METHODS OF IMPROVING SOIL FERTILITY
It is imperative that as soil nutrients are utilized to support plant growth, which eventually nourishes man and animals, this source should be constantly maintained to avoid breakdown of this all-important source for ecosystem functions. General soil fertility maintenance strategies have been suggested. These include:
- Cover cropping: It is the growing of any broad – leafy crop to cover the soil surface while another crop is growing. It also implies the growing of crops to cover the land during off seasons without leaving soil surfaces bare. Examples of cover crops include; Pueraria and Centrocema. They help to maintain soil fertility through the following mechanisms:
- Conserving soil moisture by drastically reducing evaporation from the soil surfaces;
- Smothering weeds to prevent nutrient competition with main crops;
- Fixing nitrogen in the soil through association of roots of some cover crops that are leguminous with rhizobium species of bacteria;
- Regulates soil temperature by keeping it moderate;
- Leaf fall/ litter from cover crops constitutes a good source of soil organic matter for improving soil organic carbon contents;
- Green Manuring: This involves the growing of leguminous crops such as Mucuna and ploughing them into the soil at their early flowering stage where nitrogen fixation is highest. It also constitutes a huge source of organic matter for soil fertility improvement.
- Crop Rotation: This involves the growing of two or more crops in sequence after each harvest on the same piece of land. It maintains soil fertility because different plants remove soil nutrients from different levels / zones of the soil for their use. In addition, some of the crops in such practices are leguminous which helps in improving the nitrogen levels of the soil for subsequent crops which are usually cereals.
- Inorganic fertilizer application: Fertilizer supply specific nutrients to the soil in order to replenish its fertility. Most organic matter sources lack high concentrations of micronutrients; hence, these inorganic fertilizers largely serve as micronutrient sources to augment the organic sources.
- Mixed Farming: A farming system where farmers rear animals and grow crops on the same farm at the same time. The excrements of the animals help to maintain the fertility of the soil and the crop residues also serves as food and fodder for the animals.
- Mixed cropping: This is system of farming where the farmer grows more than one type of crop plants at the same time on the same piece of land. In most cases, cereals are combined with leguminous crops. Example is maize with cowpea. On other hand, in such cropping system, shallow rooting crops may be combined with deep-rooted crops, etc.
- Alley farming/Agro-forestry: In alley farming leguminous trees/shrubs such as Leucaena is grown to improve soil fertility by supplying nitrogen. Sometimes, the leaves are also used in formulating feeds for some animals kept under this system. Litter from the trees also adds organic matter to the soil and also acts as mulch to retain soil moisture and prevents soil erosion.
Whatever method (s) that is/ are or applied to improve the fertility of the soil, the aim is to target the fertility challenge that is or are being addressed. These fertility challenges would then amend itself to strict categorization under physical, chemical or biological method.
Soil Physical challenges and their measures to improve soil fertility and productivity:
- Shallow rooting zone: This challenge restricts proper rooting of crops to access both nutrients and water for effective crop growth. Some techniques or methods of improving such impact include the use of chisel or disc plough moulds where tilth angles are lowly inclined to allow for deeper penetration to increase the effective rooting zone for root accessibility to enough water and plant nutrients.
- Hard pan or layers in the subsoil: the procedures or methods adopted in increasing effective root zones should be adopted here. So the method is by mechanically breaking hard pans with chisel plough to improve effective rooting depths.
- Poor soil structure: Here, the soil may be having loose structure which does not effectively allow for proper water and nutrient management. Or on the other hand, it might be clayey which cracks during severe drought conditions and very difficult to work in at the peak of the rainy season. The strategy for improving such soils would be the addition of organic matter, mulches and other amendments that would seek to improve the physical difficulty under these two water stress conditions for a clayey soil. For sandy soils, the additions of these amendments should help improve the structure and make it more robust towards the increase in its capacity for holding water under water stress condition.
