Funding for this column is provided by the ASCFG Research Committee.

Potassium Options for Organic Production 

Robert Mikkelsen, of the International Plant Nutrition Institute, recently reviewed the importance of potassium for plant growth and development, the typical need to supplement potassium in the soil, and the sources of potassium for organic growers.  He acknowledged that there are numerous regulatory agencies, each with slightly different interpretations of what meets the standards for organic production.  In addition to adhering to the organic standards of your production location, availability of source material and concentration of potassium available from each source are also considerations.

Potassium is the soil cation required in the largest amount by plants for overall health and vigor.  Potassium is involved in these physiological plant functions:
1.    Osmoregulation
2.    Internal cation/anion balance
3.    Enzyme activation
4.    Proper water relations
5.    Photosynthate translocation
6.    Protein synthesis
7.    Tolerance of external stresses, e.g. frost, drought, heat, high light intensity
8.    Reduced stress from disease and insect damage

Since farmers in many regions of the United States remove more potassium from the soil during harvest than is returned to the soil with fertilizer and manure, the nutrient is eventually depleted.

Even with supplemental potassium added to the soil, the availability of the nutrient is based on its solubility.  Soluble minerals include langbeinite, sylvinite and potassium sulfate. Manures are also highly soluble, but the nutrient content may vary considerably.  Some potassium supplements are less soluble, but they can serve a long-term role in building soil fertility.  The following outlines several potassium sources that have been label “allowed” or “restricted” use for organic production by the USDA’s National Organic Program.

Langbeinite:  Langbeinite is actually potassium-magnesium sulfate, typically supplying 18% potassium, 11% magnesium and 22% sulfur—all available for plant uptake.  It is allowed for organic production in its raw, crushed form.  Langbeinite is found in underground deposits in New Mexico.

Potassium Sulfate:  Generally containing 40% potassium and 17% sulfur, potassium sulfate is allowed for organic production so long as it is derived from a natural source and free from additional processing or purification.  The Great Salt Lake in Utah is one source of potassium sulfate produced for organic use.

Sylvinite:  Unprocessed sylvinite, potassium chloride, contains 17% potassium.  Because the applicator must be careful to minimize the chlorine accumulation in the soil, consultation with an organic certifying agent should precede application.  Processing removes sodium salts, but moves this potassium source to the restricted list.

Manure and Compost:  Depending on the raw material and handling, manures and composts are extremely variable in their potassium content.  A chemical analysis will allow these resources to be managed for maximum benefit to the soil and the crops.  Potassium from manures and composts are typically available for plant uptake.

Greensand:  Potassium is derived from the green mineral, glauconite, found in a sandy rock or sediment commonly called “greensand.”  Potassium content of greensand is up to 5%, but the release rate is very slow.  While some view the slow release as a management tool to avoid fertilizer burn, the slow rate doesn’t provide significant nutritional benefit to the plants growing at the time of application.  Greensand is mined in New Jersey.

Rock Powders:  Ballast, biotite, mica, feldspars, and granite are mined rocks that are known to contain varying amounts of potassium.  Since some have such as slow release rate, they are only useful for long-term soil management, not readily available plant nutrition.  In addition to insoluble properties, these minerals are often heavy and bulky to transport.

Seaweed:  Seaweed biomass contains less than 2% potassium and is readily soluble.  Seaweed can be applied directly or the potassium can be extracted.  Transportation costs for farms that are not located in proximity to the harvesting area may be prohibitive considering the potassium content.
The key to maintaining sufficient potassium availability in the soil is by conducting regular soil tests.  If potassium is deficient in the plant root zone, problems such as poor water use efficiency, increased pest problems, reduced harvest quality and lower yields may result.

Mikkelsen, R.L. 2007. Managing Potassium for Organic Crop Production. HortTechnology 17(4) pp. 455-460

How Greenhouse Sanitation Affects Insect Management

Greenhouse sanitation includes the timely removal of weed, plant and growing medium debris.  Previous research has focused more closely on the impact of sanitation on reducing the incidence of plant diseases; however, the researchers at University of Illinois, Urbana, recognize that sanitation is also important in reducing insect infestations, namely by removing the insects’ breeding and hibernating sites.  Their research efforts worked to quantify the abundance and types of insect pests emerging from plant and growing medium debris disposed of within the greenhouse.

