Maintaining Fertility for Organic Alfalfa
By Margaret Smith, PhD
Albert Lea Seed Agronomist
Maintaining soil fertility levels for alfalfa in organic rotations involves more than just these hay years. The entire rotation contributes—or may draw from—the fertility needed to produce and maintain good alfalfa and alfalfa/grass hay.
Of course, managing soil fertility is important for productivity and profitability throughout the organic crop rotation, and it’s more challenging than in conventional farming systems. Nitrogen (N) is sometimes a limiting nutrient in organic rotations, but certainly not in the alfalfa hay years. Supplemental N can come from other legume cover crops and manure. Where animal manures are added to the system, soil test Phosphorus (P) levels stay relatively stable or increase. Potassium (K) is the most challenging nutrient to supply and maintain in organic rotations that include hay crops. Sulfur may also be needed, particularly for alfalfa, and can be supplied from animal manures, gypsum, or several OMRI-listed dry fertilizer products.
Some growers, once farming organically, used to think that their diverse, extended rotation with some manure input would take care of itself. Not necessarily. If anything, organic growers need to pay MORE attention to fertility needs than their conventional counterparts.
Soil Test Levels As A Guide?
Should we rely on conventional soil tests for organic agriculture? Maybe not entirely, but they are a good place to start monitoring your fertility status. Organic rotations enhance soil biological activity and mineral cycling, and there’s some indication that this enhanced biological activity speeds nutrient release (those nutrients bound on soil organic matter and clay). Soil tests that measure respiration, such as the Haney Test, help detect increased biological activity and can also be used to monitor changes in both your soil fertility and soil biology.
Growing deep-rooted crops can help draw nutrients from below 6 inches (typical sample depth for fertility testing). All of our crops root below this depth and have this effect, to varying degrees. Perennials, including alfalfa and forage grasses, are particularly effective at moving nutrients from deeper to shallower in the soil profile. For example, soils with high subsoil K levels are much easier to manage and maintain topsoil soil test K levels than those with low subsoil K. (Subsoil K is a different measurement than made with 6-inch deep, surface soil samples. Subsoil K levels were measured when soil were mapped by SCS/NRCS. Soils in the east tend to have low subsoil K levels, while those in western regions of Minnesota and Iowa, and on west tend to have high subsoil K levels.)
Whatever soil tests you prefer, use them regularly! Test soils at least once in three-, four-, and five-year rotations. Regular tests will pick up changes in available nutrient status, whether decreasing from crop removal or increasing from nutrient additions, redistribution from deeper in the soil profile, or improved rates of mineralization and nutrient cycling. For all crops in organic grain and forage rotations, soil test K levels are best maintained in the Optimum category, 101-150 ppm for moist samples and 171-220 ppm for dry samples (see Table 1). Know which method your soil testing lab uses when conducting analyses. For best hay performance, work to maintain pH at 6.8 when alfalfa is in the rotation.
Table 1. Interpretation of soil test values for all crops for potassium (K) measured form 6-in deep samples. Samples may be either field moist, in a slurry, or dried to 95o-105oF.
| Relative Level of Potassium | Field Moist or Slurry Samples | Dried Samples |
|---|---|---|
| Very Low | 0-60 ppm | 0-125 ppm |
| Low | 61-100 ppm | 126-170 ppm |
| Optimum | 101-150 ppm | 171-220 ppm |
| High | 151-200 ppm | 221-270 ppm |
| Very High | 201+ ppm | 271+ ppm |
Abbreviated from Table 1 in A General Guide for Crop Nutrient and Limestone Recommendations in Iowa
The nutrient status of alfalfa is best measured by tissue testing. This isn’t often necessary but can be a good tool for helping solve tough production issues, particularly on eroded or sandy soils where micronutrients might be limiting. Most forage testing labs will also perform nutrient status analysis of the growing crop (see Tissue Testing for Corn, Alfalfa, and Soybeans for guidelines on taking alfalfa tissue samples and interpreting results).
Nutrient Use/Crop Removal
Each ton of pre-bloom alfalfa hay (15% moisture) contains approximately 60 lbs. of N, 13 lbs. of P2O5, 50-55 lbs. of K2O (58 lbs. on a dry matter basis), 5 lbs. of sulfur (S), and 0.5 lbs. each of zinc (ZN), iron (Fe), manganese (MN) and boron (B). Potassium (K) removal is much greater for forage crops and straw compared to grain crops, as most K remains and is active in plant vegetation, rather than in the seeds.
Table 2. Crop removal of nutrients for harvested portion of the crop
| Crop | Moisture Basis | (lbs/unit) | (lbs/unit) |
|
|---|---|---|---|---|
| Alfalfa | ||||
| Alfalfa | ||||
| Alfalfa/Grass | ||||
| Oats | ||||
| Oat Straw | ||||
| Wheat | ||||
| Wheat Straw | ||||
| Corn | ||||
| Corn Silage | ||||
| Soybeans |
From Nutrient Removal for Field Crops in Ohio and A General Guide for Crop Nutrient and Limestone Recommendations in Iowa
Nutrient Sources
Nutrient sources available for organic crop rotations include legumes and other green manures, animal manures, composted plant materials or animal manures, kelp, and fish by-products. Mined minerals available and approved for use include lime (CaCO3), gypsum (CaSO4), rock phosphate, Chilean nitrate (NaNO3—check NOP guidelines for use restrictions), potassium sulfate (K2SO4), potassium magnesium sulfate (K2SO42MgSO4), and elemental sulfur. These mined materials must be non-synthetic (unprocessed) to be approved for organic use. Before applying any branded mineral products, consult your certifier and the list of OMRI-approved materials.
