Organic Crop Production Fertility

Managing Fertility in Organic Crops

More than 90 percent of the dry matter weight of a plant is made up of three elements: carbon, hydrogen and oxygen. All three are universally available in air and water.

There are actually 16 elements essential to plant growth. Of the remaining 13, plants require large amounts of: nitrogen, phosphorus, potassium and sulphur. These are referred to as macronutrients.

Lesser amounts of the secondary nutrients and micronutrients are required, but these are still essential to plant growth. On the other hand, if too much of an essential nutrient is available, it may become toxic to plant growth (especially a micronutrient).

Table 1. The essential plant nutrients macronutrients.

The amount of nutrients required differs depending on plant species, and crop yield is a function of the law of the minimum: yield is determined by the most limiting factor.

For example, if there is only enough nitrogen available to grow a 30 bu./ac. crop, yet there is enough of all the other essential nutrients to grow a 40 bu./ ac. crop, the crop will still yield only 30 bu./ac., because yield will be limited by the supply of nitrogen. The law of the minimum also includes factors other than nutrients, such as moisture, light and heat. In the previous example, if only enough moisture was available to produce a 20 bu./ac. crop, the crop would yield 20 rather than 30 bu./ac.

Nutrient Sources

Nutrients come from two main sources, the breakdown of soil minerals and the decomposition of organic matter (mineralization). Nutrient release from organic matter breakdown begins within a few days to several years and can last for centuries. Nutrient supply from soil mineral breakdown is much slower. Depending on the mineral, it may take centuries for an appreciable amount of nutrient release.

Plants take up nutrients as inorganic ions, whether the nutrients originate from soil minerals, organic matter or approved organic fertilizers. For example, N is taken up as nitrate (N0_3-) and ammonium (NH₄₊) ions, and P is taken up mostly as inorganic orthophosphate.

Legumes, that fix N from the air via their symbiotic association with bacteria in nodules on the plant roots, are the exception.

Nutrient Balance or Nutrient Ratios

When certain nutrients are in high concentrations in the soil, they may create imbalances in other nutrients. For example, soils with very high levels of P may induce a zinc deficiency in a flax crop. However, the use of nutrient ratios may only be useful in cases where nutrients are abundant. For example, soils with low organic matter (e.g., sandy soils; eroded soils) usually have small total amounts of exchangeable cations. A deficiency may occur even though the nutrients are present in the appropriate ratios. Conversely, a clay soil with a high cation-holding capacity may not have an ideal nutrient ratio, but levels may still be adequate for crop production.

The Saskatchewan Situation

Nitrogen (N)

Nitrogen comes from the breakdown of organic matter. In Saskatchewan, soils typically have 15 to 50 lb./ac. of plant available inorganic N released per year through the breakdown of soil organic matter and crop residue. A 30 bu./ac. wheat crop with about 13 percent protein content requires about 57 to 70 lb./ac. of N. Most N in organic production results from growing properly inoculated legumes as cash crops or green manures. The amount of N supplied depends on the type of plant and the plant materials incorporated into the soil. Animal manures and compost are both good sources of N available for use on organic farms. Organic producers are cautioned to check that all inputs, input ingredients and processing methods meet certification standards.

Phosphorus (P)

Saskatchewan soils contain large amounts of total P, often 400 to 2,000 lb./ac. in the zero to six-inch depth. About half of the total P is present as organic compounds and about half as inorganic minerals.

Unfortunately, most of this P is unavailable to plants due to the relatively high pH levels in our soils and the resultant low solubility of P minerals. In fact, Saskatchewan soils are inherently low in plant available P. A survey of organic fields revealed that most had deficient levels of plant available P. As plants absorb P from the soil solution, it is replaced by the dissolution of inorganic P minerals and the breakdown of organic P compounds. However, this process is often too slow to meet the needs of a growing crop.

Furthermore, how long soil minerals and organic matter can effectively maintain enough P in the soil solution for plant growth is dependent on a number of factors.

If the soil is no longer able to supply enough P, fast enough to meet the needs of the growing crop, yields will decline unless plant available P is added.

