Tag: soil salinity

Hoophouse Solarization Ponderings

 

Our hoophouse on solarizing day, August 5. Note we kept the big shadecloth over the roof until we had done the work of installing the plastic over the soil. Photo Ken Bezilla

Solarization Benefits We Hope For

I wrote Solarization of Our Whole Hoophouse on 8/12/26. This summer we decided to solarize the whole hoophouse. We hope to reduce fungal diseases in our winter crops, draw down the weed seed bank, and kill lots of nematodes. We planned to solarize for the whole of August (4 weeks). We started that on August 5, and solarized until September 3, a full 4 weeks later.

A One-Month Break from Usual Hoophouse Work

It was a nice break to do nothing in the hoophouse for a month except run the drip irrigation twice a week. We left the drip tape in place so we could water the soil and keep it alive below the hot zone. It was an unearthly sight to just have a sea of plastic in there, and one of the crew popping in there to turn on the irrigation reported that to was quite surreal to walk barefoot on the hot plastic.

We agreed to use the time to do some infrastructure repairs, and did get a new window made and the other one repaired.

We repaired our big all-encompassing shadecloth and reset the grommets to be a better fit with the spacing of the hooks in the baseboards.

We researched horizontal airflow fans to replace our hippy-rigged box fans, but we balked at the $1000 price tag. Maybe EQIP grants would help?

We did our crop planning for the winter and made a map. We cross-checked the schedule against the map and made sure everything fit. We did some perimeter weeding and laid down landscape fabric.

A Bad Choice Narrowly Avoided

We had the notion that if we had thought of it in time, we might have spread mature compost on the beds before spreading the plastic and killed the weed seeds that landed on the compost pile while we ignored it all summer. We got behind with the mowing, and we know grass and spiny amaranth seeded somewhat on to the pile. Fortunately, we didn’t do that! Sterilized compost has most of its microorganisms killed! Instead, we re-composted the weed-seed-infested pile and got a new pile in the meantime.

Hoophouse soil thermometer showing 100F (38C). Photo Ken Bezilla

De-Solarizing Day

On de-solarizing day we allowed a little lead time (not enough!) to disperse some of the 120F (49C) heat by flinging open the doors and windows. The soil temperature was 100F (38C). The peak air temperature during the month may have reached 140F (60C) although no one wanted to linger in there long enough to really study the thermometer. It had been 80F (27C) before we solarized and is back down to that temperature in as I write this in late September.

Normally we leave the big shadecloth in place over the whole hoophouse until mid-September or so, but this year we wanted maximum heating for the solarization, so we removed it a couple of days after solarizing (when it stopped raining).

We had bought 32’x100’ (9.7×30.5m) plastic which was just right for the footprint of our 30’x 96’ (9 x 29m) gothic hoophouse. We unrolled it down the center bed (it was folded in several concertina folds from each edge to the middle). We mimicked that kind of folding to pack it away.

The first step of removing the hoophouse solarizing plastic: folding the edges to the middle. Photo Ken Bezilla

We started by removing the buckets of rocks we had set round the edges to hold the plastic down. Next we folded both sides to meet in the middle. We got old towels to wipe the underside of the plastic dry. We used some of the drying time to teach broadforking and cultivation and raking to new crew members. We noticed how warm the soil was to our hands.

Wiping the solarizing plastic dry between folds. Photo Ken Bezilla

Next we brought the outer folds in to meet in the middle again, and repeated the mopping operation. Later we thought a sponge mop might have been a useful tool. We used more drying time to start to return things we had stored in a shed during Solarization Month.

Removing hoophouse solarizing plastic: second fold of edges to the middle. Photo Ken Bezilla

We repeated the folding until a simple folding in half made a sensible size for rolling, and ended up with a fat roll of plastic tied up with rope. We wrote on it to deter anyone who might think of using that plastic for any other purpose.

The last fold of the the hoophouse solarizing plastic, making it ready to roll once dried. The dried stems are from a buckwheat cover crop we covered with the plastic. Photo Ken Bezilla.

