Growth Habits, Planting Methods and Fertilization Management of Madou White Pomelo from Taiwan, China
Madou white pomelo is a pomelo variety from Taiwan, China, and it is a very popular pomelo variety locally in Taiwan, China. So would you like to learn how this pomelo is cultivated?
Like other citrus species, mature Madou white pomelo plants sprout new shoots in three periods: spring, summer and autumn; in warm regions they also produce winter shoots. Among these, spring shoots sprout the most, followed by summer shoots and autumn shoots. Spring shoots are the important bearing shoots and vegetative shoots of the year, and vegetative shoots also develop into the main bearing mother shoots of the future. As for the sprouting position of spring shoots, terminal shoots account for the most, up to 84.6%, while the rate of spring shoots forming bearing shoots is 31.7%. As for the ratio of inflorescence forms, leafy inflorescence shoots (including leafy single terminal flower shoots) account for 41.2%, while leafless inflorescence shoots (including leafless single flower shoots) account for 51.7%. The fruiting rate of spring bearing shoots is 3.4-4.8%.
The leaves of Madou white pomelo fall in every month of the year, and during the sprouting of spring shoots and the flowering period, an average of 57.3% of the leaves on mother shoots drop heavily and are replaced by new leaves. When aging leaves fall, the leaf blade drops first, and the petiole drops several days later, and leaf fall occurs throughout the day. The maximum life span of the plant's leaves can reach 24 months, while the shortest life span is only 1 month. The average leaf life span is most commonly 11-12 months; if spring shoots sprout later, the life span is 11-13 months, and the next most common life span is 21-24 months. Madou white pomelo plants flower every year between March and April, and the fruit ripening period is October to November. By local farming custom, harvesting is carried out about two weeks after the Frost's Descent solar term (around October 22).
Cultivation Management
1. Pruning and Training
The pruning and training of young Madou white pomelo trees is mainly aimed at developing the tree form, gradually expanding the canopy year by year. When pruning and training to promote new shoot growth, the principle should be to suppress the strong and support the weak, so as to promote even branch development. Generally, a young tree can produce 3-4 flushes of shoots per year. After planting, the plant is temporarily not pruned, and the root system and tree vigor are cultivated first. If the seedling is smaller and grows more slowly, it needs to go through 3-4 growth flushes, and pruning is carried out only after the root system is vigorous. If the seedling is larger and grows more vigorously, pruning can be carried out after 2-3 growth flushes. When pruning, the principle is a single trunk, pruning at 45-60 cm above the ground to promote new shoot branching. For main branches, those with a branching angle of about 60-70 degrees to the trunk should be selected, keeping 2-3 new shoots distributed evenly in different directions to develop into 2-3 main branches.
After the main branches are selected, wait for another flush of shoot growth. When the main branch length reaches more than 80-100 cm, prune at 50-60 cm to promote the sprouting of sub-main branches, and select and retain branches with suitable angles to develop sub-main branches, so as to stagger the branching points. Likewise, when the sub-main branch length reaches more than 60 cm, prune at 40-50 cm to promote the sub-main branches to sprout new shoots at suitable positions. Once the main branches and sub-main branches are developed, the future tree shape is already determined. After that, continue pruning and training to select and retain second sub-main branches. Each sub-main branch can retain 2 side branches, and this should be combined with appropriate irrigation and frequent, light fertilizer application to promote robust and well-filled branches. Many green shoot groups then sprout on the side branches as bearing mother shoots, forming a high-yielding canopy. When pruning and training young trees, main branches with stronger growth vigor will grow upright upward and affect the tree shape; bamboo poles or ropes can be used to train and adjust their direction and angle. However, the main branches should not be trained too low, and their base should maintain a 60-70 degree angle with the ground. Young trees use ropes to train and adjust the angle and direction of shoots
Young trees use ropes to train and adjust the angle and direction of shoots
The tree shape management of mature Madou white pomelo trees mainly relies on pruning to maintain the tree shape and increase the canopy, and pruning can be divided into summer pruning and winter pruning. Summer pruning should use short cutting, light pruning and branch thinning. In adult trees in full bearing period, tree vigor growth is relatively moderate. If the plant bears many fruits or has weaker tree vigor, it generally mainly produces spring shoots, while summer and autumn shoots are very few; then only non-bearing leafless branches or dead branches should be removed. If the plant bears few fruits and soil fertility is high, or too much nitrogen fertilizer is applied, the plant grows vigorously and produces large numbers of summer and autumn shoots, or even many water sprouts. Apart from water sprouts reserved by short cutting for renewal mother shoots, the rest should be removed. As for summer and autumn shoots, they should also be short cut to promote branching into finer bearing mother shoots, and excessive ones should also be removed to avoid affecting tree shape and fruit quality. After heavy pruning of summer and autumn shoots, the fertilizer application rate should also be reviewed and adjusted to avoid producing large numbers of summer and autumn shoots year after year. However, summer pruning should not be excessive either, so as to avoid affecting the balance between canopy and root system, which may instead cause large numbers of water sprouts. Winter pruning involves a relatively larger pruning amount, but the degree of pruning depends on tree vigor. The pruning amount should in principle not exceed 15%. Around mid-December, during the low-temperature dry period, dry branches, diseased and pest-damaged branches, overlapping branches, weak slender branches or water sprouts should be pruned. Thinning pruning of large branches and shoots in stronger trees should be carried out during winter pruning, and should be implemented in stages over 2-3 years to avoid too much pruning at one time affecting canopy distribution and causing large numbers of water sprouts. When tree vigor is weak, renewal pruning is used to reduce the canopy and stimulate branch sprouting, so as to cultivate the root system and restore tree vigor. For excessively weak plants, light pruning should be carried out, combined with applying large amounts of low-nitrogen, high-fiber organic fertilizer to improve the soil, and chemical fertilizers should be applied lightly to restore tree vigor. In addition, winter pruning should not be too early, so as to avoid high temperatures after pruning causing large numbers of winter shoots and reducing the flowering amount of spring shoots the following year.
