Soil Nutrient Management on Organic Grain Farms in Montana

Department of Land Resources and Environmental Sciences Montana State University

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Soil Nutrient Management on Organic Grain Farms in Montana

Overview

Maintaining soil fertility and replacing nutrients removed at harvest are inherent challenges in organic production. Except for nitrogen (N), which can be added by legumes, nutrients removed at harvest must eventually be replaced or made more available from the soil by altering soil properties. Soil testing is the best way to know which nutrients, and how much, might need to be added for optimal benefit and use of resources (time, labor, material), and protecting the environment.

Reducing tillage and fallow and increasing crop diversity minimizes loss of soil organic matter (SOM), the key to soil health and nutrient cycling, and reduces risk of nutrient loss through erosion. Practices that support microbial populations and increase nutrient availability should be encouraged.

Legumes increase the soil’s N supplying power through N-fixation in root nodules. They should be present as a grain (pulse crop), perennial (e.g., alfalfa) or cover crop 25 to 50% of the time to provide sufficient N for small grains. Manure is an excellent source of a variety of nutrients but may not be locally available in sufficient quantities. Unless manure comes from poultry, it is low in N. Biological additives should be used with caution until more data are available.

Livestock grazing may be important for the economic and environmental stability of agricultural systems in our region. It provides more income than a traditional cover crop (also called ‘covers’) and has less removal of hard to replace nutrients (e.g., phosphorus) from the field than haying.

Integrated use of crop rotations, livestock grazing, fertilizers and amendments have the potential to improve soil quality and increase sustainability of organic crop production. Since organic standards change with time and location, be sure to check with a certifying agent before using any product or practice on an organically-managed field.

Soil Fertility Challenges for Organic Producers    

The soil’s ability to provide sufficient nutrients depends on the soil characteristics, climate, and crop demands. Growers _often focus on increasing N availability because it generally controls crop yield and quality more than any other nutrient. This can result in overlooking other nutrients and can lead to low N fixation by legumes. However, adding nutrients such as P, potassium (K) or sulfur (S) to organic fields is not easy or inexpensive.

Soil nutrient levels have decreased on some organic fields in Montana and yield and/or quality have sometimes suffered (1). For example, the soil in organic rotation at Montana State University’s Post Farm in Gallatin Valley was lower in N, P, K, and S than fertilized no-till soil after four years. After seven years of organic rotation, winter pea nodulation and yield were reduced. Nitrogen fixation had apparently decreased, most likely because of insufficient P and S, which were low in pea tissue tests. Also, low wheat grain protein in the organic rotation suggested the legume- based N management was not sufficient to maintain grain protein (1, MT).

Nutrient removal rates depend largely on the crop and its yield. For example, 3 ton/acre of alfalfa removes 33 lb P2O5/acre while a 30 bu/acre spring wheat crop removes 19 lb P₂O₅/acre. For estimates of nutrients removed by the harvested portion of crops see Fertilizer Guidelines for Montana Crops or MSU Extension Soil Fertility Nutrient Uptake and Removal. Transitioning to organic production is discussed in From Conventional to Organic Cropping: What to Expect during the Transition Years. Information on how to obtain referenced Extension publications is given at the end of this publication.

Understanding the Soil   

Organic soil fertility management must rely on biological processes in the soil that cycle nutrients between unavailable and plant-available forms (2). Soil tests can identify limiting nutrients and help with management decisions. For example, low P levels indicate that it could be beneficial to add manure, or another P source, or that the site is suited for crops that need low P or are efficient at extracting soil P (described below). Even if a grower does not plan on fertilizing, knowing soil nutrient levels can help optimize crop rotations. For example, legumes should be grown in fields low in N and wheat in areas richer in N.

A traditional soil nitrate-N test in the spring is valid for early plant growth needs (3, MT). These tests do not measure potentially mineralizable N (PMN), which is the proportion of total soil N that can become plant available N (PAN) during the growing season through decomposition of organic material. Soil health tests such as Haney (H3A) and Cornell Comprehensive Assessment of Soil Health offer estimates of PMN. However, in southwest Montana they overestimated actual PMN in the field and PMN did not correlate with crop-N uptake (3).

