Posts Tagged: leaching

It's NOt Just Water, It's Salt

Irrigated agriculture must always contend with salts. Five years of drought and its effects can magically disappear, but it will be back again. Low rainfall is the norm for California. We rely on winter rainfall to leach the salts from root zones that have accumulated salts from previous irrigations.  Salinity affects plant growth and understanding what it is and how it is measured and evaluated need to be understood.  Just having wet soil that is full of salts is not going to help a plant, it's going to add stress and eventually physiological and disease problems - https://ucanr.edu/blogs/Topics/index.cfm?start=28&tagname=disease

All waters, even rain water, have some salts dissolved in them, so all waters could be called saline.  The term saline is restricted to waters with concentrations that could cause harm to plants or people.  Seawater is highly saline, many wells are moderately saline.  But unlike humans that excrete salts, plants are often affected by salt levels that have very little health impact on humans. Well waters used for irrigation can often exceed standards for plants that are fit for human consumption.  However, with proper management many waters can be used on plants, depending on the plant species.  Domestic water supplies from cities typically have better quality than some well waters because they are monitored and often blended to meet human consumption.  Most domestic water supplies have low concentrations of salts and are not considered to be saline.  However, using even domestic water in growing subtropicals does not mean that we should not be concerned about salinity.

Before going any further it is worth remembering that salt is not just the sodium chloride that's on the table.  Salts are combinations of electrically charged ions.  These ions separate from one another when a salt dissolves in water.  Water with dissolved sodium chloride and potassium nitrate contains sodium, potassium, chloride and nitrate ions.  The most common ions in natural waters are:

 

            sodium (Na+)              chloride(Cl-)    sulfate (SO42-)

            calcium (Ca+)             boron (H3BO3)

            magnesium (Mg+)      bicarbonate (HCO3-)

                                                                                            

Different waters can have very different proportions of these ions and these proportions can change with time.  Some typical analyses of City of San Buenaventura water can be seen in the following chart (2015 Annual Report of the City of San Buenaventura).

 

Ionic composition of some wells in Ventura

 

Sample	Na+	Ca+	Mg+	Cl-	SO42-	TDS	EC
			(mg/l)				(umhos/cm)
1	200	259	70	92	839	1668	1990
2	45	92	191	44	210	645	874
3	28	59	21	20	140	316	580

 

Total dissolved solids (TDS) and electrical conductivity (EC) are two different ways of measuring the total amount of salts in water.  The old way of taking a specified volume (l for liter) of water and boiling it down to the residue which is weighed (mg for milligram) gives TDS.  The more modern technique is to measure the electrical current a water will carry (umhos/cm or micromhos/cm), which is in proportion to the number of ions in the water.

Natural waters also contain low concentrations of many other elements.  For most, the amounts are too low to be either harmful or beneficial to plants.  The main exception is boron which can be a problem for sensitive plants, such as citrus and avocado and probably for cherimoya as well, when in excess of 1 mg/l.  Many well waters in Santa Barbara and Ventura Counties contain potentially harmful levels of boron for plants.  This is not as common a problem in San Diego County.

In addition to the ions mentioned, there are also those that come from fertilizers and the soil.  The main extra ions are potassium, ammonium, nitrate and phosphate.  The concentrations of these will depend on the type of soil and the amounts and kinds of fertilizers applied, minus the amounts taken out by plants, held by the soil and lost by leaching or erosion.

In evaluating a water for its potential to harm plants, it is necessary to look at total salinity, as well as the specific ions.  Waters with a TDS in excess of 1000 mg/l or an EC greater than 1500 umhos/cm might pose problems for sensitive subtropical plants, and none at all to tolerant plants like figs, apricots or pomegranates.  Waters with an excess of sodium and/or chloride (more than 100 mg/l) can induce symptoms that are similar to high levels of salinity.

In most cases, plants respond by initially having their leaf margins turn yellow and die.  This happens first on older leaves because they have had the longest time to accumulate the ions.  Annual plants are often less affected than perennials, since they do not grow long enough to accumulate sufficient ions to cause damage.

