By Dr. Harold Willis
It is sometimes difficult to tell whether your crops are at peak health or are “slightly ill.” Severe sickness is usually easy to spot because of visible discoloration or dying of above-ground parts. The following identification chart may be of value in determining severe deficiencies (however, some leaf symptoms are general and can have other causes, such as infectious diseases, air pollution, or anaerobic soil. Also, you may have more than one element being deficient at a time). To use this chart, start with the Roman numerals (I and II), decide which of the two statements your plants fit, then go on to the capital letters (A and B) under that Roman numeral. Decide which letter your plants fit, then go on to the Arabic numerals (1, 2, etc.) under that capital letter.
Less severe deficiencies and toxicities are harder to diagnose. Often the plants look healthy, but growth may be slowed, yield may be reduced, and feed quality will probably be decreased.
Tissue analysis may be useful for spotting slight nutrient deficiencies, although it only tells you what nutrients are present in the plant tops, not the roots, and it doesn’t tell you what might be the actual cause of the deficiency, nor whether the elements are in the biologically useful forms that produce healthy plants. If you want to send a sample to a testing lab, collect the top six inches from several stems at several different locations in the field when the plants are at the pre-bloom stage, dry the sample, and take or send it to the lab in a clean paper sack or other container (not an air-tight plastic sack; plants may mold). Tissue analysis results can be misleading for some elements as far as trying to diagnose soil deficiencies, because sometimes an element will be concentrated in a plant’s tissues even when the soil is deficient. For example, special root fungi called mycorrhizae can supply adequate phosphorus to a plant growing in low-phosphorus soil.
Another tool that is easy and quick to use to check plant health is the refractometer. By measuring the sugar content of your plants at frequent intervals or in different fields, and keeping accurate records, you may be able to spot problems. Remember that the sugar content reflects the plant’s food-making activities of photosynthesis, and that photosynthesis is slower in cool or cloudy weather, and at morning and evening hours, so take your readings at approximately the same conditions and time every day. Also, the sugar content of the above-ground parts of forages fluctuates throughout the year, being high at maximum growth stage and low after cutting and in early spring.
Still another tool to spot problems is frequent soil testing, provided the testing methods measure readily available nutrients, not just the totals locked up in unavailable forms. Soil nutrient levels and pH can change considerably throughout the growing season, and frequent tests (two or three per year) can give you a better idea of what is going on in your soil, as well as help you spot problems.
What to do
Just about all the problems we have covered in this chapter can be prevented or alleviated by having good, healthy, well aerated soil. If the major nutrients (calcium, phosphorus, potassium, nitrogen) are at proper levels, the nitrogen-fixing bacteria and all the other beneficial soil microorganisms are “doing their thing,” and there are no droughts, floods, or summer frosts, your plants should be vigorous and healthy and be able to ward off diseases and pests, and your soil should not be plagued by weeds and nutrient imbalances or toxicities.
In an emergency, trace elements and some major elements can be supplied to plants through the leaves by foliar feeding, spraying a liquid solution of nutrients.
Source: How to Grow Great Alfalfa