Written by: Marge Vorndam, CMG Emeritus
If you are a gardener, one topic that is important to understand is the relationship between soil, soil pH, soil nutrients in a growing area, and the ability of the soil to release those nutrients. You also need to know how to determine if the specific soil needs pH or soil amendments to augment its health.
It all begins with pH, which means (caution: math ahead!) the negative log of the hydrogen ion concentration. The pH scale covers a range from 0 to 14 with 0 being most acidic to 14 being most basic and with 7 (neutral) in the middle. pH is used to compare the acidity or basicity of many things. Battery Acid and Stomach Acid are near a pH of 1 to 2, saliva is near normal (pH 7), and liquid drain cleaner might be pH 14. For the non-chemists among us, this might seem a bit complicated, but that’s the gist of it.
From the U.S. EPA
It turns out that soils also have different pHs. The pH level governs the amount of soil nutrients that are free for plant roots to take up. In addition to pH, soil contains the macronutrients N, P and K as well as micronutrients like Sulfur, Copper and so on. These nutrients are an important component of soil as well but not as significant as pH.
How do you know exactly how much of a nutrient to add? First, determine soil pH. There are kits available in hardware stores that can help you to determine pH, as well as current N (nitrogen), P (phosphorus), K(potassium) levels in your soil, but a more accurate reading can be obtained by sending a soil sample to a Soils Lab. Check with your County Extension specialist for processing information. The lab will return specific instructions to let you know if you need to adjust the soil pH to ensure an appropriate growing medium for the planned crop.
You are likely acquainted with the fact that N (nitrogen), P (phosphorus), and K (potassium) are indicated on fertilizer bags in percentage (the bag in the illustration provides 10 % of each nutrient). In this case, based on the crop you wish to grow, soil test results will give you an accurate recommendation as to how much macro and micronutrient to add.
This chart indicates what soil pH specific plants prefer for growth:
Alfalfa 6.5–8.4
Vegetables 6.5–8.2
Garlic 6.5–7.5
Grass for seed or pastures 5.5–8.2
Fruit trees 6.0–8.0
Highbush blueberries and cranberries 4.5–5.5
Rabbiteye blueberries 4.2–5.0
Azaleas and rhododendrons 4.5–5.5
Field or silage corn 5.5–8.4
Wheat 5.5–8.4
The chart below shows why this is significant.
Relationship between pH and availability of Soil Nutrients for Plant Growth
For instance, vegetables prefer a pH of 6.5 to 8.2. That’s because this soil pH makes the macronutrients, N, P, K, most available for these crops to grow well. They may also profit from having the micronutrients, Boron, Copper, and Zinc more available to promote the best growth.
At times, it may be more important to adjust your pH rather than adding a lot of fertilizer. That is an entirely different topic so I will leave that for another article.
A disclaimer for the organic gardeners among us: this is not the way that an organic gardener would approach this topic. There are many organic-based ways that can be used to accomplish essentially the same outcome, but without using mined fertilizers. Check online or with the several organic nurseries and gardeners in our area for their recommendations.
Happy gardening!