An alternative to using soil tests to evaluate N availability is to look at wheat protein. If average protein levels over several years were less than about 13% in spring wheat or 12% in winter wheat, then yields were likely limited by the lack of N (4, MT) and the grower should consider increasing the frequency of legumes in rotation. Spring wheat requires about 3.3 lb available N per bushel grain, and winter wheat about 2.6 lb N per bushel. The protein levels are general guidelines only, because grain protein at optimum yield varies with cultivar. In addition, moisture stress near grain fill can increase grain protein, even in N deficient fields.

For more information on soil tests and/or fertilizer calculations, see Interpretation of Soil Test Reports for Agriculture and Developing Fertilizer Recommendations for Agriculture. The National Resources Conservation Service’s Cropland In-field Soil Health Assessment Guide and Soil Quality Test Kit help assess physical and biological attributes of soil health that are not measured by standard soil tests.

Improving Soil Fertility   

INTERNAL NUTRIENT INPUTS – CROPS

Crop rotation, grain legumes, and cover crops are the foundation of soil fertility on organic farms. Additional approved materials and livestock integration may be used to supplement these practices.

Of all the nutrients required by crops, only carbon (C) and N can be added to the soil by plants. Living roots and C from plant residue are necessary to sustain thriving soil microbial communities, which are the basis for nutrient cycling. Plant residue and roots also protect against nutrient loss through soil erosion. For some nutrients such as P, plantings to minimize soil loss are an important way to maintain nutrient availability.

Crop Rotation

Ideal rotations include crops that use nutrients and water efficiently and break pest cycles (insects, weeds, pathogens). Nutrients and water are used from different areas of the soil depending on both root depth and breadth. For example, most annual legume roots are in the top 30 inches of soil. Safflower, sunflower, wheat, and perennials have roots throughout the soil down to 4 feet or deeper (5, ND). In a Saskatchewan study, barley yields were two to three times higher following canola or pea than wheat in absence of N fertilizer Figure 1. Pea provided additional benefits over canola by increasing soil N.

Figure 1. Dryland barley grain yields with no N fertilizer were two to three times greater following canola or spring pea, than spring wheat (6, SK, not organic).

Planning rotations to take advantage of N input from a legume is critical. Follow an N-building crop with a crop that has high N demands to reduce the leaching loss of soil nitrate added by the legume (7, SK). Wheat is well suited to take advantage of high soil N from legumes. Legumes should not be followed by fallow, or a crop likely limited by other resources such as water or P.

Legumes and Covers

General benefits and limitations

The benefits of grain legumes (also called pulse crops) and covers vary with species, growing conditions and management (Table 1) and can be increased by the application of good agronomic principles (8).

Only a portion of grain legume and cover benefits can be attributed to increased soil N. Long term benefits include increased SOM, improved soil aeration, aggregation, water holding capacity, and increased soil microbial activity (9). A plant cover of any type (including weeds) helps reduce erosion and can reduce leaching losses, yet uses water that will then not be available for the next crop. Cover Crops: Soil Health, Cover Crops: Soil Water and Small Grain Yield and Protein, and Cover Crops: Management for Organic Matter and Nitrogen provide regional information on covers as partial replacement of summer fallow. They contain information for organic and non-organic farming systems.

Legumes offer production system diversity which tends to increase sustainability. They are versatile as grain, hay, or cover crop. Soil N buildup from repeated legume rotations reduce reliance on outside N inputs. There is little substitute for experience in determining what works in each situation. Ask nearby producers for advice; most organic producers are more than willing to share knowledge.

Nutrient benefits

Legumes produce plant available N through N-fixation. They can also increase uptake of other nutrients (such as P and zinc) by non-legumes in rotation by stimulating soil microbial activity (9). Using legume covers to increase N will not benefit cash crop yields if P or another nutrient is limiting. In a Utah study, an overwinter grass/vetch cover supplied 55 lb N/acre for a June planted quinoa crop. Only the quinoa that also received manure compost was able to benefit from the N supplied by vetch cover. Quinoa yields were more than double on the fields that received manure compost than those that did not, likely due to P in the compost (10).

