The Soil

The soil


We all know hostas are easy to grow, very rewarding perennials.  When you put them in a spot that isn't to extremely light or dark and were they get plenty of water, they will grow into mature specimens, that can stay in that same spot for years.  Most of the time hostas perform so good without any extras it's hard to believe that with some extra soil preparation and maintenance they even grow a whole lot better.  Hostas are greedy feeders, so a good soil fertility will be very beneficial to them.
Soil fertility is the ability of the soil to provide a plant with the necessary nutrients.
As a gardener, you can control this "soil fertility" to a very large extend.
That's the reason why you should have your soil tested before you start.  Taking into account the present condition of the soil and the plants you want to grow, you'll get a tailor made advice.
You can test the soil yourself with a DIY soil testing kit, but this way you're missing the most important information, the advice.

Soil fertility - theory

In soil fertility there are three factors, clearly distinguishable, but inseparably interconnected.

  • chemical soil fertility
  • physical soil fertility
  • biological soil fertility

Chemical soil fertility

Nutritional value

Chemical soil fertility, the nutritional value, is determined by the chemical composition of the soil, in particular by the mineral nutrients. It can be altered to the desired level desired by adding fertilizers.


The acidity level (pH) of the soil is a very important factor, because every species has its own optimal pH. 
The pH scale runs from 1 to 14, where 1 is extremely acid, 7 is neutral and 14, extremely alkaline.  Each step on the scale represents a factor 10.  A soil with pH 5 is 10 times more acid than a soil with pH 6, 4 is 100 times than 6.

Hostas prefer pH 6, with a margin from 5,5 to 7.  When the values become to extremely acid or alkaline, the hair roots on the root tips will "burn", thus preventing the plant from feeding properly.  Furthermore specific chemical reactions occur in extremely acidic or alkaline soil, in which some nutrients form a chemical compound, making them unusable for the plants or, even worse, the new compounds are toxic.  The solution for this problem is not to apply more fertilizer, but to change the pH to the required level.  To achieve this, you will really need a soil analysis and professional advice on soil preparation.

Sandy soils tend to be more acid (pH 4,5 to 6,5) than clay soils (pH 6,7 and higher). pH can be raised by adding lime, it can be lowered by applying acid ammonium sulphate.

If your soil is very alkaline (8 or more) it's not a good idea to try and lower the pH; it would cost you a fortune to lower it sufficiently and for a longer time.  Go for a collection in pots or try lasagna gardening.


Composition of fertilizers

The nourishing components in fertilizers are divided into three categories:

  • primary main elements,

  • secondary main elements

  • trace elements

Primary main elements

The primary main elements are the main plant nutrients.  They are the base components for mixed (NPK) fertilizers, but can be given separately as well.

Nitrogen (chemical symbol: N)

Nitrogen stimulates the development of the vegetative parts of plants, like stems and en leaves: aka general growth. It's part of the proteins in the plants. When no extras are supplied, soil tends to have a rather low nitrogen level, especially in soil that's poor in humus, as nitrogen washes out easily.

Flowering plants shouldn't receive to much nitrogen: it causes smaller flowers, irregular development and plants that get sick more easily. During the main growth the plant needs sufficient nitrogen.

Horn meal

Horn meal contains 13% nitrogen with a slow release, prolonged activity. It's made of grinded hooves and horns. Horn meal is rich in organic compounds that stimulate growth. It can be applied during the whole season and is active for 6-8 months.

Blood meal

Contains 13 % fast working nitrogen. Blood meal also is rich in organic compounds.  Because the nitrogen is fast working it's an ideal addition is you're a bit late with the first fertilization in spring.

Phosphor (chemical symbol: P)

Phosphor (as phosphates) helps growing strong roots and flower stems with plenty of flowers and fruits. Many soils have a shortage in phosphates, as it washes out easily. To much phosphor in the soil isn't harmful for the plants; they only take what they need.

A little extra phosphor in spring will provide for good blooming an seed setting later on.

Bone meal

Bone meal is extremely beneficial in the root forming, not only in mature plants, but also with seedlings.  And, of course, a seedling with a good and large root system will grow faster and better.

