Adequate nitrogen produces a dark green colour in the leaves, caused by the high concentration of chlorophyll.  Nitrogen deficiency results in chlorosis (a yellowing) of the leaves because of the declining chlorophyll.  This yellowing starts first on oldest leaves, then develops on younger ones as the deficiency becomes more severe.  Slow growth and stunted plants are also indicators of nitrogen deficiency.  Small grains and other grass-type plants tiller less when nitrogen is in short supply.

Phosphorus deficiency is more difficult to diagnose than a deficiency of nitrogen or potassium. Crops usually display no obvious symptoms of phosphorus deficiency other than a general stunting of the plant during early growth. By the time a visual deficiency is recognized, it may be too late to correct in annual crops. Some crops, such as corn, tend to show an abnormal discoloration when phosphorus is deficient. The plants are usually dark bluish-green in color with leaves and stem becoming purplish. The degree of purple is influenced by the genetic makeup of the plant, with some hybrids showing much greater discoloration than others. The purplish color is due to accumulation of sugars that favors the synthesis of anthocyanin (a purplish-colored pigment), which occurs in the leaves of the plant.

Phosphorus is highly mobile in plants, and when deficient, it may be translocated from old plant tissue to young, actively growing areas. Consequently, early vegetative responses to phosphorus are often observed. As a plant matures, phosphorus is translocated into the fruiting areas of the plant, where high-energy requirements are needed for the formation of seeds and fruit. Phosphorus deficiencies late in the growing season affect both seed development and normal crop maturity. The percentage of the total amount of each nutrient taken up is higher for phosphorus late in the growing season than for either nitrogen or potassium.

Older leaves are affected before younger ones because of the redistribution of Phosphorus in the plant.  Corn may display a purple or reddish colour on the lower leaves and stems.  This condition is associated with accumulation of sugars in Phosphorus deficient plants, espcially during times of low temperature.

Potassium is a highly mobile element in the plant and is translocated from the older to younger tissue. Consequently, potassium deficiency symptoms usually occur first on the lower leaves of the plant, and progress toward the top as the severity of the deficiency increases. One of the most common signs of potassium deficiency is the yellow scorching, or firing (chlorosis), along the leaf margin. In severe cases, the fired margin of the leaf may fall out. However, with broadleaf crops, such as soybeans and cotton, the entire leaf may shed, resulting in premature defoliation of the crop.

Potassium-deficient crops grow slowly and have poorly developed root systems. Stalks are weak, and lodging of cereal crops such as corn and small grain is common. Legumes are not strong competitors for soil potassium and are often crowded out by grasses in a grass-legume pasture. When potassium is not sufficient, winter killing of perennial crops such as alfalfa and grasses can occur.

If Magnesium deficiences are detected in growing crops through plant tissue analyses, a soluble magnesium source may be applied and watered into the soil by irrigation or rainfall.  This will permit root access and plant uptake.  Small amount of Magnesium can also be applied to growing crops through foliar fertilization to correct or prevent developing deficiencies.  The preferred approach is to soil-apply the required amount of Magnesium before crops are planted or before they begin active growth.

In the field, sulphur deficiency and nitrogen deficiency are often easily confused. Symptoms of both deficiencies may appear as stunted plants, with a general yellowing of leaves. Sulphur is immobile within the plant and does not readily move from old to new growth. With sulphur deficiency, yellowing symptoms often first appear in younger leaves, whereas with nitrogen deficiency, the yellowing appears on the older leaves first. In less severe situations, visual symptoms may not be noticeable.

The best way to diagnose a deficiency is with a plant tissue analysis that includes an assay for both sulphur and nitrogen. Sulphur concentrations in most plants should range from about 0.2 to 0.5 percent. Desirable total nitrogen to total sulphur ratios have been considered, and range from 7:1 to 15:1. Wider ratios may point to possible sulphur deficiency, but should be considered along with actual N and S concentrations in making diagnostic interpretations.

When sulphur is deficient, nitrate-nitrogen may accumulate. This can pose significant health threats to grazing ruminants or those consuming hay high in nitrates. When nitrates accumulate in the plant, seed formation can be inhibited in some crops such as canola. Balancing sulphur with nitrogen nutrition is important to both plant and animal health.

Crops such as hybrid bermudagrass, alfalfa and corn that have a high dry- matter production generally require the greatest amount of sulphur. Also, potatoes and many other vegetables require large amounts of Sulphur, and have produced best when Sulphur is included in the fertility program. Without adequate Sulphur fertilization, crops that receive high rates of nitrogen may develop sulphur deficiencies.

Calcium deficiencies are most likely to occur in acid, sandy soils from which calcium has been leached by rain or irrigation water. It may also occur in strongly acid peat and muck soils where total calcium is low.

Calcium deficiency is not likely for most crops when the soil is properly limed to adjust soil pH to optimum levels for crop production. As soils become more acidic, crop growth is often restricted by toxic soil concentrations of aluminum and/or manganese — not a calcium shortage. Soil testing and a good liming program are the best management practices (BMPs) to prevent these problems.

Calcium deficiency can be prevented by following several BMPs such as soil testing on a regular basis and correcting soil acidity with proper liming. Balance the plant nutrition program by keeping calcium, potassium and magnesium available in a balanced supply. An overabundance of one can lead to a shortage or uptake (antagonism) of another. Also apply calcium for specific plant functions. For example, calcium applied when peanuts begin to set pods can help improve seed development.