I received this question last spring. I have old bags of fertilizer in my shed; are they still good, can I mix them together?
Yes, absolutely, don’t throw any of it away and you can mix them together. What is most confusing when trying to understand fertilizer is the word “nutrient.” “Healthy plants need lots of nutrients.” “Your plant leaves are choleric (whatever that means), it has a nutrient deficiency.” So what the heck are nutrients?
In descending order, the nutrients plants need to grow strong and healthy are; carbon, hydrogen, oxygen, nitrogen, phosphorous, potassium, calcium, magnesium, sulphur, boron, chlorine, copper, iron, manganese, molybdenum and zinc. “Hey wait, aren’t those elements?” Bingo! Elements, tiny atomic particles that are not going to trade protons or electrons (unless you have the Alchemist’s formula for changing lead into gold), or go bad sitting in the bags inside your shed.
96% of all organic life forms; plants, animals and humans are made up of only four elements; carbon, hydrogen oxygen and nitrogen. Phosphorous adds another 3%; the other eleven elements total only 1%. Bundled together, the top five elements are referred to as Macro-nutrients, and the other eleven as Micro-nutrients.
Plants obtain their macro-nutrients from the carbon dioxide (CO2) in the air, water (H2O) and nitrates (NO3) in the soil. The micro-nutrients are all available as trace elements in the soil. So, if all of the elements are available to the plants in the air and soil, why do we need to add fertilizer?
The simple answer is we don’t. Adding compost, peat and other decomposed plant material continuously to the soil will make our plants happy and healthy. BUT, if you personally are not happy with a yield of only 367 tomatoes and really want 400, then you need to provide more macro-nutrients; and that means fertilizer.
Really, the whole argument for and need to fertilize has to do with production farming. Scientists have been studying and reporting on the relationship between plant performance and the availability of elements since the late 1880’s. The evidence is conclusive that yields increase with the addition of fertilizer; and today, when you are trying to feed a planet of 7 billion people you need a lot of food. We, the home gardener have benefitted from this research.
Now back to those bags of fertilizer, what do you do with all those different kinds? There are so many; Bulb Food, Rose Food, All Purpose, Bloom Booster and Ideal Tomato Food. Then there are those numbers; 10-12-10, 15-5-13, 6-7-7, 10-16-10, 8-10-8; it is like a lottery. If I accidently side dress my tomato plants with Rose Food will it kill the plants?
My question is, how does the tomato plant know that the phosphorus in the soil came from Rose Food and it can’t absorb any of it? Is it reserved for roses only? The answer is that it doesn’t. Elements are elements are elements and it will take it, thank you very much.
Manufactures vary the percentage of nutrients according to the accepted preferred end result. We don’t want lettuce to go to seed, we do want a lot of flowers on our Dahlias. They are trying to make gardening easier and in the end make it more complicated.
Somewhere on every fertilizer package there are three numbers: 10-12-10, 15-5-13, 6-7-7, 10-16-10, 8-10-8; they correspond to the elements Nitrogen-Phosphorus-Potassium and identify the percentage amount of each in the package. These elements are also identified on the package as the N-P-K.
Nitrogen pumps up plant growth through photosynthesis by making the chlorophyll greener. Phosphorus transfers the energy produced by photosynthesis to the “Roots and Shoots”, promoting cell division and growth. Potassium is the catalyst that keeps the plant cells communicating and processing food, thus keeping the plant healthy.
The easiest way to visualize N-P-K is that if you had a 100 pound bag of 15-5-13 fertilizer; it would contain 15 pounds of nitrogen-5 pounds of phosphorus-13 pounds of potassium, and 67 pounds of filler material. Filler material contains trace amounts of micro-nutrients; traditionally they were not in sufficient levels to be identified on the package. Today some packages display lines like; “Contains Sulphur” or “Includes Iron” as a discriminator, creating separation from their competition. If it isn’t listed with the ingredients, it is still just filler material.
Fertilizer is sold to us in three formats; water soluble crystals that dissolve in water and is used as a quick feed or foliar spray; granular particles that are turned into the soil prior to planting or used as side dressing during the growing season; and slow release pellets that are typically mixed in container soil and breakdown over an extended period.
If you are going to consolidate your fertilizer, and want to feel confident that you know the blended N-P-K values; then you should not inter-mix the formats. In other words, keep the water soluble, granular and slow release pellets separate; don’t mix water soluble with granular.
So, let’s say that you are mixing that 15-5-13 fertilizer with 10-12-10. The easiest why is to combine equal amounts of each in a bowl; volume doesn’t matter, equality does. Now here is the formula:
15 5 13 67 = 100
10 12 13 65 = 100
25 17 26 132 = 200
Divide all of the numbers by 2,
12.5 8.5 13 66 = 100
Oh, you have a third bag of 6-7-7? Then:
6 7 7 80 = 100
15 5 13 67 = 100
10 12 13 65 = 100
31 24 33 212 = 300
Divide all of the numbers by 3,
10.3 8 11 70.6 = 100
Now here is the kicker. As you mix all of your fertilizer together and calculate the N-P-K, you may notice that the percentages begin to average out close to the same levels. In other words, you will have mixed what is sold as a “General” or “All Purpose” fertilizer; perfectly beneficial for your plants. So maybe you should just buying one bag of 10-10-10 and skip the alchemy.
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