Two commercial greenhouse and two university greenhouses were evaluated in the study. The four greenhouses offered a diversity of plant material and varying levels of production.  Two trash cans were placed in each greenhouse.  Each trash can lid was equipped with a binder clip to hold a 3 x 5 yellow sticky card on the interior of the closed container.  The greenhouse debris was collected weekly for 28 weeks from May through November.  The sticky cards were also collected and replaced weekly.  The insects on the cards were identified and the number of each type of insect was recorded for later analysis.

The most common insects identified from the four greenhouses were western flower thrips, whiteflies, and fungus gnats.  Based on the plant material being grown in the greenhouses, the insect types identified were not surprising.  When they analyzed the plant material according to type, and compared it to the percentage of adult insect captured on the card, they were able to conclude that only a small quantity of plant material can harbor large numbers of insect pests. Additionally, with a 1-week time period between debris removal, it is possible that pupae of western flower thrips, fungus gnats and whiteflies can develop into adults.  Without the presence of a tight-fitting lid of the trash container, the adult insects may migrate to otherwise healthy or treated crops in the greenhouse.

The best defense is to remove plant and growing medium debris from the growing area for proper disposal.  If trash containers are used in the growing area, be sure a tight-fitting lid is available and used.  In addition to reducing the incidence of insect problems, proper sanitation can also lead to a decreased need for insecticides or the introduction of natural insect enemies.

Hogendorp, B.K., and R.A. Cloyd. 2006. Insect Management in Floriculture: How Important is Sanitation in Avoiding Insect Problems? HortTechnology 16(4) pp. 633-636.

Prolonged Vase Life of Lupinus havardii   

Though Lupinus havardii may be a promising cut flower with its spike of blue flowers, its use may be limited due to its high sensitivity to ethylene.  Naturally occurring ethylene synthesis begins occurring when the flower has been open for 2-3 days.  This initial ethylene synthesis begins in the oldest, basal flowers first, before the flower stem reaches harvestable size.Once harvested, the process continues, leading to desiccation and abscission of the flowers beginning at the base of the raceme.  A collaborative research effort among universities in Mexico, New Mexico and Texas evaluated the affect of treating lupine flower stems with 1-methylcyclopropene (1-MCP).  They analyzed fresh weight and flower retention, apical flower opening and vase life longevity.

L. havardii ‘Texas Sapphire’ was grown in the greenhouse production environment. Racemes were harvested at 112, 130 and 138 days after transplanting.  At the time of harvest, no senescence-related desiccation was evident.  All cut stems were 40-55 cm (16-22 in) long with 20-30 fully opened flowers.  Fresh weight and number of  fully opened flowers was recorded immediately after cutting each stem.

Twelve hours after harvest half of the stems were treated with 1-MCP at a concentration of 160 nL/L, held at 20C for 12 hours.  The remainder of the stems did not receive the 1-MCP treatment.  The stems were immediately moved to a vase solution containing 50μM of (2-chloroethyl) phosphonic acid (CEPA).  The CEPA solution simulated postharvest exposure to exogenous ethylene.  Treatments were held in the CEPA solution or deionized water for 2, 4, or 6 days.  After the treatment time, the stems were moved to vases containing deionized water for further evaluation.  Vase life was measured beginning at the time of harvest, ending when 50% of the mature flowers abscised or wilted.

The longest vase life (8 days) was reported for lupine stems that were treated with 1-MCP, but were not exposed to CEPA.  Only a three-day vase life was reported in the absence of a 1-MCP treatment, but with 6 days in the CEPA solution.  Only mature flowers that were present at the time of harvest experienced desiccation within 6 days; however the desiccation resulted in up to a 70% loss in fresh weight and visible wilting.  Postharvest 1-MCP treatment was shown to delay desiccation, mature flower drop and newly opened flower drop by 2 days. The prolonging effects of 1-MCP were best observed in this experiment when the flowers were exposed to the exogenous ethylene, but the data also suggest some suppression of endogenous ethylene as well.

Valenzuela-Vazquez, M., G.A. Picchioni, L.W. Murray, and W.A. Mackay. 2007. Beneficial Role of 1-Methylcyclopropene for Cut Lupinus havardii Racemes Exposed to Ethephon.HortScience 42(1) pp.113-119.

Megan Bame

Megan Bame is a freelance writer in Salisbury, North Carolina. Contact her at [email protected]