Nourish to measure

Wheat is a cereal of great economic importance in the world, widely used in human and animal nutrition, as well as in crop rotation systems. Important in the direct planting system, due to the straw left on the surface, in addition to being a crop that, if well managed, becomes economically viable. Increasing competitiveness in the world wheat market necessarily involves raising productivity levels, seeking technologies that provide increases in production with adjusted costs. Among the aspects linked to the construction and maintenance of productivity through management, fertilization stands out. The mineral supply in responsive quantities and stages provides excellent yields, when there are no other limitations.

Initially, it must be considered that maximizing the use of nutrients by the plant involves soil conditions such as structure and pH, which must be adjusted to levels that allow better use of water and better availability of nutrients for wheat plants. It must be considered that the provision of water to the system, forming the soil solution, is essential so that nutrients can be absorbed by plants.

There are several mineral elements essential for the growth and development of the wheat plant. It is considered that the elements nitrogen (N), phosphorus (P) and potassium (K) are normally the nutrients that most limit crop productivity.

Considering average values, a crop that produces three tons of grains per hectare extracts 84kg/ha of nitrogen, 11,7kg/ha of phosphorus and 59,7kg/ha of potassium from the soil, of which 60,3kg of N, 9,6kg of P and 10,5kg of K are exported by the grains, and the remainder returns to the soil through the decomposition of crop residues (Pauletti, 1998).

Nitrogen is highlighted as the most limiting factor in productivity, as it has a direct influence on yield components, such as the number and size of ears and grain mass (Mundosck, 2005). It plays a fundamental role in the constitution of biomolecules and numerous enzymes, and constitutes starches, nucleic acids, nucleotides, ATP, Nadh, chlorophylls and proteins. This element is related to the growth and yield of crops, having an important role in the chlorophyll molecule, which performs regulatory functions in synthesis reactions, and the lack of nitrogen directly affects the photosynthetic capacity of plants. Its deficiency is initially observed with the yellowing and chlorosis of lower (old) leaves.

For the states of Rio Grande do Sul and Santa Catarina, the amount of N to be applied varies according to the organic matter content of the soil, determined by soil analysis. Thus, the need for N can vary from 20kg/ha to 60kg/ha if wheat is preceded by legumes and from 20kg/ha to 80kg/ha if it is preceded by grasses, considering yield expectations of two tons/ha. If the productivity expectation is higher than this, add 20kg of N/ha in wheat after legumes and 30kg of N/ha in wheat after grasses, per ton of additional grains to be produced (Soil Chemistry and Fertility Commission RS/ SC (SBCS, 2004).

Of this total amount of N, it is recommended to apply between 15kg/ha and 20kg/ha at the time of sowing wheat and the remainder as cover, between the tillering and stalk elongation stages. For higher doses, coverage can be divided into two applications, one at the beginning of tillering and the other at full tillering. Payment in installments gives the best results. The predecessor crop also influences the decision-making process, in the case of corn stubble and, when there is a lot of straw, it is advisable to anticipate nitrogen fertilization, especially in sandy soils or with low levels of organic matter.

The amount of nitrogen to be applied must also be evaluated, and this is determined according to: soil type and fertility, cultivar, crop rotation, operability, expected yield and economic return. On average, 50% of nitrogen fertilizers applied to the soil are lost through leaching and volatilization processes (Dobbelaere et al.

Based on a series of studies, the Ministry of Agriculture, Livestock and Supply (Mapa) allowed the use of the bacteria Brazilian Azospirillum in the formulation of inoculants for wheat crops. These bacteria fix nitrogen from the air, similar to the process that occurs in soybeans, however, without the formation of nodules on the roots. The amount of fixed nitrogen does not completely meet the needs of the wheat crop. In this way, the inoculant does not completely replace nitrogen fertilization, but promotes better absorption and use of that N that is in the soil or being supplied by nitrogen fertilization (Saubidet, 2002). For Didonet et al (2000), even though there are no differences in grain yield due to inoculation, there is a better reallocation of the N present in the biomass to the grains in the inoculated plants, basically producing fuller and heavier grains.

However, the efficiency of using this bacteria will only be complete if there is a guarantee of an adequate number of bacteria per seed, if there is no reduction in the presence of these bacteria due to the negative interaction of the seed treatment and the inoculant, if the pH of the soil is above 6, if there is complete coverage of the inoculant on the seed and if sowing is carried out within 24 hours after inoculation, as well as some cultivars are more responsive to interaction with the bacteria.

Phosphorus (P) is present in structural components of cells, such as nucleic acids and phospholipids in cell membranes, and also in mobile metabolic components that store energy, such as ATP. Phosphorus absorption by plants occurs essentially via the root system, depending on the supply capacity of the soil, which is often a limiting factor for a good supply, acting to compete with plants for the P available in the solution. Phosphorus-deficient plants are more sensitive to stress and disease. Young leaves of plants with this deficiency tend to darken or maintain a bluish-green color and older leaves turn reddish.

The lack of P at the beginning of development restricts growth, a limiting condition for plant development and grain production. The lack of P in the later period of the cycle has less impact on wheat grain production compared to the initial deficiency (Grant et al, 2001). These same authors found that P stress reduces the total number of seeds produced more than the seed size, with this reduction in the number of seeds occurring through a reduction in the quantity of fertile ears and grains per ear.

