When calculating phosphate fertilization in soils cultivated for more than ten years, it is first necessary to decide whether we will use “chemical” or biological reasoning. In the first case, which is more usual, it is enough to use synthetic fertilizers of high solubility and apply three or more times the quantity required by the plant (extraction), taking for granted that more than half (sometimes 80%) of the product applied, it will not reach its destination (the plant). This is due to the adsorption and edaphic immobilizations.
In agroecosystems that have been cultivated for years, but now fertilized with quality organic fertilizer, it is essential to understand that the rate of absorption of the element Phosphorus by the plant depends much more on other factors than on the quantity or solubility of the fertilizer used.
In this case, the word "solubility" should be replaced by "availability". Hey, you might ask, but to be available, doesn't it have to be soluble? From the point of view of fertilizer, the answer is yes. However, this solubility does not have to exist beforehand; we don't have to pay for it. It can happen inside the soil.
When we significantly reduce high solubility fertilizers and start to apply organic fertilization, it is always good to point out, quality, low solubility phosphate fertilizers usually deliver the best results, at a lower cost. This is due to a natural movement of the system that, “recovered” by the detoxification and revitalization of the soil, works again.
Interdependent factors, which occur in a systemic way, can be cited here, to explain in a more simplified and didactic way.
1. Increased activity of phosphate solubilizing microorganisms;
2. Increased root colonization by arbuscular mycorrhizal fungi;
3. Increased biocycling of organic phosphates;
4. Increased secretion of specific root exudates;
5. Increased root hair, and root volume and compliance;
6. Increase in the water retention capacity of the soil;
7. Decreased degree of compaction and increased oxygenation of the soil.
It is important to understand that the increase in the population, activity and diversity of the soil biota is a consequence not only of the replacement of the fertilization model, but also of the management of plant biodiversity and the reduction of the use of substances that are aggressive to life.
In turn, the physical factors of the soil, mentioned above, are both an effect and a cause of biological changes. For example: the detoxification of the soil makes the worms, larvae, beetles etc. frequent again in large numbers. The activity of these organisms revolves and unpacks the soil, forms galleries inside, facilitates the penetration of the roots. Meanwhile, much smaller organisms, such as fungi, secrete glomalin, a substance that “glues” soil particles, aggregating it and thus allowing greater aeration and water retention. These new physical conditions, in turn, favor the increase of life inside the soil.
Natural phosphoric rocks, only finely ground, unlike highly soluble fertilizers, treated with sulfuric acid, provide the necessary phosphorus stock, without significantly inhibiting the natural processes of obtaining this element by plants. There are hundreds of scientific studies proving, for example, the strong inhibition of mycorrhizal activity, when using high solubility phosphates. In addition, rates of immobilization by the soil, the phosphorus of these rocks just ground, are much smaller.
When adding remineralizers, always finely ground (> 60% passing through a 0.3mm sieve) containing phosphorus, at the beginning of the composting process, to make our enriched organic fertilizer, we will have the action of decomposing microorganisms. Their activity promotes an increase in the mass temperature (up to 70 ° C), as well as the formation of organic acids, which, after a few weeks, build chemical bonds between the carbon chains and the minerals of that rock, chelating or complexing them. increasing their efficiency in plant nutrition.
For all these reasons, and considering the lower immobilization and greater efficiency in absorption and reuse, obviously the amounts of phosphorus applied to the soil, no longer need to be as much greater than those extracted by the plants. But ... what if the levels of soluble phosphorus in the soil are low? Always thinking about soils already cultivated and fertilized in the conventional model for years, we can say that the Phosphorus is there, and in great quantity. This is proven easily, through a “total phosphorus” analysis. Of course, this does not apply to poor soils in Phosphorus, in its first years of use, such as those of a typical Cerrado, for example.
Therefore, the analysis of conventional soil, melich or resin, in these cases, is unnecessary, as it does not represent what we really want to know: will the plant have access to the Phosphorus or not?
Another caveat is the physical conditions of the soil, especially the degree of compaction, aggregation and the water holding capacity. Even knowing that Bioactivation significantly improves these attributes, some care is needed in the first year of the transition, notably in the case of short-cycle crops. In this case, it may be better to plant soybeans (for example) with a small amount of soluble phosphate fertilizer (up to 40Kg P2O5 / ha) in the planting furrow. Crops with very high populations of harmful nematodes also fit in this case, due to the reduced volume of roots.
In short: when choosing quality organic fertilizer, the producer, in order to obtain the best results, should also opt for the transition from the conventional model to an ecological one. It's what I've always called soil bioactivation. It is not about using an A or B product. We are talking about systems technology, not products. The first steps towards an agroecological transition, according to EMBRAPA itself (Landmark in Agroecology, 2006) are:
1. Reduction and rationalization of the use of aggressive inputs and handling;
2. Replacement of aggressive inputs and handling by others, favorable to life;
3. Management of plant biodiversity.
In other words, we need, on the one hand, to reduce the amount used, of substances that are aggressive to life, such as pesticides (or chemical pesticides), synthetic fertilizers, highly soluble, salinizing or acidifying. On the other hand, we need to increase the use of substances that promote life, such as a well-made organic compound, remineralizers, microbial inoculants, humic acids, algae extracts, among others. Having done this first “homework”, whose benefits normally appear since the first year, contrary to what they say out there, we can then think about redesigning the entire production system, on other bases, more appropriate to the new times.
Antonio N.S. Teixeira
Diretor Executivo IBA