Sustainable Productivity and Soil Fertility through Enhanced Soil
Biology
Former
Scientist, International Crops Research Institute for the Semi-Arid Tropics
(ICRISAT), Patancheru, 502324, Andhra Pradesh , India
In the past 20 years or
so, organic or ecological farming has emerged as an effective alternative to
the agrochemical-based modern or conventional agriculture. Ecological farming
is based on sound scientific principles. Several farmers using good agricultural
practices (without agro-chemicals) have claimed harvesting yields comparable to
those of neighboring farmers who use agro-chemicals (http://infochangeindia.org/
agenda/agricultural-revival/the-new-natural-economics-of-agriculture.html).
Several review papers and publications (http://www.indiawaterportal. org/search/node/o.p.%20rupela),
including some from UN-FAO,
(http://www.fao.org/docs/eims/upload/275960/al185e.pdf) have reported results
in favor of this type of farming. This paper discusses plausible explanations
on how such farms (without agrochemicals) may have yields comparable to farms where
agrochemicals are used, but restricts on soil fertility and plant nutrition aspects.
A
crop needs several organic elements like vitamins and growth hormones, and
inorganic (or atomic) elements and these constitute its cells (constitutive) or
participate in its metabolism (non-constitutive). Four of the vital constitutive elements are
carbon (C), oxygen (O), hydrogen (H) and nitrogen (N). Thirty elements may come
from soil and they form only 2 to 8% of body dry mass of a plant. Name of most
of these can be found in books written on soil science and plant nutrition. Claude
Bourguignon (English translation ‘Regenerating the Soil’, pages 188, published
in 1998 by Other India Press, Goa, India - original book in French) called twelve
of these as vital or essential elements for plant growth. Crop yield may suffer
if their available concentration in soil is below a threshold limit (as studied
and reported by several research publications) and their deficiency in soil shows
up readily through characteristic symptoms on different plant parts. Two of the
12 elements potash (K) and chlorine (Cl) are non-constitutive. The other ten
elements are constitutive and these are - phosphorus (P), boron (B), calcium
(Ca), magnesium (Mg), sulfur (S), iron (Fe), manganese (Mn), molybdenum (Mo),
copper (Cu) and zinc (Zn). Eighteen other elements, also called secondary
micro-nutrients or trace elements are required in very small quantities. Function
of some of these is not fully understood. Four of the 18 – lithium (Li), sodium
(Na), rubidium (Rb) and cesium (Cs) are non-constitutive. The other 14 are
constitutive. These are fluorine, (F), silicon (Si), selenium (Se), cobalt
(Co), iodine (I), strontium (Sr), barium (Ba), aluminium (Al), vanadium (V), tin
(Sn), nickel (Ni), chromium (Cr), beryllium (Be) and bromine (Br). It is
important to note that only three of the over 30 elements - nitrogen (N),
phosphorus (P), and potassium (K), widely known as major elements, are readily available
in the market and proactively promoted by the AR4D system globally.
An
important fact that gets missed, even by a large number of agricultural
scientists, but which is essential to understand, is that the major part (92 to
98%) of the body mass (dry and not wet mass) of a plant is made of carbon,
nitrogen, oxygen and hydrogen. It may be noted that all the four are gases. One
can confirm this from the fact that when the biomass of any plant is burnt, we
get only 2 to 8% of the original weight of the burnt biomass as ‘ash’. The rest
goes into air because they were gases. When we burn rice or wheat straw, we
generally get about 2% of the original mass as ‘ash’ while it is about 8% in
the case of woody plant parts, for example stems of pigeonpea plants. These
four elements also occur in soils in other forms. Carbon molecules can also
reach the plant through breakdown products of proteins and amino acids through the
carbon skeleton. Nitrogen exists in several forms in the soil – ammonium ions,
nitrates, amines etc., and can be taken up by plant roots. Hydrogen and oxygen
are part of a water molecule. A plant assembles these elements during the photosynthetic
process while growing under favorable conditions.
