Soil Characteristics and Quality

Program Contact: Tianna DuPont, Regional Specialist, Tree Fruit
(509) 663-8181 • tianna.dupont@wsu.edu

replantj1Your soil is unique. Nobody else has an orchard with the same soil as yours. Similar, but not the same. Your soil varies throughout even a small block. Texture, depth, layers, rocks, nutrient content, chemistry, and living organisms all may play a role in making your site unique. You must learn how to determine your soil qualities, and adjust them to make a better root growth environment.

There are three major aspects of soil quality that you may address as you change from an old orchard to a new one: physical, chemical, and biological.

Soil Physical Properties

replantlUnless you have deeply ripped or leveled your soil, its texture, depth, rocks and layers probably remain much as they were when you acquired the land. You see these qualities only when digging ditches or basements, planting trees, or drilling fence post holes.

Compaction of the orchard soil has occurred over the years, and it is imperative that this is addressed prior to replanting. Tree roots do not grow well from a hole dug into compacted soil.

Unless the soils is exceptionally full of stones, it should be ripped at least 1.5 to 2 feet deep, in two directions.

replantnYou may not have a good idea of the full range of variation that may exist across the block. There are often shallow, rocky and/or sandy zones in orchards with otherwise good quality soils. To avoid surprises, assess the top three feet of your soil during the years leading up to planting (this can be done any time. You probably already know the bad patches.

Soil with poor physical quality usually cannot be improved, but perhaps you can plan your new orchard to use these unpromising areas as open space for sheds, loading areas or some other purpose that does not require good soil.

Your soil may have significant layers with different textures, calcium cemented zones (caliche), rocky or gravelly layers, or compaction. If so, trees will be healthier and grow more evenly across the block if the soil is deeply cross-ripped, two or three feet deep, before planting. Ripping will mix the soil, improving potential root penetration. This ripping may also turn up many major tree roots, old pipes, car bumpers and boulders that should be removed to make tree planting quicker and easier.

Soil Chemical Properties

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There are many potential chemical imbalances that must be addressed prior to planting. The pH of the orchard soil is probably the most common problem. Orchard performance will be improved if the soil pH is between 6.2 and 7.2. If pH is lower than that, an application of agricultural lime is required. The best time to apply this product is while you can still identify the old tree rows. The pH is lowest under the trees, and that is where most of the lime should be spread. When the soil is ripped and disced prior to planting, the lime will mix with the low pH soil, react with the acid, and rapidly improve the pH. If your soil is low in magnesium (by test) dolomitic lime should be used. This is the least expensive way to add magnesium, a major nutrient, back into your orchard system.

(More lime information)

If your soil test suggests a general problem with low nutrient levels, such as phosphorus and potassium, this pre-discing period is the most effective time to apply appropriate rates. If soil P and K are already adequate, adding these fertilizers is not likely to help. Zinc fertilizer, broadcast or banded, and mixed with the soil before planting, will greatly improve the zinc nutrition of young trees for the first two or three years after planting. Boron applied in the preplant application will help for the first season. Foliar zinc and boron can be applied at low rates two or three times during the growing season during the first two years, then applied as you would to a mature tree from the third season onwards.

Arsenic and lead residues, left over from efforts to control codling moth in the 1930’s through about 1948, have been blamed for much of the tree growth restriction on old orchard sites. In many cases, arsenic in a replanted site is an excuse for poor growth, rather than a true cause. These residues can slow tree root growth, adding to the problem if other factors are not addressed properly, but trees can tolerate much higher levels than were once considered toxic.

replantqThe picture on the right is of a Gala/M7 after its first growing season, planted in an orchard with a serious replant disease problem and an soil arsenic level of over 200 ppm. As you can see, the tree is doing fine.

Many apple or pear orchards have shown almost no effect when the preplant soil arsenic test showed levels below about 150 parts per million. If your soil test shows levels near this level or higher, your apple or pear trees will probably grow better if you plant them individually, with at least the equivalent of two or three 5-gallon buckets full of “new” soil around the new tree roots.

There are some rare sites where arsenic levels are over about 200 ppm. Levels this high or higher may cause tree growth restriction unless a large amount of new soil is placed around replanted tree roots. It is too expensive to replace the amount of soil necessary to ensure good long-term tree growth in these few sites.

Stone fruits, especially peach, nectarine,¬†plum and apricot, are quite sensitive to above-normal levels of arsenic in the soil. These fruits will often¬†defoliate in the late summer if planted on arsenic levels of over 75-100 ppm. In the “stone fruit” group, Cherries are the most tolerant to soil arsenic, but less so than apples and pears. There is also quite a wide variation in tolerance to soil arsenic amongst the stone fruit varieties. For instance, “Lapins” cherry seems more sensitive than “Bing”.

Biological Factors that Must be Addressed Prior to Replanting Orchards

replanttThere has been a great deal of attention paid to the biological aspects of soil when replanting an orchard. There is no other aspect of site preparation that is as important as this one. If your site has built up the “Replant Disease”, and almost all have, these living pests will prevent healthy root growth. This disease has great potential to ruin your new orchard, even if you take care of all of the other details of site preparation. Soil pathogens have been controlled on the 1st, third and fifth trees in the picture above.

