From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Summer 1996, No. 6


In the spring of 1995, 18 bushels of grain sorghum was worth about $30. In the spring of 1996, the same amount is worth over $70! What’s 18 bushels? It is the three-year average response to starter fertilizer placed two inches to the side and two inches below the seed in a no-till system measured by Kansas researchers. The starter fertilizer used supplied 30 pounds per acre of both nitrogen and phosphate. There has never been a better time to use starter on grain sorghum.

In no-till environments both nitrogen and phosphorus are important in the starter band due to lower soil temperatures that slow the release of nutrients from organic matter and reduce absorption of nutrients by plant roots. Research has shown that the usual 1:3 nitrogen to phosphate ratio used in starters may not supply adequate nitrogen for optimum response. A nitrogen to phosphate ratio of at least 1:1 often out-performs starters with less nitrogen or containing only nitrogen or only phosphorus.

Will sorghum respond to starter if soil test phosphorus levels are high? Under high crop residue conditions, research has shown that starter phosphorus responses do occur at soil test levels where response to broadcast applications would be minimal. For example, the 18-bushels-per-acre Kansas responses occurred at a Bray-1 soil test level of 24 parts per million, an “adequate” soil test rating.

Can starter response still be expected if sorghum planting is delayed to a time when soil temperatures are likely higher? Yes, according to the Kansas studies. Yield responses for the mid-May and late June planting dates were equal across the three years of the trial.

What if the summer of 1996 is dry? Starter fertilizer could be even more important due to increased water use efficiency by the sorghum plant. Starter use can also shorten the amount of time required for sorghum to mature. If producers intentionally delay planting to avoid anticipated drought or heat stress during the reproductive phase of crop development, they increase the risk of an early frost occurring before the crop has matured. The Kansas studies showed a seven-day average reduction in the number of days to reach mid-bloom.

Although placing the starter band below and to the side of the seed is probably ideal, good responses have also been measured when the fertilizer is placed with the seed. The major disadvantage to seed placement is that the total amount of nitrogen plus potash should be kept below 10 pounds per acre for 30-inch rows to avoid salt injury, especially under dry conditions. This may not meet the starter nitrogen needs of grain sorghum in high residue environments.

Use of a starter fertilizer is a management practice grain sorghum producers can use to capitalize on good grain prices, improve water use efficiency and reduce the risk of frost injury.

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