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Alfalfa 101: Feeding Alfalfa Plants, Alfalfa Soil Types and Seeding Alfalfa

By Harold Willis

The place to begin with growing really great alfalfa and other forages is at the beginning—with establishing the stand. If the plants do not get off to a good start, they will likely be sickly, have disease and pest problems, yield poorly, and the stand may die out quickly.


Soil Types for Alfalfa

And the place to begin with establishing the stand is obviously the soil, because soil fertility and soil conditions play the major role in plant growth and crop yield—and most of all, crop quality. When you feed high quality forage to your livestock, they not only will produce more on the same quantity (or less) of feed, but they will also be healthier, and who can’t do with lower vet bills and fewer dead animals?

What kind of soil, fertility, and soil conditions do alfalfa and other forage crops need to establish a good stand?

Organic alfalfa seeds.

Alfalfa Needs

Alfalfa requires a well-drained soil for maximum produc­tion.1 Soils two feet or more in depth are also necessary for best growth, since alfalfa is capable of developing a deep root system if root growth is unrestricted. Soils in which rooting depth is limited by either a shallow hardpan, a high water table (poor drainage), or bedrock are less suitable for alfalfa production.

Hardpan and Alfalfa

Have you ever dug up an alfalfa taproot and been surprised to find it bent at a right angle six or eight inches below the surface? This is dramatic evidence that a hardpan severely restricts root penetration, the use of deep nutrients, and therefore plant growth. The vast majority of farmland today is plagued by hardpans, as evidenced by water accumulating in low spots and even in high spots and on slopes after a rain. A hardpan not only restricts water penetration (and thereby increases water runoff, erosion, and flooding), but it also seals off the lower layer of soil from air. Good soil aeration is vital for healthy soil because roots need oxygen and so do the beneficial soil microorganisms (bacteria, actinomycetes, and fungi), which are tremendously important for maintaining healthy soil and for growing healthy, high quality crops.

Aeration and Alfalfa

In order for the soil to be well aerated (and to overcome a hardpan), the soil must be loose, spongy, and crumbly. In other words, it must have good structure or tilth. Good soil structure can be obtained and maintained for the long run by having an adequate amount of humus and the beneficial soil organisms that produce it. Humus is decomposed organic matter—the plant residues and manures which should be returned to the soil. When organic matter is worked into the upper layers of the soil, a “volunteer army” of bacteria, actinomycetes, fungi, and worms should be there waiting to attack it and convert it into dark, fine-textured, rich-smelling humus.

Humus: Abundant Humus in Soil Provides Many Benefits

  1. It is a storehouse of essential plant nutrients (especially nitrogen, phosphorus, and sulfur) and growth-promoting substances (hormones and vitamins).
  2. It helps make some nutrients more soluble and available to plants. Nutrients are released slowly throughout the growing season, as the plant needs them.
  3. It contributes to good soil structure (tilth) by producing small crumbs (aggregates) of soil particles, allowing good air and water penetration. Water-holding capacity is also increased, and therefore drought resistance. Erosion from both water and wind is reduced. The soil is loose and easy to work.
  4. It protects plants from diseases, pests, toxic chemicals, high salt levels, and drastic changes in pH (acidity/alkalinity).

Soil organic matter is an important soil characteristic that improves tilth, water intake and water-holding capacity. The usual measure of humus on laboratory soil tests is percent organic matter, al­though this does not distinguish between fresh, unrotted organic matter and true humus. By digging in your soil, you can see if last year’s crop residues and manure are rotting quickly to form humus. If they are not, the problem may be due to “dead” soil without an adequate population of the humus-forming microorganisms (possibly because of toxic agricul­tural chemicals) or to tight, poorly aerated soil, causing anaerobic condi­tions (little or no oxygen). Compaction from use of heavy farm machinery is a contributing factor to anaerobic soil. Reducing or eliminating toxic chemicals and increasing humus content will alleviate these problems, but if your soil is so tight and “dead” that organic matter will not decompose quickly to form humus, then you can break out of this vicious circle by use of a soil conditioner to loosen soil and stimulate soil life. Depending on your soil’s needs, some rock fertilizers can help condition soil (calcitic lime and soft rock or colloidal phosphate) or some commercial soil con­ditioners can be beneficial (although some kinds are not so helpful or can even do long-range harm). Inoculating the soil with beneficial bacteria and other organisms may help (if the soil conditions are already fairly good, and not toxic).

Desirable levels of organic matter on soil tests are from 2 – 5%, or even up to 10%, provided the soil is loose and “alive” with organisms.

A stubborn hardpan can be broken up by subsoiling or by plowing a little deeper each year, but a good earthworm population can do a better and quicker job of it.