As trees remove water from the soil, the concentration of salts in the remaining soil water increases.  Plants adapt to moderate increases, but if the plant is sensitive (and most subtropicals are), it will slow growth in response.  If the salt increase is small, the growth reduction will be small and acceptable.  But if the level of fertilizer use is high, the water quality poor, or the soil has not been properly leached, the increased soil salinity could reduce growth seriously.

The effects of salinity are usually gradual on plants, unless too much fertilizer has been suddenly applied or strong, dry winds causes rapid drying.  Also, with some domestic water there is variation in concentration and kinds of salts in the water with time.  The 200 mg/l of sodium in water sample 1 on the chart would be a problem if this were what the homeowner continuously received.  However, according to city data, this house does get 94 mg/l at times (not on the chart).  The better quality water serves to flush out the higher concentration salts.  And this is how to practically deal with poorer quality water, occasionally leach the soil with a volume of water in excess of plant need.  When there are no leaching rains, we need to be more aware of the potential for salt accumulation in the soil.  With proper plant selection and water management even extremely saline waters can be used.

 

Water Terminology

 

The ions in water are measured as parts per million (ppm) or milligrams per liter (mg/l), terms which are interchangeable.  This is like saying a percent, but instead of the ions' weight per 100 weight of water, it is the ions' weight per million weight of water.  The ion concentration also can appear as milliequivalents per liter (meq/l).  A milliequivalent is the ppm of that ion divided by its atomic weight per charge. 

            Example:  Ca2+ with atomic weight of 40 and a solution concentration of possibly 200 ppm.  Ca2+ has two charges per atom, so it has an atomic weight of 20 per charge.  200 ppm divided by 20 = 10 meq of calcium for a liter of water.

Total Dissolved Solids (TDS): measure of total salts in solution in ppm or mg/L

Electrical Conductivity (EC):  similar to TDS but analyzed differently.

            Units: deciSiemens/meter(dS/m)=millimhos/centimeter (mmhos/cm)=

                                    1000 micromhos/cm (umhos/cm).

            Conversion TDS to EC:  640 ppm=1 dS/m=1000 umhos/cm

Hardness:  measure of calcium and magnesium in water expressed as ppm CaCO

pH:  measure of how acid or base the solution

Alkalinity:  measure of the amount of carbonate and bicarbonate controlling the pH, expressed as ppm CaCO3.

Sodium Adsorption Ratio (SAR):  describes the relative sodium hazard of water

                        SAR= (Na)/((Ca+Mg)/2)1/2, all units in meq/l

            There is also an Adjusted SAR which considers the carbonate and bicarbonate present, but does not do much better in predicting plant response.

 

General Irrigation Quality Guidelines

(U.C. Leaflet 2995, 1979)

 

Measurement                          No problem                 Increasing                   Unsuitable

Effect on plant growth

EC (dS/m)                                  3

Na+ (SAR)                                   9

Cl- (ppm)                                 140                               140-350                         >350

H3BO3 (ppm)                               2

 

Effect on soil permeability

EC (dS/m)                                    >0.5                       

SAR                                               9

 

 

1.5 feet of water with EC of 1.6 dS/m adds 10,000 # of salt per acre

WATER NEEDS TO BE APPLIED NOT ONLY FOR THE PLANT NEED,

BUT ALSO TO LEACH THE SALTS

salt pile

avocado salt damage

A Small Amount of Rain Can Cause a Lot of Damage

Thanks for the rains that leach the soils of accumulated salts and bring on new fresh growth. Or maybe not. When we apply irrigation water with salts which with few exceptions we do in irrigated agriculture, salts accumulate in the soil. They accumulate in a certain pattern depending on the type of irrigation and soil type. There's a strong tendency for drip and microsprinklers to form a pattern of salt accumulation near the margins of the wetted patterns. This pattern is stronger with drip because the source point is always pushing a front outward from the emission point. This pattern occurs with microsprinklers, as well, although not as strongly. These patterns continue to form and accumulate as long as there is no rainfall to evenly push the salt down below the root zone. The longer the period of no rain, the larger the salt concentration at the margin.

