Use of fertilizer placed in close proximity to the seed at planting is recommended in no-till or strip tillage methods of planting and if corn is planted early - i.e. before April 1 in the southern half of the state and before May 1 in the northern half of the state. In these situations, low soil temperatures slow root growth and nutrient movement from the bulk soil to the rhizosphere. Cool temperatures also reduces soil microbial activity, curtailing mineralization and the release of organically bound mineral nutrients into the plant available pool. Placing fertilizer close to the seed helps the corn plant emerge and establish. Recent research conducted in Indiana illustrates the starter response observed in reduced tillage systems (Table 1).
|Table 1. Effect of tillage on corn crop response to starter fertilizer in 11 Indiana studies.|
|Number of significant responses||Average yield increase of significant response||Overall yield response|
|Tillage system||bushels per acre|
|Conventional||1 in 11||12||0.9|
|No-Till||8 in 11||10||7.8|
|Starter fertilizer placed two inches to the side and two inches below the seed with a least a 100 lb. of fertilizer material.|
In many states the response to starter fertilizer is due to the increased localized availability of P. In Indiana, however, the observed response appears due to enrichment with N in the seedling root zone (Table 2).
|Table 2. Nutrients responsible for starter response in 6 Indiana studies|
|lb. N-P-K/a||Corn yield (bushels per acre)|
|Studies used 2 by 2 fertilizer placement and soil P levels before application were rated high to very high.|
Experimental results also indicate that not only should starter N be used in cool conditions (early planting and/or no-till) but when the N application is injected into the soil or applied in a sidedress application after the six-leaf stage. In either case, starter N application improves seedling growth and development during the interval between planting and sidedress application or until seedling roots grow down to the injected fertilizer band.
The Indiana studies on starter fertilizer used experimental sites where initial soil test levels were above that at which a response to fertilizer P or K would be expected. On these soils, responses to starter P or K would only be seen with very cool ambient soil conditions. At lower soil test levels (Bray P1-P < 10 ppm and/or exchangeable K < 75 ppm), a response to P and/or K in starter fertilizer may be expected regardless of the time of planting and tillage system used. Such a response has been observed in starter trials conducted in Michigan, Illinois and Wisconsin.
Finally, however, there are several reasons to consider including P or K in starter fertilizer, even when no response is expected. A common approach to P and K fertility management in corn and soybean production is the build-up/maintenance/drawdown characterization of soil test levels (see Tri-State Fertilizer Recommendations, E-2567). When soils tests are above the critical level, smaller amounts of P/K may be recommended to replace nutrients removed with the crop and to maintain soil test levels. Starter application may be a convenient and efficient way to apply these lower rates.
General recommendations range from 20 to 40 pounds per acre of N, P2O5 and/or K2O when placement is 2 inches to the side and 2 inches below the seed. The N application alone should not exceed 40 pound per acre, and the total amount of salts (N plus K2O) in the application should not exceed 100 pounds per acre. On sandy soils, the maximum rate for N should be reduced to 30 pounds per acre. At N and/or K2O starter rates above this level, salt damage to the germinating seed may occur. Where starter P is expected to help overcome low soil temperatures, 10 to 20 pounds P2O5 per acre are generally recommended.
Some growers place the starter fertilizer directly with the seed. When "pop-up" placement is used, the risk of salt damage increases substantially. Thus, in "pop-up" applications, the total N plus K2O rate should not exceed 8 pounds per acre. If the soil is sandy and has a low cation exchange capacity (CEC < 7), low soil buffering of salts further increases salt damage risk and rates should be reduced to a total maximum of only 5 pounds per acre N plus K2O.