iGrow Gardening: Organic Soil Amendments and Fertilizers

Wyatt Brown
SDSU Extension Horticulture Intern

By now we have established the importance of soil health and the weapons fertile soil bring to the fight. As organic growers, your responsibility is to feed the soil food web (SFW); the soil food web will feed your plants. The SFW works in conjunction with your plants to determine what types and quantities of water, nutrients, growth hormones, and natural pesticides your plants need. Microbes not only unlock elements, but in collaboration with your plants they also help establish acceptable pH ranges within your soil. With over 550 different types of soils in South Dakota, relying on natural microbial populations to decipher your plant requirements is essential as they help take the burden of guessing off of your shoulders. It is important to understand however, when it comes to feeding on available nutrients, microbes always eat first! When microbial and nutritional balances are out of whack, entire systems begin to crumble, and in extreme instances shut down completely. Thus the importance of nourishing soil microflora and microfauna with natural inputs is a necessity.

While microbes are highly resilient, there is not an organism on this planet that can survive without carbon-based energy sources or the nutrients and water needed for healthy functioning cells. The use of synthetic products, coupled with gardening practices that furnish bare soil, have disrupted the natural carbon & nutrient cycle which microbes use. Most of the soil food web members receive their daily carbon through two ways: the exudates (carbohydrates, enzymes, etc.) sloughed by plant roots, and/or through the digestion and decomposition of organic matter (OM). On the contrary, a small number of synthetic nutrients can be utilized in their ionic form by microbes, but many of them cannot.

Furthermore, and maybe most importantly, when plants receive nutrients via synthetic fertilization, the need for plant-microbe symbiosis is disrupted almost completely. Plants will rely on the readily available fertilizer instead of relying on microbes for assistance in mining these nutrients. This leads to a host of problems later on in the growing season as synthetic fertilizer sources have been depleted. Plants then must scramble to produce additional carbohydrates to feed and establish plant-microbe symbiosis when they should be focusing sugar reserves on seed and fruit production. This adds undue stress to the plant during one of the most critical times of its life cycle. It is because of this that gardeners must focus attention on delivering natural products by way of OM and natural-based organic fertilizers to these organisms. Once soil food web members have used what they need, elements in usable form are cycled and excreted for uptake by plants.

Soil amendments are materials derived from animal, mineral, and plant-based sources, and are added to soil to improve its texture, structure, and fertility. These substances promote the improvement of pore space, improved drainage, buffer soil pH, increase soil organic matter (SOM) and provide uniform distribution of water and nutrients. Such factors are needed for healthy microbial life, ease of root growth, and enhanced capability for water and nutrient absorption. While nutrients may be derived from some of these materials during natural decomposition, they technically by definition are not a fertilizer. On the contrary, in the organic world, many organic materials serve as both an amendment and a fertilizer. Amendments can be added to any soil texture, but are most commonly added as a soil conditioner to soils with high clay, high silt, or high sand content.

Before adding soil amendments, conduct a soil test so you know exactly what conditions you are trying to correct. Amendments should be thoroughly mixed into the top 6-8 inches of soil with hand tools, a broadfork, or with a garden cultivator or shallow tined rototill. Sandy soils are prone to excessive leaching of both water and nutrients. When amending sandy soils, the goal is to increase the soil’s ability to hold moisture and store nutrients. Amendments such as sphagnum peat moss, compost, or well-rotted manure will aid in water retention. Soils that exhibit salt content levels above 3.0 decisiemen per meter (dS/m) on your soil test can begin compromising your plants ability to take up water. Therefore, it is recommended to not add amendments with high salt content like well-rotted manure to these types of soils. Additionally, adding organic mulches such as grass clippings or leaf litter will not only help with water retention and weed suppression, but will eventually break down into usable nutrients for your plants.