Soil Chemical challenges and measures to improve soil fertility and productivity:
- Strong acidity: This is where there are high amounts of H ions and Al with other elements such as Fe, Zn, Cu and Mn in the soil. This arises from the use of acidifying fertilizers such as sulphate of ammonia and under certain conditions, urea. The breakdown of such fertilizers in soils to release NO3 or Ammonium ions (NH4+) which are the dominant forms for Nitrogen uptake by plants leave behind H+ ions which is implicated in soil acidity. The strategy for such condition lies with liming (CaCO3) and avoidance of use of acid forming fertilizers such as Sulphate of ammonia and Urea. If these fertilizers should be used at all, it should be under strict guidance and supervision of an expert after a thorough analysis of soil has been done.
- Strong alkalinity: This occurs through the use of irrigation under high evopotranspiration conditions. Usually, environments or agricultural production systems that rely heavily on irrigation water are prone. Under such circumstances, the application of gypsum (Ca(SO4)2, pyrite (any sulphur dominated mineral, e.g-FeS) and green manuring can alleviate this challenge.
- Strong salinity: The total concentrations of dissolved salts (TDS) which is a measure of the level of conductance of soil solution, which is sometimes referred to as the salinity hazard is expressed as electrical conductivity (EC). The causes of such condition are linked to soluble salts, evapotranspiration, drainage and irrigation water quality. Under high levels of Salinity, there is the predominance of Na+ on the soil colloidal surfaces after Ca and Mg have formed precipitates. The resultant effect of excess Na+ on the exchanger surface is dispersion leading to poor soil structure and water logging conditions. Methods of remediating such saline conditions include leaching with non-saline water, growing salt tolerant crops and green manuring.
- Nutrient toxicities: The excess availability of certain nutrients leads to a condition whereby plant growth is adversely or negatively affected. A condition referred to as nutrient toxicities. For instance, the excess availability of N would largely lead to lush vegetative growth, which in many cases leads to insect pest and disease attacks. Moreover, vegetative growth at the expense of reproductive growth and yield is common. In similar examples, excessive supply of Ca would result in P deficiencies under certain conditions in the plant as well as the soil. Methods typically employed to correct this situation includes the use of suitable soil amendments, drainage and use of tolerant crops. For instance where there is excess Fe, and Mn, one can lime with CaCO3 to help increase the soil pH which will eventually reduce the activity and high concentrations of Fe and Mn, whilst in the end making P available for crop uptake.
- Low nutrient status: This is where the essential nutrients and their required levels or concentrations are inadequate for the growth and yield of a specified crop. Under such circumstance, the application of the deficient nutrients through mineral, organic and biological sources are best options.
- Nutrient fixation: This largely applies to the situation where particularly P is unavailable to the plants grown on highly weathered soils with high acidity, low activity clay minerals, presence of oxy and hydroxide minerals (sesquioxides) and high amounts or concentrations of Fe, Al, Mn and Cu. In such situations, P is made highly unavailable due to fixation unto oxy and hydroxide minerals and also fixed by association with Al, Fe and Mn. Methods of raising such fixed nutrients levels lie with direct inorganic fertilization and amendments of such soils with lime, for instance. Liming will raise the pH of the soil and would cause the release of P bound to those ions.
Soil Biological challenges and their measures to improve soil fertility and productivity:
- Low organic matter: The factors responsible for such conditions are numerous and have been stated in previous sections. In situations of low organic matter contents, the soil tend to have low CEC, low water holding capacity and high vulnerability to erosion. Techniques to raise the organic matter contents to high levels are through manure application, compost and green manuring.
- Poor microbial activity: This parameter assess soil microbes and their respiration rates, growth, nutrient cycling and how diverse soil microbes are in a volume of soil. In addition, microbial enzymes and their activities are key indicators of how healthy a soil is and are also assessed. Soil conditions that may result in low levels of the above mentioned parameters would include high acidity, poor aeration, waterlogging conditions and low organic matter contents. In view of these, methods tailored to address these challenges would include improvement in aeration, improved drainage, acidity correction through liming and manuring to improve organic matter contents.