Nitrogen A legume’s ability to fix N relies on inoculation with host-specific bacteria and is influenced by many factors (Table 1). Spring soil tests following a legume cash or cover crop reflect only a portion of the N benefit gained from the legume. Table 2 presents estimates of the additional plant available N that will become available during the following growing season. Because only about 25% of the N in residue becomes plant available in the first year, N benefit increases with each rotation. After four cycles of spring pea cover alternated annually with wheat, soils contained 110 lb N/ acre, which was 70 lb N/acre more than soils in fallow- wheat (12, MT). Wheat grain protein tends to benefit from fewer legume rotations than wheat grain yields. It took two legume rotations for wheat protein to equal (or catch up to) till fallow with 30 lb N/acre, but three legume rotations for wheat yields to be equal (13, MT). Legumes must be present every second or third year at a minimum to supply sufficient N for wheat (1, MT).

Table 1: Crop attributes and environmental variables that benefit or limit the ability of seed legumes and green manures to enhance plant available nutrients.

Table 2. Estimated in-season plant available N (PAN) the year following legume grown to seed, as a cover, or perennial for one or more rotations (11).

Release of N from crop and cover residue, and livestock manure does not follow the same patterns as from mineral fertilizer (e.g., Chilean nitrate). The initial release of plant-available N is determined by N concentration and decomposition rate, which is fastest under warm, moist conditions.

Above ground residue high in N, from legume cover (Figure 2, vetch, 4.8% N, and faba cover, 4.0% N) or other plants that took up high amounts of N from the soil (Figure 3A , forage radish, 5% N), can release a large portion of that N rapidly once soils are moist and greater than 50?F for a couple of weeks. If no crop is in place to use that flush of N, it might not remain in the surface soil for the next crop (16, ND). Such residue that decomposes quickly provides little additional N for a crop three years later.

Residue with lower N content releases less N (Figure 3A, pea cover, 2.2% N) and at a slower rate, providing N over the next two to three years (Figure 2, pea grain, 1.6% N, faba bean, 2.8% N). Because the remaining residue can accumulate over years, after five years of annualy adding pea residue to the soil surface, the available N released during the growing season is greater (Figure 3B) than after just one year of pea residue (Figure 3A; Table 2). Low N residue provides little or no N the first year (Figure 3A, corn, small grain, 0.5% N), and with five years of annual addition, N is tied up during the growing season (Figure 3B).

The C to N ratio (C:N) of material gives a rough indication of whether N will be provided or N will be tied up (immobilized) as soil microbes borrow N from the soil, making it temporarily unavailable to plants. When organic material C:N is less than around 20 (e.g., young alfalfa or pea), decomposition supplies N. At C:N greater than 20 (e.g., small grain stubble, compost, manure solids) plant available N is very low or negative, tied up by microbial activity. The C:N does not help a producer make N management decisions. For example, winter pea grain residue (2.2%N, C:N = 18) and corn residue (0.5% N, C:N = 73) can both supply near zero plant available N the first growing season (Figure 3A). For more details and how to calculate plant available N from residue see Market Vegetable Farms: Soil Nitrogen & Sulfur. The OSU Extension Organic Fertilizer & Cover Crop Calculator helps estimate the amount of PAN provided by inputs such as fresh organic materials, cover crop residues, and compost.

The amount of N fixed generally increases the longer the crop is growing. Perennials and biennials have a longer growing season and a more established root system than annuals. They tend to produce more biomass and fix more N. However, deep-rooted plants can create a very dry soil profile that limits subsequent crop establishment and production for up to two years (17, MT).

Adding perennials to the rotation appears to limit movement of N below the rooting zone, and most likely helps retain N on the site as organic material (18, SK). The roots of perennial legumes can contribute as much soil N as the aboveground biomass; dryland alfalfa roots and crowns contained 140 to 170 lb N/acre after the third-year final harvest (19, WA). Although annual legume roots are not as substantial as perennial roots, the belowground N contribution from annual legumes can be a third or more of the N in aboveground plant parts (20, SK).

Figure 2. Above ground legume cover residue releases N more quickly than residue from pulse crops, which release N over several years. Covers were cut at full bloom. Residue was left on the soil surface (14, AB, not organic).