Organically managed soils tend to be low in plant available P because levels of replacement are usually less than that removed with harvest. However, there are several sources of P for use on organic farms, including animal manures and rock phosphate (RP).

Microbial inoculants and some green manures may also enhance P availability.

Potassium (K)

The amount of potassium in Saskatchewan soils ranges from 100 to 50,000 lb./ac. depending on the parent material from which the soil has been formed. However, usually less than two percent of the total K is plant available.

It is estimated that in Saskatchewan up to one million acres are K deficient, even though potash (K plus Cl) is mined below ground. The most severe deficiencies occur in the north and northeast, but K deficiency is possible in localized areas in all regions of the province. Sandy soils and high organic soils are the most likely to be K deficient.

Depending on the type of plant, the amount of K required can be as high as or even higher than the amount of N required. Therefore, although K nutrition may not presently be of concern over most of the province, it is very important for those soils that are or will soon be deficient. Organic producers are cautioned to check that all inputs, input ingredients and processing methods meet certification standards.

Sulphur (S)

Although S deficiencies occur most often in the Grey and Black soil zones, S deficiencies for high S using crops have been reported in other soil zones in Saskatchewan. Sulphur, like P and other nutrients, comes from decomposition of organic matter and weathering of soil minerals.

For the first half-century or more of cultivation, S deficiency was not a concern on most of our soils. This is because, even on soils with a low S parent material, a huge pool of organic S was available as the organic matter from decaying prairie grasses was mineralized.

However, decades of cropping, particularly with high S-using crops such as canola, mustard and alfalfa, has left some Saskatchewan soils S deficient, particularly the sandy Grey and Black soils in the northern grain belt.

Animal manure generally contains low levels of S as compared to other nutrients and can serve as a small source of S. A manure analysis available from a soil test laboratory can provide amount and balance of nutrients for a manure sample. Organic producers are cautioned to check that all inputs, input ingredients and processing methods meet certification standards.

Other Nutrients

At this time, deficiencies of the other essential nutrients are not widespread in Saskatchewan. However, a number of localized deficiencies have been identified and more may show up with time, so soil testing for all nutrients every few years is recommended. For example, copper (Cu) may be deficient in sandy or peaty Grey soils.

Nutrient Level Decline

Saskatchewan soils have lost approximately 41 to 53 percent of the organic matter; 31 to 56 percent of the organic N; and 40 to 67 percent of the potentially mineralizable N, depending on the soil, since cultivation began. Saskatchewan currently exports almost all the grain grown. Thus, native soil nutrient levels will continue to decline over time and nutrients need to be added to maintain yields. One of the best ways to provide adequate levels of nutrients is to build up soil organic matter content.

Soil Testing

Soil testing is an important tool in farm management and should be performed annually. It provides the producer with the approximate level of nutrients in the soil. Soil testing becomes particularly important when soil nutrient levels are deficient or marginal and informed decisions need to be made. Soil testing over a number of years can also allow the producer to measure the benefit of various agronomic practices. Each soil-test laboratory has specific sampling procedures and will provide them upon request.

Nutrient levels can vary considerably across a field, particularly in rolling terrain. Knolls are often the least fertile, whereas low areas are often the most fertile. Therefore, when taking soil samples, it is important to take them from average areas of the field to obtain an average representation of the field nutrient level. Some producers soil test the knolls, mid-slopes and low areas of a field separately for site-specific management. For example, manure can be applied to knoll areas first, then to mid-slope areas and may not be needed in the low slope areas.

Supplying Nutrients Organically

How, when and which products to use can be difficult decisions. Providing organic nutrients successfully requires a high level of management.

The products and practices available to organic producers vary somewhat depending on the organic certification association to which they belong. Producers reading this bulletin should contact their certification agency to ensure that products and practices are acceptable.

The main products and practices that will supply crop nutrients organically are the following:

• Applying animal manure;
• Green manuring or greenfallowing;
• Growing properly inoculated legumes in the rotation;
• Rotating high and low nutrient demand crops;
• Rotating deep and shallow rooted crops;
• Applying acceptable organic crop nutrient products;
• Use of registered nutrient solubilizing microbial inoculants, such as JumpStart®; and
• Summerfallow using conservation tillage.