Potassium or potash (chemical symbol: K)

Potassium generates and transports carbon hydrates. A correct dose of potash gives you plants that are more resistant to plagues and diseases.  It also provides good blooming, with bright colors and a stronger scent.  It improves the ability of the roots to take in water.

In general, the earth crust contains ample potash.  Especially in places where there used to be a sea Potassium salts occur.  This can even be a disadvantage, as in soils that are rich in potash, plants usually suffer from magnesium deficiency.  Potassium tends to be washed out in sandy soils, not in clay soils.

Yellow or dead leaves? Give your plant some extra potash.

Patent potash

A potash fertilizer with a high content in magnesium and sulphur.  The magnesium in patent potash is kieserite.  Patent potash is admissible as a fertilizer in biological agriculture and horticulture.  It's available in Europe, but I couldn't find a US equivalent.

Secondary main elements
Magnesium (chemical symbol: Mg)

Magnesium is important in getting a good green leaf color.  It's is mainly administered to plants and crops where the leaf color is important (evergreen shrubs, hedges, lawns and  ... hostas.
It's also an essential element in the plant's metabolism, providing a good quality of growth.


Kieserite is a natural product that contains about 26 % of magnesium. It's the perfect supplement for every plant's the NPK menu.  It's used in the production of Epsom salt.

Lime (chemical symbol: Ca)

Lime regulates the pH of the soil.  When the soil is to acidic, some minerals won't get converted into nutrients, thus leaving you with plants with signs of deficiency: yellowing, diminished growth rate, ...  The need to ad extra lime occurs more frequent when you use inorganic fertilizers.  When you use organic fertilizers, lime deficiency is rare.

Lime also plays a role as a nutrient, as a building substance for cell walls. 

Seaweed lime

Not only does it take care of pH regulating and cell building, it's good for improving the soil structure and activation of micro organisms.  Contains trace elements.  Only to apply on acid soils.

Sulphur (chemical symbol: Su)
When compared to the primary main elements NPK, plants only need a little sulphur. 
Nevertheless it is a necessary elements in:
  • the forming of proteins and amino acids
  • a good water balance in the plant (in the form of sulphate S04)
Trace elements

Iron (Fe), Zinc (Zn), Copper (Cu), Molybdenum (Mb), Boron (B), Cobalt (Co).

Trace elements play an important part as regulators of the metabolism processes in the plant and the conversion of minerals into nutrients.  They have a positive influence on the plant's resistance to diseases.  When you use organic substances, a shortage in trace elements almost never will occur.

Beware: some trace elements are toxic to man and animal, even in small doses.  As a gardener, you have to be extra careful in the vegetable garden, and never add trace elements if there isn't a confirmed shortage.

When leaves start yellowing, it's important to find out what's the cause: iron shortage, magnesium shortage, to much potash.

Basalt meal, Lava meal

Very fine grinded volcanic rock, that contains a high concentration of silicon, lime, magnesium and trace elements.
It gives a preventive crop ore plant protection against diseases.
It also has the positive property to hold nitrogen and prevents the washing out in sandy soils.  It's a good addition for building the ideal compost heap and it improves the quality of potting soil.


Mixed fertilizers - NPK classification

The NPK classification first was used together with the introduction of artificial fertilizers.  The chemist Van Liebig introduced NPK at the end of the 19th century. He studied ashes of burned plant material and initially only found the nitrogen (N), phosphor (P) en Potassium (K) .

Nowadays NPK classification is also used with commercial organic fertilizers.

On a bag of fertilizer, you could find the following indication:

8-4-4 (+2)

This particular fertilizer contains:

N P K Mg
8  % 4 % 4 % 2 %

Guano is a natural deposit of dried animal dung (mainly sea birds and bats), and is renowned for the very high concentration of fertilizers it contains. Guano is rich in phosphates, nitrogen and potassium.  Therefore it used to be harvested intensively in the past to be used as fertilizer.  This had a negative impact on the environment, because the nesting grounds of the sea birds were completely destroyed, so now it has been replaced by other fertilizers.

