Notes on Humus Balance

Contents:
 What is humus?
 Humus and soil fertility
 Why is the humus balance being neglected?
 Why should you do a humus balance?
 How humus balance is done
 Basic data for humus balancing 
      Humus reducing crops 
      Humus increasing crops as main crops 
      Catch crops 
      Set aside  
      Organische Dünger(Faktoren zur Umrechnung in HE) 
  What is humus?
Soil scientists define "humus" as the entire soil organic matter (SOM). Due to soil organism activity, SOM is subject to a permanent process of humification and mineralisation. But soil organisms also help to stabilize SOM. The relationship between humification and mineralisation determines in the long run the soil humus content.
Chemically speaking, humus is not a uniform substance. It is composed of stabilised phenolic compound, which's chemical composition up to now has not been possible. A rough classification of humus components is possible through their solubility in concentrated caustic soda. The organic soil fraction, which is soluble in caustic soda, is called humic acids, while fulvic acids are soluble in caustic soda, but precipitate in acids. Humins are stable in caustic soda, and thus not soluble.

The raw material for humification are energy rich, easily degradable organic primary materials, supplied mainly by plants. Their degradability and the time requested for transformation depend largely on their carbon to nitrogen (C/N) ratio. The closer this ratio is to the soil microorganisms C/N ratio, the faster transformation takes place. For this reason, e.g. legume roots, which have a C/N ratio of approx. 10/1, close to that of microorganisms, are decomposed much faster than cereal straw, with a C/N ratio of 80/1 to 100/1. To be able to decompose this kind of materials, microorganisms need nitrogen from their soil environment, to build up their own protein, which slows down considerably the decomposition process. Nevertheless, assuming that materials with a wide C/N ratio, like straw, should be the most appropriate for building up soil humus, would be a wrong conclusion. Humus itself, although showing a narrow C/N ratio of 10/1 to 15/1, is subject only to very slow decomposition. This apparent contradiction may be explained by the fact, that plant materials have to undergo multiple transformations, until forming stable humic substances. These processes, as already explained, are not well understood yet. They are, however, summarized under the term "stabilisation". Both the slowly degradable lignin (lignified plant material), as well as easily degradable plant material can be used as raw material for phenolic aldehyds and phenolic acids, which are then further transformed into polyphenols. These transformations only take place, if sufficient energy is available, which must be supplied by readily decomposable substrates. Therefore, it is very arguable, whether stable humus can really be built up by supplying mainly plant materials, which are hard to decompose (like straw), as many people believe. It is rather a balanced supply of slowly and readily degradable organic materials, which helps to build up soil humus. After the processes described above, polyphenols with the help of enzymes are transformed to chinones, which then react with amino compound to humic substances.


  Humus and soil fertility
At all times, the SOM content has been associated with natural soil fertility. Thus humus management has also received special attention by agronomists since the very beginning of scientific agricultural research. Related to this, especially farm yard manure (FYM) has been highly appreciated, due to its ability to maintain or improve natural soil fertility. This was expressed in a saying from those days: "grassland, as a producer of FYM, is the mother of arable land".


  Why does humus have so positive effects on soil fertility?

Due to its colloidal structure, humus with its huge internal surface, is able to conserve water up to several times its own weight, thus improving substantially water availability for plants.

The same colloidal structure also allows reversible nutrient adhesion to the humus surface. SOM thus functions as a soil nutrient buffer.

Humus is able to form stable unions with clay minerals in soil, the so called clay-humus complex. This complex has a highly positive impact on soil structure and soil nutrient transport capacity.

Due to its capacity to adsorb organic molecules, like e.g. pesticides, to its surface, until these compounds are being decomposed by microorganisms, humus also has a protective function for the ground water table.

Humus has a special importance for soil aeration.

Because of its dark colour, humus increases absorption of radiation, and thus soils tend to warm up earlier.

This list of positive humus effects on soil fertility could be extended largely. The few examples, however, show that a fertile soil without humus is difficult to imagine, and that it would be far too short-sighted valuating humus only through its nutrient content.


