Green | RAW Tree Care

Most people are aware that some plants and trees thrive in acidic soils and others will only grow in alkaline soils but why is this the case?

First lets touch on some basic chemistry. pH is a measurement of acidity or alkalinity on a scale of 1 to 14. A number below 7 indicates an acidic substance, above 7 indicates and alkaline and 7 represents a neutral substance. You can buy soil testing kits to find the pH of your soil from most garden centres and (in the UK) you are likely to have a pH from about 4 to 8.

The pH of your soil is determined by the amount of calcium or lime (as lime consists mainly of calcium in various forms) contained within the soil. Calcium is an alkaline element and it is usually lost from the soil over time as it is washed out of the soil. In some environments, such as soil over a bedrock of limestone, the calcium is replaced on an ongoing basis. Other soils, such as sand, are unable to hold on to the calcium and slowly turn more acidic over time.

At this point I have to take you on a tangent and look more closely at what plants need from soil. Soil is full of nutrients which plants need to grow and particles of nutrients are ‘stuck’ to the soil by adhesion at the atomic level. The nutrients stick to both inorganic matter (clay particles) and organic matter (highly decayed plant matter) within the soil known as colloids. The other inorganic parts of the soil such as sand and silt are chemically inactive so nutrients are unable to stick to them. This is why calcium is easily washed from sand and is why clay is so important in healthy soil.

So what does this mean to my plants?

Different nutrients are available at different soil pH levels. This affects the way plants grow in different soils.

Some plants are very sensitive to soil pH, the most well know example are Rhododendrons and heathers which will not tolerate lime in the soil. On the other hand there are plants which greatly prefer a lime soil. This is because of the nutrients available to the plant at a particular pH.

Soil pH affects which nutrients are available to a plants because minerals will be solid at a certain pH and soluble (able to dissolve into the water) at another pH. If the nutrient is solid it is unavailable to to plant. The nutrient needs to be dissolved before it can be absorbed by the plant or stick to the colloids (clay particles or decayed plant matter) in the soil.

Aluminium is not a plant nutrient and if the soil pH is over 5 it remains as a solid and is washed out the soil by water. However, if the pH drops below 5 aluminium becomes available to be ‘stuck’ to the colloids. If the pH were to continue to fall below pH 4.5 the amount of aluminium becomes toxic to some plants and the aluminium particles are in such great numbers they displace the other nutrients from the colloids. This reduces the amount of other nutrients available to plants.

So acidic soils are damaging because of the amount of toxic aluminium where as alkaline soils affect plants because the nutrients needed by the plant are kept lock up in solid form at the higher pH. For example plants in a high pH soil can suffer from manganese deficiency where as manganese would be in excess in low pH soils. If a plant is suffering from a Iron, Boron or Manganese deficiency in a high pH soil you are unable to correct the deficiency by adding more of the nutrient as it will remain ‘lock up’ in solid form until the pH is lowered.

The pH of acidic soils can be increased by adding lime whereas alkaline soils can have their pH lowered by adding peat or other decayed plant matter.

Soil pH also affects the organisms found within the soil. Earthworms are important as they aerate the soil and help bind it together but they prefer non-acidic soils and need high amounts of calcium available. Beneficial bacteria will generally tolerate a wide pH but most fungi prefer acidic soil.

So as you can see soil pH has a complex relationship with plants and organism within the soil and that the wrong soil pH could starve your plant (or tree) of essential nutrients or poison it with excess aluminium.

It is an age old question. Ok, its not as often asked as ‘why is the sky blue?’ but it is a question worthy of a decent answer.

Glass Prism Rainbow
by iWanders – on flickr

To find the answer we have to go back to school. Remember when, in science class, you were told light is made up of different colour light combined. If you have a prism to hand you can split white light in to red and yellow and pink and green. Orange and purple and blue. I can sing a rainbow… (sorry got side tracked there). The range of colours you can see (red through to blue) are referred to as the ‘visible spectrum’ or visible range of colours. There are other colours (such as infrared and ultraviolet) but i won’t cover these here.

Now we need to change subject to biology, don’t worry I will try and keep it simple. Many plants, trees and algae absorb the suns rays to produce their own food, it is a remarkable process and is called photosynthesis. The key ingredient for photosynthesis to work is a chemical called chlorophyll, a type of pigment. When a tree photosynthesises (the same applies to anything which photosynthesises) it absorbs carbon dioxide from the air which is combined with water in a chemical reaction which produces sugar for the tree. This chemical reaction requires energy to take place and it is chlorophyll that provides the energy needed for the reaction.

Now, back to the subject of light. When light hits an object some of the light is absorbed and some light is reflected. Some items absorb some parts of the visible spectrum better than others. In other words an item, such as my red candle, is able to absorb green and blue light but unable to absorb red light which is reflected. Because the red light is reflected it appears red in colour.Leaves are good at absorbing red and blue light and not very good at absorbing green light which is reflected, making the leaf appear green in colour. You now know why they appear green but this is only part the story.

Leaves are green because of chlorophyll, a chemical essential for photosynthesis

Many objects are the colour they are by coincidence or by man. My candle is red because it was made in a factory. Dyes and colouring agents were used to make it red but why are leaves green? We know why they appear green but we also know trees and plants need energy from light to photosynthesise. Surely it would make sense to absorb all the light, including green light) and reflect nothing (and be black in colour)?

We are starting to get a bit more technical here but please bear with me.

Remember we briefly mentioned how the light spectrum extends past the visible colours into infrared and ultraviolet? Well, chlorophyll works best with red and blue light in addition to infrared and ultraviolet light. Chlorophyll is useless at using the energy within green light which is why it is reflected. Chlorophyll is not alone within a leaf, it is joined by other pigments called carotenoids. There are over 600 know carotenoids, such as lutein (appears yellow in colour), and each carotenoid absorbs a different colour(s) of light. The presence of Lutein, and other carotenoids, is not obvious because there is so much chlorophyll but the carotenoids do change the colour of a leaf which is why we see leaves in different shades of green.

So, leaves are green because they are stuffed full of chlorophyll, a chemical which is great at absorbing red a blue light but useless at absorbing green light. Plants do contain other chemicals which do absorb green but as there is so much chlorophyll most of the green light is reflected.