Osmocote, Miracle Gro

Osmocote and Miracle Gro are resin pellets with a fertilizer that has a controlled release.  They have been developed primarily as a base fertilization for pot plants and tree nurseries.  The nutrients are released over a prolonged period of time, depending on the specific composition of the product. They contain the main fertilizing elements and trace elements in every pellet.  These product release the nutrients adapted to the plant's needs: it is the resin coating, triggered by the soil and air temperature, that determines the daily release of nutrients.  This release pattern isn't influenced by other external factors..


Physical soil fertility

The perfect balance of air and water

Plant roots get the necessary air, water and food from the soil.   If they don't get enough of any one of those elements, they will not grow well, they can get ill and, in the worst case, they will die.
The ideal soil isn't to dense, so the roots can penetrate it easily, it has good drainage and there is enough oxygen available for root growth en for the intake of water and nutrients through the roots and the evacuation of carbon dioxide produced by the root system.  It isn't to loose neither, so it can hold enough moisture and the nutrients don't wash out to easily.

There are some extreme types of soil: clay soil, very fine sandy soil and coarse sandy soil. In a pure clay soil, the roots will have a hard time penetrating into the soil, regardless whether the soil is dry or wet.  Furthermore it has, like very fine sandy soils, a very poor drainage, so it will hold to much water and not enough air. 
Coarse sandy soil doesn't hold moisture very well.  Some minerals (like nitrogen) will wash out to the deeper soil layers very soon, making them unavailable for the plants and the soil itself dries very fast.

It may seem odd, but part of the solution for solving those problems is the same for the three types of soil: adding organic matter.

If your top soil is extremely gritty, you should add 10-20 % of clay tilled in the top 12-15 inches.

Pure clay soils can be made more plant friendly by adding a gritty substance, like coarse, sharp sand.

A common mistake is to adjust the soil structure only when major landscaping is done or when plants are planted in the garden, after which the soil structure is neglected for years.  It's of great importance to keep maintaining the optimal physical soil fertility year after year.

Soil amendments

There are a lot of substances that can be used to get a better soil structure, stuff like perlite, vermiculite, sand, ..., depending on the type of soil.  In my opinion, a substance that is not only good for the physical soil fertility, but also has a positive effect on chemical and biological soil fertility is the better choice.

Compost, the gardener's black gold

Compost can contribute enormously to achieve a good soil fertility.

It adds slowly releasing nutrients to the soil, so it's beneficial to the chemical soil fertility.
It contains masses of micro (and not so micro) organisms, thus giving the biological soil fertility a tremendous boost, especially of you use compost that's not completely rotted.

The most beneficial effect however is on the physical soil fertility.  Adding compost to the top soil when landscaping a complete garden or a plant bed will leave you with a soil with a good air and water balance.  Till it in thoroughly to get a good soil - compost mix.  If you have the time, let the soil rest afterwards for a couple of weeks. To much freshly applied compost around the roots may cause the roots to burn.  Furthermore, compost needs oxygen in the decomposing process, oxygen that is also needed by the plants.  An overdose of fresh, partially decomposed compost close to the roots can result in plants that stop growing for a wile.

If you have a gritty, sandy soil, well rotted, fine compost is the thing for you. It's rich in humus and nutrients.  Well rotted compost is black and crumbly, it's almost impossible to recognize any plant parts.

For a compact clay soil, partially rotted compost is the thing to go for.  It still contains some coarser plant parts, that will bring more air into the soil.

Because compost breaks down in the soils, it's necessary to add a fresh load every year.  This can be done as a mulch layer, 2-3" thick.  It'll prevent weeds from growing, diminishes water evaporation and it keeps the soil (and the roots) cool.  Rain worms and other soil insects will take care of the dispersion in the soil.

Animal manure

It's common knowledge that fertilizers of animal origin contain plant nutrients; most people don't realize that they have even far greater value as additions to improve the soil structure.  The easiest and safest manner to use manure is in a dried and sterilized form, free of weed seeds and pathogens.
Always follow the guidelines on the bag or box.

Fresh animal manure can be added to your compost heap, where it can ripen.

