ISU Extension and Outreach
Commercial Horticulture Specialist
For audio versions of this article, see the Small Farm Sustainability Podcast where we discuss return bloom, soil sampling, and interpreting soil reports over three separate podcast. The following discussion represents a snapshot in time as a lot can happen between now and next spring.
Dry Oil Moisture Conditions
A large portion of Central and Western IA was dry during harvest this year. Fruit crops that were not physically damaged by the derecho on August 10th had ideal conditions to ripen with warm days and cool nights. Prior to the derecho, most were praising the dry conditions late summer through harvest with plants and fruit generally fairly clean with a few exceptions (black rot on grapes, scab, rust, and fire blight on susceptible cultivars) that carried through from a wet late April / early May. However, where we had very dry soil conditions leading up to the derecho and just after, we may see an impact on return bloom and crop load next year. There are a lot of factors that go into return bloom, but fruit bud development occurred under less than ideal conditions across most of Central and Western Iowa. Apples and grapes will put energy towards maturing the current crop and not towards developing buds for next year’s crop. Our midwestern grapes are more resilient at producing a steady crop from year to year than apples but I do wonder if we will see an impact on cluster size and uniformity with grapes and flower number and size with apples. This is something to keep an eye on next year when pruning and crop thinning. Some extra care, especially during crop thinning, to select uniform clusters and to not overthin apples may be especially important. Of course, perfect conditions during pollination, fruit cell division, and fruit cell enlargement can somewhat make up for less than ideal return bloom.
Coming out of a dry season for most of Iowa, I wanted to address a couple of questions.
Is there any concern with the plants themselves?
Yes. Dry soils freeze deeper and get colder. This can lead to root injury particularly on rootstocks that are marginally hardy here such as Bud9 for apples. At this point, we are not getting soil moisture recharge going into fall and winter across much of Central and Western Iowa. Soil moisture deeper into the soil profile remains dry. Thankfully, since recording the podcast and developing notes for this article, we have since received precipitation across much of the dry region which is starting to help soil moisture conditions improve at least in the upper soil profile. There is still concern but less so than a few days ago.
Is soil moisture going to get better?
It has improved a lot the past couple of weeks. We are looking at a strong La Nina going into winter which would suggest warmer and drier than normal conditions to the south and east and colder and wetter to the NW. However, where that split in the state will occur is yet to be determines. The other caveat here is that “wetter” is expected to occur late winter so soil in the NW part of the state could still freeze hard and deep. Moisture that occurs after the soil is frozen will have limited impact to soil moisture status.
Can we do anything about dry soil conditions?
For fruit bud development, no. Not anymore as apples and grapes have already set their buds for next year. We needed to irrigate back in July and August but few if any vineyards and semi dwarf orchards have an irrigation system. Rainfall during this time was very spotty. I suspect there is a lot of variability in bud development through the Central/West Central region of the state.
Can we rehydrate the soil now to prevent root injury.
Not unless you have a really effective rain dance (and to whomever is currently doing the snow dance, stop it!). In fairness, the rain/snow precipitation we have been getting in October is just what we needed. A slow rain and snow melting into the soil profile is very helpful right now. In many areas, the soil is so dry and so dry deep that we cannot effectively irrigate to rehydrate the soil.
Irrigation next year
Looking towards next year, you may be thinking about irrigating if conditions do not improve. Drip irrigation components are fairly inexpensive if you have a water source already. However, many wells across Central and Western Iowa are running dry and were not able to keep up with irrigation demands this past year. If conditions do not improve going into next season, using irrigation at timely points of the crop cycle will be essential in maturing a crop and ensuring water is available when you need it (think bud break, bloom through cell division, and at cell expansion/ripening). If need be, we will cross this bridge with more details later.
Again, these are just comments regarding a snapshot in time. A lot can and will change by next spring. What this should get you thinking about is how much crop do you want to leave on the plants next year and what does the overall bud situation look like on your plants. Are they large, well developed, and healthy? Will you make any adjustments on pruning this winter. Obviously you don’t need to answer these questions now…but do be thinking about them. You should also be thinking about water access for next year if not for irrigation, at least for pest management applications.
I also wanted to address a few fall clean up issues post derecho. Apples and grapes left hanging or on the ground need to be dealt with. That fruit is a reservoir of food for pathogens to overwinter on. Now that we have had a hard freeze across the state, leaves should be dropping. From a disease standpoint, harvesting and removing anything that was left in the field is ideal. Alternatively, you can drop clusters or apples between the row and mow/bag to remove or chop up with a flail mower. If you opt to leave in the field, chopping material up with a flail mower will get it to break down much faster than a rotary mower. You can apply a little nitrogen late in the season to help microbes break down debris faster but at this point, its too cold and we will not have a lot of warm days now for microbes to be very active.
