Correct management of phosphate fertilizer in sugarcane

The efficiency of phosphate fertilizer is influenced by several factors such as soil type, method of application, source, etc. Its management must favor the absorption of nutrients with the reduction of fixation processes in the soil, consequently increasing the use of phosphorus by plants. The doses of phosphorus applied in fertilizers are much higher than the quantities exported, and the export of this element by sugarcane is 10% to 15% of the amount of fertilizer applied.

In tropical soils, one of the main factors for this low efficiency of phosphate fertilization lies in the high levels of oxides, aluminum and iron, which promote the fixation of the elements. For the usual recommendation, the best way to apply phosphorus to sugar cane is in the total area, with phosphorus, better distributed in the area, having more rooting and a greater part of the soil explored.

In general, the efficiency of phosphate fertilizer is low, which demonstrates the importance of new application methods that take into account sources, times of application and location of the fertilizer. The appropriate combination of corrective fertilization in the total area and maintenance in the planting furrow is of great importance in order to obtain an increase in production.

Due to the importance of phosphorus for the development, productivity and longevity of sugarcane fields and its high rate of fixation in tropical soils, there has been an increase in studies of doses and forms of application of this element. The evolution of scientific knowledge in various areas of soil science is a reality and the management of phosphate fertilizers has followed this evolution. However, the efficiency of extractors in predicting available phosphorus and the use of critical levels to recommend phosphate fertilizers are topics of great interest in soil fertility due to the lack of more up-to-date information.

Phosphorus retention in the soil (adsorption) has been known for a long time and has been extensively studied. The use of phosphorus by plants is of great value when it comes to avoiding losses caused by leaching. However, it is problematic when retention is strong and the phosphorus transforms into non-labile forms. The retention of phosphorus added to the soil occurs through the precipitation of this element in solution with ionic forms of iron, aluminum, calcium and through the adsorption of phosphorus by clays.

Conventional soil preparation favors greater contact between the phosphate ion and the surface of the colloids, allowing greater adsorption of phosphorus by the soil, reducing its availability to plants. This adsorption is related to the clay fraction of the soil, especially when there is a predominance of Fe and Al oxides and hydroxides. In these colloids, the formation of negative charges depends on the pH and in acidic conditions, the affinity of the phosphate ion for the surface of inorganic colloids favors the adsorption of phosphorus.

Phosphorus, in comparison to other macronutrients, is used in smaller quantities by the plant, but fertilization is high because, depending on the characteristics of certain types of soil, most of the phosphorus is unavailable to the plant. The type and content of clay, CEC, adsorption of calcium and phosphorus, the amount of organic matter and humidity affect the solubility of this element in the soil.

Phosphorus recommendations should consider not only the available element, but also soil characteristics that reflect the phosphorus capacity factor (FCF), as they determine the variation between soils in phosphorus requirements.

 

Forms of Phosphorus in Soil

          Primary phosphate minerals are the source of phosphorus in natural systems. The disruption of these primary minerals occurs through weathering, which depends on the factors and processes of soil formation during pedogenesis. Phosphorus is then released into the soil solution and resorbed into colloids, but some of it is absorbed by organisms and plants. At this stage of soil formation, the greatest bioavailability of phosphorus occurs, since inorganic colloids are poorly weathered and the number of adsorbent sites is small; therefore, it is retained with low energy, facilitating its return to the soil solution. Concomitantly with the use of phosphorus by living organisms, its residues are deposited in the soil and a new form of phosphorus is accumulated, organic phosphorus.

As weathering progresses, phosphate minerals are degraded and their contribution to the supply of phosphorus to the system is reduced. Additionally, soil minerals also undergo weathering, decreasing their crystallinity and increasing anionic adsorption sites.

Soil phosphorus is divided into two large groups, inorganic phosphorus and organic phosphorus, depending on the nature of the compound to which it is attached. Within these two groups, the forms of phosphorus are difficult to identify due to the infinite number of reactions that the element and its resulting compounds can undergo.

In developed soils, mineralization of organic phosphorus becomes the main source of phosphorus buffering, as inorganic colloids act mainly as a sink and compete with plants for the element.

In poorer soils, that is, those with a low pH (less than 5,0), there is a greater occurrence of phosphorus in minerals that contain Fe and Al, while in soils richer in organic matter, they have high pH. This occurs, preferentially, in soils rich in Ca, and pH variation can promote the dissolution and formation of other compounds (Fixen and Ludwick, 1982).

The  fertilization and cultivation of a soil for 25 years caused an increase in the levels of organic phosphorus and inorganic phosphorus, compared to a similar soil under native forest. However, in relative values, organic phosphorus decreased from 55,55% of total phosphorus in forest soil to 25,05% in cultivated soil, showing that the accumulation of the added element occurs preferentially in inorganic forms.

 

Nutrient dynamics with soil management

As the available phosphorus levels in weathered soils of natural ecosystems are low, several mechanisms are used by plants and adapted organisms to increase the efficiency of its absorption. These strategies can be morphological in nature, such as increasing the root/shoot ratio, changes in root morphology, increased root hairs and the association with mycorrhizal fungi.

However, Malavolta et al. (1997), observed the requirement of 90 kg of N, 10 kg of P and 65 kg of K; for a production of 100 kg of sugar cane stalks.

When new areas are incorporated into the production system and when high productivity is sought, the need to correct the natural phosphorus levels in the soil is fundamental. Phosphating, as a corrective practice, aims to increase phosphorus levels in the soil, enhancing planting fertilization. The combined application of 200 kg/ha of P2O5 in the total pre-planting area, followed by 100 kg/ha of P2O5 in the planting furrow provided a 32% increase in stalk production per hectare when compared to the application of 300 kg/ha .