Applying animal manure

Applying animal manure to a field is an effective way of increasing both nutrients and organic matter content in the soil. Animals typically pass, as waste, 75 to 90 percent of the nutrients they are fed.

The level of the various nutrients in the manure varies according to the type of animal, type of feed, how the manure is stored and how it is applied. However, animal manure contains some level of all the essential nutrients. Soil testing laboratories can test manure for its nutrient content and the soil from the field where the manure will be applied so the correct amount can be applied to correct the nutrient deficiencies.

The organic certification agency may require that manure be composted before application. Properly composted materials reach internal temperatures high enough (ideal temperature range is 54 to 60 C) to kill pathogenic bacteria and weed seeds. However, temperatures should not exceed 66 C or the microbes responsible for the composting process will also be killed. Turning the pile prevents overheating. Composting stabilizes the N in raw manure, thereby reducing N losses. Nutrients from compost are released slowly and steadily for plant use throughout the season. Composting also reduces the bulk of the material reducing transportation costs.

Another benefit of composting animal manure is that the compost process helps solubilize rock phosphate (RP). The composting process produces organic acids. When RP is mixed into manure compost, the organic acids may solubilize some of the RP, producing more plant available P. Adding elemental S to the manure compost can further enhance this process. Microorganisms in the compost may convert some of the elemental S to plant available sulphate-S. Also, during the conversion of elemental S to sulphate-S, acid is produced which may also solubilize some of the RP, producing more plant available P.

Green Manuring or Greenfallowing with annual legume crops

Green manuring involves using crops as sources of nutrients. Greenfallowing, a new term to describe a specific green manuring practice, uses properly inoculated annual legume crops such as: pea, lentil and chickling vetch for green manure during summerfallow. Green manure improves soil health by increasing soil organic matter, which in turn increases soil aggregation, as well as the nutrient supplying and water-holding capacity of the soil.

Properly inoculated legumes such as sweetclover, Indianhead lentil and chickling vetch are frequently grown to supply N to the soil. Green manures are usually grown in place of summerfallowing a field, protecting the soil from wind and water erosion. However, they must be carefully managed and incorporated early enough that they don't unnecessarily deplete the soil moisture for subsequent crops, but late enough that they provide optimum amounts of organic matter and N to the soil. Other crops (e.g., fall rye) are grown to assist with weed and disease control.

During greenfallow the growth stage that the annual legume is terminated is usually determined by the amount of available moisture: if moisture is limiting the annual legume is terminated early. If moisture is adequate, the crop is terminated at about the full bloom stage to optimize biomass and N fixation.

Sweetclover is a biennial that is inoculated and usually under-seeded in a grain crop one year and incorporated at the bud to 10 percent flower stage the following year. In its second year of growth, sweetclover is a high moisture-using crop. Since most of the N fixation has occurred by the bud stage, green manuring sweetclover at the bud stage will maximize N accumulation and allow as much time as possible for recovery of moisture reserves and for residue decomposition. Sweetclover may not be an option when moisture is limiting, for example, dry areas of the Brown soil zone.

Inoculated Indianhead lentil is normally sown (at approximately 35 lb./ac. of seed) soon after the risk of severe frost has passed (i.e., just before wheat is sown). Indianhead lentil is a poor competitor with weeds. Early seeding allows the crop to get a jump on weed growth and allows time for replenishment of soil moisture reserves after it is incorporated. Indianhead lentil is green manured at full bloom, by which time most of the N fixation has occurred. In dry years, Indianhead lentil may not produce enough dry matter to supply significant organic matter and nutrients to the soil.

Inoculated chickling vetch has a high N-fixation rate and vigorous growth, making it ideal as a green manure crop. Inoculated chickling vetch can be seeded early at about 70 lb./ac. As it performs reasonably well under moisture stress conditions, chickling vetch can be grown in drier areas of the province. Chickling vetch should be incorporated at full bloom.

The main nutrient benefit of green manuring legumes is to increase soil N levels. For example, sweetclover grown at White Fox, Saskatchewan and green manured at the early bud stage, contained 68 lb./ac. of N. This N will be made available over a number of years as microbes break down the sweetclover residue. However, legume green manure is high in N, thus the N cycles to a plant available form much faster than does the straw of cereal crops.

