QUESTION #9 

When is the best time to take soil tests for stand establishment? 

EXPLANATION: Theoretically it should be possible to change soil pH with lime applied and incorporated at planting. When finely ground lime is added to dry, powdery soil and shaken in a plastic bag ("shake and bake" method), a one to two unit pH change is noted 24 hours after water is added to activate the lime. In two weeks the soil acidity will be completely neutralized.  

Under field conditions, it appears to take a year or two for lime to work. For this reason it is recommended that alfalfa fields be soil tested well in advance of planting. The reason for advanced testing is to allow sufficient time for applied lime to react with and neutralize acid soil.  

Waiting until the summer before establishment for a soil test will not allow enough time for agricultural lime to neutralize the soil. Effectiveness of liming depends on good incorporation of lime and time for it to react. When lime is applied a year in advance of alfalfa planting, there will usually be several tillage operations to help incorporate lime between application and alfalfa planting.  

This slow neutralization by lime is also the reason soil tests still show the pH to be acidic for a year or so after liming. With time, the pH will change, but it may never reach the targeted pH of 6.8. However, if an adequate amount of finely ground lime is thoroughly incorporated, alfalfa production should not be limited because of soil acidity.  

If fields are soil tested and fertilized regularly, then results of soil test pH, phosphorus, and potassium from the most recent tests can be evaluated in relation to the fertility needs of alfalfa a year or two in advance of planting. This allows selection of fields that do not need lime and that have high levels of phosphorus and potassium.  

If soil pH is known to be 6.2 or higher, then sampling before mid-summer, in the year of planting, is early enough. The only nutrient deficiencies likely to be identified in that soil test are phosphorus and potassium, and these can be added by fertilizing before the last tillage operation prior to planting.

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QUESTION #10 

What benefits are received by doubling the recommended amount of phosphorus fertilizer prior to planting alfalfa? 

EXPLANATION: Results from the first 3 years of a 6-year experiment at Chickasha have shown good alfalfa yield responses, and economic returns, from doubling currently recommended phosphate fertilizer application before stand establishment.  

Where the soil test called for about 100 lb. of P₂O₅/A/year, there was increased alfalfa yield of 1.2 ton/A during the 3-year period by fertilizing at the recommended rate. Application of 200 lb P₂O₅/A preplant and another 200 lb P₂O₅ /A prior to the third year resulted in a 2.2 tons/A increase in forage yield.  

In dollars, the doubled fertilizer rate results in an increased return of $22.55 for phosphate compared to applying 100 lb P₂O₅/A/year, assuming $80/ton for alfalfa, $0.27/lb for P₂O₅ using 18-46-0 as the fertilizer.  

In the same study, incorporating 600 lb P₂O₅/A preplant was even more profitable. Increased returns from adding 600 lb/A before planting were $43.43/A/year compared to 100 lb P₂O₅/A/year.  

These increased returns from fertilizer are profits after the additional fertilizer cost was paid. Furthermore, additional profits are over the normal profits from producing alfalfa with 100 lb of P₂O₅/A/year.

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QUESTION #11 

I really don't see a difference when I fertilize. Am I getting a response? 

EXPLANATION: Research clearly shows that application of phosphorus fertilizer to soil that is deficient pays in additional forage production. It should not be viewed as a cost. We generally show that $1 worth of fertilizer results in $3 worth of increased yield. In addition, stand life is usually extended with proper fertilization.  

Let's consider the experiment described in the previous question. Applying the 100 lb P₂O₅/A/year, recommended from the soil test, resulted in increased forage yield and economic return as discussed above. Without P fertilizer, yield averaged 5.5 tons/A/year while the fertilized alfalfa averaged 6.0 tons/A/year.  

Many producers would be happy with the 5.5 tons yield without fertilizing, and when you consider that the half-ton/year response is spread out over 4 to 6 cuttings during the year, it is easy to understand why we cannot "see" these small responses. However, this small response to 100 lb P₂O₅/A/year resulted in a profit from fertilizing (value of increasing production from fertilizing minus the cost of fertilizer) of $4.20/A/year.  

There is evidence to suggest the real value will come in added productive years of the stand. If stand life can be extended by one year with improved fertility, there would be a much larger profit.  

Without closely monitoring where fertilizer was applied and where it was not, it may be impossible to "see" a response to needed phosphorus fertilizer.

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QUESTION #12 

Does alfalfa need nitrogen for establishment? 

EXPLANATION: A small amount of soil N is necessary for the seedlings to develop leaves and roots while the rhizobium is becoming established to fix N from the air. Nitrogen deficiencies rarely interfere with alfalfa establishment, unlike establishing wheat and forage grasses.  

Many alfalfa fields are bottom land soils that have better than average soil organic matter content. When these soils are cultivated, 10 to 20 lb/A of organic nitrogen are released. Phosphate fertilizer is often needed, and the source is usually 18-46-0. Consequently, normal seed bed preparation and addition of P fertilizer raise the available N level to 20 lb/A or more.  

If there appears to be a need for some preplant N, the best source would probably be 100 lb/A of 18-46-0. The addition of 100 lb/A of 18-46-0 will provide 18 lb/A of N needed for establishment. Application of a straight N fertilizer, such as urea or ammonium nitrate, is usually unnecessary.

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QUESTION #13 

What about potash; does alfalfa need potassium fertilizer?

EXPLANATION: Potassium deficiency in alfalfa is almost as common in Oklahoma as phosphate deficiency and the need for liming. In fact, 52% of the soils tested in the OSU soils lab indicate a need for additional potassium for alfalfa production. High-yielding fields (4 to 6 tons/A/year), especially on sandy soils, may need annual applications of potassium based on soil tests.

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QUESTION #14 

Should nutrients other than P and K be added for high-yielding alfalfa? 

Potassium availability in soils is closely related to annual rainfall. Many soils in western Oklahoma were originally rich in potassium, whereas soils in eastern Oklahoma (high rainfall) are usually somewhat deficient. Current evidence shows that potassium deficiencies for alfalfa are common throughout Oklahoma. The only way to know about the need for potassium fertilizer is to have a soil test. 

EXPLANATION: Calcium, magnesium, and sulfur are used in large amounts by alfalfa. Availability of calcium and magnesium are closely linked to soil pH, as the soil becomes acidic and needs to be limed, these elements approach deficiency levels. Liming acid soils replenishes both available calcium and magnesium to adequate levels. 

Sulfur moves with water in the soil, so when alfalfa extracts water from deep in the soil, sulfur is also extracted. In addition to the abundant reserves of sulfur in most soils, Oklahoma fields receive enough sulfur in rainfall to meet the needs of 2 tons/A hay yield. Sulfur additions in research plots in Oklahoma have not increased alfalfa yields. 

Micronutrient such as boron, zinc, and iron are normally abundant in alfalfa soils in Oklahoma, and little benefit is realized with their additions. Alfalfa producers should pay much more attention to phosphorus and potassium fertilization.

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