A. 1 year of residue

B. 5 years of annual residue

Figures 3A & 3B. N released by high N plant residue (5% N, forage radish), medium N residue (2.2% N, pea cover), and low N stubble (0.5% N, corn, soybean, and wheat) during the growing season the first year left on the soil surface (A), and after annual addition of the same residue each of five years (B; 15, lab).

In our region, a legume’s water use, rather than N-fixing ability, largely determines the following small grain crop yield on dryland fields (Cover Crops: Soil Water and Small Grain Yield and Protein). Species and varieties should be selected that mature early to retain soil moisture. Because winter crops mature and can be terminated earlier than spring crops, they are less likely to become water limited and can produce more biomass, fix more N and leave more stored soil water. Their earlier termination also allows for more time for N release prior to the next crop. Wheat grain yields were greater following successful winter pea than spring pea, especially when cover was cut at bloom (Figure 4). Wheat grain protein was greatest following winter pea cut at pod. However, due to the risks of poor fall establishment or cold injury, winter legumes are only suggested for areas where winter wheat survival is highly reliable (22, MT).

Figure 4. Organic winter wheat grain yields were higher after winter pea than spring pea, especially when cover was cut at bloom. Wheat grain protein was greatest after winter pea terminated at pod. Bars with the same letter are equal with greater than 90% confidence (21).

In general, pea, lentil, and faba bean fix more N than is removed in their grain at harvest, whereas dry bean do not (23, MT, AB, SK, MB). The amount of N fixed is variable, in part because it decreases as available soil N increases, and is highly dependent on precipitation. At around 25 lb N/acre of soil N, N fixation starts to decline, and it stops somewhere between 50 and 300 lb N/acre (24, France; 25). If soils contain less than 9 lb N/acre and seedlings do not get a strong start, they also fix little N. Nitrogen fixation is slowed at pH levels less than 6.0, and when soil P, K, calcium (Ca), cobalt (Co), boron (B), iron (Fe), copper (Cu), or molybdenum (Mo) levels are low, which are easily overlooked. Sulfur is particularly important because it helps plants use N and make protein. Also, limited S reduces nodule development and function on legumes, thus N fixation (26). In S deficient soil, 5 lb S/acre as potassium sulfate resulted in 33 lb N/acre more lentil N fixation (27, MT). For more details on N contribution by covers and their impact on soil water and following small grain yields and protein see Cover Crops: Management for Organic Matter and Nitrogen and Cover Crops: Soil Water and Small Grain Yield and Protein.

Phosphorus Unlike N, P cannot be produced on the site; however, covers help reduce P losses to erosion and runoff and recycle available P. Clover and pea cover residue released 10 to 11 lb P₂O₅/acre for the following wheat crop, while pea grain, canola and wheat stubble only released 2 lb P₂O₅/ acre (28, AB).

Phosphorus availability is primarily dependent on soil chemical properties. In our region’s usually calcareous and high pH soils, P is often tied up as relatively insoluble calcium phosphate minerals. Crops such as buckwheat, legumes and some mustards acidify the area immediately around the root or secrete chemicals from their roots that help access P from the soil and rock phosphate (29). Yet, the effect does not appear to immediately carry over to a subsequent crop. In a 2-year cropping sequence, winter wheat grain yield increased with P rate, but yields were not affected by the prior crop (Figure 5), even when plant available soil P levels were low (29). The potential increase in P availability through covers is likely a long-term process (31) and a limited solution without additional P inputs.

Figure 5. Organic winter wheat grain yields increased with rate of rock phosphate (RP) applied but were not influenced by prior cover crop of buckwheat, mustard, or spring pea terminated at pod (30, MT).

Cover vs. crop harvest

Whether a crop is terminated early or harvested as a cash crop is influenced by the economic and nutrient needs of a production system. A 25 bu/acre pea or lentil crop can fix approximately 50-100 lb N/acre if the initial soil N is low. However, when the seed or legume forage is harvested, much of the fixed N is removed. Seed N can exceed the total amount of N fixed by the plant (23). Hay removes large amounts of nutrients. For example, every ton of alfalfa removes about 48, 11 and 53 lb/acre of N, P₂O₅ and K₂O, respectively. Although residue from grain legumes does not benefit subsequent wheat as much as an early terminated legume cover, grain and forage legumes help improve long- term soil fertility, especially in contrast to fallow.

