By Dr. 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.
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?
Alfalfa requires a well-drained soil for maximum production. 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.
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.
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.
Abundant humus in soil provides many benefits:
- It is a storehouse of essential plant nutrients (especially nitrogen, phosphorus, and sulfur) and growth-promoting substances (hormones and vitamins).
- It helps make some nutrients more soluble and available to plants. Nutrients are released slowly throughout the growing season, as the plant needs them.
- 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.
- It protects plants from diseases, pests, toxic chemicals, high salt levels, and drastic changes in pH (acidity/alkalinity).
What to do
“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.
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.4 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.
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.
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 (see Chapter 3). 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.
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.
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.
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.
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.
Source: How to Grow Great Alfalfa