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Phosphorus and potassium fertilization of alfalfa

Jeffrey Volenec Published on 28 February 2011

Fertilizing alfalfa with phosphorus (P) and potassium (K) can increase yield and stand longevity. However, to maximize production and profitability, it is important to adjust fertilizer rates to meet the specific nutritional needs of plants in each field.

This article provides information on P and K fertilization rates in alfalfa. First, we will review important P and K deficiency symptoms and current soil test recommendations.

Next, we will discuss Purdue University fertilizer rate studies.

P and K in alfalfa

Finally, we will address the importance of balanced fertility, the timing of fertilizer application and the influence of P and K fertility on alfalfa forage quality.

Essential for plant processes
P and K are macronutrients, which means plants require them in relatively high amounts compared to many other nutrients. Both nutrients are essential for plant growth.

Plants require K for a number of important physiological processes including the activation of several enzymes, the synthesis and degradation of carbohydrates, the synthesis of protein and the opening and closing of stomata – the pores in leaf surfaces involved in gas exchange and photosynthesis.

Plants require P for many cellular components including nucleic acids (DNA, RNA), phospholipids, ATP and other high-energy compounds. All of these compounds are necessary for photosynthesis, energy transfer, carbohydrate and protein synthesis and lipid metabolism.

Deficiency symptoms
K deficiency in alfalfa can appear as chlorotic (yellow) spots along leaf margins. These symptoms are especially evident on older leaves because K is mobile in plants, and when K availability is limited, alfalfa plants transport K from old leaves to young leaves.

These symptoms are very distinct and easy to recognize.

P deficiency appears as reduced growth, and leaves appear dark green or purple. P deficiency symptoms in alfalfa are more subtle than K deficiencies and may be difficult to recognize.

Improving stand longevity
Alfalfa stands normally provide the highest yield in their first two or three production years, and then start to decline thereafter. If a stand’s productivity decreases enough, re-establishing the stand will be necessary.

However, re-establishing a stand is costly, so it may be more profitable to improve management practices that will keep stands high-yielding for more years. Adding sufficient P and K will help alfalfa stands persist better and produce high yields longer than stands not receiving P and K.

CEC and yield affect fertilizer rates
When P soil test levels are 45 ppm or less, the current recommendations call for applying P. For very low-P soil test results (15 ppm), recommendations call for 115 to 165 pounds per acre of phosphate (P2O5), depending on yield expectations.

Recommendations for K vary according to both the soil’s cation exchange capacity (CEC) and yield expectations. At the same soil test level, a soil with a high CEC will need more K fertilizer than a soil with a low CEC. However, regardless of CEC, never apply more than 300 pounds of K20 per acre.

Current recommendations may be high
Purdue researchers have shown that the current P and K fertilizer recommendations certainly are adequate, but also may be higher than necessary for high yield and stand persistence.

With increasing fertilizer costs, a conservative approach to identifying fertilizer application rates may be more profitable than current recommendations, which tend to be more aggressive.

High rates do not always equal high yield
The amount of fertilizer an alfalfa stand needs to provide high yield and good persistence depends on the soil’s current nutrient status and yield expectations; the lower the initial soil test levels and the higher the yield expectations, the more fertilizer is needed. However, overapplying fertilizer may not always result in higher yield.

A seven-year Purdue study started out with low P (8 parts per million or ppm) and medium K (70 ppm) soil test levels. The average across all years of the study shows that the highest yields were routinely obtained with applications of:

• 50 pounds of P2O5 per acre per year and 300 pounds of K2O per acre per year; or,

• 100 pounds of P2O5 per acre per year and 200 pounds of K2O per acre per year

Higher fertilizer rates did not significantly increase yields.

Nutrient balance is critical
It is essential to consider an alfalfa plant’s P and K needs before fertilizing. Testing soil P and K and then fertilizing accordingly will ensure balanced soil fertility.

In the Purdue study, researchers found that alfalfa stands receiving P fertilizer but not K fertilizer yielded less than stands that were not fertilized at all. In fact, some of the plots receiving imbalanced fertilizer rates experienced complete stand loss, while unfertilized plots and those provided low rates of both P and K persisted, but were low-yielding.