The severity of replant disease varies from site to site, but this disease complex almost always has a measurable effect on the production potential in the replanted orchard, even if it appears unaffected. Some sites appear to grow trees well, even without special soil preparation, these sites are deceptive. Some sites grow trees moderately well, which will lead to lesser yields, which is not good enough in this competitive day and age. It is not always obvious that your trees are growing moderately, rather than well.

There are many sites that have good quality soil, but after the first orchard is removed, produce weak unproductive trees This is obvious to the grower the first time it happens, and future tree-planting planning should include treatments and/or rootstocks that will help reduce the impact of this disease complex.

replantuPlanting trees on more vigorous roots may give you a larger tree on replant disease sites, but the tree will be sick. Small and sick, or big and sick, the poor yield and fruit quality results are the same. Note the apparent poor health of the three year old Gala on seedling root in the picture to the left. The vigorous root did not overcome the replant disease problem.

There are years of research, demonstration and local grower experience that shows that

  1. replant disease is very common, it occurs on almost all sites,
  2. it is absolutely vital to your economic future that you control it, and
  3. it can be controlled, but you must make a special effort to do so.

To quickly summarize the subject:
As your orchard grows, many microorganisms that attack and damage tree roots build up over the years. These fungi, bacteria and nematodes are not always the same in type and number from one orchard to the next. Scientists continue to find new organisms that are involved and proclaim that they have identified the “one” or two that are to blame. This good research show that there is not one cause, there are many pathogens and other growing conditions that lead to poor tree growth on old orchard sites. This has made the “cause” very difficult to identify, and control of the pests that causes this disease may be variable. If you plant a young tree into soil that recently supported an orchard (or nursery), these varied pests attack the young tree roots and stunt tree growth. The stunting can be mild to severe. The tree rarely dies, but it performs poorly, and may look nutrient deficient and moisture stressed. Trees suffering from this disease may eventually grow to fill their space, but they never truly recover. Yields will not be as good as they would have been if the disease had been controlled before planting.

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See difference replant disease makes 16 years after planting.

There is no way (yet) to control this disease after planting. Sick trees can be encouraged to grow better by proper care and management, but they will never fully recover. It is vital that you “rest” the soil or treat it before planting to reduce the effect of this disease on the growth of young tree roots.

replantrWhile much research has been done on replant disease control, and very interesting efforts continue, there are now only two truly effective ways to treat this disease: First, you may rotate your block out of tree production, ripping and fallowing the land for a year, then growing a non-orchard crop for several seasons. Root sprouts allow the pathogenic organisms to survive. A “fallow” orchard with root sprouts growing is almost the same as a growing orchard, as far as the replant disease pathogen complex is concerned. After five to seven seasons without the presence of active tree roots, replant disease will not be a problem. You may be able to reduce the number of years necessary by planting specific crops during this rotation period. Research on these various crop options is under way.

If you have limited land, this long rotation, even if it is reduced by the growing of various rotational ground covers, is not an economical option. The second, and most commonly used control option, is soil fumigation. Rather than wait 4-6 years before replanting, a grower can choose to fumigate and return to full production in that same 4-6 years. This is the usual more economic choice.

For more details about fumigation for treatment of orchard replanting disease, click here.

Properly applied soil fumigants will effectively reduce the symptoms and effect of replant disease. The effective fumigant choices are presently: Telone C-17 or (C-35) (a mixture of 1,3 DD and chloropicrin), chloropicrin, and the metam sodium or metam potassium products (Vapam, Soil Prep, Busan, Metam, Nemasol, K-Pam, etc.) It is becoming increasingly difficult for anyone except a custom applicator to apply..

To date, treatments with alternatives (such as composts) have not proven effective in full-scale orchard trials.

The cost per acre for various soil fumigants and application methods ranges from about $650 to $800 per acre. If your block has replant disease, this money must be spent, but will be returned many times over during the life of the treated block. If you cannot afford to fumigate, you cannot afford to plant.

Improper soil fumigation is money and opportunity wasted, and may be dangerous to the newly planted tree. Worse yet, poorly applied fumigants may not control the disease. Replacing soil in the tree planting hole will not effectively treat this disease. Adding fertilizers (such as 11-55-0), composts or fungicides to the planting hole soil does not overcome this disease. Growing various plants such as wheat, marigolds, rape, sorghum, and Sudan grass have shown some limited effect, but, to date, tests have been limited and of short duration, and the economics of these treatments are very questionable, (or just plain bad). No silver-bullet soil treatments have been proven truly effective, and all new poorly tested treatments should be considered for test only.

In research trials, the growth effects induced by nontraditional treatments should be compared to the growth of trees properly treated with soil fumigants, as well as to untreated check trees. This is rarely done, as the soil fumigants perform much better than the alternative treatments.

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Washington State University