Ask the Agronomist by Rainbow Plant Food: Part 3

Over the years, Rainbow Plant Food has evolved into a family of three distinct brands to meet the needs of our diverse customer base: Rainbow International, Rainbow and Super Rainbow. With several different fertilizer grades designed to meet the needs of a variety of crops and soils, we understand you may have questions about which Rainbow is right for you. In Part 3 of our Ask the Agronomist series, Nutrien Senior Agronomist Mike Howell is answering some of our most recent inquiries about Rainbow fertilizer grades.

Question: Do you have recommendations for fescue/orchard grass hay?

Answer: For any crop, we would first recommend obtaining a good soil test and following the recommendations on the report. From this report, select a grade of Rainbow fertilizer that will most closely meet the needs of your soil. It is highly likely that you will need to add additional nitrogen to meet the needs of the crop, but all other nutrient needs should be able to be met with Rainbow. Rainbow fertilizer will also allow for more even distribution of micronutrients across the field. This will lead to better uptake by the plants and help ensure each plant is getting exactly what it needs. 

Q: I would like to use Super Rainbow fertilizer in my vegetable garden, but I do not have a soil analysis. What is the best Super Rainbow grade to use?

A: Super Rainbow 13-13-13 is a grade that will supply equal part N P and K, as well as necessary micronutrients. This would be a good all-around fertilizer choice. Having said that, we highly recommend that you obtain a good soil test prior to making any fertilizer application. This will ensure that you are applying what the plants need, and not applying extra fertilizer that could contaminate water supply or cost you more money.












13 13 13 2.00   9.00 .100   .10 .40 .20 9.00 .70


Q: Is there a salt index associated with your 5-8-21 rainbow product? Specifically, I have a customer looking to put a safe quantity of 5-8-21 in his air drill on 7.5" spacings. One hopper will be soybeans, one will be 5-8-21. Both products will go in the row.

A: Since we do not have an exact salt index on that Rainbow grade, I have had to extrapolate from other information. The largest component of that grade is potash, so most of the salt index contribution should come from that ingredient, followed by ammonium sulfate.  I believe the salt index of 5-8-21 will be a little less than straight potash.

Since soybeans are very sensitive to fertilizer placed with the seed, the safe rate is fairly low. My calculations estimate the safe rate of potash to place with soybean seed on 7-inch row spacing is about 11 lbs of K2O.  Using 5-8-21 to supply 11 lbs of K2O/acre, I have come up with an estimate of about 50 lbs/acre of that Rainbow grade as the safe limit for seed-placement on soybean. This may be a little on the conservative side because the salt index of 5-8-21 should be somewhat less than that of straight potash. But, with seed-placed fertilizer and possibility of stand loss, we want to err on the safe, conservative side.

Have a question of your own you would like to have answered? Visit our Ask The Agronomist page or tweet your question to us @Rainbow_Plant

Disclaimer: This information is not intended to be complete or detailed use recommendations for all geographies, crops, or applications. User assumes all responsibility for proper use and handling for specific geographies, crops, and applications. Please consult your Rainbow Plant Food representative for complete recommendations for use.


ESN eKonomics CAN17 Rainbow SmartNutrition