Legumes must be properly inoculated with the correct bacterial strain for N fixation to occur.

Some organic producers grow mycorrhizal crops for green manuring. Microbes referred to as arbuscular mycorrhizal fungi colonize the roots and produce tiny structures in the roots that allow for bi-directional nutrient exchange. The plant feeds the fungus and the fungus provides the plants with nutrients, such as P, and helps it to resist diseases better and overcome stress. Thus, increased amounts of plant nutrients, such as available P, can be brought into the organic pool by growing and green manuring these crops. Management of arbuscular mycorrhizal populations is a very important tool in nutrient cycling. Lentil, pea, bean, wheat, barley, corn, sunflower and flax are examples of mycorrhizal crops. Reducing tillage, especially during summerfallow, helps minimize the reduction of the arbuscular mycorrhizal population.

Exploiting the different rooting depths of crops can also be useful in supplying nutrients to subsequent crops. For example, deeply rooted plants like alfalfa can extract nutrients from soil depths not accessed by the shallower roots of some annual crops. Although this provides a short-term solution to nutrient supplies, in the long-term nutrients will become depleted deeper in the soil. Because grain or forage is harvested and exported, nutrients other than N (that can be fixed from the air) will eventually have to be replaced from external sources such as manure and organically approved products. Organic producers are cautioned to check that all inputs, input ingredients and processing methods meet certification standards.

Growing Legumes in the Rotation

The main benefit of growing annual legumes, such as pea and lentil, in a rotation is that if they are properly inoculated with rhizobia they will fix 50 to 90 percent of the N they require from the air (see Table 2a and 2b).

Legume residue has a higher level of N, and breaks down much faster than non-legume residue, resulting in faster cycling of N to subsequent crops (see Table 3).

Table 2a. Nitrogen fixation in inoculated legumes grown under irrigation in southern Alberta

From: Rennie, Agriculture and Agri-Food Canada Research Station, Lethbridge.

Table 2b. Nitrogen fixation by three legume green manure crops at full bloom at Swift Current, SK (three-year mean from 1984, 1986, 1987)

Table 3. Increase in the available N of Dark Brown soils due to the residue of one annual legume crop

From: Slinkard, Crop Development Centre, Saskatoon

Perennial legumes, such as alfalfa, can supply substantial amounts of N to subsequent crops. Alfalfa produces about as much root material as it does top growth, and a large part of the root system renews each year. Thus, a substantial amount of high N organic matter remains in the soil even though much of the top growth may have been removed from the field. A three to five year old hayed alfalfa stand can provide 50 to 100 lb./ac. (or more) N (in total, not per year) to subsequent crops over the next few years.

High soil N levels can inhibit N fixation by legumes, because legumes will preferentially use soil N before they fix atmospheric N. Thus, organic producers can make the most efficient use of resources by growing non-legume crops on animal manured, green manured or summerfallowed fields and growing legumes on fields with low levels of N, thereby allowing the N-fixing benefit of the legumes to be realized.

Phosphorus nutrition is very important for high N fixation in legumes.

Rotating High and Low Nutrient Demand Crops

Different crops require different amounts of the various essential nutrients. Rotating high and low nutrient demand crops may avoid placing too high of a demand on one or more of these nutrients. For example, if soil S is low, mustard should not be grown too soon after alfalfa (unless S products have been added) since both are high S users. Alternatively, if P and K are low and either pea or faba bean can be grown, pea should be a better choice because it requires only half the P and K for the same yield.

Applying Organically Approved Nutrient Products

Once again it must be emphasized that producers check with their organic certification agency regarding the specific nutrient products that are allowed.

Rock phosphate (RP) use has grown in organic production. Most of the P in RP is chemically similar to the existing slowly available forms of parent material P found in our soils. Thus, the amount of available P in RP is generally low, except in acidic soils (pH less than 7). Most Saskatchewan soils have high levels of Ca and have a neutral or higher pH, so high rates of RP must be applied to meet crop demand.

