Light, Energy and Plants

Light, Energy and Plants

While thinking about weather, particularly rain, in providing what plants need to grow well my attention has turned to light and temperature.

Permaculture keeps telling us the reduce energy, grow diverse food plants and generally observe nature to better interact but I keep coming back to the same old issue. Permaculture ideals,or ideas, are all well and good but very few Permaculture people appear to have an good understanding of some of some the issues and therefore can't act, interact, reduce or create better alternatives to achieve some of the goals.

I'm in the same position, how do you grow a wider range of food successfully if you don't know what the plants needs are other than to repeat what commercial people (farmers, seed companies and conventional books) tell you. How can you reduce energy effectively if you don't know where or how you are wasting it. How can you capture enough rain water if you don't know how much water you need.

Observation is the key word, but understanding what you are observing requires learning. The sun's energy is key to everything but apart from saying that what do we actually know about it? The plants use the sun to photosynthesise, great but how and how much? How much energy does the sun produce? I've been reading up and trying to answer some questions.

First of all how much energy does the sun give the planet?

The energy from the sun arrives at the planet in the form of different frequencies. Some of these frequencies our eyes can pick up in the form of colours of light while others are invisible. These frequencies are put into different ranges. From Ultra Violet, through Visible light to Infra Red. Ultra Violet light we can not see but this range is split further down into UV a, UV b and UV c light. We've heard about that because UV can cause skin cancer. Visible light is split into smaller ranges, Red, Orange, Yellow, Green, Blue, Indigo and Violet. After that we have Infra Red.

How much energy in these frequencies has been calculated and put in a form we can understand and compare to other forms of power. The combined power in the Sun's light that hits the earth's atmosphere is 1,366 Watts per Square Metre. There is about the same energy hitting every square metre of the earth's atmosphere to power your electric kettle.

Of that energy, plants take some and use it to make other things, some of which gets stored in the ground in the form of carbon (coal, gas and oil being the most notable) but some energy gets absorbed and heats things up which ultimately causes warm or hot spots which creates differences in temperature which then helps to drive wind and the weather, and of course we can capture some and convert into electricity.

Converting the sun's energy into electricity is something that interests me. Solar panels. The Silicon crystals are made in such a way that they respond, as well as can be made, to as many different frequencies of light as possible. Currently they can convert approximately 15% of light into electricity.

Knowing how much light reaches the atmosphere, and knowing that a solar panel can convert 15% of that into electricity suggests that I can take a solar panel and work out how much light there is at any given time on any given day. From that I could compare how well my plants grow when given certain amounts of light.

Knowing how much light there is should enable me to work out what foreign food plants will grow in my garden.

Things got complicated when I realised that some of the 1366 watts per Meter doesn't reach the ground because of haze, clouds etc but on a clear day at noon approx 1000 watts per meter reaches the ground. While doing some calculations with a solar panel I realised that things got even more complicated because of temperature. All the quoted ratings of a solar panel are based upon a solar panel running at 25 degrees Celsius. When the sun hits the solar panel it warms the panel, often to between 40 and 75 degrees. For each degree above 25 degrees C the solar panel becomes less efficient and loses approx 0.4% of its power for each degree. On a very hot day the solar panel, although supposedly 15% efficient will loose 20% of this.

The long and short of this is that although complicated I should be able to measure how much light and how much energy hits plants.

How efficient are plants?

This then led me to wonder how much energy plants take to photosynthesise. It turns out that they only need between 1 and 4% of the suns energy depending upon the plant. Not only that but plants don't use the full spectrum of light, which once I read about it makes sense. The fact that leaves are green shows that the plants don't use that part of the spectrum as they reflect those frequencies. Plants absorb light in the red and blue parts of the spectrum. This has been demonstrated by looking at how much oxygen a plant produces when different frequencies of light are applied. Plants have evolved as very inefficient energy converters because there is no pressure for them to be efficient, ie, there is plenty of light, plenty of energy in the sun.

Greenhouses

If plants only absorb and use red and blue light (broadly speaking) then why do commercial greenhouse people waste a lot of energy producing light in the full spectrum only for the plants to reject much of it. Perhaps the lighting energy bill could be halved if you only produce certain coloured light. Better testing of how well plants do could also mean that not only the colour of light can be tailored but also the intensity.

Looking deeper into observations and having a deeper understanding of the issues can and will open up many more opportunities for reduction in energy, but also a chance to grow better plants and a wider range of plants.

If I know how much energy (light) falls on my plants, and I know the temperature and rainfall I can choose the plants I grow and be more certain as to their success. Also knowing that the season has started badly (not enough light because of clouds and lower temperatures) should enable me to give up on some plants at an early stage knowing they won't ripen and still have time to plant a second crop that will mature and ripen with the time left.

The biggest problem that I see is that there seems to be little or no information on how much light a plant needs at certain temperatures but the data from solar panels around the world gives enough information about the conditions and this information can be compared against plants.

Uses of light upon disease
It occurs to me that if plants only need certain wave lengths of light to photosynthesise then perhaps there is a chance to beat some plant diseases, such as blight, by not giving the disease the wave lengths it needs. It is perfectly possible that by filtering out some frequencies you can stop fungus, such as potato and tomato blight from starting. UV is known to stop algae in ponds and hence the use of UV lamps to keep the water clear. Something similar could be used on plants.

Potato blight, the weather, and Dew Points

Potato blight, the weather, and Dew Points

For the last year and a bit I have been recording the weather for various reasons but have many plans for how this data can be used.

A blog post by Deano Martin about Potato blight and compost tea not only raised a few interesting questions but brought to my attention that the weather has a large part to play with blight. Although I was well aware that blight is caused by warm damp conditions I knew little else and his blog post lead to some reading up upon the subject as well as much thought as every good blog post should induce.

