There are a huge number of organic soil amendments on the market today. Some have been around for a while, like limestone for adding calcium and raising the pH of acid soil. Some are quite new, like specialized biostimulants or liquid complexed minerals. This page is an attempt to point out the most common amendments and their uses.
The organic amendments described here are available as certified organic.
If you are interested in how to apply amendments, check out this page.
Note that in some cases, especially with the micronutrients, OMRI requires that a deficiency be measured on a soil test before application of the amendment is allowed under the standard.
Biostimulants are substances that may be applied to the plant, seed or soil in order to stimulate nutrient uptake efficiency in the plant or otherwise helps the plant to develop or accommodate stress. It makes sense to add biostimulants when foliar feeding to help with nutrient uptake. Biostimulants may be applied to the seed or transplant roots at planting to encourage beneficial microbes to form a symbiotic partnership with plant roots. Or biostimulants may be applied to the soil to encourage overall root health.
Biostimulants are not to be confused with microbial inoculants (which are used to introduce a certain type of microbe into an environment such as a seed or soil). Biostimulants stimulate what is already present either in the soil or on the leaf.
Biostimulants sound great! So, if a little is good, a lot is better? No! In the case of biostimulants, a little goes a long ways. If you are using a manufactured product, be sure to follow the dilution rates on the label!
Humic and fulvic acids
These are carbon-based complex molecules extracted from humic substances like Leonardite or vermicompost. When added in small amounts to a foliar spray they increase the uptake of the other nutrients which means you can either reduce the amount of other nutrients used, or just get a bigger response. In the soil they can aid in water retention and absorption. We always add a fulvic/humic acid blend to foliar sprays.
Seaweed extracts
Many seaweed extracts are a good source of potassium, but they also promote more growth than can be explained by their mineral content. They enhance soil health by promoting beneficial soil microbes and by improving water holding capacity, and they promote root growth. Plants grow due to photosynthesis and seaweed enhances chlorophyll content. They can trigger early flowering and fruit set in tomatoes while increasing the size and quality of the fruit. Seaweed extracts are an important part of a foliar feeding program in vegetables and fruit.
Vermicompost
Earthworms make excellent compost due to the amazing diversity and power of the microbes in their gut. They improve plant nutrient availability. They increase the population of plant growth-promoting rhizobacteria. Earthworm microbes mineralize the organic matter and also facilitate the chelation of metal ions. They suppress pathogens. Vermicompost makes an excellent addition to potting soil and a handful dug in under transplants seems to really help. If you can’t make your own, you can purchase a top quality product. A little goes a long ways so don’t be tempted to add more than 10% of the volume to a container mix.
Soybean Meal
A good source of nitrogen, and may be obtained from organically grown soybeans. The trouble is that until it is digested by microbes it tends to set plants back and keep them from growing. It is best used in a situation where it may be applied at least 2-3 weeks before planting. For more information see this study from North Carolina State University.
Feather Meal
A good, slow release source of nitrogen. Although it is acceptable for use as an OMRI amendment, recognize that the bag you buy at the feed store is likely a product of industrial chicken farming.
Bat Guano
A potent source of nitrogen – a little goes a long way and it’s easy to overdo it. This is product that is mined from natural deposits in caves.
Amino Acids – hydrolyzed proteins
These are fairly new products on the amendment scene. Amino acids come as a soluble powder and therefore are suitable for soil drench or foliar feeding rather than as a one-time soil amendment at planting. They have the advantage of being very available and are suitable for fine tuning your nitrogen application.
Fish Fertilizer (liquid concentrate)
Not to be confused with fish hydrolysate (which can be a good source of phosphorus). Some of these fertilizers are high in nitrogen without much added phosphorus or potassium. As a liquid it must be applied fairly frequently. Despite it being “deodorized” the stuff really doesn’t smell good.
A word about fertilizer labels… Many fertilizers are labeled with three numbers, N, P and K. For example, Alaska fish fertilizer is 5-1-1. The “P” number reported is actually phosphate, P2O5, which is about 44% phosphorus. The “K” number is actually potash, K2O, which is about 82% potassium.
Soft Rock Phosphate / Calphos
Standard calphos is a mined product that is good for bringing up the baseline phosphorus level in the soil. Although it averages 20% P2O5, only 2 – 3 % is available, and then only in acid soils. Under neutral to alkaline soil environments its effect is almost non-existant. A biologically active soil increases phosphorus availability, perhaps because organic acids around biological systems decrease soil pH. Unlike MAP or DAP, the presence of soft rock phosphate in the soil does not discourage plants from forming micorrhizal associations, according to Dr. Christine Jones. In our experience it takes several years for the effect of a soil application to be fully realized.
Soft rock phosphate may also be a source of heavy metals such as cadmium, which may be taken up in crops. Commercially produced cannabis is regulated for heavy metal content and the use of certain soft rock phosphates may be problematic. In California, specific product heavy metals content may be researched at the CDFA website. Fertoz rock phosphate is recommended as a product with lower heavy metal content than soft rock phosphate.