Nutrients for Alfalfa Crops

The mineral elements that are most essential for good stand establishment are calcium (Ca), phosphorus (P), and potassium (K). Calcium is needed for cell division, cell wall formation, and root growth. Phosphorus is used for energy transfer and other metabolic func­tions in the plant, and also it increases root growth. Adequate phospho­rus is especially critical for stand establishment. Potassium is required to activate many cell enzymes and for food transport in the plant.

Nitrogen application in the nitrate form will help to establish alfalfa if your soil is low in nitrogen. The nitrogen-fixing bacteria which will later develop in legume root nodules require the trace elements molybdenum (Mo), cobalt (Co), iron (Fe), and copper (Cu). In properly fertilized soil with adequate humus and soil organisms, these trace ele­ments should not be deficient, but the soils in some parts of the country are deficient in one or more trace elements, so some may have to be added. Be sure not to supply too much, because trace elements are only required in very small amounts, and some are toxic to plants or animals in too large amounts or in out-of-balance soil. Natural fertilizer sources such as manures and rock fertilizers can often supply trace element needs, and a good microorganism population will make them available to the plant.

Test Alfalfa Fertilizers

But how can you know how much of what kinds of fertiliz­ers to apply if you have no idea of what your soil needs? So before you do anything, you should have your soil tested by a reliable testing lab. Un­fortunately, different soil testing labs differ in their testing methods and  interpretation of results, so you can send the same soil sample to two labs and get two different sets of numbers and fertilizer recommendations. Because of the prevailing beliefs about crop fertilization, most labs tend to recommend relatively too much potassium and too little calcium and phosphorus. The best soil testing methods for determining plant needs are those that test for readily available (soluble) nutrients (see Chapter 3).

Guidelines for Soil and Alfalfa

It is impossible to give definite recommendations in this book without knowing what your soil needs, but the soil should have a high level of available calcium and phosphorus. If your soil needs these elements, good sources are calcite lime plus soft rock phosphate. These plus an application of organic matter (6 to 10 tons/acre of fresh cattle manure, or 1/2 to 1/3 that amount of poultry manure, or 1 to 3 tons/acre of compost) will take care of most nutrient needs of alfalfa and other forages. The organic matter will provide enough potassium as long as calcium and phosphorus are high. Fresh organic mat­ter should not be applied in excess nor be plowed in too deeply (below 5 to 8 inches) because it may not decompose properly, but may putrify and release toxins. It should be worked into the upper several inches (the aerobic zone).

The soft rock phosphate should be applied before or at the same time as the lime, since by itself, the lime tends to leach downward. They should not be plowed under deeply, and can be left on the surface.

Liming Alfalfa Crops

If you live in a part of the country with low magnesium soils, dolomitic lime (calcium-magnesium carbonate) should still not be used; it has the disadvantage of being harder and slower to break down than calcitic lime, plus its high magnesium content can lead to tighter soil and nitrogen depletion if in excess. Calcitic limestone (cal­cite, calcium carbonate) has none of these disadvantages and should be used instead.

The more finely ground the lime is, the more rapidly it becomes available and the less that is needed. Mesh sizes of 90 – 99 or finer give al­most “instant” availability, but they are hard to spread on windy days, and special spreaders may be needed. The old “E-Z Flow” and Gandy spread­ers and the larger Stolzfus and Webster spreaders will handle fine lime.

Alfalfa pH

Standard recommendations state that alfalfa should have a soil pH of 6.5 to 7 or 7.5, which is above the average for most crops (6.2 – 6.8). Actually, not so much attention should be paid to the exact pH figure because (1) the pH of soil changes constantly, even from day to day, and (2) the pH readings produced by a soil testing lab depend on the meth­ods used. For example, if the soil samples are finely ground before test­ing, the pH readings will be somewhat higher than under field conditions because small lumps of lime will be ground up and made more available.

Perhaps one reason a higher pH is recommended for alfalfa is that alfalfa requires high levels of calcium, and large amounts of lime are ap­plied to raise pH, automatically supplying the crop’s need for calcium.

Low pH (below 6.0) can have detrimental effects in reducing or eliminating growth of beneficial soil bacteria, including nitrogen-fixing bacteria, but high quality forage can be grown on acid soil, provided it has balanced and high fertility.

Preparing Alfalfa Seedbeds

The best seedbed for forage establishment is firm and moist. Firmness will prevent loss of essential moisture; however, a crust is very detrimental to seedling emergence. Good tilth and humus content will prevent crusting. Fall plowing and spring disking and harrowing work well in most areas; however, fall plowing is not recommended in areas where erosion could be increased (steep slopes and high rainfall). Since shallow seed placement is necessary for good emergence, the use of a corrugated roller or packer will provide firmness.