I hadn't realised that there are some predefined conditions that would indicate a good chance of blight. Over the years two ways of predicting when blight may start have been developed.

The first one was called the Beaumont Period and is defined as:

Within a 48 hour period 46 hours must have been above 10 degrees C and humidity must have been over 75%

The other, and now preferred method, is called the Smith Period and is defined as:

Any 2 consecutive days where the minimum temperature was 10 degrees C or above and on each day humidity was 90% or above for 11 hours

Since I have been recording the weather details every 10 minutes I have plenty of data in which to work out previous periods when blight may have started but also predict when the next period will be. Working out what to do when you can predict when blight may start is another thing entirely.

A few hours of programming and I had knocked up a way of calculating both Smith and Beaumont methods and am able to compare both but also look back over last year and this year to see how often favourable conditions exist for Potato Blight (or Tomato blight for that matter). This should now enable me to predict, or should the word be forecast, when Blight may start. 

Running through the data I see that the conditions were met on the following days:

Possible blight days (Smith Period): 24th,25th and 25th,26th and 26th,27th of August 2013

Possible blight days (Beaumont Period): 10th,11th of September 2013

Possible blight days (Beaumont Period): 11th,12th and 21st,22nd of October 2013

Possible blight days (Beaumont Period): 27th,28th and 28th,29th of May 2014 

Possible blight days (Smith Period): 27th,28th and 28th,29th of May 2014

Possible blight days (Beaumont Period): 10th,11th of July 2014 

For a start we can see that the Smith Period occurs less often and is considered more accurate. The Smith Period has been used since the 1970's. (My data only goes back to May 2013 and is up to July 11th 2014).

What we can see, which seems to tally with the fact I didn't hear about people having blight as a problem last year, is that Blight was late and many people would have dug up their potatoes around then, leaving just the main crop to suffer, but this year it looks like we have had or might have a more early blight starting the end of May and with July being wet and humid we may well see a few more days which are favourable for blight, the Beaumont period has already been met so clearly conditions have come close.

Obviously these dates are only for my exact location.

Interestingly I have one or two plants which are turning yellow, and started looking poor around the end of May so I am wondering if it is Blight, although it also looks like a mineral deficiency. I'll need to look closer. I had put this down to them being planted directly into young manure and we have had a lot of rain and that bed got rather water logged several times. The bed that I have used for potatoes this year was hastily prepared by simply piling manure onto the ground to a depth of about 18 inches. Allowing the manure to compost in situ, ready to be dug over for an Autumn crop. I simply took advantage of the bed to see if it would grow potatoes in fresh manure, they started growing, so I planted more hoping for a bonus crop when otherwise I didn't have room for potatoes this year.

Knowing that humidity is key to when Blight may start, observing the weather in detail would be in keeping with Permaculture's principle of Observe and Interact. In Permaculture you are supposed to Observe 90% of the time and Act / Interact 10% and although I have been very critical of Permaculture, or to be more precise the way it is taught and the information provided within the Permaculture community, I won't go into this now, I do totally agree that you need to make lots of good observations, although I tend to observe and awful lot but also act much more than perhaps is considered good.

Having an enquiring mind and wanting to clarify observations and also get accurate data I started to wonder how accurate humidity readings taken from a weather station can be when applied to ground, or in this case, Potato canopy level, humidity.

The question is does a humidity reading of 90% at weather station height, also correspond to ground level? I can't take a reading at ground level easily because my weather station bits are up a pole on top of the greenhouse, 3.5 to 4 metres high. This got me thinking about Dew on the grass. 

The Dew Point

The Dew Point, which is the point that water will appear on surfaces, the ground or grass for example, is where the air can not hold any more water and will give up water and transfer it to an object or thing. When Relative Humidity reaches 100% this will happen. The Dew Point is the Temperature when Humidity is 100%

When we see Dew on the ground we know that the Humidity is 100% so, at that time, the weather station should show 100% humidity. It doesn't. At least mine doesn't. I know my readings are accurate to within a sensible margin and there is no reason why they wouldn't be so straight away I can tell something about this observation. Clearly the humidity is slightly higher at ground level, not at all surprising since there is a different micro climate in and around the grass compared to 4 metres above.

Temperature and Humidity are closely related. If the humidity is higher at grass level, as evidenced by the fact we have dew when the humidity reading I take 4 metres up suggests 90%, then the temperature is lower at grass level than we observe at weather station height. We can see that the grass level temperature is between 1 and 2 degrees C lower.

For what it is worth the Dew Point Formula is:

Dew Point Temp in Celsius = Temp - ((100 - Relative Humidity) / 5 )

If I have a reading of 15 Deg C and 90% humidity at 4 metres high but there is due on the ground the Dew Point would be 13 Deg C

This therefore raises the question, does the Smith Period of 2 days above 10 Degrees and 11 hours 90% humidity on each day take into account that readings are taken from above ground normally or should I be taking my readings from ground level? ie, does the Smith Period not mind that the micro climate within the Potato Canopy is different to the place where you measure weather? I'm guessing not.

I'm guessing not because the way the Met Office gives out Smith Period warnings will necessarily be based upon weather stations and not little sensors within potato fields.

If we therefore take into account that Blight may start when temperatures are below 10 Deg C or Humidity levels are higher than 90% at the plant level would we be able to prevent it easier or predict it better?

The other thing about taking readings from above the ground is that it takes no account of the wind. Air flow or wind, will be less around the plants than higher up, so another question to answer would be does blight happen always on these Smith Periods or is wind playing a big part?

Recording the weather in detail, observing, will allow me to look back at the conditions and answer some of these questions but first I need Blight....or perhaps not! Perhaps for me these questions are better left unanswered.