Disadvantages of Solarizing

I was sad to say goodbye to the hoophouse bell peppers as they were the best we’ve had in years. Usually we keep the peppers until October and do our winter crop planning so that the pepper area is the last planted in winter crops, but we made that sacrifice! For several years our peppers didn’t do well, as the salinity of our soil had got too high. We now do salt wash-downs in September and March. This year the peppers were spectacular! Big plants, huge peppers! Rather disappointing to have to pull them at the beginning of August.

Bullnose pepper in our hoophouse June 27. Photo Alexis Yamashita

Several days into the solarizing I realized with dismay that we had killed off all our zipper spiders and probably their egg cases. The spider corpses were still attached to the framework. Zipper spiders seem to be homebodies. Surely they could have found a way to leave? But mostly they do die at the end of summer, just not on open display. In recent years we have not had any of these spiders outdoors, but many in the hoophouse. They do good work consuming the insects they trap.

Zipper spider on the pepper plants in our hoophouse on September 10 a previous year.
Photo Pam Dawling
Zipper spider egg cases hanging from the hoophouse plastic.
Photo Wren Vile

One big disadvantage of our choice was that the work of bed prep and planting in there after de-solarizing was dreadful! We had hot sunny weather, and the allocation of each day’s prep work took 3 hours or more. By lunchtime it was 120F (49C) in there on many days. Exhausting work! But it didn’t seem worth putting the big shadecloth back in place as we normally remove it around September 20.

Hoophouse fall bed prep. Broadforking in plastic sandals is not recommended! Photo Ken Bezilla

Apart from the workers suffering the effects of the heat, the soil also dried out a lot faster, and we had to keep working (no rest!) to break up the big lumps of soil before they became bricks.

We prepped one bed and got ready to sow. We decided to use wire hoops and a 6’ (2m) roll of shadecloth like we do outdoors for lettuce in hot weather. That worked well, although the bed edges did dry out fast, maybe because the paths were still holding heat from the solarizing. So we had to handwater the bed edges daily for a week or so.

Spinach seedlings from pre-sprouted seed in the hoophouse in September.
Photo Pam Dawling

One of the first crops we sow is spinach, using seed pre-sprouted in the fridge for 6 days. No way would it have germinated in that hot hoophouse soil! On sowing day, the spinach seed showed little sign of sprouting, but we sowed, watered well, shaded it, and it came up great! The extra attention paid off. This spinach will feed us all winter until about March, as we just harvest leaves. By September 21, all the early crops in that bed had emerged (scallions, Bulls Blood beets, tatsoi, radishes as well as the spinach). We needed to remove the shadecloth from the bed once everything had emerged, so the seedlings didn’t get leggy. 12 hours of daylight isn’t so much.

The perennial weeds growing in under the baseboards did not die. Once we removed the plastic the wiregrass (Bermuda grass) raced across the edge bed and started colonizing the next bed. Fortunately juvenile wiregrass is relatively easy to pull up.

De-solarizing our hoophouse. Note the vigorous wiregrass along the south baseboard. Photo Ken Bezilla

Most years we grow summer cover crops in the hoophouse, thus keeping the soil-life active and adding organic matter. This summer offered no gain of organic matter, which I do regret.

Advantages of Solarizing

We have sowed an outdoor seedbed with things to transplant into the hoophouse in October when we have more beds prepared. Work is progressing well, although we’ve had to scramble some days to stay on track.  Because using transplants from outdoors is part of our usual plan, we did not have to make any changes to the beds after the first one, om account of high temperatures.

The advantages of solarizing will become apparent in the next 6 months. Two weeks after uncovering the soil we did find a tiny tomato seedling, but the seed could have been dropped from our clothes after the solarizing month, rather than being a survivor. It was interesting to see it.

For myself, I’m unsure if I’d want to do this again. The advantages are going to have to be huge – like no more nematodes. I’ll write more as the next year unfolds.

