Iowa Admin. Code r. 21-43.1
Additional plant nutrients, besides nitrogen, phosphorus and potassium, when mentioned in any form or manner shall be registered and shall be guaranteed. Guarantees shall be made on the elemental basis. Sources of the elements guaranteed shall be shown on the application for registration. The minimum percentages which will be accepted for registration except for those fertilizers designed to be applied and ordinarily applied directly to growing plant foliage to stimulate further growth are as follows:
| Element | Percent |
| Calcium (Ca)........................................... | 1.00 |
| Magnesium (Mg)........................................ | 0.50 |
| Sulfur (S)............................................. | 1.00 |
| Boron (B)............................................. | 0.02 |
| Chlorine (CI)........................................... | 0.10 |
| Cobalt (Co)............................................ | 0.0005 |
| Copper (Cu)........................................... | 0.05 |
| Iron (Fe).............................................. | 0.10 |
| Manganese (Mn)........................................ | 0.05 |
| Molybdenum (Mo)...................................... | 0.0005 |
| Sodium (Na)........................................... | 0.10 |
| Zinc (Zn)............................................. | 0.05 |
Guarantees or claims for the above-listed additional plant nutrients are the only ones which will be accepted. Proposed labels and directions for use of the fertilizer shall be furnished with the application for registration upon request. Any of the above-listed elements which are guaranteed shall appear in the order listed, immediately following guarantees for the primary nutrients, nitrogen, phosphorus and potassium. Warning or caution statements are required on the label for any product which contains 0.03 percent or more of boron in a water-soluble form or 0.001 percent or more of molybdenum.
Iowa Admin. Code r. 21-43.1