Soil, Fertilisers & Plant Nutrients

Plants Generally Need Water, Light & Plant Nutrients to Grow. In This Article, I’m Focusing on Plant Nutrients & How Plants Obtain Them From the Soil & Fertilisers

Without food and water, you and I will die. Plants are the same. They need food and water. Plant food is however different from the food we eat. Plants need nutrients such as Nitrogen, Potassium and Phosphates (NPK) and can use them as “straight” chemicals dissolved in water.

In fact, fertilisers used to be sold as “straights” and compounds and still can be today. The alternative to chemical (inorganic) fertilisers is organic fertilisers such as hoof & horn, dried blood,  seaweed, bone meal, poultry manure pellets and liquid feeds derived from comfrey, nettles and other plants. Compound fertilisers are a combinations of fertilisers in one “package”.  For example potassium nitrate is a simple compound that is often used to feed tomatoes. It contains both nitrogen and potassium.

Whether the fertiliser is a straight or a compound it is essentially a concentrated source of plant nutrients in either liquid, granule, prill, powder or pelleted form.

The History of Chemical Fertilisers

Straight and compound chemical fertilisers are a fairly recent gardening innovation. The first manufactured fertilisers were made by treating bones with sulphuric acid. This was in 1899 and produced superphosphate. Bones were then replaced with phosphate rock that replaced bones as the phosphate source. The K fertilizer industry started in Germany in 1861. Nitrogen production commenced in 1913 with the availability of synthetic ammonia.

Before these dates all fertilisers were organic .. the organic movement isn’t new. It just harks back to a previous age before the advent of chemical or artificial fertilisers.

Organic Or Inorganic Fertilisers … Which Should We Use?

We have two types of fertiliser and the choice is between Organic fertilisers such as bone meal, pelleted chicken manure etc and the inorganic, man made, chemical or “artificial” fertilisers. My personal choice is to use organic fertilisers wherever I can, but it’s a personal choice based on a gardening philosophy that embraces natural methods. 

Inorganic or Chemical Fertilisers

…are synthetic and manufactured OR mined from naturally occurring sources. They are commonly used and people often don’t know they are inorganic. Examples include Miracle-Grow, Tomorite  and Phostrogen, which many people use without considering how they are produced. The advantages they offer are many. For example they are manufactured to provide a given amount of plant feed in specific ratios. Example Tomorite has a plant food ratio of 4:3:8. That’s 4 parts of Nitrogen, 3 parts of Potassium and 8 parts Phosphate (NPK .. yes, I know Phosphate being symbolised by the letter K is confusing). 

Organic Fertilisers 

… are derived from animal or plant sources and contain NPK in organic form. Because of this they tend to be slower acting and they have tone broken down in the soil to become what the inorganic fertilisers provide. This is both an advantage and disadvantage. Being slow to break down means they slowly release plant nutrients over a longer period and don’t get leached out of the soil so readily. But the cost is that it takes time for the plant nutrients to become available. 

Two other types of fertiliser are often mentioned and I cover them below. Both are important. 

Controlled Release Fertilisers

…are artificially produced granular of prill fertilisers that are produced to release the chemicals slowly. The speed of release is affected by the temperature, and moisture content of the soil. 

Slow Release Fertilisers

…. are organic fertilisers that soil micro-organisms break down over time. They are also dependant on soil temperature. 

The fact that soil temperature affects the speed of release of both these types is good as it means plant nutrients are produced when the weather is warm enough for the plants to grow. 

What Are Plant Nutrients? 

I’ve already mentioned N, P & K. But they aren’t the only plant foods needed by healthy plants.  But before I go into the rest let’s look at what NPK do in plants. And why they are important. 

Nitrogen (N) is used by plants to produce leaves and other green growth. 

Phosphorus (P) is needed for roots and the growth of shoots. 

Potassium (K) is important for flowers and therefore fruit, and for general plant health and hardiness.

This is a generality as they are also used for other reasons, but in smaller amounts.

But though the above nutrients are vital they are not enough on their own. Plants need other nutrients. It’s a bit like humans. We need Protein, Carbohydrates and Fats but on their own they are not enough. We also need smaller quantities of things such as salt, iodine and calcium. 

So it is with plants. And that is what the next section deals with. 

Plant Nutrition: Macronutrients, and Micronutrients

NP&K are classified as macronutrients. 

Other macronutrients include sulphur, calcium, carbon, oxygen, hydrogen and calcium. Each is important, indeed essential. 

The micronutrients include zinc, iron, boron, nickel, copper, molybdenum, and are needed in trace amounts. Hence they are often called trace elements. But they are often included in fertilisers as accidental “contaminants” or on purpose as in boronated fertilisers. Boronated fertilisers are more common in commercial use and increase yields in some situations eg in maize (corn), alfalfa (lucerne) as indicated in research  by Mortvedt  . 

