Why rain euphoria doesn’t last

Last Thursday we were fortunate to have a very nice rain, 1.3″ that fell steadily over night. Now it’s Monday, and I’m already considering my next irrigation. Why? This graph holds the answer.

This is a data log of our soil moisture after the rain that fell on the night of December 15/16. The light blue line represents soil moisture at 12″, the dark blue line represents moisture at 36″. The range represented by the shading and dashed lines is the preferred range; a fully saturated soil would be at the top.

What we’re seeing here is that prior to last week’s rain we were quite dry… very close to the lower acceptable limit. We were ready to irrigate had the storm failed to deliver. Thankfully it delivered in line with expectations. Where are we now?

The rain had the clearest impact at 12″ (light blue), which is as you would expect, but it was insufficient to fully saturate the soil. At 36″ (dark blue), the rain barely registered at all. Shallow soil moisture has already dropped considerably as the rainwater wicks through the soil.

Hoping the rain forecast for next Monday turns up!

(Hat tip to our technology providers at Acuity Agriculture for a great tool!)

Ouch!

I am grateful for the rain last night, but I’m also frustrated. We only get so many storm systems a season… when one falls 50% short of expectations, that hurts. Last night’s system had the water we needed, but it stalled over the Santa Barbara Channel. More than an inch of badly needed rain fell directly into the ocean, while most of our agricultural areas and watersheds saw half that amount or less.

Why rain matters to avocados

First off, let’s state the obvious… all plants need water and water is expensive in California. That alone is enough to make avocado farmers jump for joy when a storm comes in. Rain can be worth as much as $20 to $100 an inch for every acre of orchard.

But there is an even more important reason. Avocados are sensitive to salt, and all irrigation water and all fertilizers (organic or conventional) carry some salt with them. Too much salt stresses the tree, slowing growth and reducing production. Rainwater cleanses this salt from the tree’s root zone.

The classic symptom of salt stress is “tip burn”, seen above. This is quite common on residential trees who often don’t get a deep watering or are planted in poorly drained soils. (The speckly discoloration on these leaves is from Persea mite, but that’s a different article.)