B Excesses of plant nutrients total dissolved solids and trace elements

The tolerance of plants to high levels of elements in plant tissue must also be accounted for in waste application to cropland. Heavy applications of waste can cause elevated levels of nitrates in plant tissue that can lead to nitrate poisoning of livestock consuming that foliage.

The ability to accumulate nitrates differs from plant to plant or even within cultivars of a species. Concentrations of nitrate nitrogen in plant dry matter less than 0.1 percent is considered safe to feed livestock. Large applications of waste material on tall fescue, orchard-grass, and sudangrass can cause nitrate buildup. Cattle grazing these plants can, thus, be poisoned. When the concentration of nitrate nitrogen in the dry harvested material exceeds 0.4 percent, the forage is toxic.

Animal manure releases ammonia gas upon drying. Urea contained in manure is unstable. As manure dries, the urea breaks down into ammonium. The release of gaseous NH3 from manure can result in ammonia toxicity. Exposure of corn seeds to ammonia during the initial stages of germination can cause significant injury to the development of seedlings. High levels of NH3 and NH4 in the soil interferes with the uptake of the calcium ion causing plants to exhibit calcium deficiency (Hensler et al. 1970; Olsen et al. 1970). Part of the ammonium released is adsorbed on the cation exchange sites of the soil, releasing calcium, potassium, and magnesium ions into solution. High levels of these ions in the soil solution contributes to an increase in the soluble salt level as well as pH.

Proper handling of manure is necessary to prevent toxicity from occurring. Manure may contain high levels of ammonium nitrogen; up to 50 percent is in the NH4 form. To prevent toxicity from occurring on young plant seedlings, the manure should be field spread and either immediately incorporated into the soil to adsorb the NH4 on the cation exchange sites of the soil or allowed to air dry on the soil surface. Surface drying greatly reduces the level of ammonia by volatilization. Direct planting into the soil surface that is covered with manure, such as with no-till planting, can lead to germination problems and seedling injury unless rainfall or surface drying has lessened the amount of ammonia in the manure.

Applying manure at rates based on nitrogen requirements of the crop helps to avoid excess NH4 buildup in the seed zone. A 0.25-inch rain or irrigation application generally is sufficient to dissipate the high concentrations of NH4 in the seed zone.

Sidedressing of manure on corn, either by injection or surface application, has been shown to be an effective way to apply the inorganic portion (NO3 and NH4) of nitrogen that is quickly made available for plant growth (Klausner and Guest 1981). Injecting manure into soil conserves more of the ammonium nitrogen during periods of warm, dry weather and prevents ammonia toxicity to the growth of plants (Sutton et al. 1982).

The soluble salt content of manure and sludge is high and must be considered when these wastes are applied to cropland. The percent salt in waste may be estimated by multiplying the combined percentages of potassium, calcium, sodium, and magnesium as determined by laboratory analysis by a factor of two (USEPA 1979).

Under conditions where only limited rainfall and irrigation are applied, salts are not adequately leached out of the root zone and can build up high enough quantities to cause plant injury. Plants that are salt sensitive or only moderately tolerant show progressive decline in growth and yields as levels of salinity increase (figs. 6-2, 6-3, 6-4).

Some plant species are tolerant to salinity yet sensitive during germination. If manure or sludge is applied to land in areas that receive moderate rainfall or irrigation water during the growing season, soluble salts in the waste will be dispersed through the profile or leached below the root zone. If manure or sludge are applied under a moisture deficit condition, salt concentrations can build up.

Figure 6-2 Effect of soil salinity on growth of field crops

Salt Tolerance of Field Crops*

ECe in millimhos per CM at 25 °C

Barley

Sugarbeets

Cotton

Safflower

Wheat

Sorghum

Soybean

Sesbania

Rice

Corn

Broadbean

Flax

Beans

Yield reduction

The indicated salt tolerances apply to the period of rapid plant growth and maturation, from the late seeding stage onward. Crops in each category are ranked in order of decreasing salt tolerance. Width of the bar next to each crop indicates the effect of increasing salinity on yield. Crosslines are placed at 10-, 25-, and 50-percent yield reductions.

Figure 6-3 Effect of soil salinity on growth of forage crops

Bermuda grass Tall wheatgrass Crested wheatgrass Tall fescue Barley hay

Perennial rye Hardinggrass Birdsfoot trefoil Beardless wildrye Alfalfa

Orchardgrass Meadow foxtail Clovers, alsike & red

Salt Tolerance of Forage Crops*

ECe in millimhos per CM at 25 °C

r~TT

100%

Yield reduction

The indicated salt tolerances apply to the period of rapid plant growth and maturation, from the late seeding stage onward. Crops in each category are ranked in order of decreasing salt tolerance. Width of the bar next to each crop indicates the effect of increasing salinity on yield. Crosslines are placed at 10-, 25-, and 50-percent yield reductions.

Figure 6-4 Effect of soil salinity on growth of vegetable crops

Beets

Spinach £

Tomato

Broccoli

Cabbage £

Potato Sweet Corn Sweetpotato

Lettuce Q

Bell pepper

Onion

Carrot

Green Beans £

ECe in millimhos per CM at 25 °C

100%

100%

Yield reduction

The indicated salt tolerances apply to the period of rapid plant growth and maturation, from the late seeding stage onward. Crops in each category are ranked in order of decreasing salt tolerance. Width of the bar next to each crop indicates the effect of increasing salinity on yield. Crosslines are placed at 10-, 25-, and 50-percent yield reductions.

A soil test, the electrical conductivity of saturated paste extract, is used to measure the total salt concentration in the soil. After prolonged application of manure, the soil electrical conductivity should be tested. Conductivity values of 2 mmhos/cm or less are considered low in salts and suitable for all crops. Above values of 4 mmhos/cm, plant growth is affected except for all but the most tolerant crops (figs. 6-2, 6-3, 6-4). At these high conductivity values, irrigation amounts need to be increased to leach salts. Added water percolating through the profile may then cause concern with leaching of nitrates. Manure application rates may have to be adjusted (Stewart 1974).

Trace element toxicity is of concern with waste application on agricultural land. Animal manure can have elevated amounts of aluminum, copper, and zinc. Sewage sludge can have elevated concentrations of several elements, most notably aluminum, cadmium, chromium, copper, iron, mercury, nickel, lead, and zinc. The element and concentration in the sludge depends on the predominant industry in the service area. If wastes that have elevated levels of trace elements are applied over a long period of time at significant rates, trace element toxicity can occur on plants. Micronutrient and trace element toxicity to animals and humans can also occur where cadmium, copper, molybdenum, and selenium levels in plant tissue become elevated.

Table 6-3 lists some general crop growth symptoms and crops most sensitive to the given trace elements. If such symptoms should occur, a plant tissue test should be done to confirm which element is at fault. Many of the symptomatic signs are similar for two or more elements, making it extremely difficult to know with certainty which element is in excess from observation of outward symptoms. Much of the toxicity of such trace elements can be because of their antagonistic action against nutrient uptake and use by plants. Table 6-4 shows the interaction among elements within plants and adjacent to the plant roots.

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