Composition of animal manure (averages)

  Nitrogen Phosphorus Potassium Calcium Magnesium Organic
  (N) (P2O5) (K2O) (Ca) (Mg)    
% % % % % % %
Cattle 0.5 0.3 0.5 0.3 0.1 16.7 81.3
Sheep 0.9 0.5 0.8 0.2 0.3 30.7 64.8
Poultry 0.9 0.5 0.8 0.4 0.2 30.7 64.8
Horse 0.5 0.3 0.6 0.3 0.12 7.0 68.8
Swine 0.6 0.5 0.4 0.2 0.03 15.5 77.6
% % % % % % %
Cattle 2.0 1.5 2.2 2.9 0.7 69.9 7.9
Sheep 1.9 1.4 2.9 3.3 0.8 53.9 11.4
Poultry 4.5 2.7 1.4 2.9 0.6 58.6 9.2

A very light sand soil will get a much better physical soil fertility by adding 10-20 % of clay to the top layer.


Bentonite is a natural clay mineral, grinded to a very fine powder, that can improve the water retaining capacities of sandy soil and (peat based) potting soil considerably.

Biological soil fertility

A healthy soil life will break the minerals and the organic substances that aren't decomposed yet down into nutrients the plants can take in, and it supplies the soil with air. Earth worms play an important role in this process, but the bulk of the work is done by micro organisms, that occur in massive numbers in healthy soil: bacteria, moulds, yeasts, algae and one cell organisms: they break down the organic matter slowly but surely, thus providing nutrients for plants for a longer period of time.


Organic or inorganic

IMHO the use of organic substances instead of inorganic has a number of benefices.

The use of organic fertilizers, like in manure, compost, seaweed extract, blood meal, fish meal, bone meal, mixed organic fertilizers is beneficial to the chemical soil fertility in the first place, but it also has a positive influence on the physical soil fertility: the water balance is much better, the soil holds the nutrients much longer. The biological soil fertility will be much better, as the soil life gets an enormous boost.

Organic fertilizers:

  • Work slowly and longer because most of the minerals in it have to be broken down in the soil into nutrients that are useable for the plants;

  • the composition sometimes is less balanced (the mineral percentage may vary)

  • Stimulates the growth of soil organisms,

  • Provide for a better soil structure; it's sometimes hard to determine if a substance should be primarily considered to be a soil amender or a fertilizer (compost, cow dung, ...). The value of these substances lies more in their ability to improve the soil structure than in their limited nutritional value.

  • It's hardly ever required to add any extra calcium.

Inorganic fertilizers are artificial substances, man made or dug in nature in a pure chemical form. They:

  • Work quickly,

  • Have a balanced composition;

  • Are less  beneficial for the soil life;

  • Don't contribute in the amelioration of the physical soil structure;

  • In a mixed form they often target a specific group of plants, like roses, fruit trees, ...

  • In time they will cause the soil to get more acid.  It will be necessary to add calcium on a regular base. to neutralize the base pH.

When the physical soil structure is improved by adding compost, manure or other substances, the plant will also receive a limited amount of nutrients and soil life will get a boost.

There is in fact no other way to improve the soil life than by adding living, organic matter.  And an optimal soil life helps to release the nutrients in minerals.  

In an ideal mix plants will not only grow better, but will also be healthier, less prone to illnesses and they will resist better to unfavorable climatologic conditions.


Soil fertility - practical

How to fertilize hostas

In my opinion, as an amateur gardener, it's best to use organic fertilizers, because they not only improve the chemical soil fertility, but also the physical en biological condition of the soil.
Furthermore the chance of getting root burn are much slimmer than with artificial fertilizers, because in artificial fertilizers the nutrients are released over a rather short period of time.

When deciding on which composition of fertilizers to use, is a good idea to take in account the specific growing season of hostas and the effect of individual nutrients.


It's important to know when hostas start to emerge in your area.  Because the organisms in the soil need some time to break down the minerals into nutrients, you should start applying the fertilizer about a fortnight before your plants start to grow.

The main activity in spring is the growing of the leaves.  This requires primarily nitrogen for the growth itself and magnesium for a good leaf coloration.