Cover termination timing & method

When to terminate The plant stage of cover at time of termination can have more influence on subsequent grain production than the cover species (32, CO). There are a few questions to ask which can help determine termination timing (see Termination Timing box).

With several species of covers, termination at bloom conserved water and increased subsequent winter wheat yield compared to termination at pod (Figure 6). Termination time did not influence grain protein after any of the covers tested in this study (21, MT). A 12-year Saskatchewan study found that seeding spring lentil cover as early as possible and terminating at bud stage produced the same spring wheat yield as after fallow (33). A gradual increase in grain protein, reduction in fertilizer N requirements, and reduced herbicide and tillage costs to control weeds all created a positive economic return. Conversely, when lentil was terminated at full bloom, the following wheat grain yield was 9 bu/acre less than on fallow.

Early termination allows more time for N to become available from the residue, although decomposition can be too slow to provide sufficient N for winter wheat (19, WA). Mid-summer rather than fall tillage of alfalfa improved spring planted crop yields primarily through reduced alfalfa volunteers and dandelion competition, rather than increased N availability (34, SK). Nitrogen available in the soil in the fall, from early terminated covers, is at risk of leaching loss in coarse or shallow soils.

Later termination produces more cover biomass and N. Nitrogen fixation peaks near bloom in dry years for pea but continues until at least pod stage for pea in wet years and lentil in dry and wet years (35, MT). Forage legumes fix N and add biomass throughout the growing season. However, each cutting removes biomass and temporarily slows down N-fixation (36, MT). If the goal of growing a cover is to increase SOM, then it is important to understand that the longer a cover grows, the more cash crop yield is reduced in water-limited systems. The amount of biomass gained by cover growth from early bloom to pod or later can be less than the loss in cash crop residue due to reduced yields because of water use by letting the cover crop grow longer. The total residue input to the system by the cover- and cash crop years can be less than if the cover had been terminated at early bloom. This is explained in Cover Crops: Management for Organic Matter and Nitrogen.

The producer needs to weigh the relative importance of long-term soil benefits to immediate cash crop yields. For maximum yield of the next crop, terminate early. Terminate late to maximize grain protein and to build soil N and possibly SOM. Wet years or irrigation should allow for later termination of cover with less negative impact on the subsequent crop.

How to terminate Tillage increases decomposition and N release (37, SK). If the soils are warm and moist, N can be released within a few weeks after tillage. On coarse, shallow soils or areas with high fall/winter precipitation, this N can be lost to leaching if there is no crop in place to take it up. In dry, cool soils, tillage may be the only way sufficient N is available for the following spring crop from summer terminated covers. Annual legume cover tilled at full bloom (mid-late June) produced higher spring wheat yields than when the residue was left on the surface at termination (38, MT). Late fall tillage just before ground freeze has a low risk of leaching loss and will provide some N at spring seeding. Spring tilled cover released N too late for that year’s crop (34, SK).

Tillage reduced soil moisture when the top soil layer was wet, but not under dry conditions (38, MT). Occasional tillage when needed for weeds or to increase plant residue decomposition is likely better for soil than frequent tillage (39, SK).

The no-till alternative of crimp rolling, which crimps plant stems to stop growth, produces mixed results. If plants are either too supple or too tough, crimping might not stop their growth. Winter pea at bloom was too supple for effective crimp rolling and started to regrow within two days. Sweet clover at mid-bloom was too woody to crimp properly and also resumed growing (40, MT). However, crimp rolling terminated winter pea at the pod stage. Winter wheat seeded directly into the crimped residue had the same yields as when seeded after pea terminated by tillage, despite increased weed densities (40). The unincorporated pea residue can also help germination and survival of wheat under dry fall conditions because it provides soil cover to slow evaporation (40). Undercutting is another option with less soil disturbance than tillage. Both undercutting and crimping likely have slower N release from the residue than tillage, but provide the benefits of soil surface cover and less soil disturbance.

