Effects of pH on Plant Growth

How soil pH affects plant growth

What is pH?

pH is a numeric scale used to specify the acidity or alkalinity of an aqueous solution. The scale goes from -0 (Extremely Acidic) to 10+ (Extreme Alkaline). Soil can usually range from a pH of <3.5 (Ultra Acid) to >9 (Very Strong Alkaline).
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The Experiment


How does pH effect plant growth? Is it bad for the soil to be too acidic or too basic? If so, at what point on the scale does it become harmful to the plant?

My hypothesis is that grass will grow best in Neutral (pH of 7-8) soil, and the others will not do very well.

Now We Wait

Let It Grow

I set the three pots of soil on the windowsill and watered them regularly, so they all received the same amount of water and sunlight. I went and recorded the number of blades of grass and their average height in each pot once a week for three weeks

Week 1

Week 2

Week 3


In the end, the Alkaline Soil with a pH of 9 did the best. It ended with 17 blades of grass averaging at 4.5 cm tall. Next came the Neutral Control Soil at pH of 7, with 11 blades and an Average Height of 3 cm. There was no growth in the Acid Soil at all.


The question for this experiment was how does pH affect plant growth. My hypothesis was that the grass would do best with neutral pH levels. I was incorrect. I found out that grass actually cannot grow at all in acidic soil, and that it actually grows best in slightly Alkaline soil rather than neutral soil.

The End