iGrow Gardening: Organic gardening

Wyatt Brown
SDSU Extension Horticulture Intern

Many professional horticulturists and hobby gardeners throughout South Dakota are considering the transition from conventional, synthetic plant production to more natural, organic methods for their production farms and at-home plant care. Although the transition from synthetic to organic can be daunting at first, readers can rest assured knowing the procedures and methods presented in the next few issues of this newsletter will help set the foundations for successful organic plant care methods for years to come. This article in no way proclaims organic methods as a cure-all, nor does it completely marginalize synthetic use, but it does advocate for a more natural approach to maintaining healthy lawns, gardens, and perennial plant growth for our readers. In fact, some plant growth or soil health disorders are so severe that a synthetic application may be the best primary treatment for timely results. However, we will usually follow these synthetic applications with organic products to re-establish the natural ecosystem after the synthetic product has done its job. This first issue in a series of organic growing methods will provide readers with an overview of organic gardening to get them started, providing follow-up publications regarding specific organic methods for combatting plant health enemies.

Improving your soil

The most important aspect of growing plants organically is starting with healthy soil. Soils are not lifeless and inert, they are alive! A single teaspoon of healthy soil can house up to a billion bacteria, several yards of fungal filaments, several thousand protozoa, 40-50 nematodes, and a variety of smaller insects and arthropods that work harmoniously throughout a complex soil food web responsible for life as we know it on this planet. The importance of maintaining fertile soil cannot be overstated. Organic producers rely on their knowledge of natural ecosystem cycles to determine the correct soil management techniques for their soil type. They regularly consider the natural inputs required for maintaining high microbial populations and nutrient cycling techniques. Growers utilizing organic practices feed the soil, which in turn feeds and defends their plants.

Up front we must acknowledge what is considered by many to be the biggest disadvantage to organic methods: they’re slow. It is true that organic production practices are devoid of overnight solutions to remediate pests and unbalanced soils. There are methods to manipulate these processes to hasten the results, and we will discuss these further throughout the season, but for now plant enthusiasts must prepare for patience and a steady transition from synthetic use to organic production over a 2-, 3-, or 4-year program, depending on your current situation. This does not mean that growers will not obtain lush, healthy growth from their plants during this time. It just means that growers will have to provide increased organic amendments and “quick release” organic fertilizers up front to ensure plant nutrient needs are met while the soil reestablishes its natural ecology.

Many gardening publications, whether they tote organic methods or not, all tend to agree on the importance of adding organic matter to your soil (lawns, gardens, etc). Organic matter (OM), also known as soil organic matter (SOM), is a dynamic mixture of living, dead, and decomposing life-forms, and is the “glue” that holds soils together. In general, 5 to 10% OM is considered optimal for healthy plant growth, however most garden soils range from 2 to 3% SOM. OM can range from kitchen wastes and shredded leaves, to well-rotted manure and compost, to dead insects and grass clippings scattered throughout the soil. According to Rodale’s Growing Fruits & Vegetables Organically, “Feeding your soil a diverse diet of organic matter is an essential part of any management program…the secret to maintaining a balance between building organic matter and growing crops intensively is to add residues continually. Mulching with organic matter, adding compost, and using cover crops (also called green manures) are the most common ways to add soil organic matter.” As microbial populations and other insects and animals consume and decompose SOM, we are left with a reasonably stable product known as humus. Humus consists of very long, hard-to-break chains of carbon molecules with a large surface area. These surfaces carry electrical charges which attract and hold mineral particles within the soil, making nutrients more available to your plants. If you were to view these molecules through a scanning electron microscope, you will notice these carbon chains resemble a sponge; capable of holding 80-90% of its weight in water, and providing housing for crucial beneficial microbes like bacteria and fungi. The gardener’s equivalent to nature’s humus production is the composting process. Compost is the organic gardener’s secret weapon! Compost is teeming with microbial populations, incorporating organisms from all six biological kingdoms throughout its highly diverse humic profile. However, there are a couple rules to follow when adding compost to your gardens, trees, shrubs, and lawns.