Vinegar is sometimes used for weed control on small organic farms but is cost prohibitive on a large scale. Vinegar did not terminate winter pea at the bloom stage despite an application rate that scorched the leaves (40, MT). The plants put out new leaves from the stem tip within a day after application (22, MT). Other termination products or methods may become available for organic growers. With any termination method, leaving residue strips that trap snow helps recharge soil water.

Non-nutrient considerations

Drought and cold hardiness combined with biomass and growth habit (tall and thin or low and bushy) affect a crop’s ability to suppress weeds or protect soil from wind erosion. Buckwheat, for example, has the growth habit to be a good cover, but is susceptible to frost kill and should be planted late. Therefore, it might not reach optimum biomass for weed suppression. Also, eating buckwheat, or small grain contaminated with buckwheat can cause anaphylaxis in some people (41). Care should be taken to prevent buckwheat from going to seed, or not be planted with or near commodity wheat (42).

Annuals are easier to terminate and turn under than perennials. Also, breakup of perennial stands often results in severe weed invasion which can limit subsequent crop growth, especially in dryland production (From Conventional to Organic Cropping: What to Expect During the Transition Years). Fall planted annuals tend to suppress weeds better than spring annuals (21, MT), provide winter soil cover and trap snow. The ability of some crops to disrupt pest cycles, such as mustards which help suppress soil-borne pathogens (43, Australia), can also influence cover crop selection. However, mustard grew very poorly in low N soils (21, MT).

Livestock Integration

Forage legume production can be less risky than seed crop production under uncertain weather patterns. Legume cover can provide more income and greater soil N and health benefit as forage than when grown to seed. On an organic farm near Stanford, Montana, wheat yields were higher following winter pea and legume cover than winter pea grain. The forage value of winter pea further increased the 2-year net return versus ungrazed winter lentil cover (Table 3).

Table 3. Yield and net return of different annual legumes in rotation with organic winter wheat 2007-2008. Net return is based on actual production costs, crop prices and forage value to the farmer (44, MT).

Improvements to soil health, water infiltration and storage, and reduced erosion increase with soil cover and the longer there are active roots in the soil. The soil health and N benefits are generally greater with perennial than annual forage legumes (45, WA), but perennials can reduce small grain yields more than annual covers due to higher water use. Covers properly grazed generally provide more benefits than when hayed (46). Livestock grazing keeps most of the nutrients on the pasture. This is especially important withP. Legumes can increase plant available P (47), but legumes cannot add additional P to the soil (48). Legume hay harvest removes about twice the P of small grain; therefore, P will likely need to be replaced using an external source more frequently in hay than grain systems.

Grazing cover might also address the often-low protein values of winter wheat because less N is removed from the field than with legume seed or hay harvest. Although one year of grazed legume, or even three years of perennial legume cover were not sufficient to increase grain protein (44, MT; 45, WA), 14 years of wheat in rotation with winter pea grazed by sheep had higher average protein (14.1%) than wheat-fallow (12.4%, 49, WY).

Identifying suitable legumes for self-seeding (ley- farming) is challenging because growing legumes for seed production reduces the following wheat yield due to water use. Also, the self-seeding legumes that provided good forage had poor winter survival or were unable to re- establish largely because of weed pressure (50, WY; 51, ND; 52, MT). Grazing grain crop residue following harvest is an option to reduce weeds.

The challenges of integrating livestock include low carrying capacity, limited fencing and water sources for animals, additional managerial skills required, complicating organic certification and lack of local markets for organic meat (53). However, integrating forages, especially grazing, might be a key to the environmental and economic sustainability of small-grain farms (46).

EXTERNAL INPUTS

Farmers striving for sustainability should consider on-site or local manure first, followed by products which are recycled, such as poultry litter or bone meal, but might require further transport. Minerals such as rock phosphate are a mined, finite resource, and can contain heavy metals.

Manure

Manure contains all the essential plant nutrients and improves soil quality, aggregation for ease of cultivation and plant emergence, and water and nutrient holding capacity. If manure is readily available, it is a good source for P, K, and micronutrients. Manure contains low amounts of S, and only a small portion of the S is readily available (5%, other than poultry with 30%; 54).

