Why Soil PH Matters

\"PH and Available Nutrient Graph\"

In my previous post The Master Gardeners Program I wrote about all of the new information I was assimilating. Being a long time gardener in South Florida I possess a bit of knowledge about all sorts of gardening, but there were some persistent mysteries for me. One particularly irritating gap in my knowledge was concerning PH and why whenever I made a soil nutrient inquiry with the Cooperative Extension, or with any educated grower the conversation immediately turned to PH. PH! Why PH? I want to ask about soil nutrients, and they just want to talk about PH! So what if my soil is alkaline, neutral, or acidic? I just wanted to know what nutrients the soil needed to grow good plants.

If this was all that I learned in the Master Gardeners program it alone would be worth the time and money spent on that class. This is such a gem! Here is why PH matters when talking about soil nutrition: Soil nutrients are only available to plants at certain and specific PH levels. The chart you see above tells it all.

You see, a plant could be showing symptoms of a phosphorus deficiency while the soil in which it grows is packed with phosphorus. If the PH is not between 5.3 and 6.8 phosphorus will not be available to the plant. You can see in that chart above how past 6.8 phosphorus and calcium bind. Above 6.8 plants will have trouble with the bound up phosphorus and calcium. Below 5.5 phosphorus availability drops off a great deal.

You can see on the chart above that Iron and Manganese don’t become available until a PH of 5.5 or lower. Plants that need a great deal of those nutrients, like Blueberries, and Ixora will benefit from an acidic soil.

So you see when we talk about a plant loving acidic or alkaline or neutral soil we are really talking about what nutrients that plant needs to find available in soil to thrive. Once you have a PH reading on your soil then you can consider or test further for what nutrition it may need.

On a recent soil test I submitted to the University I found that my compost based garden soil was off the charts for all of the nutrients for which they can test and my PH is 7.4. That is just a bit past neutral toward Alkalinity. My vegetables are growing great in that soil, and this information helps me to chart my course for the summer or fallow season. I plan to plant some cover crops like peanuts and peas on some sections which will help bring nitrogen in, and keep weeds out. I will probably add manure to the sections I let go fallow, but I will take it easy on the leaves and straw in the hopes of bringing the PH down just a little. When I am brewing compost I will go a bit heavier on the green or nitrogen heavy additives like manures and green plant wastes. The ideal PH for vegetable gardens is 5.8 to 6.5.

Adding organic materials in the form of composts and composted manures is a great way to ensure that your soil has all of the nutrients that plants need. Doing a PH test is an invaluable tool for a gardener in the know.

Happy Gardening