Fruit Orchard on the Slope: Soil Analysis and Amendments

Been thinking about that slope: will the soil support what we ultimately want to plant there, and how do we best prepare it?

This south-west facing slope, relatively sunny – somewhat shaded from the southeast and northwest, more so as you go further down the slope – will be a fruit orchard: we’ll plant blackberries and blueberries on top, and currants, gooseberries and elderberries further down. We’d also like to stick some semi-dwarf fruit trees in there if we can (cf. Garden Plans for 2013 and Beyond). We’ll coordinate all these in guilds, of course, at least at first so the guild can nurse them to maturity.

Michael Phillips’ basic recommendations for the rhizosphere (root-sphere) of an orchard are:

  1. pH in 6.3-6.7 range
  2. Calcium (Ca) between 2000-3000 lbs/acre, phopshate (P2O5) and potash (KO2) both at least 200 lbs/acre
  3. carbon-rich, fungal, porous
  4. organic matter (OM) a minimum of 3%, better 5% and above

In 2009 we had a soil test done of the soil in the vegetable patch before any plants went in. The situation in the veg garden has changed quite a bit, I should hope, and a new test is planned. We never really tested the soil on the slope, which is mainly subsoil dumped during the work on the septic system before we bought the house.  When we terraced it we added brought-in loam and spread quite a bit of compost (for the strawberries), but it wasn’t as intensively taken care of as the veg garden soil. The soil in the broad path didn’t even get that. There especially the erosion continued. So, another soil test is in order before we begin on that slope. But while waiting for the soil to defrost and dry out, I’d like to play around with the old test results and practice my “soil detective” skills.

In the following I rely heavily on Phillips’ incomparable study in Holistic Orchard (p.61-74). I also refer the undaunted reader to my Calcium in the Soil Series, a very long but (I think) valuable explanation of soil test results and some of the soil chemistry that is relevant here.  That series starts here.

  • pH and CEC

The pH at 6.4 – 6.5 looks good. But, as Michael Phillips writes, it’s the cation exchange capacity (CEC) and percent base saturation that are truly indicative (cf. Part 2 of the calcium series).

The CEC of a soil indicates how porous a soil is nutrient-wise. Our soil is 15.6 MEG/100g. That means that, in every 100 grams of our soil, 15.6 meq of soil can hold onto the goodies, both basic and acidic: calcium (Ca), potassium (K) and magnesium (Mg), that come along in the soil water, as well as hydrogen (H), and sodium (Na ) and aluminum (Al), which are not plant nutrients.  All this also indicates a fine-textured loam to clay soil and that figures with our observations of our soil.

According to Calcium in the Soil, Part 6, the percent base saturation data mean that, of the 15.6 meq that can hold on to cations, 7.9 meq is occupied, or saturated, by calcium (50.6% of 15.6 = 7.9), 1.65 meq by magnesium and 0.64 meq by potassium. So 10.19 meq/100g of soil, or 65.3% of the CEC, is saturated by bases. That leaves 35.3% of the CEC (*) for the acidic cations (hydrogen and aluminum). That explains the pH and indicates a fertile, slightly acidic soil. Acidic soils (3.5-6.0) are low in fertility because too much of the CEC is occupied by hydrogen or aluminum. Alkaline soils (8.0-9.0) are oversaturated with calcium and/or magnesium.

The fertility of this soil can be increased by adding organic matter. My soil test didn’t include an organic matter measurement, but it must be low. In any case, before contemplating this, there are more mineral considerations to be had:

  • Ratios of Ca:Mg:K

Magnesium pulls soil closer together, while calcium spreads the particles further apart. Clay soils require higher levels of calcium to improve porosity, thus drainage and aeration. The Ca:Mg ratio for us is 50.6:10, or 5:1. A clay soil that is porous enough and that is balanced (so that enough of each cation is available for plants, not tied up) should have a ratio of 7 or higher to 1.  A 5:1 ratio more resembles the nutrient holding capacity of sandy soil. Something is off here. Now enter potassium (K). According to Phillips, a good  Ca:Mg:K  ratio for clay soils  is 76:10:4-5. Ours is 50.6:10:4.1. The ratio between magnesium to potassium is spot-on for clay soils, but the main player, calcium, again throws it off.

This means one of three things: 1. either our soil lacks the calcium to make it porous, or 2. the levels of magnesium and potassium ares too high, cancelling out the effect of the calcium, or 3. both.  We’ll have to take a closer look at the absolute numbers, which we’ll do below.