First, not all compost is created equal. Compost can vary in its mineral content from producer to producer, and batch to batch. As a general rule of thumb, compost will contain the following Nitrogen (N) – Phosphorus (P) – and Potassium (K) percentages by weight. N: 1.5 to 3.5, P: 0.5 to 1, K: 1 to 2. Compost also provides secondary macronutrients like calcium (Ca), magnesium (Mg) and sulfur (S), as well as a wide variety of trace elements depending on the starting materials. Second, when applying compost to your growing areas, use compost that is well finished. Compost which has not thoroughly broken down will utilize elements in the soil to finish its decomposition process before being available to plants. This means microbes are feeding themselves instead of feeding plants, which tends to be a problem if not managed correctly. Third, don’t be afraid to apply a healthy portion of compost to these areas one to two time per year. Compost has a very low danger of burning plants, unlike its highly soluble synthetic cousins. A good rule of thumb is to apply ½ inch to 3 inches of compost as a dressing or incorporated into the soil, depending on your soil’s fertility level.

For vegetables and annuals, incorporate 1-2 inches of compost into the top couple inches of soil before seeding or transplanting. Gardeners can also apply a side-dressing of compost midway through the growing season as both a mulch and a source of slow release nutrients for their food crops and flowers. For newly planted trees and shrubs, avoid adding compost directly into a planting hole – this causes roots to ball up instead of spreading out in search of nutrients – thus leaving plants susceptible to issues like waterlogging, disease, insect problems, and/or windthrow, a condition where trees are uprooted or broken by wind. Instead, top-dress your existing woody plants or newly planted trees and shrubs with 1 to 2 inches of compost across the root zone, then cover with a mulch of wood chips or shredded bark. When considering lawn applications, users can apply compost when establishing a new lawn, or when rejuvenating your lawn in spring. You can even add fine compost as an amendment during lawn aeration so it comes in immediate contact with roots. Compost can even be used as an amendment in your potting and seedling mix. Screen out any large chunks of compost through a ¼ inch mesh screening and add it to either your store bought seedling mix or your own custom made mix. If you are making your own custom mix, be sure to include materials like peat moss, coconut coir, vermiculite, and/or perlite to ensure ample poor space, drainage, and water holding capacity. This will allow delicate seedlings to penetrate the mix and effectively mine for nutrients as they establish themselves. It is generally recommended to not use garden soil in your seedling mix unless it is of great loam texture, great tilth and little likelihood of becoming compacted

While compost can provide a good input of slow release nutrients, growers will find an even better benefit: the delivery of beneficial and diverse microbial populations to your soils. Microbes, in particular beneficial bacteria and fungi, are the organic growers #1 defense against a variety of plant health issues. In natural settings like old growth forests, grasslands, mountainous areas, and generally any area untouched by man, microbes are the primary defenders against a relentless barrage of environmental stressors. Influencing everything from drought resistance, to nutrient uptake, to defending against soil & airborne pathogenic diseases, to producing plant growth hormones, beneficial microbes are nature’s utility player for maintaining plant vigor. In fact, microbes are so important to plant survivability that plants will secrete 20-30% of the food they produce through their roots to feed and attract microbes to the root region. These secretions, known as exudates, consist of carbohydrates (sugars), organic acids, proteins, exoenzymes, and other substances. The main benefit of carbohydrate release is it ensures microbial carbon (think energy) needs are met. Microbes need roughly a 25:1 carbon to nitrogen (C:N) ratio to meet their growth needs. These needs can be met by their own decomposition of OM, and/or through the exuding of sugars by their symbiotic host’s roots.

Symbiosis is a mutually beneficial relationship between two different organisms living in close physical association. The importance of establishing a healthy plant-microbe symbiotic relationship cannot be overlooked. Nearly every known species of plant forms a relationship with several species of beneficial microbes. Microbes can inoculate every area of a plant, from the root zone below ground, to the exposed, habitable plant tissue above ground, and even tissue inside the plant as well. The job of these microbes is simple: protect and nourish the plant at all costs! Beneficial bacteria like Bacillus subtilis have been found inhabiting leaf surfaces to protect the plant from leaf diseases like powdery mildew. These leaf protecting species feed on sugar sources found secreting from the interior of the plant. Beneficial bacteria also produce several antibiotics and chemicals responsible for communication and lateral gene transfer amongst the colony, as well as fending off pathogenic bacteria, fungi and yeasts species attempting to colonize the leaf surface. Microbes used specifically as plant defenders are known as biological control agents (BCA) or antagonists.