Soil Test Results: Leaf Yellowing, High Salts

Yellowed over-wintered spinach outdoors in spring.
Photo Pam Dawling

What to do with Soil Test Results

Here I’m addressing some of the soil test results from our hoophouse, and of course the same principles apply outdoors too.

This fall we took multiple samples of soil in our 96’ x 30’ (29 x 9 m) hoophouse, because of curiosity as well as two specific concerns. The first concern was that our twice-a-year salt wash-downs maybe weren’t doing a good job of reaching the edge beds. The second was that one particular area of one of the beds produced a patch of poor yellow spinach last winter, and we wondered why. In a normal year we just take one soil sample from the whole hoophouse, mixing several cores. After all, agriculturally speaking, 2,880 square feet (268 m2) is a small area! Last year we didn’t do any soil tests. Normally it isn’t necessary to test every year, but if you think something has changed, it’s a good move.

Soluble Salt Levels

Our salt levels (after our salt wash-down) were reassuring. All were low (230-269 ppm), except the north edge bed which had 474 ppm, medium. The south edge bed had 358 ppm, but that’s still classed as low. “Soluble Salts are not high enough to cause salt injury”. We decided to do nothing right now, except buy a new heavy-duty lawn sprinkler, so we always have a spare on hand.

Why Salt Wash-Downs Become Necessary in Hoophouses

Because hoophouse soil is protected from rain and snow, soluble salts do not get washed down or away, as happens outdoors. Like many hoophouse growers, we use drip irrigation, not overhead watering. This is a much more efficient use of water, although it makes salt build up (salinization) more likely.  Think about salt pans where salt is harvested from seawater: a large flat area is covered with seawater and left undisturbed while the water evaporates in the sun. The (no-till) hoophouse soil has water (including soluble salts) spread through the surface layers. It is left undisturbed. The heat of the sun draws the water up to the surface and evaporates it, leaving salts to build up and damage crops in hoophouses, especially if your soil doesn’t drain well or if you use animal manures or synthetic fertilizers. Salinization can cause symptoms like drought stress, interfere with seed germination, and inhibit plant growth. It can also encourage some pests.

How We Do Salt Wash-Downs

Twice a year, we do a multiday salt wash-down treatment with an oscillating lawn sprinkler. In the fall we time the wash-down for October after we’ve planted the first bed of winter crops, before we start prepping the other beds, or in early November after they are all planted. We don’t want to be broadforking heavy, sticky wet soil! In the spring we do the wash-down in early March before we start transplanting the tomatoes. We don’t want overhead watering on the crops that are prone to fungal diseases.

Here is a 3-slide explanation. Click the bottom of the slide to advance to the next.

Salt Build-up

 

Our lawn sprinkler covers a rectangle about 30 ft (9 m) wide and about 24 ft (7.2 m) long. We flag 4 spots along the center bed, the first 12 ft (3.7 m) from one end wall, and then every 24 ft (7.4 m). In the simple version, we start at the easternmost flag and run the sprinkler for two hours. Then we move the sprinkler to the next flag and run it there for two hours. Doing all 4 flagged spots takes an 8-hour day. Before watering the western spot, we gather all the tools, logbook, tubs of labels, and put them outside in a garden cart (covered with a tarp) for the night.

Next day we start at the west end and once we have moved the sprinkler to the second spot we can return the tools to their accustomed hooks. In the spring we can only manage two days of wash-down, but in the fall we do three days. According to the soil tests, this is enough to give an OK reading on soluble salts.

It is tricky to get the edge beds adequately watered, so we usually run the sprinklers offset one bed either side of the center-line, giving the edge beds a better chance. This year, the south edge bed got as good a reading as the middle beds, and the north bed got a slightly poorer (higher) reading. Our hydrant gives 1” (2.5 cm) of water every hour. You can test with a shallow straight-sided container, such as a tuna can. So the soil gets 4” (10 cm) of water in the spring salt wash-down and 6” (15 cm) in the fall one.

