Roots, Shoots, Fruits & Flowers: The Down Side of Organic Gardening

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Mulch helps soils. Photo: Dr. Allen Owings, LSU AgCenter, Retired.



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Tomato & potato plants with herbicide injury. Image: LSU AgCenter.



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Phosphorus toxicity causes the iron deficiency in rhododendron leaves. Photo: Dr. Linda Chalker-Scott, Washington State Extension.



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Soil test kits enable gardeners to make decisions about their gardens. Photo: LSU AgCenter.

Even though the title of this article sounds a little negative, let us be clear: organic matter (OM) has a net positive benefit effect on improving soil health. OM will loosen heavy clay soils and minimize soil compaction. The same OM will help light sandy soils with holding more water and plant nutrients.

Organic mulches are great for landscape and gardening purposes. All sorts of materials make good mulches: pine straw, leaves, bark, etc. There is one kind of mulch that homeowners need to keep in mind. Using fresh wood chips in foundation landscaping around a house will invite termite infestations. Consider using the wood chips in the garden for paths between vegetable rows. Another option to place the chips in a compost bin, and then let the compost process reduce the chips to OM.

Another organic mulch of concern for vegetable gardeners is animal manure. Over the years, gardeners will bring in tomatoes suffering from herbicide damage. When people think of herbicide damage, they tend to think of a pesticide spray drifting off site. However, these gardeners did not use herbicide spray around their tomatoes. The herbicide came from the animal manure.

Gardeners will use uncomposted animal manure, thinking it will provide organic benefits, but these manures can be contaminated with 2,4-D, a popular herbicide to kill broad-leaved weeds in pastures. The 2,4-D safely passes through the animal, like a cow or horse, but it can end up in your garden as a uncomposted manure. Composting manures to the point where it looks like crumbly, black soil will reduce the risk of herbicide contamination.

Even compost use as an organic soil additive has its limitations. The consistent application of compost over time will raise levels of phosphorus to toxic levels. Phosphorus is generally beneficial at the correct levels, and this nutrient enables plants to set fruit. At excessive levels, phosphorus will chemically bind with iron and zinc, both essential nutrients in plants. If both iron and zinc are unavailable, then plant health will suffer.

To prevent excess levels of phosphorus from regular compost use, a soil test is necessary and will enable a conclusive diagnosis. While the diagnosis may be easy to obtain with a soil test analysis, the solution is much harder to attain. The reason for this difficulty is because phosphorus is very stable in soil and likes to stay around.

If space is available, the best solution is to move the garden to another location. A gardener should test the soil at the old garden at least every three years to check phosphorus levels. The old garden site may be out of production for many years due to the stability of phosphorus.

Another possible solution is to STOP application of OM to the old site and grow cover crops like legumes or grasses. Remove the cover crop, avoid adding compost and monitor the site with annual soil testing. Here is a useful tip. Sample your soil in the fall of each time you sample. Sampling at the same time enables gardeners to compare results of the results of soil test from different years.

Soil test kits are available at your local office of the LSU AgCenter. These kits enable homeowners to sample at home and to send the sample directly to the AgCenter’s soil testing lab in Baton Rouge. The cost for a routine test is $10 per soil sample plus $6 for the USPS Priority mailing box. Testing for OM is an additional $5.

If you want to contact Roots, Shoots, Fruits and Flowers, please send your questions and pictures to Keith Hawkins, Area Horticulture Agent (AHA), 337-463-7006 or khawkins@agcenter.lsu.edu. Also, you can be on the “green thumbs” email list by emailing your request to the address above.

“Before you buy or use an insecticide product, first read the label, and strictly follow label recommendations. Mention of trade names or commercial products in this article is for informational purposes only and does not constitute endorsement by Louisiana State University AgCenter.”

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

3/4/2021 3:31:20 PM
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