Micronutrients also
include some plant growth factors such as vitamins and enzymes, which are
organic compounds required as nutrients in tiny amounts. These are present in
soils due to microorganisms and macro-fauna (like earthworms) resident in soils
and are either excretory products of these living beings due to their normal
life cycle in soils, or are available after their death. Most of these growth
factors are also available on surfaces of roots where populations of
microorganisms are abundant.
All
the 30 mineral elements, described above, are available in most of the soils where
crops are grown and in the plant biomass (leaves, branches, flowers, fruits) of
every plant, but their composition/proportions differ between soil types,
fields and from one plant part to another. Each of the 30 mineral elements
exist in at least two forms – available or water soluble form and non-available
or bound form. Much of the concentration of each of these nutrients in the soil
and in the plant biomass is in the non-available or bound form, and only a small
fraction (generally 1 to 8% of the total concentration) is present in the
available form. Microorganisms and macro-fauna resident in agricultural fields
keep converting the non-available form of nutrients to available forms, as part
of their daily life, provided conditions are favorable for their survival and
functioning. This activity happens continuously throughout the day and night. Successful
farmers (those organic farmers producing yields comparable to chemical farms)
practice a set of good agricultural practices (GAP) to create such conditions.
These include on-farm production and use of plant biomass as a source of crop
nutrients, having the maximum possible plant biodiversity on the farm,
integration of trees and animals on fields growing annual crops, recycling of
crop residues, and soil and water conservation in a scientifically sound way.
Soil
is a source (S) of (O) infinite (I) life (L). It houses/supports several
different forms of life, starting from photosynthesizers (plants and their
roots – ‘First Trophic Level’ – the primary food formers in the soil) to
animals (predators and burrowers – ‘Fifth and higher Trophic Level’).
In-between are mutualistic decomposers and root feeders (bacteria, fungi,
nematodes - Second Trophic Level), shredders and graders (protozoa, arthropods
– Third Trophic Level), and predatory nematodes and arthropods -- Fourth
Trophic Level. All these different forms of life in the soil are intricately
woven as a food-web and make it “Living Soil” (http://faculty.ycp.edu/~kkleiner/envbio/envbiolabs/lab3_Lifeunderyourfeet_F12.pdf).
One cup of undisturbed native soil may contain up to 200 billion bacteria, 20
million protozoa, 100,000 meters of fungi, 100,000 nematodes and 50,000
arthropods (http://www.ext.colostate.edu/mg/gardennotes/212.html). All these
life forms need lot of food, and as indicated above, plants are the basic food
material for them. But soils of most agricultural farms today are heavily degraded
biologically and lack life, and therefore the capacity to produce high yields
without agro-chemicals.
To
grow plants without fertilizers and harvest a high yield, one needs to recycle
all crop residues or their converted form after their economic use, back to
soil. For example, if one has grown sorghum - grains are generally for human
consumption and stalks for cattle, the changed form of stalks (i.e. cattle
dung) should be applied to farms. But if cattle dung is needed for cooking
food, no problem, its changed form, the ash, should be evenly applied to fields.
In addition, one needs to grow fast growing trees as a source of biomass (food
for the living beings in the soil) and fruit trees (as a source of both human and
soil nutrition) on field boundaries and/or farm boundaries. All these form a local
natural resource for high yield without fertilizers. It is important to note
that one does not need a large tonnage of cattle dung (a myth among many
farmers and even scientists) for high yield without fertilizers. Cattle dung is
however needed in small quantities as a source of agriculturally beneficial microorganisms,
in the same way as we need a maximum of one spoon of old but good quality curds
to prepare more curds for say 10 or even 100 L of milk.
Cowdung
has been reported to contain five of the total six functional groups of
agriculturally beneficial microorganisms (http://www.indiawaterportal.org/node/15133,
Rupela et. al. 2006). These are nitrogen fixing microorganisms, phosphate solubilizing microorganisms,
plant-growth promoting microorganisms, cellulose degrading microorganisms and
antagonists of disease causing microorganisms. The sixth functional group – entomopathogenic
microorganisms (those with ability to kill insects) were not studied by Rupela
et al. 2006, but may also be present in cowdung, and are known to be present in
soils of most agricultural fields (eg, the bacterium Bacillus thuringiensis). Because fresh cowdung is an important
ingredient of the different liquid manures, all these agriculturally beneficial
microorganisms are present in these manures, and in larger numbers than in the
cowdung itself. Eventually, these become part of the soil in large numbers and
are not needed from market sources.