2. Strengthen Soil Fertility Management
(1) Soil pH Improvement
Madou white pomelo belongs to citrus fruit trees, and its suitable soil acidity and alkalinity (pH) is between 5.5 and 6.5. If the soil pH value is below 5.0, it is strongly acidic soil. The causes of soil acidity include the soil itself being acidic, the leaching and loss of lime components in the soil by rainwater, or the result of continuous use of acidic chemical fertilizers. In acidic soil, plants often lack calcium or magnesium, which affects fruit quality. Moreover, the stronger the acidity, the more iron, aluminum and manganese dissolve in the soil, which easily forms toxicity and is unfavorable to plant growth, and phosphorus easily forms insoluble compounds such as iron phosphate and aluminum phosphate, so that plants cannot absorb phosphorus. For orchards with strongly acidic soil, limestone powder, oyster shell powder or silicate slag can be applied. For orchards lacking magnesium, dolomite powder (containing 10-16% magnesium oxide) is suitable, as it can supplement both calcium and magnesium. Every year during the winter dormant period, about 2-3 kg per mature plant is evenly broadcast on the ground and mixed into 15-30 cm of soil. Nitrogen fertilizers such as ammonium sulfate or urea can be applied only one month after application. Applying it together with compost can prevent soil hardening. Lime materials should be applied in appropriate amounts year by year, not too much at one time, and the pH value should be checked once a year. When pH exceeds 6.0, application should be stopped to prevent excessively high pH from causing trace element deficiency, thereby affecting leaf life span and causing premature leaf fall. After acidic soil is improved, the calcium and magnesium content in the soil can be directly increased, harmful substances reduced, microbial activity enhanced to promote organic matter decomposition, and plant nutrient absorption improved, thus enhancing fruit quality.
(2) Improvement of Soil Organic Matter Content
A soil organic matter content below 2% is low, 2-3% is medium, and above 3% is high. Soil organic matter decreases due to decomposition by microorganisms, and Taiwan, China has a high-temperature, rainy climate, so organic matter also decomposes faster. Applying organic fertilizer not only allows it to be decomposed by microorganisms for crop absorption, but also promotes soil aggregate structure, improves soil drainage and aeration, and increases water and fertilizer retention capacity. Therefore, orchards with organic matter content below 2% can adopt sod culture or apply organic fertilizer to raise soil organic matter content to 3%. Apply decomposed organic fertilizer with relatively high carbon (fiber) content; the organic matter content should reach 60%, and its nitrogen, phosphorus and potassium contents are about 1% each, so it can be used in large amounts, with more than 30 kg applied per plant.
When applying soybean meal-type organic materials with relatively high nitrogen, such as soybean meal with nitrogen reaching 6.5% and peanut meal reaching as high as 7-11%, only a small amount of 3-5 kg per plant can be mixed and used, so as to avoid damaging the root system due to the fermentation heat of large amounts of soybean meal organic material (temperature reaching above 50°C) and the excessively high soil solution concentration after dissolution. Apply organic fertilizer every year in winter, by hole application or ring application in the soil around under the canopy. At the same time, after applying organic fertilizer, the amount of chemical fertilizer applied should be appropriately reduced, especially the amount of nitrogen, so as to avoid excessive nitrogen fertilizer causing the plant to produce large numbers of summer and autumn shoots, thereby affecting fruit quality.