So the way water moves is generally down. It moves in a wetting front drawn by gravity. It moves laterally too, because of the attraction water has for the soil particles. It will move laterally more in a clay soil than in a sandy soil because there are more particles in a clay soil than a sand (actually more surfaces that hold water). It also carries salt with it. Wherever the water moves, the salt moves. The more rain, the more salt is moved down. The more rain, the deeper the salt is pushed.

The problem with rain, is that if there is not enough, the salt tends to move laterally. In this wet soil solution, the salt is moving from where it is concentrated, to where there is a lower one. And if there isn't enough rain to move that salt down, it just moves back along the salt gradient, back to where the water first came from…….towards the roots. And that salt may be at such a high concentration that it can cause plant damage.

We talk about effective rainfall. This is usually about a quarter of an inch of rain. This is the amount of water to do more than just wet the dust, it's the amount to move water into the root zone. It is also moving salts into the root zone which can be a real problem. A good rain will do more than wet the dust, it will also move the salts out of harm's way in the root zone. The amount of rain necessary to do this going to depend on the salt accumulated and the soil texture. The more salt, the more rain needed. The finer the texture, the more rain. So there is no good cookbook, other than you need enough. And the first rains of the year, watch out. This is often when the highest salt accumulation and the most irregular the rains. Small amounts that can move salt into the root zone.

If there is not enough rain……………The solution !!!!!!!! Run the water to make sure there is enough to move that salt down. Crazy, but a few months ago we had just this situation. It was one of the last rains in the winter and it was not enough to move salts down, and within a week many avocados showed leaf damage. It was sad since we had all been wanting rain, and we wanted a good drenching.

So why am I bringing this up now? Well, the other night I woke up to rain, glorious rain. I enjoyed listening to it and then it stopped. I thought O NO, it's not enough. There are going to be problems. Well luckily most places didn't get and where it did, it was a dust settler. But it made me aware that with the first rains we might see this fall, growers should be on their guard.

Get ready to irrigate with the first rains if they are insufficient for adequate leaching.

Photo: Accumulated salts from irrigation water

 

soil salinity irrigation

Optimizing Leaching of Salts

Water moves in a wetting front. When irrigation water hits the soil it moves down with the pull of gravity and to the side according to the pull of soil particles (more lateral with more clay). Soil is a jumble of different sized soil particles, from clay to silt to sand sizes and then often intermixed with stones of different sizes from gravels to boulder. The different textures determine how water moves. It moves fastest through coarse textures and slowest through finer ones – the clays, the ones with the smallest pores. But soils are a jumble of particle sizes and pores.

Water first moves down the larger pores and then it slowly moves through the larger ones. As water moves through the soil it carries salts that have accumulated in the soil. At the wetting front is where the salt accumulate. As the water moves through the larger pores, salts migrate/diffuse from the small pores to the larger ones. This diffusion takes a bit of time, so typically the small pores have a larger salt concentration than the larger ones.

So an initial application of water will carry the salts from these large pores and if the irrigator were to stop in mid-application, it allows time for the salts to move out of the small pores into the larger ones. Then when the irrigation recommences, it will carry more of the salts out of the wetted area – the root zone. This technique is called “bumping” where an irrigation is stopped and then restarted in order to improve not only leaching, but also reduce runoff.

This principle also is at play when there are two or more sources of water quality. Soil salinity can be no lower than the irrigation water that is applied. Then as the soil water is removed through plant absorption or evaporation, the salinity increases. The soil salinity can easily be two to three times higher than the irrigation water.

If there are two sources of water, the initial application can be with the poorer quality water, and once that has reduced the soil salinity, then the better water quality can be applied which will then bring the soil salinity closer to that of the better quality water. By doing this two part leaching, the amount applied of the better quality water can be significantly reduced. This is a type of “bumping” to improve leaching.

Watch this U-Tube video on how water moves through soil, thanks to the work at Walla Walla Community College.

https://www.youtube.com/watch?feature=player_detailpage&v=J729VzBeI_g

Thank you Walla Walla Community College for the video

water movement

Read more