Which Seeding Method for Alfalfa?

Whether you want to use broadcast, drill, or band seeding methods may depend mainly on your situation and available equipment. With good soil conditions, any seeding method can give good results. Under less than ideal conditions (low fertility or dry weather), band seeding (placing a band of seed directly over a band of fertilizer 1-2 inches deep) has been proven superior.

Companion Crops for Alfalfa

In northern and eastern parts of the U.S., most alfalfa is sown with a companion crop (nurse crop) in spring seedings (not in summer or fall seedings). Besides providing an additional crop, companion crops protect the soil from erosion and keep out weeds before the alfalfa is established. However, companion crops can have disadvan­tages: they can compete with or inhibit the alfalfa seedlings by competing for light, moisture, and nutrients. Therefore, less leafy species or smaller seeding rates of companion crops should be used.

Commonly used companion crops are flax, peas, spring wheat, spring barley, and early maturing oats. Winter wheat, winter barley, win­ter rye, and late varieties of oats are poor companion crops for alfalfa.6 Early mowing, grazing, or harvesting of small grain companion crops before the boot stage will help reduce competition with alfalfa.

The percentage of grass in legume-grass mixtures should gener­ally be less than 25 – 40%, up to 50% in pastures, because too much grass will lower the protein content of the hay and may require more nitrogen than the legume can supply. Legume-grass mixtures that do well together include (from Univ. of Wisconsin-Extension Publication A2906,1978, p. 4):

If no companion crop is used (direct seeding, clear seeding), weeds and erosion could be problems on poor soils. On steep slopes, a thin mulch of straw or manure will help reduce erosion. If the available phosphorus level of the soil is about twice as high as potassium, and if the soil is well aerated, weeds are not generally a problem. If you wish to use a herbicide for weed control, consider that most toxic chemicals tend to upset the soil’s beneficial microorganism population, which can lead to humus de­pletion and lowered soil fertility. The use of a surfactant or wetting agent can allow you to greatly reduce the amounts of herbicides used.

Alfalfa Seeds

To get your forage crop off to the best start possible, use high quality (high test weight) seed and a suitable variety which is adapted to your climate. Yield, winter-hardiness, disease and pest resistance, and maturity time are factors to consider in choosing a variety.

Inoculating Alfalfa Seeds

Legume seed should always be inoculated with the proper strain of nitrogen-fixing bacteria to insure development of root nodules. The extra cost is small, while the benefits are great. Pre-inocu­lated seed can be purchased or you can apply the inoculant at seeding time. Inoculant or inoculated seeds should be stored in cool temperatures (below 60°F in a refrigerator is fine) and used as soon as possible (not over six months after purchase).

Generally, seed treatment with fungicides is unnecessary for small-seeded legumes and grasses.

Alfalfa Planting Depth

Optimal seeding depth for legumes and grasses is less than one inch. In fine-textured and moist soils, seeds should be planted closer to the surface, from 1/2 to 1/4 inch. In summer or drier pe­riods or in sandy soils, deeper planting (¾ to 1 inch) is recommended.

Seeding Rate for Alfalfa

There are several factors to consider regarding seed­ing rates:

1. Moisture. If the soil will not have much moisture later in the year (especially sandy soils), lower seeding rates will reduce competition for moisture among the seedlings. Adequate humus will increase available soil moisture.

2. Soil conditions. Low soil fertility or acid soils will require higher seeding rates to insure that enough seedlings survive. Proper fertilization and adequate humus will overcome these problems.

3. Species and variety. Different grasses and legumes and their varieties differ in their germination rate, number of seeds per pound, and growth-form (some spread out in growth more than others). Some useful information is provided in the following table, from Iowa State University:

University of Wisconsin recommendations for alfalfa seeding rates are 10 – 12 pounds of live, pure seed per acre for pure stands, 15 pounds per acre if quackgrass may be a problem, and 16 – 18 pounds per acre if you wish to harvest in the year of seeding.

Use the number of seeds per pound to figure seed mixtures. For example, it would take only about one-fifth the amount of orchardgrass seed to equal bromegrass.

Timing for Alfalfa Planting

The timing of stand establishment must be adjusted to your local climate and possible crop rotation schedule. In the North and Northeast, the best time is spring; otherwise dry summer weather may not allow enough growth to survive the winter (companion crops should not be used for late seedings because they compete with the leg­ume and slow the establishment). In the South, late summer is the best time for seeding.

Source: How to Grow Great Alfalfa