Use
of several different recipes of liquid manures are common among organic farmers
where fresh cowdung is recommended to be used as inoculum for their preparation
e.g. ‘Amrit Jal’ or Jeev Amrit and Gur-Jal Amrit. The quantity of dung needed for preparing 100
L of the manure is only 10 kg, and is enough for an area of one acre. The
frequency of application may be 3 to 4-times in a crop cycle of about 120 days.
One can thus manage up to 10 acres of land with just one cow. Like cowdung,
compost should also be viewed as a source of agriculturally beneficial
microorganisms and not as a source of N, P, K, and be evaluated accordingly.
Unfortunately, the modern education system of agriculture has not internalized
this knowledge of soil biology and experience of ecological farming, and still
calculates the nitrogen needs of a crop per ha (if through compost) by
measuring the N concentration in compost.
Chemical
fertilizers, when applied to soils, negatively affect the population and/or
functions of the agriculturally beneficial microorganisms. For example,
effectiveness of ‘rhizobia’ – known to convert inert nitrogen in the air to the
plant utilizable form of nitrogen (called nitrogen fixation) is adversely affected by the use of nitrogenous
fertilizers [Streeter 1988; Critical reviews in Plant Science 7:1–23]. The same
is true with the use of synthetic pesticides and herbicides. Several publications
report that fertilizers such as nitrogen and micronutrient mixtures can be
applied by spraying on plant surfaces, and that get absorbed by plants, perhaps
through the stomata on leaves. If applied through sprays, one would perhaps need
one fifth of the levels of fertilizers recommended for soil application. It is
unfortunate that this mode of meeting plant nutrient needs is not researched
sufficiently, and where known, eg, as in the case of nitrogen, it is not
promoted proactively either by industry or by agricultural institutions/universities
and extension agencies.
Plant
root system may not differentiate whether a mineral element in the soil (eg,
nitrogen) is from a bag of fertilizer or from degradation of plant biomass. Cowdung
and compost are merely changed forms of plant biomass.
It
is not only the soil of a farm, but all the plants (including weeds) growing on
a farm may have all the 30 mineral elements. Weeds are a menace but have some
positive aspects as well. Some weeds are known to be rich in a particular mineral
element. For example, Bathua or Chenopodium
album has been reported to be rich in iron (http://nopr.niscair.res.in/bitstream/123456789/7838/1/NPR%206%281%29%206-10.pdf). It is likely
that some other weeds will be rich in some other mineral elements. Up to 1.8 t
of dry weed biomass per ha has been measured in rainfed areas in a longterm
study at the International Crops Research Institute of the Semi-Arid Tropics (ICRISAT).
Weeds are thus a potential source of different mineral elements needed for
plant growth. Other noted uses of weeds are (a) enriching plant biodiversity on
a farm, and (b) serve as habitat for agriculturally beneficial insects and thus
help manage insect-pests on a farm. However, we need to manage weeds appropriately,
as they compete with crop plants for sunlight and soil moisture. They are best
removed manually or mechanically and not by use of herbicides. Herbicides, like
other agro-chemicals (fungicides, insecticides) are poisons and adversely
affect the different forms of soil life listed above, and should not be used.
Innovative farmers in Punjab are experimenting
with soft options of managing weeds – spraying with undiluted cow-urine and
soap powder (2%) solution (with 2 eggs per 15 L spray tank as sticker). Spraying
can be done on weeds growing between two plant rows (at least one foot apart),
using nozzles with hoods.
All
these are the basic facts of agricultural science and also form the core of the
science and practice of organic or ecological farming.
The regular addition of plant biomass as surface
mulch and that of microbial agents results in high organic matter in the soil,
which leads to better soil health – making plants tolerant to drought and
pests, eventually resulting in high yield.