  Why is the humus balance being neglected?

Recently, a growing number of evidences shows, that the continuous increase of crop yields, to which we had been accustomed during the past 50 years, is reaching its upper limit. While yields were steadily growing due to breeding, optimised fertilisation and plant protection, basic principles of humus management have been forgotten. Many reasons exist for this: 

Blind trust in mineral fertilisers as a tool for compensation of soil fertility.
FYM, as the archetypal fertiliser to increase SOM, has been replaced more and more by liquid manure, due to reasons of economies of labour.
Fodder crops with humus increasing effects, like hayfields, clover, lucerne, or grass-clover-leys, have been nearly completely substituted by maize, which is cheaper, but has a negative impact on SOM.
Pressure to reduce production cost, as created by the EU subsidy policy, has led to an increasing polarisation and specialisation, creating many farms without animal husbandry, which have little or virtually no possibility of a reasonable humus management.
The same policy has dramatically reduced crop diversity, increasing cereal areas, while cultivation of humus increasing crops like peas or faba beans has appeared to be little remunerative.
Up to now, no practical tools for a long term assessment of SOM contents have been available, since soil analyses reflect just a certain moment and are subject to strong variations, due to soil heterogeneity.

 

Why should you do a humus balance?

In our century, it is more important than ever to remember the old principles of humus management, aiming always at a supply level of organic materials, which is higher than the mineralisation rate. By the middle of our century, we will see, whether food production will be able to keep pace with the requirements of a rapidly growing population, under continuously deteriorating ecological conditions, due to global warming. One of the basic conditions for high yields is high natural soil fertility, based to a high degree on SOM content. Since humus accumulation on arable land is possible only in relatively long terms, while mineralisation under unfavourable conditions can take place much faster, an assessment of the rate of humus mineralisation and humus supply is highly important for sustainable soil fertility management.

 How humus balance is done ?

Current methods for humus balancing are mainly based on Rauhe & Schönmeier, 1966; Asmus & Hermann, 1977; and Kundler et al., 1981. These authors, based on the well known positive effects of SOM, tried to assess or forecast two-way effects of SOM build-up and mineralisation in different cropping systems. These methods are based on long term cropping experiments. The best known method is the HU (humus unit) method, developed by Rauhe, saying:

1 Humus Unit (HU) = 1 ton of humus, with 50 kg nitrogen (N) and 580 kg carbon (C)

Leithold and Hülsbergen further developed this method, modifying coefficients (Leithold, G.; Hülsbergen, K.J, Michel, D. & H. Schönmeier: Humusbilanzierung – Methoden und Anwendungen als Agrar-Umweltindikator. In: Schriftenreihe der Sächsischen Landesanstalt für Landwirtschaft Heft 3, 2. Jahrgang 1997 p. 19-28). Coefficients published by these authors are the basis for this humus balancing program. According to Leithold et al. (1997), a distinction is made between organic and integrated farming systems, for the following reasons: organic farms should aim at higher SOM contents, which's level depends on local soil conditions, in order to obtain sufficient yields. There is a close correlation between SOM contents and crop yields in organic farming. When no mineral nitrogen is used, crop nutrition depends essentially on SOM transformation, especially a sufficient nitrogen mineralisation. A high organic matter supply leads to a faster mineralisation of the unstable SOM fraction, of organic manure and crop residues. Microbial transformation processes are intensified. Since, in organic farming, mineral nitrogen needs to be replaced by humus mineralisation, this has to be compensated through humus increasing crops and/or a higher level of organic fertilisation. Nutrient contents of organic fertilisers are lower on organic as compared to conventional farms (Miehe, 1994; Biermann, 1995), so that their humus and nitrogen substituting effect must be considered to be lower. Intensified soil tillage for weed control further increase SOM mineralisation in organic farming. Increased organic fertilisation, however, is not a threat for the environment through excess nitrogen, because nitrogen purchase is strictly limited in organic farming.