Micronutrients are normally taken up by the plant as ions and are present in minute amounts, ranging from 0.1 ppm (parts per million) to 200ppm. Many soils have sufficient trace elements available for normal growth, but they become unavailable in some situations, especially where crops are high yieldings. 

Justus Von Liebig

Von Liebig is probably someone you’ve never heard of but he explained how plant growth is always limited not by the plant nutrients that are abundant, but by those that are scarce. It doesn’t matter how much NPK and other nutrients are available if one is in short supply. Thats why boronated fertilisers are important to maize growers. If all other nutrients are in abundance but boron is missing or short then yield is affected.  Ditto any other nutrient in short supply. 

How Do Plants Take Up Plant Nutrients?

Most plant nutrients are taken up through the roots. In these cases the nutrients are dissolved in water and the roots absorb water containing plant nutrients. It then removes the nutrients and expels surplus water through stomata which are mainly found on the leaves. The process is called transpiration. Some nutrients can be absorbed via the leaves, more on that later. 

There are three ways in which plants take up nutrients through their roots.

• Simple Diffusion 
• Facilitated Diffusion
• Active Transport

There’s more detail about these processes on Wikipedia 

Nitrogen and oxygen are mainly absorbed from the air via the stomata. The same stomata are also used to expel carbon dioxide. 

How To Use Fertilisers

How to apply fertilisers is something gardeners often ask me. So here’s a brief run down on fertiliser application methods.

Base Dressing

….is the term used to describe applying fertiliser to the soil or compost prior to planting. The fertiliser can be organic or inorganic. Often is it incorporated by rotavator or other cultivation methods. 

Top Dressing

…is the application of fertiliser to the soil surface. Usually this is around growing plants to encourage further growth whilst not disturbing the soil and hence roots. Care should be taken to avoid getting the fertiliser on the leaves as this could result in leaf scorch. Care is also needed to avoid over application as the concentrated fertiliser could also burn any roots close the the application site. Sometimes the top dressing can be lightly incorporated by hoeing. In some cases it can be watered in but often it is left to its own devices!

Irrigated Fertiliser Application

…. is where the fertilisers is dissolved or diluted in water and watered on via a watering can or via an irrigations system. Where overhead irrigation is used this means the leaves get wet so the fertiliser must be dilute enough not to scorch the leaves. In some cases drip irrigation is used to apply soluble fertilisers to the soil and hence roots without the risks attached to overhead watering. Drip irrigation also reduced evaporation and is the method I used to water and feed my commercial tomato crops.  

Foliar Feeding

…is where fertiliser is applied via the leaves. It’s good way to treat nutrient deficiencies as it is very quick acting as the nutrients are where they are needed. An example would be the use of a magnesium foliar feed where there is a magnesium deficiency in crops such as tomatoes. In this case the deficiency causes chlorosis and can be treated with a dilute solution of  magnesium sulphate dissolved in water. 

Some authorities say the problem can be eliminated in subsequent years if magnesium sulphate is added to the soil as a base dressing. However, the problem isn’t a lack of soil magnesium but the inability of the high yielding paint to mobile the magnesium. Therefore it needs to be applied as a foliar feed or Von Liebig’s Law applies.  

Nitrogen Fixing Bacteria

Legumes, such as pea, beans, clovers etc use rhizobium bacteria to fix atmospheric nitrogen.  The bacteria colonise the cells within root modules and form an endosymbiotic nitrogen-fixing relationship.
Never remove the roots of legumes, cut the stems at ground level and leave the roots and their nitrogen for the next crop.  Because leafy crops require nitrogen it’s good if you follow legumes with leafy crops  

No Dig Fertiliser Application 

…is a form of top dressing in that all the feed goes on to the soil surface. This is usually done in late autumn or winter and the compost is the source of all plant feed for the following year.  No further fertiliser addition is generally considered necessary .. except if a foliar feed is needed in similar circumstances to the magnesium deficiency cited above. 

Making Your Own Fertilisers

It’s certainly possible to make your own fertilisers. The use of a No Dig compost is one such example. People usually however think of making their own fertiliser aw being making something such as comfrey tea or using the leachate from wormeries. 

We are often told that comfrey is rich in potassium so good for tomatoes and that nettles are high in nitrogen (N). But this isn’t true, and I have a further problem with it. There’s no way of telling how much is in the tea you make for how much it needs diluting.

Manufactured fertilisers don’t suffer these problems. And nor does compost used in No Dig.  Manufactured fertilisers show the amounts of nutrients on the box. And compost is a mix of some many things that, though not normally analysed, the levels are more consistent. 

My personal feeling is that where I’m growing a plant and work hard to ensure it has the right amount of water, adequate temperatures, pest control and lots off other variables, I don’t want to take a risk on the uncertainty of the feed. I’m very happy for the fertiliser to be organic. But I want more certainty that a tea of some sort can offer.