Below ground, microbial colonies are nourishing their hosts in a variety of ways. Just as bacteria protect above ground exposed plant tissue, soil dwelling bacteria are doing the same. They produce enzymes, amino acids, organic chemicals, and other byproducts that have an enormous effect on soil and plant health. Many of these enzymes help break down minerals in soil to ionic form, making them available for root uptake. Microbial amino acids are the basis of important secondary metabolites which prevent iron from locking up with other minerals in the soil and becoming unavailable to plants. They produce plant hormones that stimulate lateral root growth, produce more flowering sites, and activate the plants natural defense and response to stress. Just as important, they produce antibiotics and anti-fungal metabolites that keep soil borne pathogens at bay.

In addition to beneficial bacteria, a powerhouse beneficial fungus known as mycorrhizae have been found associating with 95% of the world’s plants, forming symbiotic relationships with plant host’s roots. They are most well-known for providing host plants with water and nutrients, specifically hard to unlock phosphorus. Fungal hyphae extend out into the soil, mining for not only available nutrients, but decomposing SOM into usable elements, and delivering them directly to the plant root. In exchange, plants provide mycorrhizae with carbohydrates to continue their fungal growth process. Additionally fascinating is mycorrhizae’s ability to help ward off pests. They have been known to trap root enemies like nematodes in their hyphae, preventing them from decimating cash crops like tomatoes. The nematodes are then consumed by the fungus, which in turn use the nutrients obtained for growth and feed the rest to the plants.

A variety of mycorrhizae inoculants can be found on the market. Growers should spend some time shopping around for the best product for their particular plant. While adding mycorrhizae to plants won’t harm them, some plant species, like members of the Brassicaceae family (kale, broccoli, etc.) will not form associations. Many plant species will only form associations with certain fungal species. The products sold on the market today are extraordinarily capable of providing growers with a varietal abundance of different strains of mycorrhizae in every mix. Some potting mixes come pre-made with mycorrhiza inoculant mixed in. Other products come as a wettable powder or in granular form. Growers can use these products in drip irrigation systems, hydroponic systems, or as a pre-transplant inoculant added to the roots before transplanting.

Protecting the soil food web

Bacteria, mycorrhizae, and other soil food web members require nutrients in the form of either organic matter or exudates to continue their life processes. A great majority of these nutrient requirements are being disrupted by the average gardener. Bare soil during both the growing and off season is a leading disturbance for nutrient cycling within our soils. Growers should ensure they are using mulch or cover crops to avoid bare soil. A great source of mulch can be grass clippings, which provide a fairly quick source of nitrogen, or wheat straw. When using straw, be sure to incorporate a small ratio of grass or other nitrogen source, as straw has a C:N ratio of 80:1 and will tie up soil nitrogen as microbes break it down. Be sure to add no more than 2-3 inches of mulch to your garden areas between plants and across bare soil areas, while avoiding contact with the base of your plants. Your mulch serves a multifunction role. It feeds microbes, prevents weed germination, increases SOM, and prevents soil erosion and soil water evaporation. A good mulch will also lessen the need for roto-tilling.

Excessive tilling destroys fungal networks, crushes arthropods, and decimates the indispensable worms and the pore spaces they’ve created as they burrow throughout the soil. Soil structure deteriorates from the decline of these important organisms, causing closure of soil pores and leading to possible hardpan issues in the soil. As these issues compound, you’ll notice an increase in soil erosion, a decrease in water absorption, a lack of available nutrients, and overall loss of soil productivity. Gardeners can combat the need to till by developing long-term beds in their gardens. They can also use specially designed tools like broadforks. Broadforks are a handy garden tool that allows users to break up any hardpan or compacted soil areas instead of roto-tilling every season. Additionally, cover crop combinations that incorporate root crops like turnip and Daikon radish can help decrease compaction during fallow seasons. With fewer disruptions to the soil food web, additions of organic matter to our growing areas, and a better understanding of natural soil fertility, growers will begin to witness a noticeable shift in their soil’s production as they make the organic transition.

Wyatt Brown
Finished compost.
Grass mulch around 3 Sisters Garden.
Straw mulch.