Once a field is in production, analysis of plant tissues is an extremely useful tool for determining if additional fertilizer is needed for optimum production. When one considers that an alfalfa field is in production for at least 3 years (hopefully!), a method to accurately determine the nutrient status of the crop during that time is very useful.
Table 1 lists deficiency symptoms in alfalfa for several nutrients. The nutrient most commonly in short supply in Tulare County is phosphorus,and many people automatically apply approximately 100 pounds of P2O5 foreach year of production. It can be added each year or sufficient amountsmay be incorporated before planting for several years. However, it is importantto confirm that what was put on the ground is being picked up by the plant.Tissue tests can do this. Also, for those who don't want to automaticallyfertilize when it may not be needed, tissue tests can indicate whetheror not more phosphorus is needed.
|Phosphorus deficiency||No distinctive symptoms; growth is reduced, stunted with smaller thannormal leaves.|
|Potassium deficiency||White spots around the edges of the upper leaflets; with more severedeficiency there is marginal leaf chlorosis and necrosis.|
|Sulfur deficiency||Stunting, overall yellow appearance.|
|Boron deficiency||Top leaves are yellow and reddish.|
|Zinc deficiency||Rarely seen in the field because of alfalfa's ability to take up adequate amounts and its low requirement.|
Potassium and sulfur are occasionally deficient in our soils. Interestin potassium has increased the past few years because of recent research in cotton. Alfalfa uses a lot of potassium but appears to be better than cotton at taking up this nutrient. A shortage of boron is not usually a problem in this area but in areas with high levels of boron in irrigation water, growth and vigor may be reduced if the boron concentration in alfalfa is greater than 200 parts per million (ppm).
To obtain meaningful results from plant tissue analysis , it is important to sample the crop at the same stage of growth for which guidelines have been established. For alfalfa, the time to sample for plant analysis is prior to harvest when plants show regrowth shoots. This usually coincides with early bloom and harvest. (In fact, harvests ideally should be scheduled based on regrowth.) Sampling at other stages of growth provides values for nutrients, but there is no research to indicate what those values mean in relation to the nutrient status of the plant.
Once plants are in the right stage of growth, select stems from 40-60plants for each sample. If comparing an area of good growth to one of poor growth, collect 40-60 stems from each area. Cut stems off at their base near the soil.
Depending on the nutrient, different plant parts are analyzed. The lower third of the stems is discarded. The middle third of the plant is used for phosphorous (PO4-P), potassium (K), and sulfur(SO4-S). Leaves are analyzed for sulfur. The midstems(without the leaves) are tested for phosphorus and potassium. The top third of the plant is analyzed for boron.
When results are returned from the lab, compare them to the guidelines shown in Table 2. Make sure the lab results are in the same units as the guidelines. For example, phosphorous is reported as parts per million(ppm) phosphate (PO4) and potassium (K) is reported as % total K. Another important ingredient to interpreting lab reports is common sense. If a nutrient is reported as deficient in the plant sample but you think you applied sufficient fertilizer, look for other causes for the deficiency. Nematodes, root pathogens, and root eating insects can interfere with nutrient uptake.
When one nutrient is deficient, growth is affected and the concentration of other nutrients in the plant may not represent what is available in the soil. When mor e than one nutrient is low in an analysis, correct the most severe deficiency first. After it is corrected, take new samples to determine if other nutrients are still deficient.
November 5, 1999