Organic Fertilizers In Greenhouse And Nursery Production

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Windy Meadow Nursery Scott Titus

Chemicals that have the potential to contaminate may become contingent environmental liabilities for businesses that are identified to potentially cause harm to the environment. The potential expenses associated with cleaning up pollution may weigh heavily as more agricultural enterprises come under new EPA clean-up regulations and are scrutinized for pesticide and nutrient violations. The issue of how horticulture grows crops in the future can be seen as a threat to an ossified paradigm, or as a challenge and opportunity to better serve the needs of society.

One of the tenets of sustainability is continuous improvement. Whatever noble strides growers have made at present will seem inadequate in years to come. Growers interested in developing more sustainable growing practices will eventually adopt organic nutrients into their fertility management programs.

While few growers are interested in organic certification today, exploring the beneficial possibilities that organics provide is well worth the effort. I started Windy Meadow Nursery with the belief that if my crops performed better on the retailer’s bench and in their customer’s landscape, I could be successful in a market that was already crowded with large producers.

This consumer-centric focus has worked well for me. The 100 independent garden centers I serve know our plants are quality and arguably a cut above the ones from other suppliers. The organic nutrient charge provides ample nutrition for the crop to sustain growth for the consumer, ensuring a successful gardening experience. I will share some of the independent research that has proved very profitable and sustainable on several levels.


For the first several years of production, I trialed many organic amendments and fertilizers in formulating commercial potting soil. My original choices were dictated by cost and 25 years of using compost in the vegetable garden. I started by combining locally sourced composts of mink, chicken and cow manures in different ratios because composted cow manure tests higher in potassium, and chicken and mink manure tests higher in nitrogen. Composts may also provide micronutrients at concentrations that are unpredictable without thorough chemical analysis.

Composts can provide satisfactory nutrition in the initial period of the crop cycle in the greenhouse. Newly transplanted crops generally need less than optimum levels of nutrition. Incorporating 10 to 20 percent compost in a mix in the dark days of early spring may lead to luxury consumption of nitrogen and develop soft growth that may be more susceptible to diseases and insects. Additional inputs of pesticides and plant growth regulators may be required to maintain quality in the greenhouse.

Using more than 5 percent compost in a mix may eventually cause the crop to stall – if not in the greenhouse then on the retail bench or in the consumer’s yard. Nitrogen draw occurs when the bulk of the compost carrying the manure – usually sawdust or straw – starts to decompose at accelerated rates. As the soil temperature rises in the greenhouse, so does the population of decomposing bacteria that was introduced by the compost.

As the crop matures, the carbon-to-nitrogen ratio in the soil rises to unsustainably high levels, requiring the grower to make large adjustments to the liquid-feed program. From a nutritional standpoint, composts proved to be too inconsistent from batch to batch and proper chemical analysis was expensive. Simple pH and electrical conductivity tests do not indicate if the high pH and EC values are attributable to high levels of the cation. Potassium as found in cow manure or other factors like quick lime (calcium oxide) used as a disinfectant is responsible. Composts will raise the bulk density of most potting soils, increasing the workload on the labor force and transportation. Sacrifices in porosity and drainage occur in crops with long production cycles. As the compost decays, the soil shrinks in volume and settles over time. The broad leaf herbicides Aminopyrolid and Chlorpyrolid can survive digestion by ruminants and months of composting and still cause productivity losses to dicotyledonous crops at concentrations of less than 10 parts per billion. Logistically, managing bulky compost amendments proves cumbersome.

In a pre-plant nutrient charge, the synthetic nutrients most easily substituted by slow-release organics are nitrogen and phosphorus. Both nutrients are subject to leaching from nursery containers, requiring additional inputs. They may present environmental contamination issues for growers.

Controlled Nutrient Cycling

Understanding how insoluble organics like soya bean meal and Bone meal impart nutrients to crops will improve your understanding of how to calculate the amount of organics to add to a commercial potting soil. Bacteria and fungi exude specific enzymes to assist in the absorption of proteins, amino acids and many minerals. These soil microbes are then consumed by the microscopic animals in the soil called amoebae. This mineralization process is referred to as nutrient cycling.

Some strains of soil microbes produce the enzyme protease that is responsible for dissolving proteins, enabling bacteria to assimilate the nitrogen from the amino acids that make up protein. Other strains of microbes require different diets to complete their lifecycles. Some produce phytase, an enzyme that assists in the digestion of insoluble phosphorus in the soil.

The rate at which nutrient cycling occurs depends on a number of cultural, environmental and biotic factors. Generally, the same temperatures and moisture levels plants require for active growth are also conducive for soil microbe activity and nutrient cycling. When soil temperature and available water decline, nutrient cycling and availability also declines. During cold, wet weather, the costly effects of nutrients leaching from the container during extended periods of cold rain in the field are greatly restricted. The water-insoluble nature of many feed-grade organic components allows the nutrients to stay in the immediate root zone, available for the crop to utilize when favorable growing conditions for plants and soil microbes resume.

Certified Organic Water Insoluble Nutrients (WINs)

As the nursery became more successful and revenues increased, I tested many of the more expensive certified organic meals and bat guano. Each 40-yard load of potting soil was formulated slightly differently and analyzed for eight years of commercial production involving thousands of crops of annuals and perennials. Research focused on a pre-plant nutrient charge formulated with a combination of WINs that provided controlled and sustained release rates similar to conventional polymer-coated controlled-release fertilizer (CRF) under ideal conditions.

