Soil Fertility

Soil Fertility:

As you probably know, farmers grow crops on their land to produce food. Important food crops worldwide are maize, rice, wheat, potatoes, and cassava. What is important to farmers - is crop yield and also crop quality. Farmers want to realize an optimal crop yield, with a good crop quality. The key to this, is to have a fertile soil.

So, what is a fertile soil?

When plants are growing, they form roots in the soil. Soil is composed of minerals, organic matter, air, and water. Plants use their roots to take up nutrients, which they need to grow. But plants can only take up nutrients from water. In other words, plants do not eat soil, rather their roots take up nutrients that are dissolved in what we call the soil solution. One important aspect of a fertile soil: its capacity to supply nutrients that are essential for plant growth to the plant roots, in adequate amounts at the correct time. The soil’s capacity to do this, supply nutrients to plants is what we call chemical soil fertility. Another important aspect of a fertile soil is a good soil structure, so that plants can penetrate and explore the soil with their roots. A fertile soil also has to supply sufficient water to plants. Soil organic matter and soil life are very important for soil structure and soil water retention.

When talking about essential nutrients, what do I mean exactly? A nutrient is a chemical element and it is essential for plant growth when a plant is unable to finish a normal life cycle without it

and it has a direct role in the plant metabolism and cannot be replaced by any other element.

In this table, you can see a list of the sixteen nutrients that are essential for plant growth. Oxygen, Hydrogen and Carbon are taken up by plants from the air. The other thirteen nutrients that are listed in this table have to be supplied to plants by the soil. Let’s now have a look at the numbers that you see for each nutrient in this table. Each number represents the relative content of a nutrient in an average plant. With the term relative, I mean that all nutrient contents are calculated as a fraction of the nitrogen content. So nitrogen is the most important nutrient. Nitrogen, potassium, calcium, magnesium, phosphorus and sulphur are called macro nutrients, because they are needed in large amounts. Plants also need other nutrients to grow, like chloride and boron. They are called micro nutrients, because plants only require very small amounts of these nutrients. When the ability of a soil to supply these fourteen nutrients to plant roots falls short, plant growth will be limited. I will illustrate this with the so-called “Law of the Minimum” from Justus von Liebig, a German chemist who lived in the 19th century.

This picture shows what is called “Liebig’s barrel”: the staves of this barrel have an unequal length

and the water volume in the barrel depends on the length of the shortest stave. Just like the water volume of the barrel, plant growth is limited by the nutrient that is shortest in supply. Only one nutrient can limit plant growth at any given time. Most often this is nitrogen or phosphorus.

 Nutrient Deficits and Effects on Crop Production:

In many places around the world yields are low, much lower than what farmers hope to realize. The consequences of low yields are well known: pictures of populations that are undernourished are just one example. Cases of persistent hunger have grave consequences, for the people themselves, for the next generation, and for societal well-being. Such situations can easily lead to societal unrest. Next to the amount of food produced, there is a further issue of food quality. Low yields often go together with a shortage of essential nutrients in the food, and this may negatively impact health, especially of women and children. For instance, zinc deficiencies in food slow down the intellectual development of young children, so that future generation also suffer from low yields and low food quality.

The causes for low yields are variable. In some cases low yields are due to unpredictable weather, for instance a prolonged dry period. In other cases external plagues are the problem, for instance when massive swarms of migratory locusts consume the crops. In many cases the root cause is low soil fertility, where nutrient supply seriously limits plant growth and productivity of crops.

If we look at plants under conditions of low soil fertility we can observe symptoms of shortage of essential elements. For instance, leaves with purple margins indicate that phosphorus is limiting growth. Similarly, yellow-green leaves often suggest a lack of nitrogen, as nitrogen is a major requirement for the photosynthetic machinery. That is why healthy plants usually have a dark green color. However, other nutrient deficiencies can also result in yellowish leaves, for instance iron shortage that often occurs on calcareous soils, a phenomenon known as lime chlorosis.

Other symptoms of nutrient deficiencies are more difficult to observe, and training is required to recognize which essential element is limiting productivity. Simply adding sufficient quantities of an element that is limiting can result in a dramatic demonstration of how important soil fertility is.

In this picture, zinc fertilization was applied on a soil in Turkey where barley was grown. The pattern of fertilization, in the form of the letters Zn, the symbol for zinc, is immediately visible in the growth of the barley. In order to assess which element is limiting, the technique of omission trials can be applied. In such a trial, plots are fertilized with a mixture of all essential nutrients except one, the potentially limiting nutrient. There are as many treatments as there are elements that are potentially limiting. Based on the law of the Minimum, proposed by Von Liebig, plant performance and biomass production immediately reveal which nutrient is limiting, as the plots where that nutrient was omitted have much poorer plant growth and yields. For practical purposes it is not only important to understand which element is limiting, but also to understand why this element is deficient.

On that basis the best corrective measures can be selected, in almost all cases the problem of nutrient deficiency is not the plants ability to take up these nutrients, so genetic modification of the uptake system will rarely work. The problem of nutrient limitation is in the soil. In the soil either the total amount of the element is limiting, or the available amount is limiting. In that latter case the total amount can be adequate, but the soil might be too acidic, that is that pH is too low, to release enough phosphorus to plants. This happens in many tropical red clay soils.

Or, under conditions of a high soil pH, on calcareous soils, the availability of iron or zinc may be

limiting, because of strong interactions in the soil matrix. In such cases adding nutrients to the soil is not what is needed. Rather, in the example of phosphorus deficiency, additional liming, which increases the pH, is required. This increases phosphorus availability. On calcareous soils, application of iron and zinc as a spray on the leaves works better than soil application, because lowering the pH is not easy in these well-buffered soils.