Manure nutrient content and the amount of nutrient that becomes available during the first growing season vary greatly, depending on animal type and diet, type and amount of bedding, and handling and storage methods. It is best to have manure tested for nutrient content. Table 4 provides approximate amounts of N in manure and other selected amendments to make rough calculations of application rates.

Manure provides a flush of plant available N from ammonium-N. Poultry and pig manure are relatively high in ammonium, and cow manure slurry (2 to 8% total solids) typically has more immediately available N than solid cow manure (56). Sprayed liquid manure can lose up to 50- 70% of the ammonium-N to the atmosphere within hours of application (Using Manure as Fertilizer) and solid cow manure can lose 80% of its ammonium-N to the atmosphere within a week if not incorporated.

Table 4. Approximate amount of N per ton of material (as-is, not dry matter basis), percent of N that becomes available the first growing season, and the amount of material needed to provide 100 lb N/acre the first growing season. For additional materials and references see Market Vegetable Farms: Soil Nitrogen & Sulfur.

Plant available N is two to three times greater from fresh than composted manure (57), therefore, so is the potential loss of N (58, UT). After the readily available N is released from fresh manure, plant available N release averages 2 to 7% per year following the application year (56). Available N by the third or fourth year after application is the same whether the manure was fresh or composted (57). The plant available N release rate can increase as the soil microbes adapt with repeated manure applications (56).

In a Utah study, the increase in grain yield from a single manure compost application in dry conditions (10- inch annual precipitation) was more due to non-nutrient benefits (e.g., soil water holding capacity) than the addition of nutrients (e.g., N, P). In wet years or locations (22-inch annual precipitation) the addition of nutrients was relatively more important to increased grain yields than the other benefits of the added manure compost (59). However, once crop nutrient requirements are met, the non-nutrient benefits outweigh the nutrient benefits. Grain yields peaked at lower (11 ton/acre) manure compost rates in wet versus dry locations/years (22 ton/acre).

In the Gallatin Valley, Montana (16-inch average annual precipitation), a nine-year organic rotation with no external nutrient inputs, 9 dry ton/acre of 2-year-old steer manure increased winter wheat grain yields by 37% (Figure 7). In this study, there was an immediate grain yield increase, however, year-to-year variation in growing conditions can hide an immediate benefit of manure application. There is potential long-term carryover benefit of manure application which might only be realized when all nutrients and water are sufficient for high crop yields. In a Utah study, yields only increased four years after manure application when a vetch cover supplied necessary N (61). The small grain crop was not able to benefit from P added by manure compost because it was N deficient. The vetch cover crop was not N limited but benefited from P supplied by the manure compost.

Figure 7. Aged steer manure increased wheat grain yield but not weed biomass in Gallatin Valley, MT, organic plots. Bars with the same letter are equal with 90% confidence (60).

Manure increases yield for several years. Yield more than doubled in the first two years after composted cow manure was applied once to dryland wheat fields at 22 tons/ acre (660 lb N/acre). Yields were still greater after six years (Figure 8). Phosphorus, K, zinc, microbial activity and soil organic carbon were still higher on the manured than non- manured fields after 16 years of organic winter wheat-fallow (62, UT). Given the cost of manure application, manure is not suggested more often than every few years applied at rates that meet crop P needs and provide a boost in soil health benefits. Low rates do not necessarily provide soil health benefits (63, SK).

Figure 8. Organic wheat grain yield was much higher for several years with 22.2 ton/acre cattle manure compost (650 lb N/acre) applied once and incorporated (62, UT).

Half, to all, of the P in cattle and pig manure is plant available. The conversion to plant available P happens quickly and a high portion of the P is taken up by plants if soil P is low. Plant available P in following years depends on whether it gets bound to soil particles or is lost in erosion or runoff (64). Organic material from manure or crop residue helps keep P plant available by slowing processes that create relatively insoluble phosphates and increasing the rate of P moving into solution (65).