Merits and demerits of Physical methods of improving soil fertility
- Shallow rooting zones are improved to allow for more water and nutrient reach by plant roots.
- Hardpans are broken with the help of chisel based ploughs to increase the depth of good soil.
- Addition of organic matter to improve poor soil structure enhances the workability of soils.
- The organic matter addition also enhances the water holding capacity of sandy soils.
- Agricultural extensification is reduced as a result of making marginal lands productive again.
- The use of mechanical plough may further lead to soil compaction.
- The initial costs of such technology such as plough are prohibitive to smallholder farmers.
- It is only suitable where the landscape is flat.
Merits and demerits of Chemical methods of improving soil fertility
- Liming reduces the acidity of soils to moderate levels which enhances soil fertility and productivity.
- Liming makes other nutrients such as B, Ca and in addition P available.
- Over liming leads to deficiencies in some other nutrients.
- Draining soil to reduce salinity when not done properly leads to leaching of essential nutrients.
- Over supplication of low nutrients through direct inorganic application of nutrients sometimes causes nutrient imbalance which leads to poor crop quality.
- Over supply of low nutrients to improve fertility may lead to environmental pollution through eutrophication.
Merits and demerits of biological methods of improving soil fertility
- The application amends not just lost nutrients, but also soil chemical and biological properties such as bulk density, aggregate stability, enzyme activity, etc.
- Micronutrients are supplied through organic matter application.
- Unintended environmental consequence such as eutrophication (loss of N and P into surface and underground water bodies through leaching leading to algae-bloom) is minimal.
- Adoption of biological methods requires the removal of other biophysical challenges such as drought and P for instance.
- The initial cost is usually prohibitive due to labour and bulkiness of organic residues.
Nutrients are indispensable as plant constituents, for biochemical reactions, and for the production of organic materials referred to as photosynthates (carbohydrates, proteins, fats, vitamins, etc.) by photosynthesis. The nutrients required are obtained by plants both from soil reserves and external nutrient sources (fertilizers, organic manures, the atmosphere, etc). Almost all of the 90 natural elements can be found in green plants although most of them have no function (e.g. the heavy metal, gold). More than 100 chemical elements are known to man today. However, only 16 have proven to be essential for plant growth and development. The essential plant nutrients may be grouped into three categories. They are as follows:
- Primary nutrients – nitrogen, phosphorus and potassium
- Secondary nutrients – calcium, magnesium and sulfur
- Micronutrients – iron, manganese, zinc, copper, boron, molybdenum, and chlorine
The bases of a nutrient being classified as being essential or not depends on the following:
- if a deficiency prevents the plant from completing its vegetative or reproductive cycle.
- if the deficiency in question can be prevented or corrected only by supplying the element.
- if it is directly involved in the nutrition of the plant and is not a result of correcting some microbiological or chemical condition in the soil or culture media.
SOURCES OF PLANT NUTRIENTS AND SOIL AMENDMENTS
A large number of diverse materials can serve as sources of plant nutrients. These can be natural, synthetic, recycled wastes or a range of biological products including microbial inoculants (biofertilizers). Nutrient sources are generally classified as organic, mineral or biological.
Organic nutrient sources
These sources are often described as manures, bulky organic manures or organic fertilizers. Most organic nutrient sources, including waste materials, have widely varying composition and often only a low concentration of nutrients, which differ in their availability. Some of these, such as cereal straw, release nutrients only slowly (owing to a wide C:N ratio) while others such as the N-rich leguminous green manures or oilcakes decompose rapidly and release nutrients quickly. Residues from processed products of plant or animal origin are increasingly important as nutrient sources and lead to nutrient saving by recycling. A significant amount of N is made available through BNF by a number of micro-organisms in soils either independently or in symbiosis with certain plants. The inocula of such micro-organisms are commonly referred to as biofertilizers, which are used to enhance the N supply for crops.
