Organic Fertilizer Elemental Contents: Do You Know What You’re Feeding Your Plants?

A colleague who owns both a couple hydroponic stores called me with an inquiry. He and his customers had tried to wade through the current maze of organic fertilizer products in order to make a selection that would meet the elemental requirements of the plant species that they were growing. They didn’t know what to do.

State statutes that regulate the labeling and sale of materials as fertilizer do not fit well those substances that are identified as containing organically-sourced essential plant nutrient elements.

A material marketed as fertilizer, in terms of its elemental content, requires an analysis that identifies its percent content.

For nitrogen (N), the amount on the label is the elemental total; for the element phosphorus (P), the number represents the citrate-soluble form (expressed as the oxide, P₂O₅); and for potassium (K), it is the water-soluble form (expressed as the oxide, K₂O).

The other essential plant nutrient elements, if guaranteed, are identified as their elemental total content, although some might use their oxide form.

However, labeling is not the primary concern for the user; when using an organic fertilizer as a source for a particular element, it is the accompanying elements—identified or not—that can result in an elemental insufficiency.

A recent correspondent asked me about various organic-based sources for N, in particular wondering if blood meal would be suitable (it has relatively high N content). I responded with another question: What about the companion elements?

In blood meal, P has been reported to be as high as 1.0% (2.2% P₂O₅), depending on the blood source. With repeated use of blood meal as an N source, the accumulating P could eventually result in an excess that could adversely impact plant growth.

I was sent a sample of a blood meal material that had excellent physical properties, finding that its N content was 14.0%, the high end of that reported in the literature. Another blood meal product I saw listed only N (at 12.0%).

This range in N content raises an interesting question: what is the expected natural range in N content among blood meal products? It may seem like an inconsequential question, but could have significant importance if blood meal is the only available N source.

The other element of interest would be P, reported to be 1.0% in some literature, but was found to be 0.6% in the sample that I assayed. I decided to purchase a package of blood meal from a local fertilizer store for analysis. The results of the two blood meal products are given in Table 1.

When I was preparing the samples for elemental analysis, I found that blood meal product A went into solution when concentrated nitric acid was added to the digestion flask, while blood meal product B did not.

The physical form of the blood meal product A is a small pill, while blood meal product B was a mix of fine and small particles of varying size. Would the differences in solubility and particle characteristics have an effect on the reactivity of blood meal when added to a rooting medium? It’s a question that needs to be determined by testing.

The differences in elemental content between in the two blood meal products is considerable (see Table 1), and many of the element contents are not in agreement with values found in the literature for blood meal (N higher, P and K lower).

So, the next question is: what would be the range in elemental content among batches of the same blood meal product, as well as between different blood meal sources? For a grower assuming that the blood meal product being using has a 12% N content, what plant growth effect would occur if the actual product contains 16.6% N?

The other element in blood meal that can affect a plant’s physiology is iron (Fe), 0.42% in product A and 0.47% in product B. Depending on the growing conditions and method of use, high Fe availability can interfere with zinc (Zn) metabolism in the plant by inducing a Zn deficiency, a deficiency that might not be visually evident, but will result in reduced plant growth.

The elements in an organic fertilizer are not in that form required for root absorption, therefore requiring decomposition of the organic matrix. Such decomposition might not readily occur and, in addition, decomposition might then result in either the transformation of an element into another form not available for root absorption, or be absorbed by the micro-organisms present in the rooting media.

Those organisms that carry out decomposition are also plant-like, having the same elemental requirements as the green-leaf plant growing in the rooting medium. Since these organisms are strong competitors, they are in position to absorb released elements and thereby deprive the plant.

A good example is what occurs when a highly carbonaceous substance is added to a soil, as the growing plant might become N deficient as the organisms in the rooting medium absorb whatever N is available in order to decompose the added material, thereby depriving the plant of N.

There are three physical phases within a rooting medium: organic, inorganic and a solution phase. The factors that will determine which equilibrium exists within the solution phase are determined by the physical and chemical characteristics of the mineral and organic components plus the factors of temperature, moisture level and pH.

With the addition of an organic substance to a rooting medium, a dynamic and ever-changing environment will evolve, which could significantly alter the biochemical activity within the rooting media. However, plants do not grow well when an active biological environment exists within a rooting medium. Such activity consumes oxygen and interferes with the movement of ions within the water solution that surrounds plant roots.

When an element is not sufficient in the rooting medium, the trick is to apply only what is required by the plant, adding either an inorganic or organic fertilizer source of that element. In order to do so, three things need to be known:

• The nutrient element requirement of the plant.
• The available concentration of that nutrient element already existing in the rooting medium.
• The quantity of nutrient element being supplied by the applied inorganic or organic fertilizer.

For most inorganic fertilizers, the elemental content is usually known for all the elements in the fertilizer.

For most organic fertilizers, on the other hand, the primary element content is known and the accompanying elements are not.

For the user, this can result in an elemental insufficiency of a companion element when using an organic fertilizer.

The remedy: have the organic fertilizer assayed for its elemental constituents.

The next step is to determine what the rate of application should be to provide the required element while not adding an unneeded element that can lead to an excess, creating a nutrient element insufficiency in the growing plant.