Dossier Soil processes and fertilization

Table of contents

1. Spatially separated soil processes
2. Importance of the food web
3. Turning knowledge into soil management
   1. Greater crop diversity
   2. Reduced tillage
   3. Smart fertilization
4. Most common misconceptions

Natural processes in the soil have a significant impact on soil health and quality, and are therefore important for soil function and crop growth and yield. Understanding these processes makes it easier for growers to make decisions about soil management (timing and method of fertilization and tillage, sowing time, etc.).

The soil is a living organism that must be properly nourished for optimal metabolism and functioning of the soil-plant system. With healthy, well-timed metabolism in the soil, you close the plant nutrient cycle and make them available to the crop. If you do not take these natural processes into account, you can reduce your yield, for example by incorporating large amounts of fresh organic material just before sowing or planting, thereby inhibiting the early growth of your main crop. In this video, soil expert Koen Willekens gives an initial idea of how this works.

Spatially separated soil processes

To understand the impact of soil processes on crop growth, you first need to know that there are two forms of soil biology: decomposition biology and symbiotic biology. They are naturally spatially separated, and the activity of symbiotic biology can only follow the work of decomposition biology.

What is happening where? And when?

The growth cycle of the plants is also decisive. After the growth phase (vegetative phase), plants form flowers, fruits, and seeds (generative phase), after which they die. Each of these phases is linked to a biological process in the soil.

Degradation biology

in the litter layer ● when dying

In a natural ecosystem, decomposition biology takes place in the litter layer where dead above-ground plant material ends up. The decomposing organisms are microorganisms (fungi and bacteria) and small soil animals (earthworms, millipedes, woodlice, etc.). They break down fresh organic material, including plant debris and excrement, and convert it into nutrient-rich humus (humification).

The mineral nutrients released during this process (mineralization) are consumed by the decomposing organisms, but can also be a source of plant nutrition.

An important insight is that decomposition activity can inhibit crop development due to competition for nutrients between the decomposing organisms and the crop, and due to possible oxygen depletion during intense decomposition. A slower start to growth cannot be reversed later on and therefore reduces the yield.

In an agricultural ecosystem, recommendations are:

• allow crop residues to decompose as much as possible on the soil surface;
• After working green manure or fresh manure into the soil (shallowly), allow time for it to decompose before sowing or planting your main crop.

Symbiotic biology

in the root environment ● during the growth phase

Other soil organisms, namely symbiotic microorganisms, are found around plant roots. These are fungi and bacteria that live together with the plants and vary depending on the plant species. This coexistence is beneficial for both parties (symbiosis): they feed each other.

As they grow, plants produce substances through photosynthesis that they excrete through their roots (exudates). These exudates are essential nutrients for soil life in the root environment.

In turn, these soil fungi and bacteria make nutrients that are not or are more difficult to access available to the plant. These are nutrients that are trapped in stable organic matter (nutrient humus) and in soil minerals. Through the production of acids (such as organic acids) or bases (such as ammonia), microorganisms have a direct influence on the acidity of the soil. Some bacteria also fix nitrogen from the air.

The potential of this additional source of nutrients for crops therefore depends on 'active' rooting. The mutual benefits for both plants and microorganisms are also greater after the decomposition process has taken place and thus after the production of nutrient-rich humus. This humus is produced from crop residues or soil-improving fertilizers, and therefore with little or no use of nutrient salts with fast-acting fertilizers.

Importance of the soil food web

Both during their growth and after they die, plants nourish the soil biology, allowing an extensive soil food web to develop. Below is an illustrative representation by soil scientist Ron de Goede, adapted by ILVO.

This web is essential for good soil quality and ensures:

• improved soil structure
  Decomposition and symbiotic processes require oxygen, and good soil structure ensures that oxygen penetrates deep enough into the soil. It is mainly soil organisms that create favorable soil structure by forming soil aggregates (soil structure elements, e.g., soil crumbs), secreting mucilage (bacteria), and forming mycelium threads (fungi).
  Rooting also contributes to soil structure, both physically and by stimulating soil biology.
• preservation of nutrients
  A well-developed soil food web preserves and circulates nutrients to the maximum extent possible.
• greater resistance to disease
  Every crop lives alongside its own community of microorganisms. Crop diversity therefore increases the variety of organisms in the soil, and that variety is just as important for plants' natural resistance to diseases and pests.

