Fertilizing Alfalfa

Alfalfa Establishment

Soil Sampling

It is much more effective to address nutrient deficiencies prior to planting than after a stand is established. For new seedings, the soil should be tested well before field preparation to identify possible nutrient deficiencies and obtain fertility recommendations that will enhance seedling establishment. By doing so, nutrients can be incorporated prior to planting, which is especially important if phosphorus (P) additions are required. 

Proper sampling of a field is vital to obtaining accurate test results. A good sample consists of at least 15 to 20 soil cores taken 8 inches deep from randomly selected areas of the field. Avoid sampling areas that may have misleading pockets of high or low fertility such as near old barns or feedyards, where fertilizers were previously banded, where soil types vary from the rest of the field, or where field history or management have differed. If such areas are large enough, they can be sampled and tested separately. Soil samples should be combined into one uniform, composite sample per field and allowed to air-dry by spreading the soil on clean paper or any other clean surface where it will not be contaminated. Do not oven dry the soil because this will change the test results. Once the sample has air-dried, ship the soil in a clean container to the soil testing laboratory. 

Soil testing can be done at Colorado State University’s Soil, Water, and Plant Testing Laboratory, or by a number of private labs, for a modest fee. For alfalfa establishment, the most relevant tests will include pH, soluble salts, phosphorus, potassium, zinc, iron, and sulfur, all of which are included in a routine test package. It is important to provide the soil testing lab with information on the crop to be grown and realistic yield goals so that accurate recommendations can be made. Irrigation water often contains sulfur and boron, and, therefore, when considering fertilizer applications of these nutrients, it is important to analyze irrigation water, as well. 

Nutrient Additions and Placement 

Nitrogen

Despite using large quantities of nitrogen, N fertilization is usually unnecessary with alfalfa and other legumes. In a process known as nitrogen fixation, symbiotic bacteria attached to the roots of legumes convert nitrogen gas from the atmosphere into nitrogen compounds that are usable by plants. This process can only take place if the nitrogen fixing bacteria are present in the soil; this is why all seed should be inoculated with the species-specific strain of bacteria before planting.  

Because bacteria must be alive for the symbiotic relationship to occur, care should be taken to protect bacterial inoculants from heat or drying. Additionally, fresh inoculants should be used with each seeding. Liquid and peat inoculants should be stored in a cool, dark place (refrigerated if possible) and replaced after 6 months. Most commercial alfalfa seed comes already coated with the appropriate bacterial inoculant and should be stored in a cool, dark place until planted. Even pre-inoculated seed should be re-inoculated with liquid- or peat-based inoculants if not used within 6 months.  

Some preplant N may be needed if a companion crop is used during alfalfa establishment or if residual nitrate levels in the soil are low (Table 1). Any necessary preplant N fertilizers are generally broadcast and incorporated in combination with P fertilizers. Application of N fertilizers in the establishment of alfalfa is not recommended under dryland conditions.   

Table 1. Suggested N rates for new seedings of irrigated alfalfa. 

ppm NO₃–N in soil	Companion crop: With (Fertilizer rate, lb N/A)	Companion crop: Without (Fertilizer rate, lb N/A)
0–3	60	20
4–6	30	10
>6	0	0

Phosphorus

Phosphorus is the nutrient most likely to limit alfalfa yields in Colorado. Phosphorus moves very slowly through the soil profile, so P fertilizers should be banded at planting or broadcast and incorporated into the root zone prior to planting to optimize their availability. Because young plant roots need a ready supply of available phosphorus, banding a starter fertilizer an inch below the seed (even in addition to a broadcast application) can often give seedlings a head-start.  

Soil test recommendations for new seedings usually cover the first three years of production. However, soil pH must be considered when deciding on how much P to apply at time of seeding. Phosphorus is most available between a pH of 6.5 and 7.5. As soil pH becomes either more acidic or more alkaline, some of the applied P will be chemically fixed by the soil which means it is not available for plant growth. High pH soils are most problematic in Colorado. If your soil pH tests close to 8 or higher, you should consider applying smaller amounts of P every year instead of applying a 3-year supply. If your soil pH is not too high, alfalfa may require added P every 2 to 3 years following establishment to account for crop removal. This will be discussed in the “Maintenance” section below. Fertilizer recommendations for dryland alfalfa are about one-third of the amount recommended for irrigated alfalfa. This difference is based on the lower yield capability of dryland alfalfa.