Nutrients become more and more limiting when farmers harvest plants annually, which removes nutrients, without adequate replenishment. This practice is called nutrient mining. It is evident that agricultural practices that remove more nutrients from the soil than are naturally replenished will be unsustainable. This is like spending more than your bank account is earning.

If you keep doing that and do not add more money, your bank account will become exhausted.

Important to note:

(1) Both crop yield and nutritional value are reduced by nutrient deficits.

(2) It’s important to know both what nutrients are limiting and also why.

(3) Long-term nutrient deficits are not a sustainable way to manage soil for life.

Measures to prevent nutrient losses:

We all know that nutrient losses from agricultural land to the environment are not a good thing, but can they be prevented? The answer is yes, at least to a large degree.

Before talking about prevention measures, it’s important that you realize that in many regions with intensively managed agricultural soils, the input of phosphorus by fertilization has been higher than crop removal for decades. This means that large amounts of phosphorus have accumulated in these soils. For example, on average more than 2000 kg of phosphorus per hectare has accumulated over time in the upper 50 cm in different agricultural land. Now, imagine that we can remove 26 kg of phosphorus per hectare per year through harvesting of arable crops, it would still take more than 70 years before this phosphorus stock would be fully depleted.

What does this have to do with preventing phosphorus losses?

It means that even if we balance the input of phosphorus better with phosphorus output by crop removal, the current problem of phosphorus losses to ground- and surface waters will persist for a very long time. For this reason, there is a need to not only reduce phosphorus input to soils with high phosphorus levels, but also to develop measures to mitigate current phosphorus losses to the environment. I am about to give you two examples of such measures. The first example is a measure that is called “phytomining” or “phytoremediation”.

With phytomining, no phosphorus is applied at all, meaning that the input of phosphorus is zero. All other nutrients are applied in adequate amounts to ensure that crop growth is optimal. The idea behind this measure is that crops take up the phosphorus they need from the excess phosphorus in the soil, and that by continuous crop harvesting, phosphorus is removed from soil. The second example that I want to give is a measure that is called “the enveloped pipe drain”. In many agricultural fields, water discharge takes place via pipe drains.

This picture shows an example of agricultural land that is drained by pipe drains.

This picture shows the concept of the enveloped pipe drain:

when a pipe drain is enveloped with a reactive material that binds phosphorus, water that passes through this layer is cleaned. After passing through the reactive material, the clean water is discharged to surface waters via the pipe drain. The reactive material, commonly iron-coated sand in an experiment in Netherlands is used. A picture is shown here.

It consists of coarse sand, and the sand grains are coated with a thin layer of iron oxide. Iron oxide is a natural component of soils, and it has a high capacity to bind phosphorus. First an excavator dug a trench between two existing pipe drains. In this trench, we installed a new pipe drain with a 10-cm thick envelope of iron-coated sand. And the phosphorus concentration in the water was monitored that left the pipe drain at the outlet for more than a year. This figure shows that the iron-coated sand was very effective in reducing the phosphorus concentration in the water.

As you can see from the green symbols in this figure, the phosphorus concentration in the water from the enveloped pipe drain was much lower than the phosphorus concentration from the two reference or “normal” pipe drains, which are indicated by the red and blue symbols.

So you see that it is possible to develop innovative strategies for reducing phosphorus losses. But obviously, we should not only develop measures that solve the problem of current phosphorus losses to surface waters. We really need to address the cause of this problem: over application of nutrients, and in this case phosphorus. And that can be done by doing a better job of balancing phosphorus application to agricultural fields with phosphorus removal by crops.

The importance of soil carbon:

If you're like most people you're probably feeling a little hopeless about climate change and the damage we've done to our planet and now there's a new way to look at climate change. As we know is all about too much carbon in our atmosphere but carbon is not our enemy it's the building block of life everything alive is made it even hurts the problem and the solution are simply a matter of practice. Let's step back and look at the five pools were carbon is stored starting about 500 million years ago when plants first appeared on land carbon began to cycle in an amazing balance between these pools. A balance that allowed for life as we know it to evolve then one life that would be us figured out how to extract carbon from the fossil pool which was pretty much a time out so we've been burning it for energy putting into play and disrupting, the way we manage land into agriculture is moving even more carbon into the atmosphere. Specifically, we have eight hundred and eighty billion tons of carbon dioxide into the atmosphere which is heating up the planet and destabilizing our climate, the oceans have absorbed a lot of this excess carbon throwing off the ocean. This results in ocean acidification and accelerating a mass extinction of sea life. So in order to save life as we know of course we need to stop burning fossil carbon.

The big question is where do we put this excess carbon to get the cycle back in balance the good?

The answer is literally right under its the soil, plants using sunlight and water naturally perform photosynthesis, they pull carbon in frontier and turn it into carbohydrates sugars. Then they pump some of these sugars down through the roots to feed microorganisms who use that carbon to build healthy soil carbon. Scientists have recently discovered that applying a thin layer of compost can help regenerate healthy social setting up an ongoing feedback that brings more and more carbon into the soil. Each year together with other regenerative practices like not tilling soil planting trees and cover crops and plant grazing, we can build and retain billions of tons of soil carbon this is Carbon Farming. This is regenerative agriculture, unlike more carbon in the atmosphere more carbon in the ground is good for us, it makes healthy soil which is nutrient rich and full of life and hold sway more water.

This means more nutritious food and crops that are more resilient in the face of drunk, that's good news for farmers families and everyone who eat remember this. The way we grow our food fiber and fuel either puts carbon into our atmosphere or pull down into the ground. The regeneration of soil is the task of our generation our health the health of our soils and the health of our planet are one in the same.