During fallow periods, part of the soil life dies off, especially the symbiotic organisms, because they are no longer fed by active roots. Lack of soil life activity (including root activity) is bad for soil structure and the preservation and availability of nutrients. That is why permanent ground cover and living roots are very important.

Turning knowledge into soil management

To optimize your soil management, you can focus on three areas: greater crop diversity, less tillage, and smart fertilization.

Greater crop diversity

You can broaden your crop rotation not only by growing a larger number of crops and cover crops in succession (diversification over time), but also by growing mixed crops or multi-species cover crop mixtures (diversification in space).

Mixed cropping poses challenges in terms of planting/sowing, weed control, and harvesting, and is easier to implement in arable farming than in horticulture. Mixed crops often consist of grass-like and leguminous species, such as corn with runner beans, or grains with beans or peas, but also grass (herbs) with clover.

Through their symbiosis with Rhizobium bacteria, which bind nitrogen from the air, legumes provide a nitrogen supply to the system without additional phosphorus input. This nitrogen fixation is enhanced by combining a legume with a grass. The grassy plant is more competitive in absorbing soil nitrogen, which stimulates the legumes to bind nitrogen from the air.

Legumes also add value to diverse ground cover mixtures, as does maximizing the number of plant families.

Green cover crops not only act as catch crops for residual nitrogen, they also make part of the nitrogen they have absorbed available for the subsequent crop after dying off and undergoing (light) incorporation. The extent to which this plays a role depends on soil conditions (oxygen, temperature, and moisture content), the type and stage of development of the green cover crop, and the timing and method of incorporation.

ILVO research showed that incorporated species-rich mixtures (without legumes) had a better influence on the main crop than two-part mixtures. Another trial showed that nitrogen uptake by the main crop was higher, without additional nitrate residues, in a species-rich mixture with legumes compared to a two-component mixture without legumes. And a green cover crop with legumes did contribute to the nitrogen supply of a subsequent pumpkin crop at low mineralization potential, but not of leeks in overly wet soil.

You can read more details about this in the Dutch-language publication Optimaliseren van bemestingsstrategieën vanuit de principes van de biologische landbouw - Vlaamse Landmaatschappij

Less tillage

If a crop has developed strongly together with the soil life, the soil structure is usually good and digging or plowing is unnecessary, even undesirable. You can, however, work the soil for seedbed or planting bed preparation to a maximum of the sowing or planting depth.

Non-inversion tillage respects the natural stratification of the soil and minimizes the incorporation of crop residues and organic fertilizers into the soil. Compaction caused by machine operation or other circumstances can be eliminated by deeper non-inversion, non-tilling cultivation with a suitable fixed tine cultivator, to just below the depth of the compaction.

Non-inversion tillage:

• preserves soil structure and moisture because the large pores for drainage and aeration remain intact, while the fine pores on the soil surface, through which moisture rises and evaporates, are broken down by shallow tillage;
• maintains the load-bearing capacity of the soil;
• reduces erosion by keeping crop residues on the soil surface;
• retains more soil organic matter in the top layer;
• prevents acidification by supplying sufficient oxygen during the decomposition of crop residues;
• preserves nutrients by reducing acidification and facilitating the absorption of mineral nitrogen by crops.

Read more about it (in Dutch) here: Gereduceerde bodembewerking voor biologische teelten - Wageningen UR

Smart fertilization

The type of manure determines the time of application.

Straw-rich manure, immature compost, or other carbon-rich organic material that has been little or not at all decomposed (crop residues, wood chips, etc.) is best worked into the soil superficially during the fall.

This timing is important because the decomposition process, which starts after the fresh carbon-rich material has been applied, immobilizes nitrogen and thus competes with the crop for nitrogen and possibly other nutrients.
It is best to apply it in good soil conditions just before sowing the green cover crop. In this way, decomposition and conversion take place during the winter, so that this soil-improving basic fertilization becomes plant-feeding in (early) spring.

Mature compost can be spread on the field throughout the year because the decomposition and conversion process is complete. You can therefore also apply it in the spring, just before planting or sowing. It is best to do this regularly and in smaller quantities rather than in large doses, especially on light soils.