The amount of P to be applied will depend on the phosphorus content in the soil (Mehlich-1) and the textural class of the soil (amount of clay), quantified through soil analysis. Thus, the P content in the soil is classified into ranges of very low, low, medium, high and very high. From this, it can be recommended the amount of fertilizer to be applied to meet the plant's demand and the need to correct levels in the soil, in situations with very low or low levels, which can occur in a single crop or divided into two crops. Considering soil correction in two crops, the amount of P to be applied in the first varies from 30kg/ha for soils with high levels (maintenance fertilizer) to 110kg/ha for soils with very low levels. If the soil is in the very high range, there is no need to apply the nutrient, as the soil itself is capable of supplying it sufficiently to the plant. In the second cultivation, the amount applied varies from 30kg/ha to 70kg/ha for the very high to very low ranges. These quantities are necessary for a productivity of two tons per hectare, if the expectation is higher, apply 15kg/ha per additional ton of grain that you want to produce (Commission of Soil Chemistry and Fertility RS/SC (SBCS, 2004).

As for how to apply phosphate fertilizer, it is recommended in recommendation bulletins from several regions of Brazil that it be done in the sowing furrow, a little below and next to the seed, being the most efficient way of providing this nutrient to the crop, with better use by plants of the applied P, this is because P is relatively immobile in the soil and thus remains close to the place where the fertilizer was placed. Broadcast fertilization allows greater contact between the fertilizer and the soil, resulting in high adsorption of P, which reduces the use of the element by the plant, due to the low mobility of the mineral in the soil profile, especially in clayey soils. In general, positive responses to P broadcast application are found in soil conditions with high levels of available P, as in this case fertilization would only be to maintain the level of P in the soil. In soils with low P levels, greater productivity and P recovery were obtained with application in the sowing furrow (Guareschi et al.

Potassium (K) is the most abundant cation in plants, not being part of the cell structure, but it is fundamentally important in numerous processes, such as photosynthesis, regulation of stomatal opening, maintenance of cell turgor and is a constituent of several enzymes. Lack of K is more common in intensively cultivated sandy soils. When there are low levels of K and high levels of N, the stems become weak, leading to a greater occurrence of plant lodging. In case of deficiency, older leaves show yellowing and weakening at the tips and margins, followed by chlorosis, which progresses until it reaches the vein, causing the leaf to curl downwards. Reduced plant growth and shortening of the internodes generally occur.

Potassium is an element with good mobility in the soil, and its behavior is very different from that of P. However, its arrival at the roots and absorption by plants are highly dependent on diffusion, as is P, due to the low concentration in solution. Around 95% of the total K is found in the solid fraction of the soil, which is in equilibrium with the liquid phase (solution), which easily passes from one fraction to another. When the plant absorbs the nutrient from the soil solution, or it is lost through leaching, a new fraction is released from the solid phase to the liquid phase, maintaining balance. The relationship between K in the solid and liquid phases of the soil depends mainly on the CEC of the soil, which varies depending on the organic matter content, the amount and type of clay and the pH (Sanzonowicz & Mielniczuk, 1985).

The amount of K in crop residues is very large, and because it is not a structural component of plant cells, after harvest or plant senescence it quickly returns to the soil, in a form readily available for subsequent crops, making straw a significant reservoir. of K in the short term (Rosolem et al, 2007). This K cycling becomes more efficient when there is crop rotation, so that plants with different capacities for extracting the element from the soil and high dry matter production become an important source of K for the next crops to be implemented. Another fact to be noted is that the constant maintenance of plants in the field is very important, in order to avoid losses due to the leaching process, considering that the element becomes incorporated into the straw and, as soon as it is released, there is already another layer in the field. plant capable of absorbing it.

The amount of K to be applied will depend on the element content in the soil and the cation exchange capacity (CEC) of the soil. Thus, in the same way as P, the K content in the soil is classified into bands, and the amount of fertilizer to be applied, when at low levels in the soil, can be in a single crop or divided into two crops. If you are correcting the soil in two crops, the amount of K to be applied in the first varies from 20kg/ha for soils with high levels (maintenance fertilizer) to 100kg/ha for soils with very low levels. If the soil is in the very high range, there is no need for fertilization. In the second cultivation, the amount applied varies from 20kg/ha to 60kg/ha for very high to very low levels. These quantities are necessary for a productivity of two tons/ha, if the expectation is higher, apply 10kg/ha per additional ton of grain that you want to produce (Commission of Soil Chemistry and Fertility RS/SC (SBCS, 2004).

As for how potassium is applied to the soil, it can be in the sowing furrow, deposited below and next to the seed, or broadcast. If the application is in the furrow, care must be taken when it is necessary to apply very high doses, which can cause a saline effect on the seed, due to the higher concentration of the nutrient in the fertilized areas. At the time of sowing, doses greater than 80-100kg/ha may harm the germination and/or initial growth of the plant. In these cases, an alternative is broadcast or split fertilization (Vilela et al, 2004). Currently, many producers adopt the potassium fertilizer installment system, with one application being carried out in the sowing furrow and the other as a cover, during tillering.

The management of fertilization of wheat crops is essential to obtain satisfactory productivity, but it is necessary to diagnose the situation of the soil that will be cultivated, through analysis. From this procedure, associated with some factors such as expected yield, previous cultivation and type of soil preparation, it is possible to stipulate the quantities of nutrients to be supplied and which method to use. It is emphasized that fertilization must always follow the recommendations in technical bulletins for each region and crop, and that the producer is aware that the amount applied is normally above the crop's needs, due to the potential retention of this nutrient in the soil and its losses through leaching and/or volatilization processes. As well as carrying out conservation practices such as direct planting and crop rotation.

This article was published in issue 183 of Cultivar Grandes Culturas magazine. Click here to read the edition.