Mineral sources of nutrients (fertilizers)
The term fertilizer is derived from the Latin word fertilis, which means fruit bearing. Fertilizer can be defined as a mined, refined or manufactured product containing one or more essential plant nutrients in available or potentially available forms and in commercially valuable amounts without carrying any harmful substance above permissible limits. Many prefixes such as synthetic, mineral, inorganic, artificial or chemical are often used to describe fertilizers and these are used interchangeably. Fertilizer grade is an expression used in extension and the fertilizer trade referring to the legal guarantee of the available plant nutrients expressed as a percentage by weight in a fertilizer, e.g. a 12–32–16 grade of NPK complex fertilizer indicates the presence of 12 percent nitrogen (N), 32 percent phosphorous pentoxide (P2O5) and 16 percent potash (K2O) in it. On a fertilizer bag, the NPK content is always written in the sequence N, P2O5 and K2O.
Chemical fertilizers are grouped into two, namely; mined/compound/complex, and straight/simple single.
This is a fertilizer, which contains more than one primary element. Those that contain only two elements are described as incomplete. Examples NPK 20: 20: 0, while those that contain all the three elements are described as complete fertilizers. Examples are NPK 15: 15: 15, NPK 17: 17:17 and NPK 20: 20:20, NPK 23: 10: 5
Straight fertilizer is that type of fertilizer containing only one of the three (3) primary nutrients. Examples are Urea (N), Single/Triple super phosphate (S), Muriate of potash (K), and Sulphate of ammonia (N).
NUTRIENTS, FUNCTIONS AND THEIR MOVEMENT IN SOILS
Some knowledge of the properties and functions of plant nutrients is helpful for their efficient management and, thus, for good plant growth and high yields.
Available nutrients in the soil solution can be taken up by the roots, transported to the leaves and used according to their functions in plant metabolism. Most plant nutrients are taken up as positively or negatively charged ions (cations and anions, respectively) from the soil solution. However, some nutrients may be taken up as entire molecules, e.g. boric acid and amino acids, or organic complexes such as metal chelates and to a very small extent urea. One should bear in mind that whether the original sources of nutrient ions in the soil solution are from organic substances or inorganic fertilizers, ultimately, the plants absorb them only in mineral forms.
Nitrogen: N is the most abundant mineral nutrient in plants. It constitutes 2–4 percent of plant dry matter. Apart from the process of N fixation that occurs in legumes, plants absorb N either as the nitrate ion (NO3–) or the ammonium ion (NH4+).
N function: N is a part of the chlorophyll (the green pigment in leaves) and is an essential constituent of all proteins. It is responsible for the dark green colour of stem and leaves, vigorous growth, branching/tillering, leaf production, size enlargement,
and yield formation.
N mobility and form: Absorbed N is transported through the xylem (in stem) to the leaf canopy as nitrate ions, or it may be reduced in the root region and transported in an organic form, such as amino acids or amides. N is mobile in the phloem (the plant tissue through which the sap containing dissolved food materials passes downwards to the stem, roots, etc.); as such, it can be re-translocated from older to younger leaves under N deficiency and translocated from leaves to the developing seed or fruit. The principal organic forms of N in phloem sap are amides, amino acids and ureides. Nitrate and ammonium ions are not present in this sap.
N deficiency: N deficiency in plants results in a marked reduction in growth rate. N-deficient plants have a short and spindly appearance. Tillering is poor, and leaf area is small.
As N is a constituent of chlorophyll, its deficiency appears as a yellowing or chlorosis of the leaves. This yellowness usually appears first on the lower leaves while upper leaves remain green as they receive some N from older leaves. In a case of severe deficiency, leaves turn brown and die.
Phosphorus (P): P is much less abundant in plants (as compared with N and K) having a concentration of about one-fifth to one-tenth that of N in plant dry matter.
P function: P is essential for growth, cell division, root lengthening, seed and fruit development, and early ripening. It is a part of several compounds including oils and amino acids. The P compounds adenosine diphosphate (ADP) and adenosine triphosphate (ATP) act as energy carriers within the plants.
