Planting corn on soils colder than optimum requires special attention. In addition, fertilizer prices continue to be higher than normal. Therefore, starter fertilization can be useful to supplement pre-plant primary fertilization for corn. This article will summarize the key concepts of using corn starter and the conditions under which it may be more profitable this spring.
Start-up effects on early season corn growth
Common starter fertilizers contain nitrogen (N), phosphorus (P), potassium (K), and sometimes other nutrients. Placing small amounts of nutrients in the seed furrow or in a strip to the side and under the seed row increases the nutrient concentration where the seedling roots are growing. Early season corn growth responses to starter fertilizer are more prevalent in soils that are low in P and K (no preplant P and K application), no preplant nitrogen fertilization, and if corn is planted in cooler soils than usual. With cold soil, seedling root growth is slowed, the diffusion of nutrients through the soil to the root surface is slowed, and the plant’s ability to take up nutrients is reduced. These effects are more likely to occur in soils with moderately poor to low drainage and no-till with a thick residue cover (particularly corn stalks) because these conditions maintain cool soil temperatures longer in the spring.
Starter P has a much greater and more frequent effect on early season plant growth than other nutrients. Phosphorus is essential for early cell multiplication, its diffusion through the soil to the root surface is slower than for N and K, and its uptake by seedlings is limited in cold soil. Early season corn growth responses to starter P are greater and more frequent with no-till versus fall strip tillage and when annual rate P is applied in the fall compared to spring. Early season growth responses are unlikely when the two-year P rate for the corn-soybean rotation is broadcast before corn in the fall or spring, as long as applied rates are not lower than recommended according to soil analysis. (see Iowa State University Extension and Outreach publication PM 1688, A General Guide for Crop Nutrient and Limestone Recommendations in Iowa).
Early season maize responses to starter N are only slightly less frequent than to P. Nitrogen is essential for cell multiplication, and since nitrate is highly mobile in soil, it can be low in the first centimeters of the ground. Early corn growth responses to starter nitrogen are seen primarily without preplant nitrogen fertilization or with preplant fertilization in the fall and when soil nitrate in the top 6 inches of soil is low due to limited mineralization. soil N in cold soils, leaching or deeper injection of fertilizer. or manure.
Early season crop growth rarely responds to starting K unless the soil test K is extremely deficient as it is much less critical for early cell multiplication than N or P and its diffusion through the soil is faster than for P but slower than for K.
What about the grain yield response to the starting application?
While it is common to see increased early season corn growth with starter fertilizer, research in Iowa and the north-central region has shown that grain yield increases are much less common. Indeed, good growing conditions after planting can overcome slow early growth.
Figure 1 shows the results of a corn study in which early growth (plant air weight at growth stage V6) and grain yield were measured when starter PK fertilizer (5-25 lbs. P2O5 and K2O/acre), broadcast PK fertilizer (100-160 lbs P2O5/acre and 100-180 lbs K2O/acre) or both broadcast and starter were applied. High, non-limiting N levels were applied at all sites and in all treatments. The results were grouped into four initial categories of soil test P (colorimetric Bray-1 or Mehlich-3) which were very low or low, optimal (for which only removal-based P is suggested), high (for which only choke is suggested under certain conditions) and very high (for which no P is suggested).
Figure 1. Relative early season corn growth and grain yield responses to application of starter PK fertilizer, preplant broadcast PK fertilizer, and broadcast plus starter in 31 trials of the ‘Iowa in fields managed with no-till or with a chisel/disc plow. Proportionally, early growth responses of maize to PK mixtures were much higher than for maize grain yield. Early increases in growth were almost always largest for broadcast and start-up, smallest for broadcast, and intermediate for start-up. These early growth responses varied inconsistently with initial soil test P levels. Grain yield responses decreased as soil test P increased, and treatments differed only for the lowest soil test P range (very low/low), with an increase 7% for starter only and 15% for jump start or jump start plus start. Yield increases were 12% for the Optimum category, 4-5% for the High category and 0.5-2% for the Very High category. Research results shared in an ICM News article last fall show that economic returns to P or K fertilization are generally negative in high-test soils when kill-based rates are applied.
Early season corn growth and grain yield responses in Figure 1 did not correlate well with the initial soil test K which ranged from low to very high in the P categories of the soil test (not shown). This result is not surprising since potassium affects the early growth of maize only in cases of extreme deficiency. As an example, Figure 2 summarizes the results of a maize study in which starter K, broadcast K, and broadcast plus starter K were compared to uniformly high rates of N and P fertilizer. On average across all sites, applied K decreased early maize growth (V6 growth stage) slightly with starter or broadcast K, but significantly with both broadcast K and applied starter K. Diffused K alone resulted in the earliest K uptake and the highest grain yield.
Figure 2. Relative early season corn growth and grain yield responses to starter K, broadcast K, and broadcast plus starter K in eight Iowa fields. Starter N and K rates applied to the seed furrow should not be too high as the effects of ammonia or salt can damage the seedlings. The traditional rule of thumb for in-furrow starter application is to apply less than 10-12 lbs. of N plus K2O/acre, mainly with fertilizers containing urea (which converts to ammonia gas before converting to ammonium), potash (KCl), potassium nitrate and ammonium thiosulfates or potassium. Several available starter fertilizers (often called low salt starter fertilizers) avoid using these compounds or include very low concentrations. Scientists at South Dakota State University have developed a tool that helps make decisions for in-furrow starter application (decision support for in-seed fertilizers). Seedling damage is more likely in dry soils or with insufficient rainfall. Despite much study and development of this tool, however, the question of how much higher in-furrow start rates are than recommended start rates cannot be answered with certainty due to many unpredictable factors.
What about Starter P and K in high test soils?
Although grain corn yield responses to P and K are unlikely in high-test soils, many farmers still apply kill-based rates as an “insurance” practice. This does not make economic sense with currently high fertilizer prices. As shown by the results in Figure 1, a lower and more economical application of starting P is sufficient to avoid any unlikely and low yield loss.
Additionally, infrequent responses of corn kernel yield to starter NPK in high test soils are often due to N in the starter. As an example, Figure 3 summarizes the results of a corn study conducted in eight farmer fields managed without tillage and tested at high or very high soil P and K. The measures were early season growth (growth stage V6) and grain yield. responses to liquid starter fertilizers NPK (5-10, 15-25 and 0-7 lb N, P2O5 and K2O/acre), N alone (UAN to provide 23-30 lb N/acre) and starter rate mixed with UAN to provide a total of 23-30 lbs N/acre. All fertilizers were applied approximately 2 inches to the side and below the seed row. There were significant increases in early growth of all starter treatments (26-36%) which were explained by N in starter or synergy between higher N level with PK in starter. In contrast, grain yield responses were much lower (2–3%) and were explained by applied nitrogen.
Figure 3. Early season no-till corn growth and grain yield responses to liquid starter NPK, starter N, and mixed starter with UAN in eight Iowa fields with P and K high or very high.
In summary, the conditions under which the use of corn starter is likely to pay off are:
- Lower than recommended N, P or K broadcast application rates.
- Little or no spring application of preplant nitrogen, mainly for corn after corn.
- Plant in cooler than normal soils.
- Direct seeding with high residue coverage, especially in maize after maize.
- Moderately to poorly drained soils, often cooler than other soils.
- High test soils instead of applying much higher removal based P and K rates.
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