amount of germination damage caused by the application of fertilizer with the seed at planting depends somewhat on the crop species. Some crop seeds are more sensitive to NH3 and salt injury as a result of their size, seed coat type and water content.

Small-grain crops (wheat, barley and oats) are able to tolerate higher rates of fertilizer with the seed than corn or soybeans, which are more sensitive. The tolerance of crops other than small grains can be found in the individual crop fertility circulars, and rate applied with their seed should not be guided by the values in this publication.

The amount of germination damage associated with applications of fertilizer with the seed at planting will vary with seasonal or yearly climatic conditions and among locations. Growers should consider all factors that influence germination damage when determining the amount of N fertilizer to apply with the seed at planting.

Tables 1 and 2 are provided as general guidelines for the amount of N fertilizer to apply with the seed at planting based on only soil texture and seedbed utilization factors. Rates may need to be adjusted upward or downward based on the grower’s specific conditions at planting related to soil water, fertilizer material and crop seeded.

Selected References

Deibert, E.J. 1986. One-pass pneumatic fertilizing-seeding with various N sources and N rates. p. 120-125.

Franzen, D.W. 2009. Fertilizing hard red spring wheat and durm. NDSU Ext. Circ. SF-712(2).

In J. Havlin (ed.) Proc. Great Plains Soil Fertility Workshop. Denver, Colo. March 4-5, 1986. Department of Agronomy, Kansas State University. Manhattan, Kansas 66506.

Deibert, E.J., D.A. Lizotte and B.R. Bock. 1985. Wheat seed germination as influenced by fertilizer rate, fertilizer source and spreader type with one-pass pneumatic seeding-fertilizing. North Dakota Farm Research 42(6):14-20.

Hofman, V., C. Fanning and E. Deibert. 1988. Reduced tillage seeding equipment for small grains. AE826 (Revised). NDSU Extension Service. North Dakota State University, Fargo, ND 58105.