One advantage to WINs is that loss from leaching is averted almost entirely, as availability is dependent on microbial activity. Organic meals afford a more precise calculation of nutrient inputs compared to widely variable composts that don’t carry readily available guaranteed analyses. Duplicating uniform mixes with WINs is accomplished with the same ease as synthetic nutrients and offers several benefits.

Determining WIN Longevity

When formulating organics, a determination of how long the grower needs the WIN charge to last can be determined by at least three criteria. The first is the biotic factor mentioned earlier in this article. The second factor is the selection of the organic fertilizer components and their rate of decomposition. This short list of WINs is ranked from longest lasting to more quickly available: coarse bone meal, hoof and horn meal, feather meal, fish meal, bone meal flour, cotton seed meal, blood meal, soya bean meal/flour.

Selecting only components from the beginning of the list will release nutrients very slowly and may compensate for nitrogen draw but contribute little immediate nutrition to short-cycle crops when soils are cold. Selecting and over-applying only components from the end of the list will give an immediate response with increasing rates of nitrogen as the season gets warmer. The ability to “hold” a crop of annuals once it becomes marketable may be difficult in the heat of summer. By choosing WINs with balanced rates of solubility and biological availability, plants receive a consistent and predictable supply of nitrogen for the entire growing season.

The third factor regulating nutrient longevity is the relative particle size of the organic WINs. Whole soya beans may require 30 to 60 days to decay and impart all their nutrition depending on soil temperature. Soya bean flour may release all of its nutrients in only two to four weeks, depending on the temperature of the soil.

Guaranteed Analysis & Use Rates

Particle size also plays a role in determining the guaranteed analysis and use rates of some organic WINs. Steamed Bone meal is often available in a granule or chip, with a particle size similar to very course sand. It has a guaranteed analysis of 12 to 14 percent phosphorus. The same material, when pulverized to the consistency of cake flour, qualifies for a guaranteed analysis of 20 percent phosphorous. The insoluble complex of calcium phosphate in bone meal affords lower rates of application than ordinary superphosphate that leaches readily from soilless container mixes.

When calculating use rates of very high WINs, factors other than guaranteed analysis must be considered. Depending on the material selected, some or even most of these nutrients may not be available during the relatively short cropping cycle, but will become available over the course of the entire season and beyond. The “WIN advantage” is the reservoir of nutrients responsible for enhancing both shelf life at retail and garden performance for the consumer.

Using the coarse grade of steamed bone meal as an example, 100 pounds will supply 14 pounds of phosphorus. The rate of mineralization is determined by the relatively small surface area of the large particle. Bone fragments may still be detected in soil five years after incorporation. If 20 percent of the 14 pounds of phosphorous is available per year, then it stands to reason 100 pounds of coarse bone meal will supply 2.8 pounds of phosphorus per year. The same bone meal in flour form has particles that are perhaps 100 to 300 times smaller and has more than 1,000 times greater surface area for soil microbes to access. One hundred pounds of bone meal flour will supply 20 pounds of phosphorus.

This finely pulverized product will only release nutrients for two years, supplying 10 pounds of phosphorus each year. The difference is significant between the two products, the latter supplying more than 350 percent more phosphorus per year.

Organic WINs As Organic CRF

Phosphoric acid was eliminated from our liquid-feed program in 2001. Plants rely solely on micronized bone meal flour as the source of phosphorus and have superior root development and flower production because of the extremely small size and excellent distribution in the soil. The flour form of WINs reaches a much higher level of homogeneity than granular fertilizers and is especially suitable for use in propagation soils designed for the smallest of plug trays. The newest miniature resin-coated, controlled-release fertilizer (CRF) contains 250 prills per gram. The particle size of organic flours can be measured on the scale of nanometers, affording much greater distribution than synthetic CRF.

When soil temperatures spike, coated CRFs release more nutrients than the crop requires, increasing salinity and loss of productivity from root necrosis and may lead to environmental concerns. The cost of labor and water needed to correct high soil salinity is conserved with organic WINs because they do not have this undesirable “Flash” characteristic. Utilizing organic WINs as CRF eliminates the potential for nutrient and crop losses when temperatures spike and places less emphasis on the liquid-feed program, freeing the grower to make only slight adjustments in nutrition when needed. Balanced nutrition relies on using multiple sources of WINs that are paired to the growing climate, duration of crop cycle and what proportion of the crop’s total nutrition is desired to be supplied by the soil. An integrated fertility management system utilizing organic WINs as CRF may supply 50 to 80 percent of a crop’s total nitrogen requirement per season and 100 percent of the phosphorous required for crops with a minimum two-year crop cycle.

Liquid-feed concentrations may be reduced by the same 50 to 80 percent compared to crops grown without a slow-release nitrogen charge. Reduced reliance on liquid feeding renders the soil more forgiving by reducing the level of management required to maintain quality and support crop growth. Shifts in pH and wide variations of the crops nutritional status that often results in reduced yields is less reliant on the liquid feed program because of the nutrient buffering capacity the organics impart to the soil.

Scott Titus is the owner of Windy Meadow Nursery in the Bellingham, Wash., area. Learn more about his operation at


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One comment on “Organic Fertilizers In Greenhouse And Nursery Production

  1. Norm

    Too complicated for me to understand. Simple gardener with about 800 square feet of garden space growing typical vegetables. soil test shows zero N, zero K, and very high P. So, assuming I don't have the equipment to feed liquid but want to grow organic…..what do I use and how much?