Lately, I have been corresponding with growers and consultants about slow spring growth in their alfalfa fields. There are several reasons why growth may resume slowly this spring. I describe them below and discuss some way we may be able to manage for them.
Wet winter and spring. The rain keeps falling! According to the California Irrigation Management Information System (CIMIS), Manteca has received approximately 11 inches of rain this year (as of April 2nd), and Staten Island has received approximately 19 inches, just to name a couple sites in the region. With that rain has come associated cool, possibly anoxic (lack of oxygen) soil conditions. While the amount of standing water has probably been less than what we saw in the winter of 2016-17, it still might be wise to keep an eye on fields in the coming growing season. In this article, Dan Putnam (UC Alfalfa and Forage Specialist) describes things to look for in alfalfa fields where there has been standing water over the winter. Of note, one of the issues that can result from saturated conditions is damage to root nodules where nitrogen fixation occurs (Fig. 1). This can cause lower nitrogen availability for the crop and reduced growth. While we generally never recommend applying nitrogen to alfalfa fields, there are a few rare situations where it may be beneficial, as noted in this article by Farm Advisor Rachael Long. A field with poor nodulation may be one situation where starter N (11-52-0) can help to regrow roots and reestablish nodulation.
Figure 1. Root nodules containing nitrogen fixing bacteria.
Roland Meyer, UC Soils Specialist Emeritus, provided this information regarding water-run inoculum: it does not work well because the inoculum is not water soluble and floats to the surface. Rather, better success with re-inoculating fields comes with applying additional alfalfa seed coated with inoculant. The field needs to be irrigated soon after broadcasting the seed to get the inoculant into the soil.
Dormancy. I think we have a tendency to look over the figurative fence at neighboring fields and make comparisons. Keep in mind that the dormancy rating of a variety will have an influence on whether the field “wakes up” early in the season or tends to start growing a little bit later.
Stem Nematode. There are areas of San Joaquin County where stem nematode is a perennial problem, particularly in the northwest part of the county. Stem nematode reduces alfalfa growth in the spring (Fig. 2), and unfortunately, there isn't much that can be done to manage it, aside from variety selection. In areas prone to stem nematode, it is essential to pick varieties that are highly-resistant (HR). The National Alfalfa and Forage Association produces this leaflet with the resistance ratings of alfalfa varieties. I have also heard from the seed industry that selecting a lower dormancy rating (4 or 5) might help in the battle against stem nematode because alfalfa plants will delay “waking up” when the stem nematodes are most active. Remember, once conditions warm up, stem nematodes recede deeper into the soil and have less impact on alfalfa growth. So, in addition to HR varieties, consider lower dormancy varieties where stem nematodes are present.
Figure 2. Shortened internodes and swollen buds of alfalfa with stem nematode.
Nutrients. Nutrient management involves complex decision making and an understanding of agronomy, soils, and economics! When commodity prices are low, it can be hard to justify input expenses, but keep in mind that alfalfa is a perennial crop with perennial nutrient needs for maintaining yield and quality. Fall is the best season for addressing alfalfa fertilizer needs, particularly phosphorus (P) and potassium (K). There are soils in this region, especially in the Delta, that are low in K. We suggest soil sampling in the fall to gain an understanding for nutrient availability and then, as needed, applying fertilizer between October and February because it could take 60-90 days for the crop to fully respond to fertilizer application.
For the rest of this discussion, let's focus on K. While we generally recommend applying fertilizer in the fall, K fertilizer can be applied anytime, so it's not too late! (Keep in mind that in-season applications may not produce immediate results.) Alfalfa K deficiency is distinct (Fig. 3), but some varieties may not show these symptoms as readily. Either soil or plant tissue tests can help determine whether to apply K fertilizer. If a soil test (ammonium acetate extract) has a K value less than 40 ppm, then the soil is deficient, and a fertilizer application would likely improve plant growth. A value between 40-80 ppm is considered marginal, and applying fertilizer may improve yield. A value between 80-125 ppm is considered adequate. Muriate of potash (0-0-52) applied to the soil surface will be the most economical choice, but potassium sulfate (0-0-52, 18% sulfur) is another option if sulfur is also deficient. For soil K in the deficient range, correct it by applying 300-400 lbs K2O/acre (if yield was around 8 tons/acre) or 400-600 lbs K2O/acre (if yield was around 12 tons/acre). For soils that test in the marginal range, apply a rate of 150-200 K2O/acre (if yield was around 8 tons/acre) and at a rate of 200-300 lbs K2O/acre (if yield was around 12 tons/acre). Single applications of K should not exceed 200-300 lbs K2O. If tests indicate that higher rates are needed, then split the application.
Figure. 3. Potassium deficiency in alfalfa.
A couple other considerations for K nutrient management:
1) In new stands where the taproots may not yet be deep, soil sample in the top 12 inches to determine K availability. I have heard that some folks may be sampling down to 24 inches in alfalfa fields because they know alfalfa grows long taproots. While a mature stand will have developed taproots and may be able to scavenge for nutrients that deep, a younger stand probably cannot, and sampling too deep may give a false impression for nutrient availability.
2) Even when the soil test indicates adequate K, some K fertilizer may be needed in high-yielding crops. Alfalfa has a long growing season, and therefore, a long season of nutrient demand. Each cutting removes large amounts of nutrients with the plant tissue.
Use these rates to guide your K fertilizer applications – remembering that soil type, climate, and yield will influence fertilizer needs – and keep good records of all laboratory results, fertilizer applications, and crop observations. These records will be helpful in developing a long-term, economical fertilization program that maintains alfalfa yield and quality year after year.
Sending everyone best wishes for the season, and don't hesitate to reach out if you have questions or comments.