The main P soil tests used in Colorado are the sodium bicarbonate (NaHCO₃) and Mehlich-3 tests. The sodium bicarbonate test is also known as Olsen’s test. Because fertilizer recommendations are based on the amount of extracted P, it is important to know which test has been used. Values for both tests are given in this fact sheet (Table 2). Recommendations given by soil testing labs must correspond with the appropriate test.

Most phosphorus is applied as one of three granular fertilizer sources: triple superphosphate (TSP, 0-45-0), monoammonium phosphate (MAP, 11-52-0), or diammonium phosphate (DAP, 18-46-0). Some high-P starter fertilizers, such as 8-32-16 and 10-34-0, can be used as well. Most phosphorus containing fertilizers also contain nitrogen. While this is undesirable for alfalfa, it is also unavoidable in most instances. Manures and composts can contain appreciable amounts of phosphorus; however, care should be taken in their use (see the subsection titled “Manures and Composts”). 

Table 2. Suggested P rates (lb P₂O₅/A) for broadcast application and incorporation prior to seeding alfalfa. Fertilizer rates can be cut in half if fertilizer is banded. 

ppm P in soil		Relative level	Fertilizer rate (lb P₂O₅/A)
Mehlich-3 test	NaHCO₃ test		Irrigated	Dryland
0–10	0–6	Very low	200	60
11–22	7–14	Low	150	45
23–35	15–22	Medium	50	0
>35	>22	High	0	0

Potassium

Most Colorado soils are relatively high in extractable potassium. Consequently, few crop responses to K fertilizers have been reported. It is important, however, to realize that hay and silage production remove large quantities of potassium due to harvesting of the whole plant. This could result in K deficiencies in soils with a long history of forage production.  

Soil testing is the only way to know for sure whether or not soil K levels are adequate. Suggested K rates for alfalfa under irrigated and dryland conditions are given in Table 3.  

The main K fertilizer is potash (KCl). Broadcasting and incorporating K into the soil prior to planting is the usual method of application. 

Table 3. Suggested K rates for irrigated and dryland alfalfa. 

		Fertilizer rate (lb K₂O/A)
ppm K in soil (NH₄OAc)	Relative level	Irrigated alfalfa	Dryland alfalfa
0–60	Low	200	45
61–120	Medium	100	0
>120	High	0	0

In a recent study near Fruita, Colorado, K fertilizer was applied at different rates for 3 consecutive years to a soil measuring between 80 and 100 ppm of extractable K. Alfalfa was raised over the 3-year study. Only minor and inconsistent yield increases were observed, which were not enough to even cover the cost of the fertilizer. Based on previous research, we would have expected alfalfa yields to increase in response to K fertilizer in this field, due to its low level of extractable K. To optimize K fertilizer recommendations for alfalfa, additional research is needed in low soil K testing fields around the state. Other factors that affect soil K availability to plants include the amount of clay and clay type, subsoil K levels, calcium and magnesium levels, and soil moisture. 

If your alfalfa field tests below 120 ppm extractable K, we recommend following the suggestions in Table 3 above to prevent yield loss. However, if you are interested in doing your own on-farm research, you could apply K to parts of the field and not to others and then measure yield in both areas. Ideally, a series of alternating strips, three with potash and three without potash, would give you a good indication of the effect of potash on alfalfa yield.  

Micronutrients

Iron and zinc availability decreases with an increase in soil pH; therefore, these two nutrients are the most commonly deficient micronutrients in Colorado. Current soil test methods are good at determining whether micronutrient levels are adequate, so always refer to soil test results before applying any micronutrient. Most micronutrient fertilizers, including zinc, are usually broadcast and incorporated preplant in combination with P fertilizers. Iron, however, rapidly precipitates into an unavailable form if applied in most granulated salt forms. Some chelated iron fertilizers are effective, but they are often expensive. As a result, most iron is added as a foliar application, which must be done every few weeks and may not be economical for alfalfa.  

Alfalfa Stand Maintenance 

Soil and Plant Sampling

Soil sampling protocols during the maintenance period are identical to that of the establishment period. It is especially important to take soil cores to the 8 inch depth, as surface soils in undisturbed crop fields can develop differences in pH and nutrient levels, and alfalfa tends to be deep-rooted. Soil test recommendations during the maintenance period mainly aim to replace nutrients removed by the crop (Table 4). For most established alfalfa stands, sampling every three years is sufficient. 

If nutrient related concerns arise that are not explained by soil testing, plant tissue sampling should be considered. Plant tissue tests can identify problems where soil nutrients may be adequate, but where other factors may compromise nutrient uptake. Additionally, plant tissue analysis can also be used to diagnose possible micronutrient deficiencies. 

