Garden Soil pH and Amendments

Compost & Soil
Garden Soil Amendments

After familiarizing yourself with soil texture and soil structure (and ways to test them), you can learn more about how to amend your soil and improve it. It's important to understand that soil amendments are not fertilizers, even though some have fertilizer value. Fertilizers are applied to supply nutrients to the soil. Amendments, on the other hand, serve to improve the soil's structure and texture, which in turns improves its drainage, water retention, aeration and other parameters.

Before moving to soil amendments, it's important to learn something about soil's pH values:

Soil pH

Soil pH is a very important feature of your garden soil, and it's important to know its value. What is pH? The "p" in the name stands for potential, while "H" stands for Hydrogen. It refers to soil's acidity or alkalinity. Sometimes, this is referred to as "sweet" or "sour" soils. Sweet soils are alkaline, while sour soils are acidic.

pH values of the soil are very important for plant health. pH values directly affect soil fertility because they control how well nutrients are dissolved. It means you can fertilize your soil, but if pH values of the soil are of range, the plants won't be able to access those nutrients. Also, if plant nutrients are being rendered insoluble due to inappropriate pH levels, while toxic elements become more soluble, which can damage the roots and kill the plant. Also, if pH is not adequate, beneficial bacteria will not be able to grow in the soil and without microorganisms, the soil's structure gets poor.

If you choose to grow your plants in soil that's either too acidic or alkaline, you will notice your plants developing yellow leaves, lack of proper growth and flowering and sick looking plant.

However, there are certain plants that thrive in acidic soils, while some prefer more alkaline environments. It all depends on the plant. You need to inform about the best care required for any plant you wish to grow in your garden.

For all these reasons, it's important to determine your soil's pH values. For home gardening, it's ok to use a simple testing kit you can find in most garden centers. To test your soil's pH values, collect a soil sample and follow the instructions provided with the kit. Alternatively, you might use litmus paper. Dip the tip of the paper into a bit of garden soil mixed with water. To read the results, compare the color change with the chart provided with the litmus paper.

Other easy home tests include using baking soda and vinegar. Alkaline soils fizz when vinegar is added while acidic soils fizz when soda is added.

Finally, there are many pH meters available for purchase, but they are a more expensive option. These meters are portable and easy to use. They usually measure the hydrogen ion activity.

Generally speaking, pH level of 7 is considered neutral. Most plants perform best at a pH level between 6.3 and 6.8. If you want to raise the pH levels of acidic soil, add calcium carbonate, ground dolomite limestone or wood ashes. Make sure to switch to fertilizers and amendments that are not acidic. To lower the pH values, you may add pine needless and agricultural sulphur. Add organic matter and use only acidifying soil amendments and fertilizers.

Using Soil Amendments

When choosing a soil amendment for your garden, it's important to think about your soil's pH level. Just like the soil, the amendments can be either acidifying or alkalizing. In case your garden soil has a high pH, it means it's highly alkaline, so it means you shouldn't add wood ashes as an amendment, since it also has high pH value. Instead, you should add a more acidic amendment, such as peat moss and compost.

Here are some of the best soil amendments you may use to improve your garden soil:

  • Shredded tree bark (slightly acidic)
  • Compost - Decomposed plant material (typically slightly acidic)
  • Sphagnum peat moss (acidic)
  • Manure - you may use sheep, cow, horse, rabbit and chicken manure (sheep manure is typically slightly acidic, while cow, horse and chicken manure is usually alkaline)
  • Leaf mold (acidic)
  • Pine needle (acidic)
  • Wood ash (highly alkaline)

It's best to add your pH amendments about one month before fertilizing and also well in advance of planting. After this one month, retest your soil. In case pH is still too high or low, give soil a second treatment. However, make sure to never apply more than recommended dosage. Remember, it may take years to fully amend your soil and change its pH levels.

Correcting Problematic Soils

Finally, it's important to know there are some soils that come with special challenges when it comes to structure, texture or pH imbalances. These problems need to be addressed and corrected before you start planting.

Here are some of the most common problems and how to solve them:

  • Hardpan. This is an impervious layer of soil near surface. Nutrients, roots and water can barely penetrate it. It makes the soil structure very bad. To correct this, add a lot of organic matter to the soil. Make sure not to walk on the soil when its wet or operate any machinery on it (including lawn mower).
  • Poor drainage. Poor drainage is problematic, because it makes water replace air in the soil. Air is vital for root growth, so too much water can kill the plant. To improve drainage, add some organic matter and gypsum. For more serious cases, you may consider installing drain piles or tiles beneath the soil surface so you can carry excess water away from this area.
  • Salty soils. Some soils are very salty. Excessive salt is bad for plant growth so it's best to be avoided. You will easily recognize salty soils by a typical white deposit on the surface. To correct this problem, add gypsum to the soil. Water the are using a deep slow watering method to flush the salt out of the root zone.

Photo credit: Nenad Stojkovic

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