Read more about using compost as fertilizer in this Dutch-language article: Compost levert complete bemesting - Wageningen UR

Soil condition determines the dose

Adjust the dose of your organic fertilizer to the nitrogen requirements of your crop and the extent to which your soil's nitrogen-supplying capacity (through the release of nitrogen from organic matter by soil life) already fulfills this requirement. If there is a nitrogen deficiency due to insufficient mineralization from the soil organic matter, you should also opt for fast-acting organic fertilizers with a low C:N ratio.

Your organic fertilization is also determined by the need to restore or maintain the levels of other plant nutrients. In the event of structural deficiencies, you can use targeted mineral fertilizers.

What about nitrate residues?

To prevent excessive nitrate nitrogen residues, adjust the nitrogen input via (organic or mineral) fertilization to

• the nitrogen requirement of your crop;
• the mineral nitrogen reserves present in the soil and;
• nitrogen mineralization potential: the availability of nitrogen through the mineralization of soil organic matter, incorporated crop residues, and applied organic fertilizers.

The nitrogen mineralization potential is not easy to estimate. An incubation test in the laboratory differs from the field situation. A better method is to draw up balances of the nitrogen available to plants in certain years and derive the nitrogen mineralization potential from these. You can use this to determine a future fertilization strategy. Also keep in mind that regular application of large doses of animal manure can (excessively) increase the mineralization potential of the soil.

Combined effect

It is a good idea to combine slow- and fast-acting forms of nitrogen fertilization. Slurry works faster than farmyard manure, and farmyard manure has a greater nitrogen effect than compost. Young grass clover with a high proportion of clover is also a relatively fast-acting form of fertilization if you transfer the clippings from one plot to another as mulch.

Avoiding nitrogen losses during manure storage

Stored manure loses a lot of nitrogen through volatilization (up to 40%), especially when dumped loose, where there is plenty of oxygen available for the decomposing organisms. Loss through leaching is much lower (up to 4%), which means that storing manure on the headland during the winter is not a problem if you cover the manure heap. This storage is subject to certain conditions (more info at VLM-Mestopslag).

To keep nitrogen in the manure during storage, you can:

• compost it with the addition of brown waste streams (see also farm compost dossier);
• compress it by pressing it down (similar to compacting grass during ensiling). This reduces the oxygen content of the manure and limits the decomposition process. The more air you exclude through compaction, the less nitrogen loss there will be.

Most common misconceptions

Spreading manure at the wrong time

Fresh manure is often spread in the spring, relatively shortly before planting or sowing. For an early start to the growing season with early crops, it is better to apply raw manure in the fall before sowing a frost-sensitive green cover crop. This allows the manure to decompose over the winter and act as plant food in the following spring.

Working manure into the soil in poor conditions

Applying manure to soil that is too dry or too wet hinders the decomposition and conversion process, which is detrimental to the growth of the next crop.

Plowing

Soil life creates a good soil structure. Plowing destroys the natural layering of the soil and the beneficial fungi that, as food for higher soil organisms, ensure a complete soil food web. In addition, plowing pushes fresh organic material deeper into the soil where there is insufficient oxygen for rapid decomposition and transformation. This increases the risk of rotting and acidification.

Short-term thinking

Administering only mineral salts via artificial fertilizers may provide sufficient plant nutrition in the short term, but it reduces soil quality. If you give plants directly absorbable nutrients, they will invest less in soil life. The crop secretes fewer exudates to obtain the necessary nutrients through symbiosis with soil life, resulting in less soil life.

Less activity in the soil means a greater risk of nutrient loss. Revitalizing your soil with living plant roots and organic fertilizers is the best guarantee of healthy soil for healthy crop development.

Good soil management is an investment that also ensures long-term yields. Motivated farmers can achieve the transition to a well-functioning living soil relatively quickly through the right combination of measures.

Sources (in Dutch)

• Presentation: Fotosynthese: de groene motor ontleed
• Fotosynthese: De groene motor ontleed – functies en noodzaak van plantengroei voor het behoud van een gezonde bodem

Links (in Dutch)

• Read more about the project: Optimaliseren van bemestingsstrategieën vanuit de principes van de biologische landbouw - Vlaamse Landmaatschappij
• Read more about the BOPACT trial at ILVO, part of the European EJP SOIL project: 13 jaar compost toevoegen: wat doet dat met je bodem? (Hoofdstuk 4.4)

Also read these dossiers (in English)

• Soil improvement with organic fertilizers
• Soil: carbon sequestration
• Erosion
• Soil health
• On-farm composting