Why are some orchards seriously deficient in magnesium? Magnesium is an important component of chlorophyll, magnesium deficiency will lead to leaf yellowing, affect photosynthesis, reduce the synthesis of organic matter, reduce fruit set rate. As a result, the citrus fruits grow are small and thick- and have poor color, the fruit is not sweet, the quality is poor, and the yield is low, which greatly reduces the economic benefits of planting. 
1. Deliberately pursue low cost, digging trenches have not been dug, every time fertilization is sprinkled with compound fertilizer, no organic fertilizer, no trace elements, or compound fertilizer is not used, urea + calcium superphosphate + potassium sulfate (because of the serious lack of magnesium in the two wide areas, so most of the compound fertilizer has calcium and magnesium).
2. Some mountainous land, or just planted on thin soil, this kind of soil is not in the original soil, so it is more likely to be missing than the existing soil. Either acidic or strongly acidic soil. Magnesium in sandy soils is susceptible to rain. Lack of magnesium due Mono Ammonium Phosphate MAP to wetting of irrigation water;
3. Potassium fertilizer or phosphate fertilizer application too much, will also inhibit the absorption of magnesium, the expansion of the fruit stage everyone to apply puffed fruit fertilizer with more phosphorus and potassium, will inhibit the absorption of magnesium, the occurrence of magnesium deficiency symptoms. The best thing to do is to supplement together.
4. The amount of organic fertilizer used is small. The soil is compacted, there is a problem with absorption, anyway, it is necessary to plant citrus and dig a ditch once a year. Or you have long-term integration of water and fertilizer.
5. The sensitivity of different species and varieties of citrus magnesium varies, and the order of sensitivity strength is generally grapefruit> sweet orange> wide-skinned citrus.
6. Other factors: improper selection of rootstock. Nematodes are infested and stagnant potassium sulphate fertilizer water causes rotten roots. Factors such as rotten roots can also cause various deficiency symptoms in citrus due to difficult nutrient absorption.

What exactly does magnesium do? 60% of the magnesium absorbed by citrus in that year is used for tree growth such as branches and other trees during the full fruiting period, 24% for fruit development and yield building, and another 15% accumulates in old leaves through leaf fall loss. Therefore, the role of magnesium diammonium phosphate fertilizer is also in the growth of branches and fruit development, and magnesium is an essential element for citrus photosynthesis. Magnesium is one of the essential nutrients for plant growth and development, is the only metal element of the chlorophyll molecule, involved in the composition of chlorophyll, pigment, involved in the synthesis of proteins, nucleic acids and fats, when magnesium is sufficient, the fruit surface of citrus, the leaf surface wax is thick, the fruit oil cell is full, and the color is good. The demand for magnesium during the expansion of the fruit is very high. During the expansion and branch growth phases of the fruit, physiological activity is very active and requires a lot of magnesium:
1. Enhance leaf photosynthesis and increase the accumulation of photosynthetic products, improve fruit dap diammonium phosphate set rate, promote the expansion of young fruits, increase fruit sugar content and promote fruit coloring.
2. Improve the synthesis and accumulation of organic matter in trees and fruits.
3. Promote the synthesis of vitamin A and vitamin C in the fruit, which is conducive to improving the flavor and taste of citrus fruits and improving fruit quality.