“Fertoz” Rock Phosphate
Fertoz rock phosphate contains phosphorus that is more highly available than in soft rock phosphate. Depending on the product, it may be as high as 7% available phosphate compared to the 2-3% availability of standard calphos soft rock phosphate. It is also low in heavy metals.
Bone Char
Bone char is produced by heating animal bones in the absence of oxygen, in a pyrolisis process similar to that for making biochar. It is a highly porous material with highly available phosphorus: 16% phosphorus availability compared to 2-3% for calphos. The micro structure is similar to that of biochar, with many tiny micropores. It is also used in water filters to remove fluoride and metal ions from water, and is used in the sugar industry to remove sulfates and the ions of magnesium and calcium. This “removal” of nutrients sounds similar to how raw biochar reacts in soil – it draws nutrients unless it is “charged” before soil application. We have not used this particular material but if we were to, we would want to charge it first.
Bone Meal
A product of industrial agriculture, bone meal is a source of phosphorus. It also contains sodium in relatively high quantities. We have used bone meal as an amendment under fruit trees, only to find that voles were attracted to it (perhaps to eat the sodium) and undermined the tree to get at it!
Seabird Guano
Unlike rock phosphate, the phosphorus in seabird guano is relatively available (since it is already bound to organic molecules) making it a good choice for higher pH soils. It is a pelleted product, mined from seabird colonies. Some products also contain considerable nitrogen and some potassium while others supply mostly phosphorus.
High P Fish Hydrolysate
This is a liquid fish concentrate that has been cold processed using phosphoric acid (e.g. Neptune’s Harvest Fish Fertilizer). As with all liquid products it would be applied through the growing season rather than as a bulk application before planting.
A word about fertilizer labels… Many fertilizers are labeled with three numbers, N, P and K. For example, Alaska fish fertilizer is 5-1-1. The “P” number reported is actually phosphate, P2O5, which is about 44% phosphorus. The “K” number is actually potash, K2O, which is about 82% potassium.
Compost
Compost usually has a large amount of plant available potassium, and repeated applications of compost can result in a buildup of excess potassium in the soil. That said, compost is the best source of potassium for a soil, due to all of its other benefits. Our potassium targets are on the low side so that compost may be continued to be applied.
Potassium Sulfate
This is a good source of potassium for soils where compost applications or SulPoMag are not appropriate. Much of the North American production comes from the Great Salt Lake in Utah.
K-Mag, Langbienite or SulPoMag
This is a mined product that has a high amount of potassium, magnesium and sulfur in a form that is readily available to plants. It is our understanding that if your soil needs these three elements, this is a good choice.
Greensand
Greensand is a mined product that contains a high percentage of the green mineral glauconite. Although it has a fairly high potassium content, the potassium solubility is extremely low, <0.1% of the total potassium present, and thus is not a good source of potassium.
Kelp
Kelp is a good source of potassium along with balanced micro-nutrients, and is a bio-stimulant. It may be found in liquid concentrate form for application throughout the growing season. It is a good product to add to other foliar nutrient sprays, since it helps increase biological activity. Bagged powdered kelp, although expensive, is also available.
Wood ashes
Ashes are the white powder left by complete combustion, and are not to be confused with biochar or charcoal. The nutrients in wood ash vary according to their source, and have a fertilizer value of perhaps 0-1-7, and are 25% to 45% calcium compounds. They are extremely alkaline, impact soil biology, and will raise pH. Our ashes go into the greenwaste container, never into our neutral/alkaline soil. It is better to raise soil pH and increase calcium with lime and increase potassium with compost or potassium sulfate (if potassium is deficient) than it is to use wood ashes as a soil amendment.
Ag Lime (Calcium Carbonate)
Agricultural (ag) limestone (lime) is a mined material primarily made up of calcium carbonate. While it supplies plenty of calcium, it also raises the pH of the soil due to the carbonate component. Some soils need this liming agency plus the calcium. In those cases it is a very appropriate amendment.
You may be offered a choice of grades – the grade is the fineness of grind of the limestone. In general finer grades will be faster acting.
Ag lime takes up to three years to become fully available in the soil, with relatively equal amounts of calcium becoming available each year. During this time the standard soil test, Mehlich 3, will give erroneous results for CEC and the amount of available calcium. Instead of using the M3 test, if ag lime has been applied in the last three years the AA8.2 test should be used.
Dolomite / Dolomitic Limestone
The existence of dolomite is a good reason to get a soil test before applying any mineral. Dolomitic limestone contains calcium along with a healthy dose of magnesium. If your soil does not need magnesium, dolomite is not an appropriate amendment. Magnesium, while an essential plant nutrient, is rather hard to remove from soil when in excess and tends to tighten soils. Many sticky clay soils have excess magnesium.
