Subject: What are the numbers on the label?

Date: May 23, 1997

Dear SWF:

When I look at all of the products on the shelves I am confused.  I have a small vegetable garden and would like to use only organic products.  What are the numbers on the label?  How can I decide which product to use?  Why are the numbers on the various labels so different?  They seem to range from 0-0-1 all the way to 20-20-20. Help!

Cecile T.

The numbers on the labels are required by law and indicate the content percentage of the big three nutrients: Nitrogen (N); Phosphorous (P); and Potassium (K), respectively.  The numbers given are the guaranteed minimum percentage.  When lab tested, the nutrient content must equal or exceed the numbers on the label.  In the commercial world this is necessary so the grower can make the calculations as to how much of each product would be necessary to apply the requisite amount of each nutrient per acre.  It also cuts down on the sale of “snake oil.”  In the homeowner world, the numbers also inform and protect the buyer, but present huge marketing opportunities.

There are many factors that affect nutrient requirements of plants.  Location, environment, soil, growth stage, etc…  A farmer that has grown sweet peas for twenty years has a pretty good idea of the N-P-K requirements of the sweet pea plant.  But if he changed to potatoes, then obviously the nutrient requirements would also change.  There is test data galore that helps determine the specific requirements of the various plants, but in the homeowner world, we mostly use averages and good practices.  It is most difficult to translate the nitrogen requirement per acre to a single 10’ row of tomatoes.  To compound this situation, a plant has different requirements at different growth stages.  For instance, some plants would require less nitrogen and more phosphorous during times of flowering.  As the fruit is developing, less nitrogen would also be required.

Taking advantage of this ambiguity, some companies take the “more is better” approach.  Many people have ruined, stunted, or burned their plants thinking the high test is better than the regular strength.  One company offers the 10-10-10 and another offers the 20-20-20 and a third offers the 20-10-20.  Remember, these are percentages.  The numbers do not mean that one product is more powerful than another; it means you would use less of a product to apply the same amount of nutrients.  If you were comparing a product with 1% nitrogen and one with 3% nitrogen (Seaweed/Fish Fertilizer) and one with 10% nitrogen, the mass merchandisers would have you believe the 10% product is superior to the other products.  That is not the case.  The reality is that in order to get the same amount of nitrogen to a plant it would take three times as much of the 3% product than the 10% product and ten times as much of the 1% product.  So if you use one scoop (1 Tbsp.) of the 10% blue crystals per gallon of water, you would use three Tbsps. of the 3% product and ten Tbsps. of the 1% product.  It’s all mathematics and not magic.

A plant can only take advantage of a certain amount of nutrients at a time, so clearly more is not better.  The remainder either goes unutilized or harms the plant.  I once heard this process compared to an “all you can eat” buffet.  In the end it does not matter how much food is on the buffet table, you can still only eat so much.  Plants need more than N-P-K.  It is important to read the label to see what else is contained in the product.  Research has shown that the micronutrients, vitamins, and plant growth hormones found in seaweed are most beneficial to the growth and good health of plants.

The key is to choose quality products and develop a feeding schedule for your plants based on knowledge, experimentation, and experience.


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