Anthocyanin pigments have several colors that vary depending on their influence on a range of factors, including acidity of the medium, temperature, level of plant nutrition, agricultural servicing ..etc.?
The hydrangea plant is something of a litmus test for soil pH. In the acid soil environment the flowers are deep blue in the alkaline soil environment they are pink. This color reation will show the sectoring of areas underneath the plant with areas showing blue and pink depending on the subterrean pH. The pigments can be sensitive to pH and color can be useful for inferring environmental pH values. Since plants are generally most productive at the neutral pH the liming according to blue flower pattern might be very useful. Plants can be wonderful bioindicators and most of our world soils are probably not optimized for critical values of soil pH.
Prof. Paul Reed Hepperly already added details explanation. Following !
The red areas of our global soil map are plagued by acid soil infertility. The acidity predominates in older soil families such as ultisols and oxisols which are red in color acid in pH and are high in metal toxicity Manganese, Aluminum and Iron and generally have multiple mineral deficiencies. When these issues are addressed they can recover their productivity. These soils are also many times very low in soil organic matter and fix Phosphorus in forms which are not easily taken up by plants.
The blue alkaline soil areas are found in dry areas where evaporation exceeds precipitation. They are many times both salty and alkaline and the remediation of these soils depends on replacing sodium salt and acidifying to near pH neutral. These soils can display physical issues from the salt and low micronutrient availability.
The majority of all soils are not optimized for their pH reaction and indicator plants are excellent means of diagnosing the underlying issues and the cost is modest compared to laboratory analysis.
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