Managing Saline Soils in North Dakota (continued)

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Saline Soil Management (continued)

Lower the water table and lower salinity risks

The key to managing saline soils is to control the flow of saline water into the crop root zone. When the source of saline water is a shallow water table, the management tool is to lower the water table. Since drainage is not a common option in North Dakota, the solution is to continuously crop, using late-maturing, deep-rooted crops in the rotation.

A crucial element in successful salt reduction in a continuously cropped system is to eliminate bare or black summer fallow. Water use efficiency of fallow ranged from only 0 to 18 percent of rainfall during a five-year study. The researchers found that some water evaporated, but some contributed to ground-water below four feet in depth. If the soil profile is dry enough, however, the loss to groundwater is minimal and certain soils would retain more infiltrated water in the upper four feet in the spring.

The study found that fallowing in a loam-textured soil when soil moisture before planting was less than four inches in the top four feet did not contribute excess water to groundwater. Soil moisture levels of four inches of available water in the upper four feet in a loam soil is about 50 percent of field capacity. Extending this principle to a sandy loam would not be appropriate, since the maximum water holding capacity of coarser soils are often not much more than four inches, so significant rainfall is rapidly moved to deeper depths.

It would be rare to have soil moisture levels low enough in the spring that fallow would not result in seasonal losses of added precipitation to groundwater. When spring moisture levels are sufficient for crop production, the chances of salts reaching the rooting zone are very high and fallow should not be used.

A late-maturing, deep-rooted crop with salt tolerance would be a good choice to help lower the water table. Deep-rooted, salt-tolerant crops can utilize saline groundwater. Figure 7 shows that crops can root deep to utilize saline groundwater, depending on the soil texture.

Figure 7. Evapotranspiration supplied by a saline water table as affected by water table depth. (Gismer and Gates, 1988).

Several studies have shown that alfalfa is an excellent choice to help lower the water table. Alfalfa should be used as a part of a rotation or as a permanent water barrier when it is necessary to control the flow of salt water from one soil to another. Along ditches, potholes and intermittent streams, a 30-foot strip of alfalfa will use enough water that salts are kept from approaching the surface (Figure 2b). In situations where the water table is too high, alfalfa will lower it better than any other crop. In recharge areas, alfalfa can use a large amount of water before it has a chance to discharge farther down slope.

Other possible rotational crops are sunflower and safflower. However, they are not as good as alfalfa in using water because of their annual growth habit. Sweet clover would be an excellent green manure crop which would help on fallow by lowering the water table and supplying nitrogen for the next crop. Water use by sweet clover is often great enough to reduce yields the following season. Proper management will reduce this risk. If green manures are used, shallow tillage instead of plowing is recommended, so that salts are not returned to the surface.

There may be years when, despite the best water table management, excessive rainfall could raise the water table close to the surface. However, the chances of this event would be greatly reduced if the water table was lower initially. Lowering the water table should be viewed as a long-term management tool, and not a quick nor permanent renovation technique.

Late-maturing crops with deep rooting properties are important for saline soil management for the following reasons:

1. Late-maturing crops provide a mulching soil cover until frost, reducing the potential for late summer and early fall surface evaporation.
2. Deep-rooted crops leave the soil drier at deeper depths going into the winter, increasing the potential for salts to leach away from the soil surface.
3. Deep-rooted crops can use more water at the capillary water boundary, preventing further upward movement.

In a recharge area, which is the source of the water that carries salts to a discharge site, a perennial, deep-rooted crop is best at limiting discharge. The next choice is a deep-rooted, long season annual. The third choice is any annual crop.

The following crops are ranked by their potential contribution to limiting salt water discharge from a recharge area: alfalfa>sweet clover>sunflower, safflower, sugarbeet>barley, wheat, soybean, durum wheat and canola.

A crop rotation could be designed so that a combination of perennial and annual crops could be used to diversify the system to meet goals of improved soil quality and profitability. The most important point, no matter what cropping system is used, is to continuously crop the recharge area with something green for as long a period as possible.