  • Recommended absolute levels for macro-nutrients

Phillips’ recommendations for good orchard soil indicate optimal lbs/acre, but my soil test gives me those numbers but in ppm (parts per million). Luckily Phillips addresses this in a footnote (chapter 3, footnote 47 in case you’re curious).  The conversion formula (called the Cornell equivalent) is (Ca in ppm x 0.75) x 2 = Ca in lbs./acre.

CALCIUM. Calcium benefits the fruit’s skin and cell strength, which leads to lower bruising susceptibility, better keeping ability and better pathogenic fungi resistance. Phillips’ bare minimum total Ca for an orchard = 2,000 lbs/acre for a lower-CEC-value soil (below 25 CEC). Ours is 1548 ppm, so 2322 lbs./acre [(1548 ppm x 0.75) x 2].  Our calcium level is good. (The ppm bar chart on the soil test say it is too high – actually, off the charts – but this interpretation was for vegetable garden soil, not for orchards.)

NITROGEN. Phillips explains this so well. Most nitrogen in any soil is locked up in organic form (as protein) and needs to be converted into mineral nitrogen that can be taken up by plants. This conversion start with the protein form of nitrogen being ammonified, and a portion of the ammonified nitrogen can then be nitrified. This is done by bacteria and fungi who constantly immobilize (take up) mineralize (release) it by digesting it and the other soil microorganisms who have absorbed it. In a soil dominated by bacteria, nitrifying bacteria rapidly convert the ammonified nitrogen into nitrates. However, in a fungally dominated soil, the acidic enzymes produced by the fungi will lower the pH, making it unfavorable to nitrifying bacteria.  More of the ammonium therefore remains available. It is this kind of nitrogen (ammonified, not nitrified) that is preferred by woody perennials like berries and fruit trees. Too much soluble nitrogen causes problems with calcium and other mineral uptake. High levels of nitrogen, particularly as nitrate, encourages fungal diseases like powdery mildew and rust, as well as bacterial diseases. That our soil is fungal is indicated by the low level of nitrate (NO3-N) on the soil test, but…

PHOSPHORUS (P).   The right amount of phosphorus determines the nutrient density (Brix) of the fruit as well as root development. Phosphorus too is  a very fungal affair. It is made available by fungi that feed and then die and decompose and delivered to the plant by mycorrhizae. In biologically managed soils, potassium is constantly replenished by the decomposition of organic matter. Phillips recommends phosphate (P2O5) to be at 200 lbs/acre, or P levels at 43 ppm. Our P is only 12 ppm, a marked deficiency in phosphorus. This indicates something wrong with the “fungal machine”  in my soil, no doubt because it was at the time of the test so disturbed and eroded. Phillips writes that getting this phosphate system working is challenging. You kind of have to already have in order to get it. The trick here seems to be organic matter: a good quantity of that with a good population of beneficial fungi in balance with bacteria (brought in by enough, not too much nitrogen) should do the trick. Ha! I will have to do some more research here. Maybe now, after several years of non-disturbance and checked erosion, the phosphate levels are up again?

POTASSIUM (K). Phillips recommends 20o lbs/acre of potash (KO2) or P levels at 83 ppm. Our potassium level is very high at a whopping 243 ppm. As we saw, potassium plays a large role in the cation balancing act. Our high levels of K  are which is reflected in the skewed  Ca:K ratio and the recommended 1:1 to 1:2  ratio for P:K  is also well off.

CONCLUSION. If the new test on the soil on the slope comes back looking like this, then it seems like we will need to bring the Mg and the K down, the P up. The Ca and pH can remain the same.

One recommendation I found was to add gypsum to leach out the excess potassium and magnesium. Gypsum (calcium sulfate) would also up the calcium without changing the pH (which is fine).  It also helps slow the nitrate release of decomposing organic matter. However, Phillips warns that the calcium cation saturation needs to be over 60% before adding gypsum  to lower excess magnesium, otherwise the sulfur in the gypsum will take out the calcium first. Mmm. Then the potassium will need to be increased. Wood ash seems a possible candidate for this: it is 20-30% calcium, with 4% potassium, but only 2%  phosphorus, magnesium, aluminum and sodium. It may, however, increase the pH, and also, because of its potassium content it should be applied only when active growth has engaged, so wood ash could be my liming agent after planting…

A new soil test is in order, because these numbers are just too out of whack for me to make sense of. One thing I know for sure, though: we will also want to add lots of organic matter. That’s where the hugelswales come in. And that’s another post.

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