As a base fertilizer, a mixed fertilizer 14-10-8 (+4) or 7-5-4 (+2) will do nicely.  It should be applied following the instructions on the bag or box.  If your mixed fertilizer doesn't contain any magnesium, use some extra kieserite.

Summer - Autumn

Starting in June or July, not only a second flush of leaves appears, but, more important, the root system expands and a little later the flower appear.  Therefore it's wise to use less nitrogen and some extra phosphates.  This can be achieved by applying half the dose of mixed fertilizer, completed with a gift of gift bone meal en some patent kali for desert.  This fertilizing scheme can start in June.

Towards the end of the growing season, it's  not a good idea to give much nutrients, especially nitrogen.  The plants need to prepare for winter.


A better soil structure

This is to me the key to get great hostas. 

Hostas need lots of water, so a good, balanced soil structure is extremely important to them.  They need a soil that will retain water very well, without water logging, and from which nutrients aren't washed out easily.  I hope I didn't forget to mention hostas need a lot of water.

They will do well in heavy clay soils, provided a large amount of organic matter and some coarse sand is added.  This allows the roots to penetrate the soil well, they get the air they need and the water balance is perfect.

It's possible to transform a dry, sandy soil into hosta paradise in the same way: add loads of organic matter and a portion of clay if possible.

A fresh mulch layer should be spread out at least once a year, if possible twice.  It's a good insulation, keeping the plants warmer during the cold months of winter and cooler when the summer heat tries to burn them.

Important: be careful with "green compost"

The compost I use is "green compost".  It's produced in large composting installations from collected garden and kitchen waste.  It contains a high percentage of green waste, like vegetable leftovers, grass cuttings, ...  This produces a very rich compost, that can be rather aggressive to plant roots when it's fresh.  "Brown compost" is made mainly from wood chippings, bark, tree leaves, etc.  Its nutritional value is lower, but is much safer to use.  It can even be used straight as potting soil.

As I experienced myself, hostas are likely to stop growing or even die when you use to much "green compost" with container grown plants.  My advice is never to use more then 25 % "green compost" mixed in the soil, and to additionally use it as a mulch layer. 


A healthy soil life

When you make sure your soil has a good chemical and physical fertility, the ideal biological fertility will follow.

Adding organic matter that isn't fully decomposed is ideal and recommended to get and maintain a healthy soil fauna and flora.


Hostas on steroids

There are a number of substances that have an extremely beneficial influence on hostas in particular.
I have to admit I haven't tested these substances myself (yet); that something to do in the years to come.


Alfalfa is a member of the family Fabaceae, that contains a chemical substance, triacontanol, a plant hormone.

Using alfalfa will:

  • get the plant out of winter dormancy earlier (it seems to me that isn't always a desired property);

  • double a plant's weight in one growing season;

  • realize the normal growth over a period of three years in one year (this one sounds better to me);

  • result in a much larger root system;

  • double number and size of flower buds, flowers and seeds.

The original article can be found here.

Alfalfa is sold as straw or as pellets.  It's often used as horse food.
It can also be found in tablet form as a food supplement.
The straw, pellets or tablets can be used to make a "tea", that is poured around the plant about 5 times every growing season.

You can add the pellets to the top soil or use chopped straw as a mulch.

I bought a small bottle of concentrated alfalfa tea last year (2006) and started watering my plants with it in 2007.  I'm hoping to show you the fabulous results in the "fertilizing experiment".



BENZYL AMINO PURINE (BAP) is a chemical plant hormone. 
It's usually known as BAP-10, a 10 % solution.

Application: spraying the leaves (1000 to 3000 parts BAP to 1.000.000 parts water).

Effect on the plants:

  1. the dormant buds will start to grow immediately;

  2. within a few weeks, new eyes will form, that will produce leaves a couple of weeks later;

Possible disadvantages:

  1. doesn't seem to work with all types of hosta;

  2. the newly formed shoots can be smaller and weaker;

  3. when applied during late summer or autumn, it will reduce hardiness;

  4. spraying seedlings to early can cause the roots to burn and total loss of the plant.

I haven't tried it out myself (yet), but the reactions of addicts are very positive.

When used correctly, it's an ideal aid for fast growing of seedlings and sports.

The original article can be found here.