Soil Moisture and Dendrometer Sampling of Kent Trees Irrigated with Micro-Sprinklers

Mexico is the leading mango exporter worldwide and requires to optimize yield; quality and water use. Water shortage due to climatic changes can affect yield production and quality by increasing fruit drop and size. In the period 2018-2020 decrease in raining events reduced mango yields. Sustained deficit irrigation (SDI) techniques have been applied worldwide and four holes were dig to wet each tree by means of a micro-sprinkler irrigation system. Soil moisture content around the trees were acquired and compared against those irrigated all around the tree trunk. Also dendrometer measurements at the branches help to understand water dynamics within the tree; and high yields were obtained when branches had good water uptake. It was found that roots are closer to the surface and that dry soil areas beneath branches did not provided enough water to produce mangoes over 350 grams.

to see the effect of zone irrigation in fruit yield. Dendrometers measured leaf uptake in branches, and soil moisture sensors the water in the soil area beneath the canopy.

Materials and Methods
The experiment was carried out during the year 2021 in a mango orchard located in Loma Bonita, Guerrero (17 ° 25 '47 "N, -101° 11' 19" W, 17 m ASL). The apparent density of the red loam clay soil was obtained from 30 random samples, being its average density of 1.1 gcm -3 . The samples were extracted from holes at depths between 10 and 40 cm. Twenty mature 'Kent' trees with similar pruning and fertilization practices were selected for the experiment. Half of the trees were irrigated with a hose and the other ten with a micro-sprinkler system. The low energy micro-sprinkler system provided a discharge of 120 l h -1 , and used a low pressure of 5 kg cm -2 . A wetting circular area with a radius of 1 m is always found after spraying (Figure 1 a, 1b, 1c), but if the soil is compact the water will evaporate easily.
With a 10-cm drill bit driven by a gasoline engine, holes 10-cm deep were made. The holes were filled with coarse gravel, allowing the water from the sprinklers to run down towards the roots. Water was applied for half an hour every two days, providing 200 liters per tree every event. The same water was applied with a hose.

Results and Discussion
In both (Figures 2 & 3), measurements of one tree with microsprinkler control are shown; Monitoring with hose irrigation are not shown. Trees were irrigated reaching a field capacity of 25% volumetric soil water content (VSWC) at a depth of 30 cm. There were some events of rain during May 2021, so all the sensors measured a VSWC of 25%, and irrigation was suspended for 5 days and reached 10%. After irrigation was applied again the 15th of May 2021, SS1 and SS4 regained field capacity ( Figure 2).
This values correspond to the ones reported in Irvin mango trees [12]. At lower depth than 10 cm, water losses were found due to atmosphere demands and root absorption [13]. The first micro sprinkler didn't turn on the 28th of May so the water soil content decreased more. The place where both sensors SS2 and SS3 were placed didn't receive water from the irrigation system. Soil water content became of 15% on May 30th for SS2 ( Figure 2), but leaves in the top did not wilt, as they got water from the humid air during sunrise. Mangifera indica L. does not grow, leaves wilt and fell-off after a severe drought stress. Relative soil water content has to be maintained over 45% in soil to ensure tree normal growth and photosynthesis [1].   Different studies have been conducted to investigate tree water use in drylands after monitoring stem water content with dielectric capacitance sensors, and dielectric ECH2O moisture probes drilled within the soil at different depths [14]. Daily variation in stem water content of trees caused by transpiration or stem storage was too small to be detected by the GS3 probe [14]. Diurnal patterns of sap flow follow a similar pattern at the base of the trunk and in branches, increasing in the morning and decreasing in the afternoon [15,16]. A lag between the stem volumetric water content reduction and stem contraction exists and was caused by the relatively rigid tissue and water movement from the bark to the xylem [17].

Conclusions
It can be concluded from this experiment that water should be applied in all the circumference beneath the tree canopy, so that it is available for excellent yields. A better water management requires of more micro-sprinklers. The holes with gravel provided a deeper water movement, but roots are superficial. This is the reason why the leaves on the dry-zone branch did not wilt.