​Water in the safrinha

The decision regarding the corn sowing time is based on climatic risk factors that directly affect grain yield through changes in the metabolic efficiency of plants. Therefore, there is a need to plan corn sowing considering all its phenological phases, which is extremely difficult, as climatic factors are events that are predicted to occur with relative reliability.

In Brazil, the corn sowing time is defined, in general, by the distribution of rainfall, which directly influences the water supply in the soil, the consumption of which for this crop, during its complete development cycle, is between 500mm and 800mm.

“Safrinha” corn is widely cultivated in succession with the summer crop, a fact that makes it dependent on the harvest time of the predecessor crop and, in general, sown in the months of January, February and March. Its productivity in relation to corn "summer" period is naturally lower, hampered by limitations such as reduced rainfall, radiation and temperatures.

The water available in the soil to corn plants is related to their different phenological stages, with the period between pre-flowering, bolting and maturation of the corn kernels being the most critical for water deficiency.

On this subject, we can mention the work carried out by professors Fábio Régis de Souza and Raphael Maia Aveiro Cessa, together with academics José Luiz Faccin and Luiz Miguel Kadar, in the experimental area of ​​the Faculty of Agronomy of the Centro Universitário da Grande Dourados (Unigran), in Dourados, Mato Grosso do Sul, with the aim of evaluating the development of “off-season” corn based on the water supply in the soil. The work addresses the importance of possible productive losses of corn grains when the plant “goes” through periods of hydrical stress.

The study concluded that corn cultivation in the irrigated system provided better plant development and grain productivity, due to the greater frequency and quantity of water at lower negative tension in the soil between the limits of field capacity and the permanent wilting point, reducing risks related to water stress on plant development. Furthermore, the work highlighted that monitoring soil water using tensiometers is a great tool for monitoring and/or controlling soil water supply in irrigated systems in corn crops.

The work was conducted in a clayey Red Distroferric Latosol sown in rows spaced 0,90m apart, with five corn seeds per linear meter. The fertilization adopted was that recommended for “off-season” corn, considering the soil analysis data. The experimental area was divided into irrigated and non-irrigated, where mercury tensiometers were installed at a depth of 0,2 m from the soil surface (Figure 1) in order to monitor the water matric potential in the soil with the help of equation 1.

Figure 1 - On the left, the installation diagram of a mercury tensiometer. On the right, the experimental area containing corn where mercury tensiometers were installed

SOURCE: Adapted from Santos, C. R. Dos. Use tensiometer in coconut tree irrigation. Embrapa Semiarid (CPATSA). Petrolina, 2001. (Technical Instructions from Embrapa Semiarid n^O 51)

Equation 1:

on what,

Ø_m - matric potential of water in the soil, given in meters of water column; h_Hg - height of the Hg column (tensiometer reading), given in meters; h_c - height of the Hg level in the cuvette in relation to the soil surface, given in meters; z - installation depth of the center of the tensiometer's porous capsule, given in meters. The result must be converted to bar.

Whenever the tensiometer installed in the irrigated area registered water tension in the soil between -0,40bar and -0,70bar, an irrigation blade was supplied to restore the amount of water in the soil to field capacity. To carry out the calculation of water replacement in the soil, use was therefore made of data recording water tension in the soil and its characteristic moisture curve for a depth of 0m – 0,20m (Figure 2).

Figure 2 - Soil moisture retention curve (Dystroferric Red Oxisol) in the experimental area. Depth 0 – 0,20m

*The voltage values ​​-15bar and -0,10bar corresponding to humidity 27,88% and 32,62% respectively represented the permanent wilting point (PMP) and field capacity (CC)

The corn characteristics evaluated at seven-day intervals were: fresh mass of shoots and roots, plant height, number of ears and final productivity at 14% humidity. In each examination, average values ​​of the evaluated characteristics were obtained from the sampling of ten plants in the experimental area.

According to Figure 3, it can be seen that corn grown under an irrigation system had higher values ​​of fresh mass of the shoot and root. The fresh mass of the shoot is a characteristic that probably correlates positively with thicker and heavier leaves per area. Such leaves, therefore, will have a more efficient photosynthetic apparatus for the production of metabolites.

Figure 3 - Fresh mass of the shoot (MFPA) and root (MFR) of “safrinha” corn grown in Dourados-MS. (n=10)

The results presented demonstrate that the water deficit in the reproductive period and in the plant development process results in losses in area size and grain yield due to the water deficit. In Figure 4 it can also be seen that the corn plants grown in the irrigated area had a taller height throughout most of the development period.

Figure 4 - Plant height of “safrinha” corn grown in Dourados-MS. (n=10)

The better development of corn in the irrigated system was related to the greater frequency in which water remained in the soil in greater quantities and lower negative tension between the limits of field capacity and permanent wilting point for the depth of 0m to 0,20m (Figure 5), this fact determines the main phenological stages, tassel emission, flowering and pollination, milky grain and mealy grain.

Figure 5 - Water tension in the soil at a depth of 0-0,20m in the area cultivated with corn in an irrigated and non-irrigated system

According to the soil moisture curve, between tensions of -0,40bar and –0,70bar, it was possible to maintain the soil moisture content above 30% in the irrigated system. In the non-irrigated system, soil water tension values ​​between -0,90 bar and -1bar were observed. In this case, soil moisture remained below 30%.

The presence of adequate water levels in the period from bolting to the beginning of grain filling provides an increase in corn productivity, with water deficit therefore limiting corn yield. Furthermore, water stress during bolting, especially during the flowering and maturation of the corn plant, causes decreases in grain production.

The better development of corn grown under an irrigation system provided a higher average final number of ears compared to corn grown without irrigation (Figure 6). Thus, greater productivity of corn grains in the irrigated system (81,50 bags of 60kg) was already expected, with a difference of 30,85 bags of 60kg more per hectare (increase of approximately 30%) when compared to the non-irrigated system. irrigated.

Figure 6 - Number of ears of “safrinha” corn grown in Dourados-MS. (n=10)

Click here to read the article in issue 178 of Cultivar Grandes Culturas.