Reading fertilizer labels for an informed choice

Below are some insights on this topic, with particular reference to fertilizers containing organic matter.

Fertilizers: mineral, organic or organo-mineral products capable of supplying nutrients to plants (fertilizers) or improving soil properties (soil improvers/correctives).

Fertilizers: products whose main function is to supply nutrients to plants (Nitrogen, Phosphorus, Potassium, micronutrients). They are divided into:

• mineral fertilizers, of inorganic origin, single (one nutrient) or compound (NP, NK, PK, NPK);
• organic fertilizers: derived from materials of biological origin, containing carbon bound to nutrients;
• organo-mineral fertilizers: mixtures of mineral and organic fertilizers.

Soil improvers: natural or synthetic substances, mineral or organic, capable of modifying and improving the chemical, physical and biological properties of a soil.

Correctives: substances capable of modifying the chemical characteristics of the soil, in particular pH (correction of acidity or alkalinity).

The most “noble” matrices are represented by manures: cattle, sheep, horse, poultry and rabbit manure. However, there are also organic soil improvers produced from different matrices, such as simple vegetal soil improver, mixed composted soil improver, green composted soil improver and peat.
The agronomic performance of these products is not equivalent due to different parameters, such as the humification rate and the C/N ratio. For this reason, reading the label is essential in order to select the type of soil improver best suited to the needs of a specific soil.

However, other considerations must also be made.

The first concerns Nitrogen present in the organic fraction. The Nitrogen actually available for absorption by plant roots is that which is soluble and present in the soil solution, and for certain matrices the label must state how much of the Nitrogen contained is effectively soluble.
But what is the nature of the remaining Nitrogen present in the organic matrix? Is it slow-release, and therefore able to contribute to crop nutrition over more or less extended periods, or is it simply insoluble? Clearly, this parameter makes a substantial difference and must be known.

With regard to the other two macronutrients, it is important to verify the water solubility of Phosphorus and, in the case of organo-mineral fertilizers, the presence of Potassium derived from sulphate or chloride.

Another important aspect to verify on the label of an organo-mineral fertilizer is the organic carbon content. Since this is a fundamental factor in determining soil fertility and the nutrient use efficiency of the elements contained in the fertilizer, a carbon content equal to or higher than 7.5%, which is the legal minimum, will clearly make a significant difference.

• mycorrhizal fungi
• rhizosphere bacteria
• Trichoderma spp.

Simply declaring the generic presence of microorganisms in an organic fertilizer is not formally correct: the presence of microorganisms in organic matter is normal, but these are not necessarily beneficial microorganisms for plants.
Even when the generic product description mentions the presence of microorganisms, what is legally binding and must be verified is what is stated on the label.

Paying attention to fertilizer labels is essential in order to make the correct choice, based on effective soil and crop management, and to avoid unnecessary and harmful farm costs and environmental impacts.