Guidelines for Interpreting Water Test Results

What's in the irrigation water can affect your crops. This is especially true in the San Luis Valley of Colorado. With the sandy soils, with low organic matter and cation exchange capacity (CEC) the water used to irrigate plays a big role in the crops nutrition and health.

Water Sampling and Analysis

Run the well without any fertilizer being injected for at least 10 minutes (2 hours would be better) to get the well to draw down the water table to normal conditions. Have a clean labeled container ready - most labs supply these if you ask. Rinse out the container and the cap with the irrigation water and fill the container. Keep the container in a cool place and ship to the lab as soon as possible to avoid any chemical reactions that may change the water's elements. Request a complete analysis that includes: pH; Cations like Calcium, Magnesium, Potassium, Iron, and Sodium; Anions like Nitrate-nitrogen, Sulfate-sulfur, Chloride, Boron and Bicarbonate; EC (salts); and SARa (adjusted sodium absorption ratio).

Interpreting Water Test Results

Use the following table to interpret the water test results. Units need to be the same to compare results with this table. A brief explanation of each test results follows the table. This table is very general and some guidelines change with crop and soil conditions. To change units:

• ppm is 1 part element to 1,000,000 parts water and is essentially the same as milligrams per liter (mg/l)
• 1% is 10,000 ppm
• Pounds per acre-foot water (lb./A-ft) is calculated by multiplying ppm by 2.72
• Milliequivalents per liter (meq/l) is another form to express results which requires one to know the molecular weight of each element to convert to ppm. See the table at the end of this page on conversion factors.

pH

pH is a measure of the soils acidity or alkalinity and is on a log scale (meaning 5.0 is 100 times more acid than 6.0). 7.0 is neutral and below is more acid and above is more alkaline. Crops differ if they want acid or alkaline conditions. Irrigating with acidic or alkaline water will change the pH of the soil very slowly over time. Generally you want a moderate test result around 7.0. Alkaline water can interfere with the activity of some pesticides - especially organophasphate insecticides, Gyphosate herbicide (Round-up) and some pyrethroid insecticides.

EC (salts)

Electrical conductivity is a measure of the soluble salts in the water. Some crops are very sensitive to high salts in the irrigation water and some are very tolerant. Irrigating with salty water, over time, can increase the salt level in your soil. If you have salty water you need to increase your irrigation amounts greatly to try to overcome the competition the salts in the water have with your crops for the soil water since salts in the soil water increase the energy a plant must use to extract water from the soil. For more information on soil salinity click here. A table of relative salt tolerance of different crops follows:

SARa

This ratio expresses the relationship between calcium, magnesium and sodium ions in the water. The adjusted ratio factors the effect of bicarbonate. High SAR value means sodium ions are in a greater percentage than calcium and magnesium. Too much sodium in the soil is very detrimental to the soil. Sodium is a very small ions and if sodium occupies too many sites in the soil particles the soil will collapse on itself and water penetration problems will follow. This permeability hazard of high SAR water can be partially corrected by adding calcium to the soil (or sulfur to free up soil lime if present) and leaching.

Nitrate-Nitrogen

For many cropping situations high water nitrates is "free" nitrogen for your crops. Multiply the ppm by 2.72 to get lbs./Acre-foot nitrate added to the field in the irrigation water. Figure this nitrogen in your fertilizer recommendations except in very shallow rooted crops that have high nitrogen needs like lettuce. Excess nitrogen in the irrigation water may be a problem with crops where you don't want to apply nitrogen at certain times like malt barley late season. High nitrate levels are dangerous to human consumption and livestock. Nitrate-nitrogen levels over 10 ppm are considered dangerous for drinking water. Nitrogen in the water is sometimes expressed as only nitrate levels. To convert nitrate levels to nitrate-nitrogen levels divide the nitrate level by 4.5

Sulfate-Sulfur (SO4-S)

Sulfate-sulfur has no effect on the soil and crops except increasing the overall saltiness of the water. Some well water contains high levels of sulfate-sulfur and can supply all or part of a crops needs. In drinking water sulfate-sulfur levels greater than 75 ppm can have a laxative effect of people not accustomed to it.

Chloride (Cl)

Under sprinkler irrigated fields excess chloride in the water will accumulate on the leaves as the water evaporates and can burn leaves. Very high levels of chloride in the water can also lead to toxic levels for sensitive crops like: bans, alfalfa, rice and sorghum.

Boron (B)

High levels of boron in the irrigation water can lead to toxic effects since plants only b=need very small amounts of boron. Take boron in account in fertilizer recommendations. Boron sensitive crops include: beans, oats, sweet potato, potato, tomato, and peas. Boron tolerant crops include: asparagus, sugarbeet, alfalfa, onion, lettuce, turnips and carrots.

Sodium (Na)

Sodium can have two bad effects on crops and soils irrigated with water high in sodium. Sodium can decrease soil water penetration as discussed above in the SAR discussion. Sodium in the water is more serious if no accompanied by calcium in the water. Generally sodium is ok if the ppm of calcium is twice the ppm of sodium up to a point. Excess sodium can also lead to toxicity problems at high levels in sensitive crops like: Some fruit trees, beans and oats. Sodium tolerant crops include: wheat, cotton, alfalfa, barley, tomatoes and beets.

Calcium (Ca)

Calcium is generally good to have in your irrigation water unless it is at such high levels that it increases the total salt levels of water. Calcium can also offset the bad effects of sodium in the water as discussed above.

Potassium (K)

Potassium in the irrigation water is readily available as a plant nutrient and the lbs. per acre-foot can be subtracted from the fertilizer recommendations.

Magnesium (Mg)

Magnesium in the irrigation water is readily available as a plant nutrient and the lbs. per acre-foot can be subtracted from the fertilizer recommendations. Magnesium can also offset the bad effects of sodium in the water as discussed above. Magnesium fertilization is often not needed if water levels exceed 8 ppm.

Iron (Fe)

Iron in the irrigation water is readily available as a plant nutrient and the lbs. per acre-foot can be subtracted from the fertilizer recommendations. Excess iron in the water can lead to deposits in water pipes.

Bicarbonate (HCO3)

Bicarbonate in the water can combine with calcium and magnesium to form generally insoluble lime and remove calcium and magnesium from the soil water. Thus bicarbonate can increase sodium permeability hazards. High bicarbonate levels in the water can also precipitate phosphate fertilizers injected in the irrigation water. A quick way to test if injecting phosphate fertilizers in your irrigation water is a problem is to add one tablespoon of a phosphate fertilizer like 10-34-0 to 1 quart of irrigation water. If the water turn milky you will have problems.

Table of factors for converting milliequivalents per liter (meq/l) to ppm

Element	Conversion Factor (Multiply meq/l by this number to get ppm.
Calcium	20
Magnesium	12.2
Potassium	39
Sodium	23
Nitrate-nitrogen	13.6
Magnesium	12.2
Chloride	35.4
Sulfate-sulfur	16

Note: This information should only be used as a guide. Adjustments for local conditions must always be made.

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