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Beyond Nitrates: Understanding Anions in Soil Fertility in the Northeast

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This article comes from the NOFA/Massachusetts 2013 March Issue Newsletter

By Luke Pryjma

The presenter of this workshop, Derek Christianson, runs Brix Bounty Farm in Dartmouth, Massachusetts and has twelve years of farming experience in the Northeast.  He farms on six acres of leased land, and sells at one market, a successful farm stand, and through a Community Supported Agriculture (CSA) program. His philosophy is: “Grow food with respect for future generations.”   Though he admits this philosophy changes as he develops as a farmer, health has always been a priority for him.  His customers know his farm as “the farm that grows food that tastes good.”

There are three general conditions necessary for healthy crops: mineral availability, soil biology, and adequate air and moisture in the soil.  Proper soil investments can increase farm viability and allow the soil to deliver crop health on its own.  Using a soil test to measure the strength of the soil’s fertility is a preventive health measure.  As covered in this workshop, some yearly inputs are necessary based on the conditions found on farmland in the Northeast.  On the whole, investing in soil fertility is investing to increase the soil health potential, and therefore increase the crop health and our health, too. 

An anion is a negatively charged ion.  Most Northeast soils are negatively charged because clay particles, which increase nutrient adsorption, have mostly negative exchange sites.  Due to the large amount of rainfall in this region, anion exchange capacity (AEC), the ability of soils to hold anions, is generally very low in Northeast soils.  Unlike positively charged cations, anions cannot cling to the soil; the weak hold anions have in the soil is easily broken and they wash out with the rain, unless the soil is under cover with a greenhouse or hoop house.  Higher AECs are built into soils through adding humus or soil organic matter and increasing the bioavailability of minerals cycling through the plant and soil system (the circulation of minerals). The more our soil biology creates biological mineral complexes, by incorporating minerals into their forms and functions, the more bio-available these minerals become to our plants.

There are four steps to mineralization cycling.  First, the appropriate minerals in proper amounts need to be applied based on a soil test.  Second, the crop uses some of the minerals for growth.  These minerals are now sequestered into a biological form.  Third, the crop residue is broken down and these minerals are circulated back into the soil system through microbial processes.  Microbes, through the use of microbial foods and stimulants, can more easily break down the residue.  Fish hydroslate, kelp, molasses, and compost tea all feed and stimulate microbial breakdown.  Fourth, other microbes, including fungi, enhance the building of root systems with the more biologically available minerals.  

Soils should be tested in the late summer for fall application of minerals.  This will allow the soil biology adequate time over the winter to work on the availability of those minerals.  Note that individual soil labs can offer differing recommendations for mineral needs as they may use variable methods for testing soils.  For example, weak acid extractions will yield a different mineral profile than strong base extractions.

Sulfur is an often underappreciated mineral, yet plant health depends on it.  In the 1970s, the Northeast received a third of its soil sulfur from western factory pollution coming from smoke stacks and deficiencies were relatively rare.  Current emission standards have greatly reduced atmospheric sulfur.  Northeast soils now need to be amended with sulfur.  Neal Kinsey, renowned soil agronomist and crop consultant, believes sulfur is the key element in the flavor profile of a crop.  In plants, human perception of taste comes from the act of chewing and thereby volatilizing the aromatic compounds in the plant for noses to smell.  Healthier plants produce more aromatics, called plant secondary metabolites.  Certain crops are sulfur rich and many of their secondary plant metabolites are sulfur-based. Garlic, other alliums, and brassicas are examples of sulfur-rich crops. 

Sulfur comes in many forms.  Gypsum is calcium sulfate.  Sul-po-mag is a combination of sulfur, potassium, and magnesium.  Other sulfur sources include potassium sulfate and elemental sulfur.  Elemental sulfur is 90% sulfur and requires more microbial activity to make it available, providing a slower release throughout the growing season.  Sulfate forms of sulfur tend to more readily leach. As a result of annual rainfall and reduced deposition from air “pollution”, yearly applications of sulfur will most likely be needed.  Sulfur is affordable.  Its benefits - better taste, building disease and insect resistance, and overall plant health - more than pay for themselves.  

Boron is another essential anion and enables plants to better utilize calcium.  Boron as borate (H4BO4) is highly leachable.  Since boron has an affinity for nitrogen, surplus nitrogen levels in soil will react with and deplete boron levels.  Boron is taken up through the mass flow of the plant, and its availability to plants is dependent on soil moisture and humidity levels which impact plant respiration and the flow of nutrients up through the xylem.  Therefore in times of drought, even with adequate levels of boron in the soil, plants may have hollow stems due to a lack of its accessibility.  Keeping boron in your soil and available is dependent on yearly applications and adequate levels of organic matter.

Certified organic growers who wish to comply with regulations can only apply boron to their soil with a soil test showing a deficiency.  Boron increases plant sap pressure and too much will cause the leaf tips to burn.  Borax and Solubor are common forms of boron.  Solubor can be applied with a backpack sprayer for more uniform coverage.  Many growers follow Michael Astera’s advice that boron levels should be 1/1000th the amount of calcium in the soil.  On average, 1ppm of boron should be the target level for low calcium and low CEC soils.  On soils with high calcium and high CECs 2ppm, or even 3ppm, of boron is the target.  Total boron application should be spread over at least three applications.  Boron costs $9 for an acre per year.  Consult a crop specialist before applying.

Molybdenum is a trace anion deficient in most Northeast soils.  It is essential in the nitrate reductase enzyme reaction and stimulates biology to convert nitrate nitrogen to the nitrite form.  Molybdenum is more available at a higher pH and deficiencies are less common in well-limed soils.  Molybdenum is expensive. Deficiency can be addressed through kelp applications or as concentrated sodium molybdate, which is used widely in the industry by soybean and legume producers.  Molybdenum benefits brassicas, cucurbits, and legumes.  

Selenium is a trace anion more broadly recognized to be essential for human health.  In Finland and New Zealand there are incentives and mandates to apply selenium because of deficiency in soils.  Heart disease is linked to consuming foods from selenium deficient soils.  Too much selenium, along with all the anions, can be toxic to the soil and humans.  Applications should follow detailed and carefully taken soil tests.

Phosphorous is an essential macro anion for driving plant energy.  Phosphorous on most Northeast soils has a history of being taken off and not returned.  Old hay fields are notoriously deficient.  A recommended program to address a severe deficiency would be an application of 2000 lbs of soft rock phosphate or 600 lbs of bone char per acre.  In conventional agriculture, phosphorous is applied in forms which “lock up” quickly and become unavailable to plants.  Soil testing for phosphorous is tricky.  The University of Massachusetts’ lab uses a weak acid test which shows available phosphorous.  A strong acid test may indicate adequate levels of phosphorous in the soil but it may be locked up.  Encouraging soil biology is crucial to phosphorous availability. 

An understanding of the role of anions on the farm is crucial to being a successful farmer of truly healthy food. A license isn’t needed to grow food, which may be surprising considering the impact food can have on human health, whether to harm or to help.  Crop health, human health, and the taste of the food grown are all connected. Taste has great marketability because it is inherently sought after and is a measure of nutrition.  Through a balanced soil fertility plan and healthy soil biology Northeast farmers can achieve better yields, taste, and plant and human health.



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