Purdue researchers showed that an alfalfa stand fertilized with 50 to 150 pounds of P2O5 per acre per year and 200 to 400 pounds of K2O per acre per year had higher yields than unfertilized stands. Providing adequate P and K fertilizer slowed yield reduction over time, resulting in progressively greater yield advantages due to P and K fertility as stands became older.

Apply after first and last harvests
At the test site, the soils released considerable amounts of K over the winter. Plants generally use these released nutrients during their initial growth in spring, so applying additional P and K after the first harvest is recommended.

After the last harvest of the growing season, make a second application to increase the availability of K in fall, which may improve the alfalfa stand’s winter hardiness and survival.

If the soil receives more fertilizer than what the plants can remove, then there is an increased risk that nutrients will move to surface waters. Soil P levels can especially increase if more P is added than what plants need.

Soil K levels are more difficult to increase because alfalfa plants can take up excess K from the soil – this is sometimes called “luxury consumption”. Luxury consumption often results in elevated K concentrations in plant tissues, increased removal of K from the field, and reduced economic returns.

While fertilizer placement can be important in other crops, applying fertilizer with deep placement or tillage is not possible in established alfalfa stands due to the risk of damaging the plants’ roots and crowns.

Broadcast applications of P and K keep most of the nutrients in the top few inches of the soil because these nutrients do not move vertically in most soils.

Purdue research indicates that this is not a concern because most of the fine roots active in nutrient uptake are in the top two inches of the soil. This root density pattern was similar in all fertility treatments.

Applications may reduce quality
Adding fertilizer to alfalfa may increase yields, but the plants’ morphology and physiology may be altered in ways that impact forage quality.

P and K applications can cause shoots to grow taller and thicker, which reduces the amount of leaves relative to the amount of stem tissue (the leaf-to-stem ratio). Low leaf-to-stem ratios often mean reduced forage quality.

Digestibility decreases as yields increase due to P and K application. To compare the benefits and constraints caused by P and K fertilization, researchers calculated the digestible nutrient yield (DNY, in pounds per acre) by multiplying the percent digestibility of the forage by the forage yield (in pounds per acre):

DNY = (Percent Digestibility ÷ 100) x Forage Yield

Higher yields easily compensated for slightly reduced digestibility in the high-fertility plots.

Fertilizer applications may lower protein
Animals differ in their protein requirements, so it may be worthwhile to regulate the amount of protein in the forage (depending on how it is used).

In fields with low P and K fertility, protein concentrations were slightly higher than in fields with high P and K fertility.

Despite these differences, Purdue research shows that high-fertility, high-yield stands provide sufficient protein concentration to satisfy the requirements of dairy cows in lactation and only slightly less than the requirements for dairy cows in early lactation.

Small drops in protein concentrations resulting from P and K fertilization were more than offset by the large difference in forage yields. Other factors, such as cutting management, will influence forage quality more than fertility.

Excessive P and K applications can reduce economic returns because of wasted fertilizer as well as lower forage value.

If the forage K concentration exceeds 3 percent of its dry weight, animals may be stricken with milk fever, which is caused by hypocalcemia (low blood calcium). This potentially fatal condition is most prevalent in cows shortly after calving.

Monitoring soil test levels and applying fertilizers as split applications can reduce the risk for excess K in the forage and milk fever.

Forage can be sampled and tested for P and K concentrations, which is a good idea if low or high P and K levels are suspected. Table 1 provides general guidelines regarding deficient, sufficient and excessive tissue P and K concentrations.

Key points to remember
Before starting any fertilization program, remember the following:

• P and K positively influence alfalfa yield and stand persistence.

• Alfalfa requires balanced nutrition for high yield and persistence. Split applications of P and (especially) K after the first and last harvests enhance productivity and avoid luxury consumption of K.

• Broadcast applications of P and K fertilizer work well, since fine roots are abundant near the soil surface.

• Fertilize for high yield; do not worry about forage quality. Higher yields will compensate for slight reductions in forage quality.

• Do not overapply K. Plants can engage in luxury consumption, leading to increased tissue K concentrations and the risk of milk fever.  FG

References omitted due to space but are available upon request.

—Excerpts from Purdue University Extension newsletter, March 2009