As stated, only a small fraction (about 1 to 4%
in case of nitrogen) of all the 30 mineral elements in the soil may occur in
available form and rest in bound form. Interestingly, all the soil testing laboratories
that are stated to help farmers (but are designed to help industry) measure
only the available form of elements. Most, or all such laboratories in India, lack
a facility to measure the total or bound form of any mineral element. All these
laboratories are designed to promote the use of fertilizers and the soil-test
results are used for generating advisories on quantity of a given fertilizer a farmer
should apply to soil.
Typical soil of a farm may contain 0.1% of soil
mass as total or bound form of nitrogen. Roots of a plant can access nutrients from
up to 30 cm of soil depth, and this upper 30 cm layer of soil weighs about 4
million t (for every ha) and thus translates to 4000 kg of bound N in every ha.
And 1.0% of the 4000 kg (ie, 400 kg) is in available form. The root system of a
plant is therefore exposed to a large quantity of nitrogen. Typically less than
1% of dry mass of a cereal grain is nitrogen. If yield, say of sorghum, is 3000
kg per ha, it would remove maximum of 30 kg N per ha; but if yield is 5000 kg,
it will remove about 50 kg N per ha, and this quantity is much smaller than its
high quantity of nitrogen in soil (400 kg available and 4000 kg bound form). The
same is true for the other mineral elements. We, however, need to facilitate a
plant to access this small quantity of N (30 to 50 kg) through use of liquid
manures rich in agriculturally beneficial microorganisms or through making soil
– a living soil of the kind indicated above.
A plant
accesses nutrients largely through its root system, and microorganisms and soil
macro-fauna play an important role in this function. Published literature
indicates that roots of a plant have the major role in selecting microbial life
around it. For example, rhizobium of chickpea can only enter into the roots of a
chickpea plant, and no other type of rhizobium species enters its roots. Some
species of rhizobia have been reported to enter the root system of rice, travel
to the leaves through its stem, and enhance plant growth (Applied
and Environ. Microbiol. 71:7271-7278,
2005). The
population of microorganisms in the soil in close proximity of roots
(called the root rhizosphere) is about 10 to 100 times bigger than that in the
neighboring soil mass. There is a whole world of endophytic microorganisms
(those living and functioning inside a plant system, rhizobium is only one type)
that are expected to help a plant in various ways and are yet to be explored
substantially. GAP promoted under ecological farming help plants access
adequate nutrients needed for good plant growth and yield.
There is no doubt that conventional agriculture
is based on science, but its focus is on developing research outputs that make
farmers depend on the purchased inputs. Moreover, these research outputs are
tuned mostly to serve the interests of input-providers/corporates. On the contrary,
if we are serious in addressing the distress widely observed in the
farm-sector, we need agro-technologies that empower farmers to produce inputs
on-farm, which would help them reduce the cost of crop production. The
challenge for us is to make the AR4D cater to this need, and also to ensure
that science is articulated to the agri-practices of the organic farmers. This
should help in scaling-up with confidence.
The Soil-Health Card program of the Government of
India (www.agricoop.nic.in/Comsoilhealth28612.pdf)
and of the states working on this program is presently targeted to analyse pH,
EC, organic carbon, available form of nitrogen, phosphorus and potassium. As
stated earlier, this program too is targeted to promote use of fertilizers and
only does a lip service to ‘soil health’, while in practice it will still spoil
soil health but at a slower pace than when fertilizers are used
indiscriminately.
In conclusion, the present
scenario, where AR4D works more in favor of external input providers and less
on the interests of farmers, every farmer should become an experimenter and
grow crops using GAP (all or most of the methods stated earlier) to meet the
need for crop nutrients on a small area (say one acre), and compare the results
with the neighboring area where fertilizers have been applied. If yield in the
experimental area without agro-chemicals is lower, he/she must visit some
successful farmer of their area to find out what additional practices the
successful farmer of the area is employing to get high yields. It should be
noted that experience and confidence that high yields are possible without
agro-chemicals lies with successful organic or ecological farmers; they generally
do not lie with most agricultural scientists and the agricultural institutions/universities
mandated with AR4D.
Acknowledgments:
Interactions with a large number of successful organic farmers of India that
shaped my understanding of organic farming/ecological farming, and English
language editing by Ms Lydia Flynn, former Senior Manager, Scientific Editing
and Publishing, ICRISAT, are gratefully acknowledged.
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