Working in the Landscape: Questions about Soil Amendments


A graph showing the best pH for plant nutrients. Image: digitalcommons.

Soil amendments

John of Alexandria contacted the AgCenter to ask several questions about soil amendments:

  1. What is the optimum pH to enable plants to take up the most nutrients?
  2. What is the optimal particle size for haydite in containers?
  3. How does a vendor determine the pore space of bark mulch in containers?

Here are some answers to John’s questions about using soil amendments.

Optimum pH

An AgCenter article entitled, “Properly prepare beds for landscape plants” by several writers discusses the best pH for plants, ‘Optimum soil pH is critical for landscape success. Louisiana has soils that are somewhat variable in pH ranges. Ideally, a perfect soil pH for most ornamental plants in Louisiana is 5.5-6.5. Soil pH is a measurement of its acidity or alkalinity. A pH value of 7 is neutral while a pH value less than 7 is acid and a pH value greater than 7 is alkaline or basic. Soil pH is raised by using lime (normally dolomitic lime in landscape situations) and is lowered by using sulfur. Always adjust pH based on the results of a soil test.’

‘Some plants in Louisiana landscapes and home gardens are classified as acid-loving. These plants do best with a soil pH slightly lower than other plants that we commonly grow. A soil pH in the 5.0-5.5 range is preferable for plants that require more acid growing conditions. Common examples are blueberries, camellias, sasanquas, dogwoods, azaleas, periwinkle, petunias, and pansies. In turfgrass, centipede grass prefers acid soil, while St. Augustine grass prefers neutral to slightly alkaline soil.’

‘The LSU AgCenter Soil Testing and Plant Analysis Lab can conduct soil testing for you. It will provide a report with information on soil pH and the levels of many essential nutrients present in your soil. A routine test is $10. You can access information from the LSU AgCenter Soil Testing Lab at .’


An image of haydite. Photo:

Optimal Particle Size for Haydite in Containers

AHA consulted with Dr. Jeb Fields, the AgCenter’s specialist in commercial horticulture, on the last two questions. Dr. Fields shared via email his comments, “I don’t think there is an optimal particle size. Instead it is very dependent on the rest of the mix. Haydite is likely just a small portion of the mix. Are they looking to increase drainage or retain more water? Also, what is the reason they are looking to use Haydite? Some more information will be a great help.”


Landscape mulches being research at the Hammond Research Station. Photo: Dr. Allen Owings, LSU AgCenter.

How to Determine the Pore Space of Bark Mulch in Containers

Dr. Fields shared his thoughts on the question of the pore size of bark mulch, “As far as pore space, that is a good question. There is a rough method that you can do in the field and maybe I can make a video or fact sheet. Basically, you fill the container and slowly submerge the container in water. This needs to be done slowly to force all the air out. Seal the bottom holes and remove the container from the water. Then open the drain holes and collect all the water that drains out (99% of the water that will drain, will drain in about 2 min.). Divide the volume of the water that drained by the container volume and you have the Air-filled porosity.

Weigh the remaining wet bark, put in the oven, and dry (or you can spread it out and air dry the bark for very rough results). The difference in the weight (wet – dry) is the volume of water. Divide the difference in weight (wet-dry) by the container volume and that is the Container Capacity or water filled porosity.

Air filled porosity + water filled porosity = total pore space. Notice I did air by volume and water by weight. You must convert both to the same if you use gallons or lbs. However, if they use grams then there is no conversion needed as 1g = 1 mL for H20. I would always recommend using mL and g for convenience.

Feel free to call me on my cell 863-412-7959 if this is confusing and I can talk you through it.”

If you want to contact Working in the Landscape, please send your questions and pictures to Keith Hawkins, Area Horticulture Agent (AHA), 337-463-7006 or Also, you can be on the “commercial horticulture” email list by emailing your request to the address above.

“This work has been supported, in part, by the USDA National Institute of Food and Agriculture, Renewable Resources Extension Act Award, Accession Number 1011417.”

7/24/2020 3:30:33 PM
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