Soil-Borne Pests and Diseases

By John Wallace, SIFT Farm Manager

 Bone meal, blood meal, and kelp meal are beneficial ingredients in potting soil mixes.
Bone meal, blood meal, and kelp meal
are beneficial ingredients in potting
soil mixes. Photo: NCAT

Commercial potting soil mixes can cause a lot of problems down the road. Often, the cheaper they are, the higher the chance there are diseases and weed seeds embedded in them. One thing we do each year on the SIFT farm is prepare our own potting soil. This minimizes the risk of introducing soil-borne pathogens to our plant starts and reduces costs compared to buying bulk potting soil. We have been using a recipe that has enough nutrients to keep the plants healthy and fertilized from our first seeding in the middle of March until they are transplanted the second week of June.

Potting soil needs to have enough food for the plant. Organic fertilizers are scaled on a ratio of Nitrogen, Phosphorus, and Potassium (N-P-K). For instance, blood meal is high in Nitrogen and the product may show 12-0-0. Bone meal is high in Phosphorus and may show 3-15-0. Kelp meal has a scale of 1-0.1-2. We try to have enough of each to keep the plants well fed through their beginning phases. Peat moss helps retain moisture in the soil while perlite offers a substrate to keep soils loose and well-draining. Lime acts as a pH buffer and will prevent acidic soils that can be caused from using homemade composts.

The recipe for our soil mix is:

5 gallons sifted compost
5-10 gallons peat moss
5 gallons perlite
1-2 cups blood meal
1-2 cups bone meal
1-2 cups kelp meal
1 cup lime

Compost can contain pathogens and weed seeds if not “cooked” correctly. The first line of defense is to not put infected plants or weeds into your compost. Compost is full of probiotics that will help unleash nutrients in the soil and combat overpopulation of undesirable bacteria and fungi. You want enough moisture to keep the metabolic activity constant and enough oxygen to prevent anaerobic activity. Compost needs to be turned at the right time to keep the temperature of the pile in a range that is most effective. Weed seeds are killed at a temperature above 135 degrees F. If it gets too hot, the aerobic bacteria will be killed off and anaerobic bacteria will take over. This happens when the compost reaches temperatures above 160 degrees F. The best plan of action is to turn the compost and water it each time it spikes up to around 140-150 degrees F. Generally, the compost will be complete after three temperature spikes. Do not add compost that has not completed cooking because it can burn the plants.

 Bleach can help sterilize starter pots, but be sure to use it correctly.
Bleach can help sterilize starter pots,
but be sure to use it correctly.
Photo: NCAT
 Neem oil or Spinosad can help kill pests such as spider mites.
Neem oil or Spinosad can help kill pests
such as spider mites. Photo: NCAT.

Another way pathogens can ride along into the plant starts is if you reuse starter pots. Excess soil on the pots needs to be wiped away. You can then sterilize the pots by dunking them in a bin of water and bleach. Bleach will kill both good and bad bacteria so make sure it is rinsed well to remove excess bleach. Once they are dry you can begin filling the pots with your potting mix. Spider mite populations can hide on all kinds of surfaces. When growing intensively in a greenhouse, these pest populations can explode very quickly. We use organic pesticides to clean all the surfaces before we begin seeding. We typically use Spinosad or neem oil, both of which are OMRI listed. Even though these products are allowed under organic standards, they do kill beneficial bacteria as well. It is best to limit the use of such insecticides. We try to use these as a preventative measure so that we aren’t using them once the starts have been seeded.

By being proactive during the seeding of plant starts, pest and disease populations can be kept at bay. This year we will be actively supporting beneficial insects as well. I will be seeding rows of flowers in-between our vegetable starts that will help feed ladybugs and green lacewing. These bugs will help eat aphids. We will purchase larvae and introduce them to the greenhouse before the destructive pests take hold. This will add one more line of defense and support our integrated pest management efforts to grow healthy organic plant starts.

Posted on: March 19th, 2018