Organic Farmer Techniques and Practices
Craig Anderson in south central Minnesota implements an organic rotation of Corn, Soybeans, Oats or Wheat (seeded with alfalfa), and Hay for one or two years. They have a small beef herd and finish and direct-market their calves. All of the hay on the farm is used for the cows and finishers. The beef manure generated on the farm does include small grain straw and reed canarygrass hay used as bedding and is applied in the fall on some of his alfalfa acres. Craig uses a Melroe Multi-Weeder after spreading the beef manure to break up any clods or clumps due to the bedding component.
Alfalfa is harvested three times a season and averages 4.5 tons/acre/year. This hay contains about 250 lbs. of K2O (Table 2). The beef manure doesn’t fully supply this amount of K2O, but Craig also feeds the entire rotation and applies chicken layer litter. Before his corn crop, he applies 3 to 3.5 tons/acre and before his small grains he applies 1.5 – 2 tons/acre. These manure additions, growing only one or two years of hay in the rotation, and feeding the hay on the farm keeps his soil test K levels in the Optimum range.
Bryce and Brian Irlbeck in southwest Iowa operate an organic grain and alfalfa operation. They implement a fairly long rotation with two years of corn followed by four years of alfalfa. Occasionally they also grow other crops for niche markets. Their hay is marketed to nearby organic dairies—moving nutrients off the farm. For their corn crops, they apply about three tons/acre/year of either broiler or layer litter. They adjust the amount applied slightly each year depending on the source and nutrient analysis. Their target, following alfalfa, is to deliver 125 lbs. N/acre/year from the chicken litter. The Irlbecks also have access to beef feedlot manure from a neighbor. They apply 10-15 tons/acre/year in the winter to their alfalfa with a target of delivering 300 lbs./acre K2O to those fields (Table 3), though not all becomes available in the first year. Because the beef manure doesn’t contain bedding, and they use a TEBBE spreader that beats the manure into small particles, it spreads evenly, without the clumps that in-bedded manure could be picked up in the next year’s first cutting of hay.
Ross Nelson of Nelson Organic Dairy in southeast Minnesota manages a three- or four-year rotation of Corn, Oat/Pea with an Alfalfa/Grass underseeding, and Hay for one year. Ross seeds 3 lbs. total of Meadow fescue, tall fescue, and timothy with his alfalfa. Hay is harvested usually four times the year after seeding and yields about 6 tons/acre. After his oat-pea forage is harvested, Ross fertilizes the new hay seeding with 250 lbs. of a dry, custom fertilizer mix from Midwest BioAg with an analysis of 0-1-33. Before corn, he applies 6,000 to 8,000 gallons/acre of liquid dairy manure, which supplies approximately 162 to 216 lbs./acre of K2O (Table 3). Occasionally, Ross keeps the hay stand for a second year, but yields tend to drop that year and he thinks that may be due to the slightly low potassium status of the farm. Ross feels that his current system leaves him a bit short of potassium in the rotation. He has applied potassium sulfate (K2SO4) in the past at 250 lbs./acre. With the analysis of 0-0-50-17, that supplied 125 lbs. of K2O and 42 lbs. of sulfur. The product, at $900 to $960/ton, is expensive and Ross doesn’t use it every year. With his current management system, he’s maintaining his soil test K levels on the farm in the Low to Optimum range.
Ross does watch the K levels in his hay, particularly for his dry cows, with an eye on avoiding milk fever at calving. He uses a TMR to dilute and moderate K levels in their feed, a superior strategy compared to avoiding fertilizing the hay stand to achieve low K levels in the forage.
Ross manages another rotation on the farm on about 40 acres: one year of corn followed by four to five years of grazing mixed perennial grasses and legumes. This is the only rotation he’s experienced where both P and K levels actually start to climb. He is able to maintain his pH at 6.8 to 7.2 in both rotations. This American version of the European ley farming system is a great way to maintain fertility and build soil.
Table 3. Estimated nutrient content of liquid and solid manures
| Animal Source | (lbs/1,000 gal) | (lbs/1,000 gal) | (lbs/1,000 gal) | (lbs/ton) | (lbs/ton) | (lbs/ton) |
|---|---|---|---|---|---|---|
| Beef (Feeders) | ||||||
| Beef (Cow) | ||||||
| Dairy (Heifer) | ||||||
| Dairy (Cow) | ||||||
| Swine (Grow-Finisher) | ||||||
| Poultry Layers | ||||||
| Poultry Broilers |
Abbreviated from Table A3 in Manure Management Plan: A Step-by-Step Guide for Minnesota Feedlot Operators
*Values will vary from farm to farm. Use actual farm manure analyses when available.
†Available nutrients will vary based on manure handling techniques and associated potential losses.
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Resources:
- Alfalfa Management Guide, American Society of Agronomy
- Alfalfa Nutrient Management Guide, Utah State University
- Fertilizing Alfalfa in Minnesota, University of Minnesota
- A General Guide for Crop Nutrient and Limestone Recommendations in Iowa, Iowa State University
- Leys in Sustainable Farming Systems, Grassland Science in Europe
- Manure and Compost: Nitrogen Availability in Organic Production, Ohio State University
- Manure Management Plan: A Step-by Step Guide for Minnesota Feedlot Operators, University of Minnesota
- Nutrient Removal for Field Crops in Ohio, Ohio State University
- Oats as a Cover Crop for Alfalfa, Midwest Forage
- Organic Alfalfa Management Guide, Washington State University
- Tissue Testing for Corn, Alfalfa, and Soybeans, Cornell University