Dolomitic lime, like ag lime, will raise soil pH.
OrganiCalc carefully calculates the amount of dolomite a soil needs to balance magnesium with the other cations. Additional ag lime may also be recommended and it is important to include it as ag lime rather than substituting dolomite.
Gypsum / Calcium Sulfate
Gypsum is calcium sulfate and so it is a source of both calcium and plant-available sulfur. Unlike ag lime, it does not change the pH of the soil. It is a very appropriate amendment for supplying calcium in cases where the pH is already over 6.6, even if there is adequate sulfur in the soil.
Gypsum is relatively soluble in the soil and is used in cases where there is an excess of sodium or potassium or magnesium, along with enough water to move the excesses down through the soil profile. The sulfate in the gypsum will solubilize the excesses and allow them to move out with the water.
Gypsum takes about 6 months to become fully available in the soil. If gypsum has been applied in the last 6 months the AA 8.2 soil test should be used instead of the Mehlich 3.
Spanish River Carbonitite
Carbonitite is a mined mineral which is a combination of highly available calcium carbonate, biotite (a source of potassium), apatite (a source of highly available phosphorus) and rare earth elements such as manganese, zinc, copper, cobalt, molybdenum and boron. It is reputed to be low in heavy metals. Due to the carbonate in the mineral, carbonitite will raise soil pH. For more information see the manufacturer’s website.
The BioNutrient Food Association can supply carbonitite to its members in the US Northeast. It is suitable for low pH soils with low calcium levels.
Oyster Shell Flour
This product is also available as a fast acting source of calcium. It has about the same amount of calcium as ag lime and may be substituted for ag lime. Like ag lime it will raise pH.
Magnesium Sulfate (Epsom Salts)
A very soluble source of magnesium. This is the same powder you may have dissolved in water to soak a sprained ankle. Because of its solubility this product is best applied in small doses over time or as a drench or foliar spray.
Dolomite / Dolomitic lime
See the discussion above under calcium amendments
SulPoMag
See the discussion above under potassium amendments
Gypsum
Gypsum is a good source of sulfur as well as calcium (see discussion under calcium amendments). In cases where a soil is in need of sulfur and does not have excess calcium, gypsum is a good amendment. The sulfur in it is readily available but is not toxic to soil biology.
Ag Sulfur (Tiger-90)
This is elemental sulfur and should be used in a limited manner to supply sulfur when gypsum is not appropriate but sulfur is needed. In large doses sulfur can be toxic to soil biology and because of this OrganiCalc limits the amount that can be applied at any one time to 300 pounds per acre or less. Sulfur is a necessary element for plant growth and some soils tend to be very low in it. Sulfur also reduces the pH of the soil and it can be used to remove excesses of cations such as calcium, magnesium.
Ag sulfur is also useful in cases where the soil pH is too high. See our page about how to lower soil pH using sulfur.
Borax
This is a mined product that may be found in the laundry detergent section the grocery store. The white granular powder is somewhat soluble. Boron is a necessary nutrient but it is toxic in excess, so it is worth considering how it will be applied. One method of application is to dissolve borax in water and spray it evenly on the soil. Another method is to mix the powder evenly with bulkier amendments and spread the result evenly. Borax lends itself to the latter method.
Solubor
Solubor is a more soluble form of boron with twice the elemental boron as borax. If using it as a replacement for borax, use 1/2 as much. It is best used dissolved in water and either sprayed evenly on the soil, or as a foliar.
All the trace minerals listed above are available as sulfates (e.g. iron sulfate). The sulfates may be applied to the soil but they are not generally plant available form. They must be metabolized by the soil biology before being transformed into a form that plants can take up.
Manganese in particular is difficult to get into the plant by broadcasting manganese sulfate due to manganese forming tight bonds with other elements in the soil that cannot be broken by soil biology. The manganese becomes unavailable. It is recommended that if manganese is deficient, it is also applied as a foliar fertilizer right before flowering as well as being soil applied.
Sea mineral fertilizers are a good source of balanced trace minerals but only in soils that also need sodium. Testing shows that sea minerals contain very significant amounts of sodium chloride (table salt) which will inhibit plant growth if it applied in excess.
The silicon in sand is tightly bound up. Despite silica, SiO2, being the main component of sandy soil, it is not very soluble in the soil solution.
Wollastonite is a mined source of silicon in the form of calcium inosilicate, CaSiO3. Silicon uptake in the plant has been found to correlate to low susceptibility to powdery mildew and rusts and wollastonite has been found to be a good source of silicon for the plant. This link reports a field trial of various silicon and liming sources on powdery mildew in pumpkins.
Basalt rock dust has been found to release silicon in low pH, weathered soils. Basalt rock dust has other advantages of increasing nutrient availability in these soils and in areas where basalt is found, such as the northeast US, it may be obtained relatively inexpensively.
