Background: Alfalfa is a plant in the Legume (bean) family which originated in South West Asia, in what is now Iraq. Alfalfa was introduced to Europe in the 5th century. And made its way to south America via the Spanish Colonization. Eventually, Alfalfa was introduced to California by way of Chile in the mid 19th century. Alfalfa is well adapted to limestone soils (alkaline pH). Consequently, it produced well in the western United States and is grown now on over 1 million acres in California alone.
Root-zone: Alfalfa has a deep root system. The effective root-zone of an alfalfa plant is over 1 meter deep. Alfalfa does well in a variety of soils types that range from Silt-loam to very sandy coarse texture. The key to good production is good drainage. Alfalfa does not grow well in areas where water lingers for very long. Alfalfa likes alkaline pH, and moderate soil salinity, but growth can be limited by excessive soil salinity. Stanworth laboratory regularly consults on the feasibility of growing alfalfa on prospective farms. This is accomplished by testing the soil. Because soil drainage and root-zones are important, soils are sampled to a 2 meter depth (see photo). Soils are sampled in multiple areas of the field with a pit dug for every 5 hectares of area. Soils are sampled in the hole down to a 2 meter depth. Each 33 cm depth section of the hole is analyzed separately. This helps us understand if there are any soil layers that may restrict the growth of the roots. Soil analysis includes tests for: soil salinity, moisture (drainage), soil texture, and nutrients. Special attention is made to look for restrictive layers that may limit water movement.
Soil Preparation: Good soil preparation is essential to successful growing of this crop. The seedbed must be prepared well to be loose and not cloddy. Good soil-seed contact is essential to good root establishment. Cloddy soil can cause non-uniform germination and thin plant populations. The alfalfa seed is small and does not have significant resources to grow through difficult environments. So good land preparation is essential. Commonly, growers prepare the soil by deeply cultivating soils prior to planting. This creates an excellent soil environment for the tap-root to penetrate. Tractors pull deep steel shanks up to 1 meter deep though the subsoil to break up any hard soil layers that may restrict roots or water flow into the soil.
Irrigation Preparation: It is important to provide ample water to the field before the seed is planted. After the soil has been prepared to accept the water, it is best to re-fill the profile with water to wet the entire root-zone. If the soil is very dry, this can be a considerable amount of water and may take days of irrigations to fill the soil back up. Amounts of water can be estimated. For example, if a farmer is preparing a loam soil, filling up the top meter takes about 13 cm of water (imaging ponding 13 cm of water on the surface of the field). If the field is under center pivot irrigation, this means irrigating over the period of 10 days! This accomplishes two things: 1. it gives the young plant good moisture to penetrate down into to explore the soil and 2. It germinates competitive weeds so that they can be controlled prior to establishment. After planting, sometimes a light irrigation may be needed to soften the surface crust to allow the young plants to emerge. Alfalfa roots can grow quickly and surprisingly deep. During establishment a young alfalfa plant, less than 10 mm tall will commonly have a root that is 250 mm deep. After establishment, high producing alfalfa uses a considerable amount of water to maintain good yields. It is a good practice to irrigate as close as possible to the cutting as good moisture is essential for regrowth of the plants. A price is paid for water stressed plants waiting for irrigation after the bales are picked up.
Fertilizer: Alfalfa responds well to phosphorus and potassium fertilizer where soil tests reports show need. Since alfalfa is a legume, applications of Nitrogen are not normally needed, however, it is important that the seed be inoculated with Rhizobium bacteria to help the plant fix Nitrogen. The most important time to apply phosphorus for establishment is prior to planting. Phosphorus is important in development of the root system, so the seed-zone must be rich in P as the seed germinates to allow for good uptake. Typical application rates of phosphorus prior to planting range from 50 to 150 kg/ha Phosphate (P2O5). After the field is established, additional amounts of P and K are normally applied to maintain good production. Rates of fertilizer vary upon yield potential, the higher the yields, the more fertilizer that is required. The best way to estimate fertilizer needs is to base them upon soil analysis. After establishment, the best time to apply these nutrients is after a cutting. The reason is that alfalfa roots grow quickly and proliferate the soil after the field has been harvested, so the timing is good for plants to pick up nutrients in the first irrigation after the cutting.