Basic data for humus balancing in integrated and organic farming, expressed in humus units (HU)

   Humus reducing crops

Crop

HU integrated farms

HU organic farms
Sugar beets and fodder beet

-2,3

-3,4
Potatoes and 1st vegetable group

-1,80

-2,75

Maize for silage and 2nd vegetable group

-1,35 

-2,05

Cereals, oilseeds1), sunflowers, maize for grain harvest, fibre crops, 3rd vegetable group

-0,70 

-1,05

according to Baumann and Schmidt (1979):
1st vegetable group: leeks, asparagus, rhubarb, cucumbers
2nd vegetable group: witloof chicory, swede, carrots, horseradish, black salsify, tomatoes
3rd vegetable group: corn salad, kale, kohlrabi, lettuce, little radish, beetroot, spinach, onions.

1) In the case of cereals and rapeseed, coefficients are calculated when harvesting grain and straw, for all other crops, it is supposed that crop residues remain on the field.

Humus increasing crops as main crops

Main crop Cropping pattern HU
Hayfield per main cropping year +1,05
Hayfield in year of sowing as summer open sawing +0,20
Hayfield in year of sowing as underseed +0,35
Alfalfa in year of sowing as spring open sowing +1,2
Alfalfa in year of sowing as green cover crop +0,6
Alfalfa for seed harvesting in year of sowing as underseed in cereals +0,5
Alfalfa in year of sowing as summer open sawing +0,3
Alfalfa in 1st main cropping year +1,8
Alfalfa in 2nd main cropping year +1,4
Alfalfa in 3rd main cropping year +0,8
Leguminous-grass mixtures in year of sowing as spring open sowing +1,4
Leguminous-grass mixtures in year of sowing as green cover crop +0,6
Leguminous-grass mixtures in year of sowing as underseed in cereals +0,5
Leguminous-grass mixtures in year of sowing as summer open sawing +0,3
Leguminous-grass mixtures in 1st main cropping year +2,1
Leguminous-grass mixtures in 2nd main cropping year +1,8
Leguminous-grass mixtures in 3rd main cropping year +1,0
Grain legumes (straw being harvested) +0,35

 Humus increasing crops as intermedium crops

Cropping pattern Crop HU
Winter intermedium crops winter rye -0,30
leguminous-/nonleguminous mixture (vetch-rye, ryegrass) +0,30
Landsberg mixture +0,50

Stubble crops (ploughed under in autumn, or freezing during winter)

 leguminous-/nonleguminous mixture +0,20
grass mixtures +0,20
rapeseed, bird rape, mustard, phacelia, perco +0,15
Underseeds ploughed under in autumn grass mixtures +0,30
leguminous-/nonleguminous mixture +0,60
Underseeds ploughed under in spring grass mixtures +0,50
leguminous-/nonleguminous mixture +0,70
Oats and other non-leguminous crops for fodder harvest (spring sowing) -0,20

 Set aside

Fallow pattern Management HU
Set-aside 1 year, spontaneous vegetation from autumn of previous year on +0,20
from spring of fallow year on +0,10
Set-aside 1 year, sowing: leguminous-non-leguminous mixture sown in late summer of previous year +1,50
ab Frühjahr des Brachejahres +1,20
Set-aside 5 years spontaneous vegetation +2,50
sowing +4,50

  Organic Fertilizers

Formula: Fresh matter in tons * factor = Humus units (HU)

Fertilizer   Factor
Fresh farm yard manure (FYM) 0,05
Slightly decomposed FYM 0,07
Compost from FYM 0,10
Semi-iquid cattle manure (per 10% dry matter) 0,022
Semi-liquid pig manure (per 10% dry matter) 0,018
Straw 0,12
Green manure (per 10% dry matter) 0,013
Compost from crop residues 0,14

Source of data: Tagungsband‚ 4. Wiss. -Tagung Ökol. Landbau’, 1997, Bonn, S. 56 - 62