Posts Tagged: salt burn

A Little Rain Can Cause Salt Problems

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.

S-AV-CULT-IR[1]

When the Rains Don't Do Their Thing

"We don't need to irrigate, it's winter."  This is a commonly held idea, and many years it is true.  Adequately timed rains will often meet the needs of avocado trees during the winter period, and in times like last year, even satisfy much of the spring requirement.  And the calls are coming in – “What’s wrong with my trees, they have all these brown leaves?”.  This from San Diego to San Luis Obispo.

 

In a low rainfall year, irrigation can be as necessary as at other times of the year.  This is because a subtropical evergreen like avocado continues to use water regardless of rainfall patterns.  At the time of writing this article in March, we have had a scant 4 inches in Ventura and this is on top of a low rainfall year in 2011-12.  Rain is necessary to leach the salts that have accumulated from the last irrigation season.

 

The driving forces for plant water use are light intensity, wind and relative humidity, as well as temperature.  Remember how cold, dry winds can dry your skin or freeze-dry backpack food.  Even during the winter, the trees are quite capable of losing large amounts of water with clear skies and cold winds.   

 

Dry Santa Ana conditions are also more common in winter than in the past.  This winter, a time of drought, I went out to see an orchard to evaluate it for pruning.  On arrival, my first concern was for the water stress in the trees.  The grower, however, was unconcerned.  The trees had been dutifully irrigated the previous Friday.  But over the weekend, a Santa Ana had blown for three days and completely dried the soil in the top 10 inches.  Digging around the roots convinced the grower of water stress.  Do not take irrigation for granted.

Contributing to the problem is the determination of what amount of rainfall is effective.  Effective rainfall is defined as the amount of water that is retained in the root zone after rain.  Avocados, especially on shallow soils, do not have much of a root zone.  Most soils can be expected to hold about 2 inches of available water in the top 2 feet, less the more sandy, more the more heavy. 

 

If rainfall exceeds the holding capacity within the root zone, it is lost to the plant.  Just imagine if all the year's expected rain fell during one storm.  It would not be long before irrigation would be required with no more rain coming.  The extra water may, however, perform the all-necessary function of leaching accumulated salts from the root zone.  When the rain gauge says that 2 inches fell, it is quite possible that all that rain will not be available to the tree.  This also goes for the quarter inch storms we get that do not even make it through the leaf litter.  It is not effective rainfall, even though it may wash the persea mite off the leaves.

 

One of the best ways to assess the effectiveness of rainfall within the root zone is with tensiometers.  These trusty instruments are most commonly used to schedule irrigations.  A good rainfall should return the 8- and 18-inch depth gauges to close to 0 cbars.  This will tell you whether the rain thoroughly wetted the root zone.  It will not tell you how much may have passed through the root zone, however.

 

If you are using soil sampling to assess the depth of rain infiltration, simply squeezing a handful of soil can help.  Regardless of soil texture, a wetted soil will form a ball or cast when thoroughly wetted.  Water moves as a front through the soil.  After a rain, take soil samples with depth to find where the potential to form a ball abruptly ends.  This will tell you the depth of effective rain.

How well a soil holds together can also be an indication of when to irrigate.  Even a sandy loam texture will retain a ball that does not hold together well when there is still adequate moisture for the tree.  The possibility of forming a ball decreases with water content.  When visible cracking of a soil ball is obvious, it is time to irrigate.

 

Winter irrigation is something we do not commonly perform, but in low rainfall years it is an activity we need to consider, especially for controlling the salts that accumulate from our previous irrigation season.

Copy of avocado irrigation

When Winter Rains Don't Do Their Thing

"We don't need to irrigate, it's winter."  This is a commonly held idea, and many years it is true.  Adequately timed rains will often meet the needs of avocado trees during the winter period, and in times like last year, even satisfy much of the spring requirement.  And the calls are coming in – “What’s wrong with my trees, they have all these brown leaves?”.  This from San Diego to San Luis Obispo.

 

In a low rainfall year, irrigation can be as necessary as at other times of the year.  This is because a subtropical evergreen like avocado continues to use water regardless of rainfall patterns.  At the time of writing this article in March, we have had a scant 4 inches in Ventura and this is on top of a low rainfall year in 2011-12.  Rain is necessary to leach the salts that have accumulated from the last irrigation season.

 

The driving forces for plant water use are light intensity, wind and relative humidity, as well as temperature.  Remember how cold, dry winds can dry your skin or freeze-dry backpack food.  Even during the winter, the trees are quite capable of losing large amounts of water with clear skies and cold winds.   

 

Dry Santa Ana conditions are also more common in winter than in the past.  This winter, a time of drought, I went out to see an orchard to evaluate it for pruning.  On arrival, my first concern was for the water stress in the trees.  The grower, however, was unconcerned.  The trees had been dutifully irrigated the previous Friday.  But over the weekend, a Santa Ana had blown for three days and completely dried the soil in the top 10 inches.  Digging around the roots convinced the grower of water stress.  Do not take irrigation for granted.

Contributing to the problem is the determination of what amount of rainfall is effective.  Effective rainfall is defined as the amount of water that is retained in the root zone after rain.  Avocados, especially on shallow soils, do not have much of a root zone.  Most soils can be expected to hold about 2 inches of available water in the top 2 feet, less the more sandy, more the more heavy. 

 

If rainfall exceeds the holding capacity within the root zone, it is lost to the plant.  Just imagine if all the year's expected rain fell during one storm.  It would not be long before irrigation would be required with no more rain coming.  The extra water may, however, perform the all-necessary function of leaching accumulated salts from the root zone.  When the rain gauge says that 2 inches fell, it is quite possible that all that rain will not be available to the tree.  This also goes for the quarter inch storms we get that do not even make it through the leaf litter.  It is not effective rainfall, even though it may wash the persea mite off the leaves.

 

One of the best ways to assess the effectiveness of rainfall within the root zone is with tensiometers.  These trusty instruments are most commonly used to schedule irrigations.  A good rainfall should return the 8- and 18-inch depth gauges to close to 0 cbars.  This will tell you whether the rain thoroughly wetted the root zone.  It will not tell you how much may have passed through the root zone, however.

 

If you are using soil sampling to assess the depth of rain infiltration, simply squeezing a handful of soil can help.  Regardless of soil texture, a wetted soil will form a ball or cast when thoroughly wetted.  Water moves as a front through the soil.  After a rain, take soil samples with depth to find where the potential to form a ball abruptly ends.  This will tell you the depth of effective rain.

How well a soil holds together can also be an indication of when to irrigate.  Even a sandy loam texture will retain a ball that does not hold together well when there is still adequate moisture for the tree.  The possibility of forming a ball decreases with water content.  When visible cracking of a soil ball is obvious, it is time to irrigate.

 

Winter irrigation is something we do not commonly perform, but in low rainfall years it is an activity we need to consider, especially for controlling the salts that accumulate from our previous irrigation season.

 

Salt damage due to lack of leaching

 

avocado tip burn

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