Our March Tip of the Month explores the connection between the fertilizers you use and the pest insect problems those choices can create.If you want to seriously geek out on where synthetic nitrogen fertilizer comes from and what its global impacts are, we suggest you read our article 5 Facts You Should Know About Synthetic Nitrogen Fertilizer.
In this article, we’re going to briefly discuss a small side-effect of using synthetic nitrogen fertilizer (the stuff the friendly employee at the garden center hands you) that you probably don’t know about. We’ll also recommend a different, better soil fertility approach for you to consider.
Is Your Fertilizer Causing Pest Insect Infestations?
If you’re not familiar with how soil microorganisms and plants interact, the notion that fertilizer could cause pest insect infestations might seem very strange. What mechanism(s) would explain the causality?
Let’s take a quick look at what the research says on this matter:
A review of 50 years of research identified 135 studies showing more plant damage and/or greater numbers of leaf-chewing insects or mites in nitrogen-fertilized crops, while fewer than 50 studies reported less pest damage. Researchers have demonstrated that high nitrogen levels in plant tissue can decrease resistance and increase susceptibility to pest attacks. Although more research is needed to clarify the relationships between crop nutrition and pests, most studies assessing the response of aphids and mites to nitrogen fertilizer have documented dramatic expansion in pest numbers with increases in fertilizer rates.
How does feeding your plants synthetic nitrogen fertilizer cause them to attract pest insects?
Cultural methods such as crop fertilization can affect susceptibility of plants to insect pests by altering plant tissue nutrient levels. Research shows that the ability of a crop plant to resist or tolerate insect pests and diseases is tied to optimal physical, chemical and mainly biological properties of soils. Soils with high organic matter and active soil biology generally exhibit good soil fertility. Crops grown in such soils generally exhibit lower abundance of several insect herbivores, reductions that may be attributed to a lower nitrogen content in organically farmed crops. On the other hand, farming practices, such as excessive use of inorganic fertilizers, can cause nutrient imbalances and lower pest resistance. More studies comparing pest populations on plants treated with synthetic versus organic fertilizers are needed. Understanding the underlying effects of why organic fertilization appears to improve plant health may lead us to new and better integrated pest management and integrated soil fertility management designs.
-Altieri & Nicholls, “Soil fertility management and insect pests: harmonizing soil and plant health in agroecosystems.” Elsevier’s Soil & Tillage Research Journal, 2003
In short, if you want to create ideal conditions for a pest insect infestation, feed your plants with synthetic/inorganic nitrogen fertilizer. Then, you can spend lots of time, money, and effort applying pesticides to try to get rid of the problem.
However, if you want to grow healthy garden plants with strong immune systems and robust communities of symbiont microorganisms, we suggest you take a different approach: biological soil fertility.
With biological soil fertility, your plants won’t be a magnet for pest insects and will be better equipped to fend off whatever pest insects they might encounter (especially if aided by predatory insects in your garden/farm system).
5 Ways to Build Biological Soil Fertility In Your Garden or Farm
There’s no one-size fits all model that will work exactly the same way for every garden or farm on earth. Each of us has a different climate, different soil types, different plants, different resources available, etc.. A rural farm in south Texas is going to be quite different than an organic garden in Portland, Maine.
However, below are 5 methods that are universally applicable for building biological soil fertility. The more of these you can check off your list, the healthier your growing system is likely to be (soil organisms, plants, and macro organisms):
1. Avoid tilling your soil.
Instead, use no-till methods which minimize or eliminate soil disruption, leaving your microbial communities undisturbed. No-till also supports native pollinators which overwinter and lay eggs in your soil, many of which are now extinct or critically endangered. Leave your crop debris to decompose on the soil surface, helping to feed the beneficial fungi, bacteria, and other soil microorganisms in your soil system, while increasing soil organic matter/carbon.
2. Use microbe-dense worm castings or hot compost instead of synthetic or mineral fertilizers.
Not surprisingly, the quickest way to increase biological soil fertility is to add biology to your soil. Top-dressing your soil with worm castings and/or finished hot compost is the best way we know of to drastically boost biological soil fertility. Yes, there is nutrition in these media, but their near-magical effect is primarily due to the huge loads of beneficial microbes present. If you’re starting garden beds from scratch, you can also consider methods such as hugelkultur which can provide decades of biological soil fertility.
If you have a large growing area and not a whole lot of castings/compost, make actively aerated compost tea (AACT) and apply it as a soil drench and/or foliar spray several times a year (or more if you have poor soil or your plants are showing signs of foliar diseases).
*Note: We top-dress 2″ of castings/compost on new beds and/or when our mulch has been broken down to soil, but we use an AACT soil drench whenever we want to add biological fertility to a bed that still has a thick layer of mulch on it. If you want to get high quality worm castings at a great price, get them here!
3. Avoid pesticides.
Measuring the impacts of various pesticides (insecticides, herbicides, fungicides, etc) on soil microbial communities in every type of soil and planting system is nigh impossible, especially when you start combining and applying different types of pesticides throughout the year.
Our general philosophy (based on the Precautionary Principle) is that it’s a good idea for humans to avoid ingesting endocrine and neurotoxic pesticides, especially if you’re a pregnant mother, an infant, or a child. (The American Academy of Pediatrics happens to agree with our position.)
We think the same philosophy holds true for applying these substances to your soil microbial communities as well. If you have to use a pesticide, make sure it’s OMRI listed. Most pesticides used in organic gardens and farms (all of which are OMRI listed) are completely harmless and are actually just strains of beneficial bacteria that you can spray on your plants – they’ll outcompete or consume the pathogens! A range or OMRI listed organic pesticides that we recommend are available here in our organic gardening supplies store.
4. Always cover your soil.
Do you have a garden? Always keep your soil covered. Wood chips, mulch, or chopped leaves all work great, in addition to trying to keep living roots in the soil throughout the year (plant roots feed microbes and microbes feed roots). If you’re a farmer, cover crops will work wonders. There’s lots of research going on right now via the USDA and various universities around the country as to exactly what types of cover crops work best with the aim of helping more farmers get off the “chemical treadmill” and reduce pollution.
5. Utilize biodiversity.
In large farms, this might mean crop rotation and intercropping. On smaller farms, it might mean developing high-yielding food forests with hundreds of species of plants and animals mixed in. If you’re a gardener, you can actually develop far more biodiverse, resilient agroecosystems than a large farmer could ever dream of.
Research has shown again and again, that the more plant diversity you have, the more microbial diversity in your soil system, the more (and more varied) your beneficial insect communities will be, and the less likely you are to have severe infestations of pest insects or plant pathogens. Best of all, you’ll always get a yield! If one type of plant has an off-year, you’ll have 10, 20, 50 other plants that will fill your harvest basket.
Now, put the fertilizer down and start focusing on the biological fertility of your soil!