By Dr. Harold Willis & Neal Kinsey
If a plant is not getting the proper amounts of nutrient elements, it may develop certain symptoms, abnormal colors or growth deformities. By the time these symptoms appear, it is often too late to do much to alleviate the problem (unless plants are still small or you can foliar feed them), but you can try to trace the cause and overcome it for the future.
Nutrient deficiency symptoms may not necessarily mean one or more elements are deficient in the soil. The nutrient may be adequate but the plant may not be able to take it up, perhaps because of high or low pH, or too much or too little of some other soil element. Or perhaps stress on the plant from drought, wet soil, cold weather or toxic soil conditions causes roots not to absorb the element. Nutrient deficiency symptoms in soybeans are listed in the accompanying box.
Using commercial fertilizers that are toxic or that cause nutrient imbalance can lead to poor soil structure and weed problems. In addition, highly soluble fertilizers leave excess soluble nutrients to fertilize the weeds. Types of fertilizers used and careful placement can greatly reduce the fertilization of weeds. Anhydrous ammonia and fertilizers that release high amounts of ammonia (solid urea and diammonium phosphate, or DAP) can kill crop seedlings and cause worsening soil conditions (acidity, loss of humus and denser soil). High-salt fertilizers such as muriate of potash (or Kalium potash) can also injure seedlings and cause salt build-up which may favor weeds.
Encourage a high earthworm population. With their tunneling and production of casts, earthworms can eliminate compaction and increase granulation of soil. They need a supply of fresh organic matter (manure, crop residues, etc.) for food and as little disturbance as possible from tillage (at least until their population builds up).
Other beneficial soil life also helps improve soil structure — the microscopic bacteria and fungi that decompose organic matter. They need aerated soil and occasional additions of raw organic matter for food. Usually if earthworms are common, the microscopic organisms will be too. Earthworms often eat weed seeds and either destroy them or lower their germination capability. Some weed seeds may be destroyed by microorganisms. Composting manure or other materials is a good way to kill weed seeds.
MINERAL DEFICIENCY SYMPTOMS IN SOYBEANS
(from Modern Soybean Production, 1983, p. 171-73)
- Nitrogen. Pale green or yellowish leaves. Seldom a problem if root nodule bacteria are present. Can be due to a molybdenum deficiency.
- Phosphorus. Plants stunted; leaves blue-green and sometimes cupped.
- Potassium. Irregular yellow border around leaves.
- Calcium. Few nitrogen-fixing root nodules, causing nitrogen deficiency symptoms.
- Magnesium. Leaves turning yellow or brown between veins; leaf tip curled down.
- Sulfur. Slow growth; leaves becoming yellowish.
- Iron. Slow growth; new leaves yellow or brown between veins.
- Manganese. Leaves light green to white between veins.
- Molybdenum. Reduced growth; leaves with nitrogen deficiency symptoms.
- Zinc. Plants stunted; lower leaves turning yellow to brown to gray and dropping off; young plants with pale green leaves. Few flowers and pods; pods mature slowly.
Sick Root Nodules
Healthy, active nitrogen-fixing root nodules will have a pink or reddish internal color. You should monitor the health of these furnishers of free nitrogen by occasionally digging up a plant and examining its root nodules. They should be abundant and pink when cut open (until the plant begins its period of senescence). If the nodules are few in number or greenish or yellowish inside, something is wrong in the soil. Perhaps there are toxic substances (from pesticides or too much raw organic matter) in the soil which are harming the bacteria. Perhaps the soil is poorly aerated, since nitrogen-fixing bacteria need oxygen and nitrogen from the air. Perhaps there is a molybdenum deficiency or low pH (acid).
Perhaps the soil already has plenty of nitrogen, either from past fertilizers or from heavy manure application (this is not necessarily a problem, however). Sick root nodules are just one more symptom that you may be able to use to trace down and solve a problem.
Source: How to Grow Super Soybeans