Organic products are complex in terms of their elemental compositions. An unknown substance in an applied organic product may turn out to be detrimental to plants. The damage can sometimes be seen immediately, or might appear after continued use of the organic product. Growers need to know the total composition of the organic products they are applying to their plants, whether they are applying them to seeds, directly on the plant roots, as a foliar spray or adding them to a rooting medium.
A home gardener I once worked with, who was supplying his neighbors with vegetables from his garden, decided to grow organically. He contacted the state agency that grants such a designation and began to make the necessary changes in his cultural practices to comply with the requirements so his products could carry the organic label. Finding a suitable nitrogen source for a fertilizer was a major challenge, but he was able to find a chicken litter compost that had the required nitrogen content and also met state requirements.
After several years of producing good yields of vegetables of excellent quality, visual symptoms of plant stress began to appear in the garden. The grower thought the symptoms were due to a possible nutrient element insufficiency, so he contacted his local agricultural extension agent, who took both soil and plant tissue samples and submitted them to the state soil testing and plant analysis laboratory. The assay results were sent to me for interpretation.
The soil test levels for both phosphorus and potassium were in the excessive range and the tissue results reflected the adverse effect these high soil elemental contents were having on the plants. The corrective recommendations were to abandon the garden site, start over and not use the chicken litter compost unless applied on the basis of its total elemental content, not nitrogen alone. The home gardener had not considered the relatively high phosphorus and potassium contents in the compost; his focus was mainly on supplying sufficient nitrogen.
Vermicompost is generated by the decomposition of organic substances by worms, with the final compost containing essential elements. It therefore meets the requirements for organic designation. Store-bought vermicomposts will normally carry an N-P-K fertilizer ratio label, but I have found that the number is usually less than the actual content of these three elements. The results of an elemental analysis of six commercially available vermicomposts is given in Table 1:
Fertilizer Element Sample Designation
A B C D E F
Nitrogen (%) 1.25 1.78 1.07 3.54 3.70 0.77
Phosphorus (%) 0.11 1.48 0.77 1.76 0.16 2.10
Potassium (%) 0.08 0.79 0.89 4.16 0.22 0.69
In addition to a wide variance in fertilizer element contents, there were equally significant differences for several other essential plant nutrient elements such as calcium, magnesium, copper and zinc in these six vermicomposts.
The variance of nutrient levels in various vermicomposts reflects the range in elemental content of the source material composted by the worms, as well as the extent of composting. Generally, the longer the composting takes, the higher the elemental content of the end result.
From Table 1, it can be determined that growers selecting vermicompost A, for example, could expect a considerable difference in plant response as compared to what would occur if vermicompost D had been selected.
In additional experiments, more research found varying responses of plants germinated in various rooting media with added vermicompost. Compost teas generated from various vermicomposts also gave varying, inconsistent results. Unless growers know the actual N-P-K contents of a vermicompost, they will not be able to make a correct decision as to its proper use.
It has been suggested that humic acid adds beneficial effects when brought into contact with plants. Humic acid, which encompasses any of the various acids obtained from humus, is not a specific substance, but a mixture of acids whose forms and concentrations are determined by the source (soil, compost, organic soil, etc.) from which they are extracted, and by the method of extraction and techniques used for purification.
Humic acid products will have properties that reflect their acid composition, which is determined by their derived sources and the methods used for extraction and purification. To properly identify what substances exist in a humic acid product, and at what concentrations, an infrared spectrum is required.
Therefore, humic acid products and the effects they have on plants vary considerably, and can either be beneficial or detrimental. I recall one grower adding humic acid to the nutrient solution for a greenhouse tomato crop only to find the plants began to wilt and blossom end rot began to appear on generating fruit, a completely unexpected response from that given by the humic acid supplier who said that fruit yield and quality would be enhanced.
As evidenced in these three examples, growers selecting organic-based products need to know the actual compositions of the products and their actual concentrations, not just generally accepted or approximate concentrations. Growing plants organically rather than using procedures based on a mix of chemical and biological treatments is a greater challenge in terms of obtaining and sustaining sufficiency.
The potential for insufficiencies when growing plants organically is due to the characteristics of applied substances for meeting plants’ nutrient element requirements, as well as based on speculated needs other than nutrient element requirements.
Hydro shop owners should be able to provide actual content data of all the substances included in organic-based products, along with specific instructions regarding product use. Be sure to also ask them to identify the potential detrimental effects that may occur with use.