Now that we received moisture and the upper soil profile is starting to rehydrate, it is time to collect soil samples. If you have not soil sampled in 3 to 5 years, it is a good idea to do so. Here are a few tips for success.
- Collect one sample per soil series
- If you have a large field, there can be quite a bit of variability of the underlying soil across your field. Use the Web Soil Survey to identify soil types across your field if you are unsure where they are.
- One sample per cultivar
- Cultivars can have tremendous variability in nutrient uptake. Do not collect samples that cover two or more cultivars in one area.
- Sample soil at a depth of 0-6”
- Put the soil probe into the ground to a depth of 6 inches (or 8 if that has been what you are doing). Remove the probe from the ground, remove the grassy material at the top of the soil core, and place soil core into a clean bucket. There is no need to sample below 6 (8) inches in an established orchard or vineyard…there is nothing you can do anyway at that depth.
- Collect 10-15 cores per sample
- Put the soil probe into the ground at 10 to 15 unique, random places across the field.
- Avoid oddities
- Don’t collect soil cores from low lying wet areas, near gravel roads, old building sites, etc. The soil cores you collect should be representative of the overall field.
- Avoid Saturated/bone dry Soils
- The analysis by the lab is worthless if the ground is saturated or bone dry. The potassium and pH will be off.
When in a pinch, sometimes I will sample just from the primary cultivar or two and apply those results to the entire field. You don’t necessarily need to sample from every soil type / cultivar combination especially if those smaller areas are not going to influence your management decisions. When soil sampling, ask the lab to analyze for soil pH, electrical conductivity, organic matter, potassium and phosphorus. Petiole samples will tell you what you need to know for nitrogen, calcium, magnesium, and micronutrients. When it comes to selecting a lab, any certified lab (see list of IDALS certified labs) should give you consistent results though the interpretation of those results may vary. Be realistic about expectations when soil sampling in a mature orchard or vineyard. Soil samples are part of the information (along with petiole samples) to help make decisions on fertility plans.
An Example of Why Foliar Samples are Needed
When making fertility decisions on established orchards and vineyards, soil samples should always be paired with leaf/petiole samples. I just had a perfect example cross my desk last week illustrating why this is important. A soil report for a vineyard came back as adequate potassium, phosphorus, and micronutrients except boron with pH a bit high. Based on that info alone, you would think it would be pretty easy to deal with. A little sulfur broadcast applied to the soil surface to try to pull down the pH each year in hopes that it slowly moves into the soil profile in addition to foliar applying a little boron in the spring.
However, the petiole sample indicated that the high soil pH was much more of a problem than indicated in the soil sample report. The petiole sample report indicated that in addition to boron, potassium was also clearly deficient and marginally deficient in the two cultivars sampled. We would not have known that potassium was deficient based on the soil report alone. The low potassium in this case is due to the high soil pH (actually excessive calcium and magnesium but same thing here) inhibiting potassium uptake. This is a very common issue I see with grapes, and to a lesser extent apples, along the Mississippi and Missouri river valleys.
Interpreting Soil Sample Reports for Apples and Grapes
See the following articles for assistance in working out the math in determining application rates based on soil sample reports.
- Interpreting Soil Reports
- Managing Soil pH in Horticultural Crops
It is extremely difficult to adjust soil pH, phosphorus, or potassium availability in established orchards and vineyards. Phosphorus, potassium and soil pH adjustment materials (sulfur and limestone) are not water soluble (like nitrogen) and will not move down into the root zone on their own quickly or easily. These nutrients must be physically put into the soil through tillage for them to really be effective. Surface applications have little value in the short term though repeated applications over a period of years may have some impact.
In the example above, the low potassium in the plants is due to the high soil pH (and thereby high calcium and magnesium in the soil). You can attack this a couple of different ways (none of which are a 100% solution). The reality is, if this is your situation, you will need to implement a variety of practices to deal with the issue.
- Surface apply potassium. Potassium, like sulfur, does not leach so surface applications have little overall effect on K uptake in the short term.
- Apply liquid potassium through a drip irrigation system. Similar challenges as applying granular potassium with somewhat better results.
- Reduce soil pH with surface applications of sulfur.
- Foliar feed potassium. You cannot apply 100% of potassium needs via foliar application but it can help. Potassium uptake by grapes is highly dependent on cultivar and form of potassium.
- Foliar feed micronutrients. Rotate adding boron and iron to your tank mixes or consider a boron, iron, potassium (and maybe a blank week) rotation in the tank mix throughout the season.
There is a lot to be thinking about going into winter as you plan for next season. Bring on 2021!
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