It is easy to over-apply P and K if manure is applied to match the crop’s N need (66, CO). When soil test levels approach Olsen P 30 ppm or 800 ppm K2O, further manure applications should be limited to minimize potential for water pollution and interference with uptake of other nutrients. Legume covers rather than manure can then be used to supply N if needed until P and K levels fall below the critical level (Olsen P = 16 ppm, K₂O = 250 ppm), at which point manure could again be applied. If P and K are in excess, legume hay can be used to remove high amounts of P and K while adding N through N-fixation.

Manure application must be made without jeopardizing water quality. State regulations may include a field assessment for potential N and P loss, soil and manure nutrient tests, and the nutrient requirements of the subsequent crop (available in Fertilizer Guidelines for Montana Crops). If there is risk of leaching or denitrification loss (higher when soil is near saturation) of the readily available N, manure should be applied as close to the time of crop nutrient uptake (Nutrient Uptake Timing by Crops: to Assist with Fertilizing Decisions; https://landresources.montana.edu/soilfertility/nutuptake. html) as possible under NOP guidelines, and not when the ground is frozen, snow-covered, or saturated. Manure incorporation, reduced tillage, cover crops, vegetated waterways, field buffers and SOM to increase water infiltration rates all help reduce the risk of environmental P and N contamination. Many resources help producers properly use manure (56; 67; 68).

Manure can increase weed populations due to undigested weed seeds, but not always (Figure 7). Subsurface banded liquid manure tends to produce fewer weeds than broadcast applications, and composted manure usually produces fewer weeds than fresh (69, AB).

Fertilizers and Amendments

Fertilizers generally target particular nutrients (e.g., langbeinite for K and S) and should come with a guaranteed analysis of total N, and available P₂O₅ and K₂O, whereas organic materials (e.g., manure) generally contain a diversity of nutrients, but in low amounts, and they often provide a variety of non-nutrient benefits. Many materials, such as canola meal, fall somewhere in between high concentrations of specific nutrients and low concentrations of a diversity of nutrients.

To selectively add P, producers can use bone meal or rock phosphate (Table 5). In alkaline soils (pH > 7), rock phosphate from sedimentary sources provides plant available P, igneous sources do not (70, ON). Rock phosphate mined in Utah and Idaho is from sedimentary sources. Bone meal is similar to many calcium phosphates already in the soil; P availability increases as pH decreases. There is likely little benefit from bone meal at pH 7.5 to 8.5 for this reason, but P should be highly available below pH 6.5. A finer grind helps bone meal and sedimentary rock phosphate solubility but does not help solubility of igneous rock phosphate (71). Rock phosphate contains heavy metals such as lead and cadmium. Heavy metals become more available as pH decreases, therefore, rock phosphate should be used with more caution in low pH soils. Bonemeal contains fewer heavy metals and is a renewable resource.

Table 5. Average N, P, K and S concentrations for various fertilizers and amendments.

Bone meal and rock phosphate break down more quickly when incorporated. In P deficient soils, it is possible to increase plant available P from rock phosphate by adding elemental S (up to 355 lb/acre; 72, Australia). The S needs to be in close contact with the rock phosphate, but in high pH and calcareous soils, the effect is short-lived and may not increase yields (73). High soil microbial activity increases the rate P converts to soluble, plant-available P (47). Combining compost with rock phosphate or bone meal is more likely to increase P availability in sandy than clay soils (74, greenhouse).

Rock phosphate is not always effective at enhancing P nutrition (30, MT) or cost-effective in increasing yields the year of application (75). If rock phosphate is used, it should be added well before the growing season, built up over time, or banded with the seed. The Phosphate Rock Decision Support System can help determine the relative agronomic effectiveness of rock phosphate based on the source, field soil characteristics and climate, and the crop to be planted (76). Bone meal adds N as well as P, and if used to meet N requirements then crop P needs are generally met (77).

Potassium is generally abundant in Montana soils but can be limiting in sandy, dry, or cold conditions. Greensand is often mentioned as a source of K; however, the K is in an unavailable form and greensand mines in North America are now closed. Alternative materials with readily available K are listed in Table 5. Micronutrients rarely limit crop growth in soils with high SOM or that have received compost, manure or other organic material such as feather meal. Micronutrients can be temporarily unavailable for example, in cold soils.

Sulfur deficiency is becoming