P mobility and form: P is absorbed as the orthophosphate ion (either as H2PO4 – or HPO4 2-) depending on soil pH. As the soil pH increases, the relative proportion of H2PO4 – decreases and that of HPO4 2- increases. P is readily mobile within the plant (unlike in the soil) both in the xylem and phloem tissues. When the plant faces P shortage (stress), P from the old leaves is readily translocated to young tissue. With such a mobile element, the pattern of redistribution seems to be determined by the properties of the source (old leaves, and stems) and the sink (shoot tip, root tip, expanding leaves and later into the developing seed).
P deficiency: Plant growth is markedly restricted under P deficiency, which retards growth, tillering and root development and delays ripening. The deficiency symptoms usually start on older leaves. A bluish-green to reddish colour develops, which can lead to bronze tints and red colour. A shortage of inorganic phosphate in the chloroplast reduces photosynthesis. Because ribonucleic acid (RNA) synthesis is reduced, protein synthesis is also reduced. A decreased shoot/root ratio is a feature of P deficiency, as is the overall lower growth of tops.
Potassium (K): K is the second most abundant mineral nutrient in plants after N. It is 4–6 times more abundant than the macronutrients P, Ca, Mg and S.
Potassium function: K is involved in the working of more than 60 enzymes, in photosynthesis and the movement of its products (photosynthates) to storage organs (seeds, tubers, roots and fruits), water economy and providing resistance against a number of pests, diseases and stresses (frost and drought). It plays a role in regulating stomatal opening and, therefore, in the internal water relations of plants.
Potassium mobility: K is absorbed as the monovalent cation K+ and it is mobile in the phloem tissue of the plants.
Potassium deficiency: The general symptom of K deficiency is chlorosis along the leaf boundary followed by scorching and browning of tips of older leaves. The affected area moves inwards as the severity of deficiency increases. K-deficiency symptoms show on the older tissues because of the mobility of K. Affected plants are generally stunted and have shortened internodes. Such plants have: slow and stunted growth; weak stalks and susceptibility to lodging; greater incidence of pests and diseases; low yield; shrivelled grains; and, in general, poor crop quality.
The secondary nutrients of interest are Calcium (Ca), Magnesium (Mg) and sulphur (S). Information on their nature, functions in plants and mobility both in soil and plants and finally their deficiency symptoms are discussed below:
Calcium (Ca): Calcium (Ca) ranks with Mg, P and S in the group of least abundant macronutrients in plants.
Calcium function: Ca is a part of the architecture of cell walls and membranes. It is involved in cell division, growth, root lengthening and activation or inhibition of enzymes. Ca is immobile in the phloem.
Calcium mobility: It is absorbed by plant roots as the divalent cation Ca2+
Calcium deficiency: Ca deficiency is seen first on growing tips and the youngest leaves. This is the case with all nutrients that are not very mobile in the plants. The Ca-deficiency problems are often related to the inability of Ca to be transported in the phloem. The problems occur in organs that do not transpire readily, i.e. large, fleshy developing fruits. Ca-deficient leaves become small, distorted, cup-shaped, crinkled and dark green. They cease growing, become disorganized, twisted and, under severe deficiency, die.
Magnesium (Mg): Mg ranks with Ca, P and S in the group of least abundant macronutrients in
Magnesium (Mg) function: Mg occupies the centre-spot in the chlorophyll molecule and, thus, is vital for photosynthesis. It is associated with the activation of enzymes, energy transfer, maintenance of electrical balance, production of proteins, metabolism of carbohydrates, etc. Mg is mobile within the plants.
Magnesium (Mg) mobility: Plants take up Mg in the form of Mg2+
Magnesium (Mg) deficiency: As Mg is readily translocated from older to younger plant parts, its deficiency symptoms first appear in the older parts of the plant. A typical symptom of Mg deficiency is the interveinal chlorosis of older leaves in which the veins remain green but the area between them turns yellow. As the deficiency becomes more severe, the leaf tissue becomes uniformly pale, then brown and necrotic.