Although the addition of RP will not significantly improve crop growth (except at high application rates) on most Saskatchewan soils, note that:

• The more finely ground the RP, the more plant available the P;
• P availability depends on the origin (chemical make-up) of the RP;
• P from RP is more available on low ph soils; and
• A combination of RP and a P solubilizing microbial inoculant may enhance the P availability.

RP sources should be compared on the basis of the percent of available-phosphate, not total-phosphate. Soil testing laboratories can test RP for percent available phosphate.

Elemental S (90% to 99% S) and gypsum (0-0-0-17)

Elemental S must be oxidized to sulphate-S before the plant can make use of it. This process takes time so elemental S should be applied one year or more (depending on the product) in advance of crop demand. Alternatively, depending on the product, it can be applied at up to two to three times the recommended rate (recommendations are made on the basis of sulphate-S use) the year it is needed. The smaller the particle size of elemental S (i.e. the size of the particles that are used to make the S fertilizer granule) the more rapidly it will be oxidized to sulphate-S. Surface broadcasting in the fall may allow time for the granules to disperse and further reduction of particle size may result from wet-dry and freeze-thaw cycling.

Gypsum contains sulphate-S. The finer the particle size of the gypsum, the sooner the S will be available to plants. Like elemental S it can be applied a year or more in advance of crop demand.

Finely ground elemental S and gypsum can be difficult to handle and apply, particularly when the product is wet.  

Other products derived from natural sources (e.g., seaweed) that supply one or more nutrients may be acceptable nutrient sources. Some of these products are for soil application, while others are for foliar application. When considering these products, care should be taken to calculate the cost per pound of actual nutrient and the pounds per acre of nutrient required. It is recommended that producers test a small amount of a new product on a small area of land, making sure to leave an untreated check strip for comparison.

Also, care should be taken when considering product claims. If a soil test requires 40 lb./ac. of a nutrient to meet a target yield, then 40 lb./ac. of that nutrient will be needed. The nutrient can come from the addition of an approved nutrient product or result from managing the soil system using strategies such as green manuring with legumes.

Use of Registered Nutrient Solubilizing Microbial Inoculants (such as JumpStart®)

JumpStart® is a microbial seed inoculant registered for use on several crops. The active ingredient in JumpStart® is the spores of a naturally occurring soil fungus (Penicillium bilaiae).

JumpStart® can increase the availability of soil P or RP to crops during the growing season. When soil test P level is low, JumpStart® can make available the equivalent of at least 10 lb./ac. of applied P when applied with at least 10 lb./ac. of inorganic P product (e.g., RP). When soil test P level is high and only 10 to 15 lb./ac. of additional P is recommended, JumpStart® can replace the application of this amount of P.

JumpStart® aids P utilization efficiency, but is not a substitute for a long term P nutrient products program. Fields should be monitored by soil testing to ensure that adequate background levels of P are available.

Summerfallowing

Summerfallowing exposes organic matter in the soil to air and under good soil moisture conditions stimulates the activity of microorganisms that break down organic matter, thus speeding organic matter breakdown. Also, summerfallowing prevents plant growth, and thus, nutrient use during the summerfallow period. Accordingly, summerfallowing does increase the availability of nutrients to the following crop. If tillage summerfallowing is being practised, conservation tillage methods should be used to keep the crop residues on the surface for soil and moisture conservation. However, it does so at the expense of soil organic matter, and thus, the soil nutrient pool. Summerfallow also puts the soil at increased risk of erosion, and thus, further nutrient loss. Consider greenfallowing with an inoculated Indianhead lentil, pea or chickling vetch instead of tillage-based summerfallow.

General Rules

Many organic producers will tell you there are a few general rules they always try to practise:

• Build organic matter levels through crop rotations that include legumes. This builds the organic pool of nutrients to reduce the need to rely on other ways of supplying nutrients organically,
• Prevent the loss of nutrients by preventing soil erosion,
• Always try something new on a small acreage, and
• Check that all inputs, input ingredients and processing methods meet certification standards. Crop nutrient and supplement products sold in Canada must follow the regulations set out under The Fertilizer Act of Canada.