(3) Rational Fertilization
The amount of fertilizer applied in an orchard varies with tree age, fruit load, soil fertility and tree nutritional status. The fertilization amount can be based on the recommended fertilization amount for citrus orchards. For example, calculated at a yield of 60 kg per plant, each plant needs 600 g of nitrogen, 300 g of phosphorus pentoxide and 600 g of potassium oxide per year, and afterwards soil and leaf analysis results are used to adjust fertilizer types and amounts. Madou white pomelo has a relatively high fertilizer requirement. If soil fertility is low and the fertilizer amount is insufficient, the plant will flower and fruit less and fruit quality will be poor, so the recommended citrus fertilization amount should be appropriately increased by 20%-30%. The base fertilizer of chemical fertilizer should in principle be applied deeply in winter together with organic fertilizer, while spring fertilizer and summer fertilizer should in principle be broadcast. For the distribution ratio of chemical fertilizer application, in terms of base fertilizer, spring fertilizer and summer fertilizer, nitrogen fertilizer is 40%, 40% and 20% respectively; phosphorus fertilizer is 50%, 50% and 0%, or it can be applied all at once as base fertilizer; potassium fertilizer is divided into 30%, 30% and 40%. The base fertilizer application period is December to January, the spring fertilizer application period is April, and the summer fertilizer application period is June to July. If nitrogen fertilizer is applied excessively in the soil and the leaf nitrogen content is above the standard value (2.9-3.1%), the plant leaves will be dark green and produce large numbers of summer and autumn shoots, making the fruit larger and the peel thicker, while sugar content and juice amount decrease, affecting fruit quality, so nitrogen fertilizer should be reduced. If available potassium content in the soil is insufficient and leaf potassium content is below the standard value (1.4-1.7%), the fruit will be smaller and both sugar and acidity lower, so the potassium fertilizer amount should be appropriately increased. If available phosphorus content in the soil is 600-1,000 ppm and available potassium content is 600-800 ppm, then the phosphorus and potassium fertilizer content in the soil is already quite sufficient. When the pH value reaches 6.0, soil available calcium content can reach above 1,000 ppm. If soil available magnesium content also reaches above 100 ppm and the calcium-to-magnesium ratio is within 6, then the plant usually will not lack calcium and magnesium. However, if potassium fertilizer is applied excessively and its available potassium content exceeds 1,000 ppm, even if the soil contains a large amount of available magnesium, antagonism will cause magnesium deficiency in the plant leaves, and the leaf magnesium content will be below the standard value (0.26-0.50%). Magnesium-deficient yellowing leaves will fall prematurely, so potassium fertilizer should be reduced during fertilization.
3. Appropriate Soil Moisture Management
During the growth period of Madou white pomelo, the amount of soil moisture supplied has a great influence on plant growth, yield and quality. Because annual rainfall in Taiwan, China is unevenly distributed, with distinct dry and rainy seasons, in order to stabilize the yield and quality of Madou white pomelo, flatland orchards need orchard drainage during the rainy season to prevent root waterlogging that leads to leaf and fruit drop, while sloping orchards should prevent soil erosion and supplement water timely and appropriately during the dry season. Madou white pomelo belongs to subtropical citrus fruit trees, and the soil also needs moderate drying during winter. However, if soil moisture content is too low, the plant is prone to severe leaf drop due to water shortage, while excessive irrigation instead easily produces large numbers of winter shoots and reduces the flowering amount the following year. Therefore, during the winter dry period, micro-sprinkler irrigation should be used to supplement a small amount of surface soil moisture, so that the soil at 60 cm depth maintains a relatively low moisture content of 10%, in order to maintain normal plant growth and favor flower bud differentiation. During spring, if the soil is too dry and soil moisture content is below 10%, the plant is prone to severe leaf drop, flower drop and fruit drop. If the soil is dry in winter and then frequently over-irrigated during spring, with soil moisture content often reaching as high as 20% (-8 to -12 centibars) of field capacity, the root system's wet-dry variation is too great, which will also cause severe flower and fruit drop. Therefore, before spring shoots sprout, around the Start of Spring solar term on February 4, the soil should be fully irrigated. Then, 1-2 weeks before the plant flowers, the soil should be moderately and lightly irrigated again to supplement moisture, maintaining soil moisture content at about 15% (-30 to -40 centibars) of sufficient moisture, so as to facilitate root growth, spring shoot sprouting, flowering and fruit set. During the fruit enlargement period, soil moisture content can be maintained at about 15% of sufficient moisture to facilitate fruit growth and development, while irrigation should be stopped one month before harvest and soil moisture content maintained at 10% (-50 to -70 centibars) in a relatively dry environment, so as to accumulate fruit sugar and improve quality. If rain occurs before harvest and soil moisture is excessive, fruit cracking easily occurs, so drainage facilities should be strengthened.