As a guideline, to leach out soluble salts from the top 12” (30 cm) of soil, apply 6” (15 cm) of water to wash down 50% of the salts, apply 12” (30 cm) to wash down 80% of the salts and 24% to wash down about 90% of the salts (California Fertilizer Association, Western Fertilizer Handbook, 8th Ed.)

See Dealing With High Soluble Salt levels in High Tunnels PA

 Soil Nutrient Levels

Our levels of nutrients were all Sufficient, High or Very High, except potassium in one test, which was Medium. Various nutrient deficiencies can cause leaf yellowing and other symptoms. Nothing I saw in the nutrient test results worried me. I’ve written previously about high phosphorus levels. I did note that the phosphorus was highest in the area where we’d had the very yellow spinach last winter (and where the pH was highest). More on this next week.

Causes of Leaf Yellowing (Chlorosis)

I wrote about causes of yellow spinach here in 2019. Here’s a recap:

Yellow over-wintered spinach in spring. Note that it is the outer leaves that are yellow, while the younger inner ones are green. Note that even the veins are yellow.
Photo Pam Dawling

Possible causes of yellow spinach leaves include poor drainage, compacted soil, damaged roots/poor root growth, high soil pH, too much or too little water, too low or too high a temperature, or perhaps cold temperatures followed abruptly by very warm temperatures, 80°F (26.7°C) or greater; nutrient deficiencies or disease. Our roots grew OK, and we don’t tend towards alkaline soil. But this year’s soil test revealed exactly that! The problem area had a pH of 7.2! Other areas within the hoophouse had 6.6-7.1, all buffered to about 6.6. I’ll say more about pH and what we did to improve it in a week or so. Today I’ll focus on salinity and causes of leaf yellowing.

Nutrient deficiencies can occur due to low amounts in the soil or because high soil pH has made those nutrients unavailable. The most common nutrient problem associated with yellowing is lack of iron, but it may also be caused by deficiencies of manganese, magnesium, boron, zinc, or nitrogen. Iron deficiency starts on young leaves and works towards the older leaves (which initially had enough iron, as a transplant). It can occur in water-logged soil. The veins can remain green. Not the problem we have – our older leaves were yellower.

Deficiencies in manganese, zinc or nitrogen develop on older leaves first and then progress upward/inward. Within older leaves, magnesium is transported from the leaf’s interstitial areas to the veins, resulting in yellowing of the areas between leaf veins. This creates a marbled appearance, a typical symptom of magnesium deficiency. Our leaves were yellow all over.

Nitrogen deficiency. Overall yellowing (including veins). The lower, older leaves appear yellow first as the plant moves the available nitrogen to the more important newer leaves. Spinach is sensitive to inadequate nitrogen. Our main suspect.

Soil auger, form and sample box from Virginia Tech and our soil tests results file.
Photo Pam Dawling

Boron deficiency also yellows the leaves and stunts spinach plants. We do tend to run short of boron, and my approach was to add boron before brassicas each winter. We haven’t added any for several years and the only brassicas in these beds were turnips in spring 2017 and fall 2018. I don’t remember if we added boron in 2016/2017, and the records are tucked away.

Spinach is a heavy feeder. Recommendations are to feed with compost tea, manure tea, or fish emulsion when plants have four true leaves. Side-dress with compost tea every 10 to 15 days. Mix 1 tablespoon of fish emulsion and 2 tablespoons of kelp extract per gallon of water; use about one cup per foot (30 cm) of row on a weekly basis until plants are about 4″ (10 cm) tall; then feed twice more before harvest. Add mature compost to planting beds twice each year.

Fusarium wilt or fusarium yellows, Fusarium oxysporum f. sp. Spinaciae (also called spinach yellows) is a fungal disease which infects plant vascular tissues. Fungal spores live in the soil for at least 10-15 years and can be carried by cucumber beetles. We certainly have lots of striped cucumber beetles! But our plants that year were not wilting. Liming to ensure the soil is not acidic can help prevent this disease.

See Identifying nutrient deficiency in plants and Guide to Symptoms of Plant Nutrient Deficiencies

Nematodes (we do have peanut root-knot nematodes) can cause yellow leaves.