Sulphur (S): S is required by crops in amounts comparable with phosphorus (P).
Sulphur fucntions: S is a part of amino acids cysteine, cystine and methionine. Hence, it is essential for protein production. S is involved in the formation of chlorophyll and in the activation of enzymes. It is a part of the vitamins biotin and thiamine (B1), and it is needed for the formation of mustard oils, and the sulphydryl linkages that are the source of pungency in onion, oils, etc.
Sulphur (S)mobility: S moves upwards in the plant as inorganic sulphate anion (SO4 2-). Under low S conditions, mobility is low as the S in structural compounds cannot be translocated. As the S status of the plant rises, so does its mobility. This pattern of mobility means that in plants with adequate S, sulphate is preferentially translocated to young, actively growing leaves.
Sulphur (S) deficiency: In many ways, S deficiency resembles that of N. It starts with the appearance of pale yellow or light-green leaves. Unlike N deficiency, S-deficiency symptoms in most cases appear first on the younger leaves, and are present even after N application. Plants deficient in S are small and spindly with short and slender stalks. Their growth is retarded, and maturity in cereals is delayed. Nodulation in legumes is poor and N fixation is reduced. Fruits often do not mature fully and remain light green in colour.
A micronutrient is a nutrient that constitutes less than 0.1% of plant dry matter. The micronutrients are boron, chlorine, copper, iron, manganese, molybdenum, and zinc. However, our focus would be on boron (B), molybdenum (Mo) and Zinc (Zn).
Boron (B): Boron (B) is taken up by plants as the undissociated boric acid (H3BO3).
It appears that much of the B uptake mainly follows water flow through roots. B in a plant is like the mortar in a brick wall, the bricks being the cells of growing parts such as tips (meristems). Key roles of B relate to: (i) membrane integrity and cell-wall development, which affect permeability, cell division and extension; and (ii) pollen tube growth, which affects seed/fruit set and, hence, yield. B is relatively immobile in plants and, frequently, the B content increases from the lower to the upper parts of plants.
B deficiency: B deficiency usually appears on the growing points of roots, shoots and youngest leaves. Young leaves are deformed and arranged in the form of a rosette. There may be cracking and cork formation in the stalks, stem and fruits; thickening of stem and leaves; shortened internodes, withering or dying of growing points and reduced bud, flower and seed production.
Molybdenum (Mo): Mo is absorbed as the molybdate anion MoO42- and its uptake is controlled metabolically. Mo is involved in several enzyme systems, particularly nitrate reductase, which is needed for the reduction of nitrate, and nitrogenase, which is involved in BNF. Thus, it is involved directly in protein synthesis and N fixation by legumes. Mo appears to be moderately mobile in plants. This is suggested by the relatively high levels of Mo in seeds, and because deficiency symptoms appear in the middle and older leaves.
Molybdenum (Mo) deficiency: Mo deficiency in legumes can resemble N deficiency because of its role in N fixation. Mo deficiency can cause marginal scorching and rolling or cupping of leaves and yellowing and stunting in plants. Yellow spot disease in citrus and whip tail in cauliflower are commonly associated with Mo deficiency.
Zinc (Zn): Zn is taken up as the divalent cation Zn2+. Early work suggested that Zn uptake was passive, but more recent work indicates that it is active (energy-dependent). Zn is required directly or indirectly by several enzymes systems, auxins and in protein synthesis, seed production and rate of maturity. Zn is believed to promote RNA synthesis, which in turn is needed for protein production. The mobility of Zn is low. The rate of Zn mobility to younger tissue is particularly depressed in Zn-deficient plants.
Zinc (Zn) deficiency: Common symptoms of Zn deficiency are: stunted plant growth; poor tillering; development of light green, yellowish, bleached spots; chlorotic bands on either side of the midrib in monocots (particularly maize); brown rusty spots on leaves in some crops, which in acute Zn deficiency as in rice may cover the lower leaves; and in fruit trees the shoots may fail to extend and the small leaves may bunch together at the tip in a rosette-type cluster.