Table 4. Alfalfa nutrient removal per ton of yield. 

Nutrient	lbs removed / ton alfalfa
Nitrogen	45
P as P₂O₅	10
K as K₂O	45

Nutrient Additions and Placement 

Nitrogen 

Applying N to an established alfalfa stand will increase the competitiveness of grasses that tend to encroach over time as well as reduce the nitrogen fixing capability of the bacteria on the roots. Therefore, this practice is discouraged in alfalfa production. 

Phosphorus

Established stands that will be maintained for at least three more years may need topdressing of phosphate fertilizer. Always base P application rates on current soil test recommendations. Test soils in early fall so P fertilizers can be applied prior to winter if needed. Phosphorus moves slowly in soil under most conditions; therefore, applying P fertilizers during this time allows P to move into soil through snowmelt and winter freezing and thawing.  

Three important factors should be considered in managing P fertility in established stands of alfalfa. First, the probability of forage response to topdressed P is greater on soils testing very low to low in extractable P. Second, the probability of obtaining a yield response to topdressed P declines as stands age. For this reason, maintenance recommendations for P are usually about half of what is recommended for establishing alfalfa. Third, as mentioned in the “Establishment” section, if soil pH tests in the range of 8 or higher, yearly applications at smaller rates should be considered to avoid some of the P becoming chemically fixed due to the high pH. 

Table 5. Suggested P rates (lb P₂O₅/A) for broadcast application to established stands of alfalfa.

extractable P in soil (ppm) 0–8 in.	Relative level	Fertilizer rate1 (lbs P₂O₅/ac)

NaHCO3 test	Mehlich-3 test		Grass only	Grass/legume mixtures
0–12	0–6	Very low	100	45
13-24	7-14	Low	75	30
25-30	15-22	Medium	0	0
>40	>22	High	0	0

Potassium

Potassium levels are easily monitored by a routine soil test. Suggested K rates for alfalfa under irrigated and dryland conditions are given in Table 3.  

Micronutrients

Any suspected micronutrient deficiency should be diagnosed using a combination of soil and plant testing and symptom evaluation. A soil-based micronutrient deficiency is hard to correct in an established alfalfa stand. Granular topdressings of most micronutrients are slow to move into the root zone and so can fail to correct the problem in the short-term.

Foliar fertilizers can rapidly correct micronutrient deficiencies, but they are often expensive and temporary. Iron, a micronutrient of concern in Colorado, rapidly precipitates into unavailable forms if applied in most granulated salt forms. Some chelated iron fertilizers are effective, but they are often expensive. Most iron is added as a foliar application, which may need to be done every few weeks.  

A more effective method of increasing available soil supplies of iron is to add organic material such as manure or compost. This is most efficiently accomplished by adding manures or composts during the non-legume portion of a crop rotation.  

Notes on nutrient sources

Zinc

Zinc sulfate (ZnSO₄, 35% Zn) is the most used zinc-containing fertilizer. A few other non-chelated fertilizer sources, such as zinc oxide and zinc phosphate, exist, and all are satisfactorily soluble in soils. 

Iron

In high pH soils, inorganic, soil-applied iron fertilizers quickly precipitate into unavailable forms. Because of this, iron is either applied in foliar or chelated form. Foliar applications of FeSO₄ are sometimes applied one or more times to remedy Fe deficiencies in high value crops. Chelated forms, which are protected from precipitation, may be applied to foliage or soil, but are often expensive. Manures and composts can be used to provide significant amounts of iron. 

A principle of sustainable landscape is to limit the amount of irrigated turf to areas of high use by the homeowner. Select turf species adapted for your location and use.

Manures and Composts

Manures are generally not recommended for use on alfalfa and other legumes. While manures often contain appreciable amounts of phosphorus, zinc, and iron, they also contain N and are, therefore, of more value when applied to a non-legume crop. In a legume/non-legume rotation, manures applied during the non-legume portion of the rotation can make use of the N contained in manures while still providing valuable nutrients to the legume portion of the rotation. The nitrogen in manures will not harm alfalfa or other legumes; the symbiotic bacteria on the plant roots will simply reduce or stop fixing N. If manure is to be used, it is essential that it be tested prior to determining application rates. Many factors, such as animal species, diet, and bedding, can affect manure nutrient levels and availability. Composts also contain P, Zn, and Fe, and can be topdressed in the fall after the final cutting. Although N is present in composts, its mineralization rate is very slow and is not likely to create problems in an alfalfa stand.