On the opposite side of the spectrum, clay soils have a tendency to become excessively compacted due to their unique structure. Clay particles stack on top of each other much like a deck of cards, or like clay platelets. This phenomenon of stacking together disperses pore space, and in drastic situations eradicates air and water space all together, making root penetration impossible. Amending clay soils is not an easy task and may take years to witness beneficial results for your efforts. When amending clay soils, the goal is to improve soil aggregation, improve drainage and aeration, and increase porosity and permeability. It is important to avoid working clay soils while they are wet as this will only increase compaction. Best amendment selection for clay soils again would be compost or well-rotted manure. Contrary to popular belief, do NOT add sand to clay soils. Sand, clay, straw, and water mixed together result in a substance known as cob; a material used to build cement-like blocks and earth buildings.

Soil amendments used to raise or lower pH are varied in their intensity, residual effects, and cost effectiveness. Most of the soils in South Dakota provide a relatively alkaline pH, and altering alkaline soil can take time as most acidifying amendments take a season or more to take effect. Furthermore, the leaching effect of rain and irrigation can wash acidifiers out of the soil, bringing alkaline elements back into place. Yearly applications of acidifying amendments may be required. Amendments used to neutralize alkaline soils include garden sulfur, and uncomposted peat moss, pine needles, and oak leaves. Since our soils are generally NOT acidic, we rarely need to add amendments like ground limestone, wood ash, manure-based compost, or biosolids.

Organic fertilizers differ from their synthetic counterparts in that they are a source of nutrients for both plants AND the soil food web. According to Rodale’s Ultimate Encyclopedia of Organic Gardening, “Over time, soils treated only with synthetic chemical fertilizers lose organic matter and the all-important living organisms that help to build a quality soil…For organic growers, creating a living soil, rich in humus and nutrients, is the key to growing great fruits and vegetables, abundant flowers, and long-lived ornamental trees and shrubs. The overall fertility and viability of the soil, rather than the application of (synthetic) fertilizers as quick fixes, is at the very heart of organic gardening.” Organic fertilizers are derived from natural sources like plants, animals, and mined minerals. They come in a variety of delivery methods, but the main two methods include dry organic fertilizers and liquid organic fertilizers. Dry fertilizers may consist of a single material or a blend of many ingredients. Most organic fertilizers provide a wide array of nutrients, but blends are specially formulated to provide a well-balanced profile of macro- and micronutrients, as well as trace minerals in some respects.

The most common way to incorporate dry fertilizers into your gardens is to broadcast materials across the growing area, then work fertilizer into the top 4 to 6 inches of soil. This can be done with a hoe, a rake, a broadfork, or a light cultivator. Side-dressing your plants during the growing season can add a boost to your plants by incorporating your fertilizer into the top inch of soil after it has been applied. Dry fertilizers need to be digested and cycled by soil food web members first before they become available for plant uptake, thus the delay in availability to your plants. Cycling is dependent upon the population of soil food web members, soil temperature, and water availability. Microbial activity operates at temperatures between 50°-90°F, with optimal temperatures between 77°-86°F. As both ambient and soil temperatures increase, so do plant nutrient needs. Microbial activity mimics this increase by enlarging population size, which in turn enhances nutrient cycling capabilities and availability for plants.

Examples of dry fertilizer and typical analysis

• Alfalfa meal: 5-1-2

• Blood meal: 11-0-0

• Bone meal: 1-11-0

• Kelp meal: 1.0-0.5-2.5 plus 60 trace minerals

• Rock phosphate: 0-3-0. 32% total phosphate. 32% total calcium. Contains 11 trace minerals.

In addition to these fertilizers, certain materials are used to improve organic matter and used as a mulch, and breakdown overtime into usable nutrients.

• Grass clippings: 0.5-0.2-0.5

• Leaves: 0.8-0.4-0.1

• Wheat straw: 0.7-0.2-1.2

Liquid organic fertilizers (LOFs) are a more readily available delivery method for organic nutrients. While some components of liquid fertilizers must still be cycled, the majority of these components are readily available for plant uptake both through the roots, and through the stomatal pores when used as a foliar spray. LOFs also provide a great supply of microbial variety and populations to plants, and can be used as a quick boost for plants showing signs of deficiency throughout the growing season. LOFs can be applied around the root zone by hand watering or through drip irrigation systems. Be sure to follow instructions for these fertilizers as some products can clog drip irrigation holes and emitters.

