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Vegetables may need iron Calcareous soil with high pH susceptible

Nov 4, 2000 12:00 PM, Joe Nunez

Iron deficiency is usually not a problem in vegetables because iron is almost always available in plentiful quantities in most soils. However, vegetables grown on calcareous soils or soils with high pH values in arid regions may exhibit iron deficiency symptoms. These are soil conditions that describe some of the vegetable growing areas in the San Joaquin Valley.

Iron deficiency symptoms first appear on the younger leaves as a yellowing of the leaf blades with the veins and petioles remaining green (interveinal chlorosis). On some vegetable crops such as carrots, small grayish-brown leaf spots may develop on the leaves while the veins and stems remain green. These older fields may have an overall yellow to bronze coloration that gives the field an appearance of early senescence.

Affected plants will remain small and will not respond well to normal fertilizer treatments. Whole leaf or petiole tissue analysis may not reveal low iron levels in the plants even though the iron deficiency symptoms are present.

Because iron is normally found in abundant quantities in most soils, low iron content in the soil is rarely the cause of iron deficiency in most plants. Low iron availability or mobility due to environmental and soil conditions are most often the cause of iron deficiency problems in plants grown in arid regions. Iron salts that have low solubility such as iron oxides, carbonates, phosphates, hydroxides and some forms of insoluble chelates are created in certain soil types that make iron not readily available to plants.

Iron availability is controlled by soil pH. Iron is readily available in acidic soil solutions but is tied up in basic soils. Iron is least soluble at pH of between 7.4 and 8.5. In acidic soils iron is easily dissolved into the soil solution, which the plant can then use, while it is relatively insoluble in basic soil solutions. Availability of iron in the soil can be decreased up to 1,000 fold for each unit increase in pH.

High levels of carbonate, bicarbonate, and phosphates in the soil or irrigation water also may lower the availability of iron because low solubility salts are formed. Besides reducing iron solubility, bicarbonate ions also reduce the mobility of iron in the plant's vascular tissue. Plant tissue analysis may indicate there is sufficient iron in the stems and petioles, but the lack of mobility of iron in the presence of bicarbonate will cause deficiency to occur in the leaves. Even though the plant may have adequate iron in petioles and veins, it is not able to move from the vascular tissue into the leaf blades.

Soils of arid regions are often composed of calcareous soil types that are high in carbonates and bicarbonates. These soils also tend to have high pH values, which in itself limits iron availability. Therefore, iron deficiency problems are most likely to occur in arid growing regions because of the calcareous soils and high pH associated with these soils.

Bicarbonate levels in the soil are increased by high levels of C02 in the soil. Soil C02 is a normal by-product of respiration by soil microbes and plant roots. However, soils that are waterlogged or that are poorly aerated tend to have increased levels of C02. Therefore, calcareous soils with high pH are especially susceptible to iron deficiency when they are waterlogged or poorly aerated.

Most iron deficiency problems are corrected with foliar applications of ferrous sulfate or fast acting soluble iron chelates. Although the response is quick with foliar sprays, several repeat applications may be required during the season because there is not a reservoir of soluble iron available for the plant to pull from as the crop grows. Soil applications of soluble forms of iron last longer but the plant response is slower because it has to be picked up by the plant roots and translocated up to the leaves. Also, soluble iron applied to soil with iron deficiency problems are often quickly oxidized to more insoluble forms, thus making them unavailable.

If irrigation water has high levels of bicarbonates, then it should be treated to lower the water pH to 6. Soils also may be treated with sulfur or gypsum to lower soil pH.

Record veggie use forecast by USDA Vegetable and melon consumption this year is projected to exceed the 1999 record high. On a per capita basis, USDA says the total is expected to remain near last year's level.

Reduced fresh-market use is expected to be offset by increases in canning vegetables and potatoes. Fresh-market use will likely decline about 1 percent as growers and shippers reduced production in response to financial losses caused by low shipping-point prices the previous year. However, canning use is forecast to rise about 3 percent, led by an increase in processed tomato products.

Per capita use of all vegetables and melons totaled 454 pounds in 1999 up 8 pounds from a year earlier. Large supplies and much lower prices led to a 5 percent increase in fresh vegetable use (excluding potatoes).

On the fresh-market side, significant increases in 1999 per capita use were experienced in cauliflower (up 40 percent), head lettuce (15 percent), broccoli (15 percent), cantaloupe (9 percent), and watermelon (8 percent).

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