In the discharge area, a salt-tolerant crop will be the only crop which can be grown. A list of crops and general crop tolerances are given in Tables 3-5. These lists are very general. There may be situations when the most salt-tolerant crops do not perform well in these areas. There may be other situations in which sensitive crops do quite well. There will also be differences between varieties of the same crop. Information concerning the salt tolerance of specific varieties should be obtained from a commercial seed source before making a selection. It will also be important to note Table 1, which shows that there are differences in the ability of crops to tolerate salt at germination and later on. Sugarbeet, once established, is one of the most salt-tolerant crops available, but it is very sensitive to salt levels at germination.

Managing Sodic Soils

Many saline soils in North Dakota also have elevated levels of sodium. High levels of sodium restrict water-holding capacity in two ways. First, sodium prevents soil clay particles from gathering together into small aggregates. This process of gathering together is called flocculation. Flocculation allows water to penetrate between the groups of soil particles and provide moisture at deeper depths. When sodium levels are high enough to prevent flocculation, the individual clay particles overlap each other randomly during wet conditions, preventing water penetration through the high sodium layer.

Secondly, when the soil dries out, areas within high sodium soils form hard massive structures which look like round-topped columns. These columns do not allow roots to penetrate, so the only water and nutrients which are available to plant roots come from the small surface area surrounding these structures. The plants are therefore allowed only a small percentage of the total possible volume of soil in which to grow.

Areas of high sodium in glacial till soils can be suspected when soil pH is greater than 8.4. However, many high sodium soils in southwest North Dakota have pH values less than 6 and may tend to be even more acidic in parts of some fields. Suspicions of sodium affected soils can be confirmed by requesting a sodium soil test, along with calcium and magnesium. The concentrations of all three elements can be used to calculate the SAR (sodium absorption ratio) or the ESP (exchangeable sodium percentage), which indicates the level of sodicity present. Most laboratories equipped to analyze for potassium are also equipped to analyze for sodium.

The spread of high sodium areas can be checked by following the same management plan as for any salt problem. Decreasing the level of sodium may be much more difficult, however. Because of the restriction of water movement within the soil, leaching is more difficult.

Use of gypsum as a sodium remediation amendment

If high levels of gypsum are present in the soils with high sodium, addition of gypsum will not help replace sodium in the soil. In these soils, deep tillage may help to mix the gypsum already present in the soil with the sodium bearing soil horizons. If the soils do not already contain gypsum, addition of gypsum will replace sodium with calcium in the profile. Amounts of gypsum required to amend the upper foot of soil may be four-eight tons/acre. The material needs to be mixed into the layer of soil which needs the amendment. In order for the application to work, sulfate should not be the dominant ion in the soil. If sulfate levels are low, or other anions such as chloride are proportionally high, then gypsum amendments will be able to dissolve and replace sodium ions. For the application to be successful, it is important that good drainage be present. Drainage can be either natural or tile. Water is also needed to flush the sodium out of the soil once the application is made. Without good drainage, any amendment will not work as needed.

In soils with high sulfate levels and relatively low levels of chloride, calcium chloride will perform an even faster remediation than gypsum at about 85 percent of the gypsum rate. Calcium chloride is more soluble than gypsum, therefore it needs less water to become active.

Summary of Saline Soil Management Tools

1. Soil test for salinity levels and the extent of the problem in each field.
2. Select the right crop and variety for the situation.
3. Use shallow tillage.
4. Schedule seeding in saline areas when salt levels are lowest, from snowmelt or spring rains.
5. Do not fallow if available water in the top four feet of soil is sufficient to grow a minimal crop, or if the soil texture is sandy loam or coarser.
6. Use long growing season, deep rooted crops to control the water table depth.

Summary of Sodic Soil Management Tools

1. Soil test of sodium, have the laboratory determine the sodium index (SAR or ESP).
2. Determine if gypsum is present at deeper soil layers, and if so, deep tillage may be helpful.
3. Improve drainage within the site.
4. If sulfates are low, gypsum applications from four-eight tons per acre combined with adequate soil mixing and drainage would improve the soil.
5. If sulfates are high, calcium chloride, at rates about 85 percent of gypsum requirement, combined with adequate soil mixing and drainage would improve the soil.
6. If amendments and drainage are cost-prohibitive, growing more drought tolerant crops, more timely tillage to avoid making clods and attention to inputs would improve profitability. If areas of sodium are extensive, the field may be better off in pasture, with drought tolerant, sodium/salt tolerant grasses.

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