Foliar sprays are most beneficial during critical periods such as transplanting or fruit set. In addition to nutrient uptake, some foliar sprays also provide leaf surfaces with beneficial microbes that help fight off pathogens. Diluting the solution and spraying in the early morning or late evening when the sun is not as intense are a MUST as this decreases the chance of leaf scorch. It is recommended to use a surfactant such as yucca extract, coconut oil, or a mild soap (1/4 teaspoon per gallon of spray) to ensure better coverage of the leaves. Set your spray emitter to the finest spray as possible, and never use a sprayer that has been used to apply herbicides. You’ll want to ensure your spray is delivered to both the tops and bottoms of your leaves. Depending on the product used and the plant being grown, you’ll want to stop using your foliar spray about a month before harvest; wash your produce well before consuming.

Examples of liquid fertilizer and typical analysis

• Liquid bone meal: 0-12-0

• Compost tea: NPK varies. Enriches soil and plant growing areas with beneficial microbes.

• Fish emulsion: 4-1-1 plus 5% sulfur.

• Worm casting tea: 1-0-0 plus extremely high microbial diversity.

In addition to the fertilizer options listed above, many commercial fertilizer companies have produced a number of outstanding pre-made fertilizers that help take guessing out of the equation. Many products come with a mixture of two or more of the fertilizer components listed above and have been blended in a manner that helps deliver both slow and quick release nutrients to your plants. If growers are unsure of what amendments or fertilizers to add to their soil, commercial products can help ease the confusion of a large diversity of single material fertilizers.

Compost tea

One of the best concoctions available to organic growers is the use of compost tea, or actively aerated compost tea (AACT). AACT allows growers to multiply and concentrate compost microbes and nutrients into a liquid solution that can be delivered directly to the root zone and plant surfaces. In fact, the microbial population of compost grows from 1 billion in a teaspoon of compost, to 4 billion in a teaspoon of AACT. To brew AACT, start with a 5-gallon bucket, thoroughly composted material, and chlorine-free water. To de-chlorinate water, simply add an aquarium air stone or two to the bucket and bubble for 2-4 hours. We want to ensure adequate air is moving through the solution. To do this, purchase a quality air pump and stones for the operation. The smallest pump available which is still effective in aerating the solution is a two-outlet air pump with 4 feet of plastic tubing. Both air stones will need to be used during the operation. If you plan on using your AACT in a sprayer, you will want to strain the concoction either before or after the solution is made to ensure sprayer emitters do not clog. To do so, users can utilize either a set of pantyhose (size Q works best for 5-gallon buckets) or cheesecloth to keep large particles out of the solution. Straining can also be done in a similar manner after the brewing process is complete.

Water temperature is a crucial factor for the brewing process. As water temperature increases, dissolved oxygen decreases. We want as much O2 as possible in our mix, therefore, room temperature water (60-70°F) is best. Once water requirements have been met, the introduction of compost, earthworm castings, bat guano, or similar materials can be added to the brew. Typically 4-5 cups (~1 pound) of materials is added to 5 gallons of water and allowed to brew for 24-48 hours. Guano and casting providers usually have tea mix instructions on their bag, so follow them accordingly. Furthermore, many growers add no more than 2 tablespoons of unsulphured blackstrap molasses to their brew. This brix addition acts as a carbon source for microbial populations to feed on. Warning: If you are worried about pathogenic microbes in your AACT, and are applying AACT to food crops, leave extra ingredients like molasses, kelp, etc. out of the mix. Additionally, don’t brew AACT with manure if you are adding it to food crops.

Mark your calendars

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Wyatt Brown, SDSU Extension horticulture intern.