The use of micro-fertilizers to increase the productivity of crops in intensive farming

The intensive agricultural system involving the cultivation of the most productive cultivars, the application of highly concentrated non-ballast nitrogen-phosphorus-potassium fertilizers has led to a decrease in the content of plant-available forms of trace elements in soils and, consequently, to the need for widespread use of micronutrient fertilizers, i.e., fertilizers whose active ingredient consists of trace elements. In the Russian Federation and neighboring countries, up to 59.5% of arable land requires the application of boron fertilizers, 90.8% cobalt, 41.3% manganese, 64.5% copper, 75.3% molybdenum, and 83.0% zinc. The current demand of these states for boron fertilizers is 12,350 t, cobalt – 310, manganese – 11,550, copper – 8,740, molybdenum – 2,100, and zinc – 7,450 t in terms of pure element. These calculations are based on agrochemical soil analyses and the level of yield of major agricultural crops prevailing at this stage. With further increases in the productivity of cultivated plants, the demand for micronutrient fertilizers will grow.

The need for micronutrient fertilizer application is established by the content of mobile forms of trace elements in the soil, which is determined by the nature of the parent rocks and vegetation, the type and biological activity of the soil (Table 74; Kurkaev V.T., Sheudzhen A.Kh., 2000).

Table 74 – Content of plant-available forms of trace elements in soils, mg/kg (Peive-Rinkis extracts) Soils B Co Mn Cu Mo Zn

 Sod-
               0.08–0.38   0.12–3.0   50–150    0.05–5.0   0.04–0.97 0.12–20.0
 podzolic
 Chernozems     0.38–1.58 1.10–2.2      1.0–75   4.5–10.0   0.02–0.33   0.10–0.25
 Sierozems      0.23–0.62 0.90–1.5     1.5–125   2.5–10.0   0.03–0.15   0.09–1.12
 Chestnut       0.30–0.90 0.10–6.0      1.5–75   8.0–14.0   0.09–0.62   0.06–0.14
 Brown          0.38–1.95 0.57–2.25     1.5–75   6.0–12.0   0.06–0.12   0.03–0.20

Plant-available (mobile) forms of trace elements in the soil are subdivided into low-mobility ones, which are determined in strong acid extracts; medium-mobility ones, extracted by solutions of weak acids, as well as alkalis and acid buffers; and readily soluble ones, determined in water and carbonic acid extracts. It is important that the chosen extractant, when determining the mobile form of a particular trace element, corresponds as much as possible to the uptake capacity of the root system of a specific plant and objectively reflects the degree of the plant's need for micronutrient fertilizer.

The determining criterion for judging the soil nutrient status regarding plant nutrients, including trace elements, is field experimentation. In it, the correspondence between the content of mobile forms of elements in the soil, their quantity in plants, and their removal with the harvest, as well as the correspondence between the element reserve in the soil and the effectiveness of fertilizers, is experimentally established. There is a differentiated approach to choosing methods for determining the mobile forms of trace elements in soil depending on its type, properties, and agrochemical characteristics.

For sod-podzolic type soils, the system of extracts proposed by Ya.V. Peive and G.Ya. Rinkis has found the most widespread use. An acetate-ammonium buffer solution with pH 4.8, proposed by N.K. Krupsky and A.M. Aleksandrova, is used in the analysis of chernozems, chestnut, and grey forest soils to determine the mobile forms of manganese, copper, cobalt, and zinc. Boron in all soils is determined in a water extract, and molybdenum is extracted using an oxalate buffer solution with pH 3.3 according to J.C. Grigg.

Based on their supply of individual trace elements and the need for applying micronutrient fertilizers, soils are divided into three groups: low, medium, and high status (Table 75; Kurkaev V.T., Sheudzhen A.Kh., 2000).

Table 75 – Grouping of soils by plant trace element status, mg/kg Status B Co Mn Cu Mo Zn

                             Peive-Rinkis extracts
 Low 1           <0.1        <0.3      <15       <0.5      <0.05        <0.3
 Medium 1         0.1–0.3     0.3–1     15–30     0.5–1.5  0.05–0.15     0.3–1.5
 High 1          >0.3         >1       >30       >1.5      >0.15        >1.5
 Low 2           <0.3         <1       <45        <2        <0.2        <1.5
 Medium 2         0.3–0.5      1–3      45–70       2–4     0.2–0.3       1.5–3
 High 2          >0.5         >3       >70        >4        >0.3         >3
 Low 3           <0.5         <3      <100        <5        <0.3         <3
 Medium 3          0.5–1       3–5     100–150      5–7     0.3–0.5        3–5
 High 3           >1          >5      >150        >7        >0.5         >5
                        Krupsky-Aleksandrova extracts
 Low 1           <0.1       <0.07       <5       <0.1      <0.05         <1
 Medium 1         0.1–0.3   00.7–0.15    5–10     0.1–0.2  0.05–0.15       1–2
 High 1          >0.3       >0.15      >10       >0.2      >0.15         >2
 Low 2           <0.3       <0.15      <10       <0.2       <0.2         <2
 Medium 2         0.3–0.5   0.15–0.30   10–20     0.2–0.5   0.2–0.3        2–5
 High 2          >0.5        >0.3      >20       >0.5       >0.3         >5
 Low 3           <0.5        <0.3      <20       <0.5       <0.3         <5
 Medium 3          0.5–1     0.3–0.7    20–40      0.5–1    0.3–0.5       5–10
 High 3           >1         >0.7      >40        >1        >0.5        >10

Note: 1 – for plants with low trace element removal;

2 – for plants with high trace element removal;

Crop demand for trace elements and specific features of soil analysis

The demand of agricultural crops for trace elements varies significantly. Potatoes, cereals, and grain legumes are characterized by low trace element removal while having a relatively high ability to assimilate them from the soil. Root crops, vegetables, sunflowers, cotton, forage crops, fruit trees, and vineyards require increased amounts of trace elements. Furthermore, under irrigation conditions and when using intensive technologies, the removal of trace elements from the soil increases sharply.

When planning a fertilization system, it is important to take into account that the results of soil analysis for trace element status depend on the method used. Different extractants extract unequal amounts of mobile forms. Because of this, the grouping of soils based on different analysis results often does not coincide.

• Manganese (acetate-ammonium extract pH 4.8 to 0.1 N H2SO4 extract) — 3–4 times less
• Zinc (acetate-ammonium extract to 1 N KCl) — 2–4 times more
• Copper and cobalt (buffer solution to 1 N HCl and 1 N HNO3) — 6–8 times less

Effect of microfertilizers on crop yield and immunity

The application of microfertilizers on soils with low nutrient content increases yield by 10–15%. When cultivating intensive cultivars, top dressing is effective not only on poor soils but also on those with average nutrient supply. However, microfertilizers will work to their full potential only with a high overall level of agronomic practices. It is unacceptable to attempt to compensate for flaws in ploughing and general soil preparation solely by applying microelements.

Microelements also increase plant resistance to pests and diseases. For example, seed treatment of peas with molybdenum, zinc, and cobalt reduces the number of pea leaf weevil larvae on the roots. The application of manganese, copper, and boron increases the resistance of cereal crops to the Hessian fly. Boron, molybdenum, copper, and zinc fertilizers reduce the damage caused by rust, polysporosis, and anthracnose, while cobalt and manganese ones are effective against powdery mildew of cereals and tomato late blight.

The use of microfertilizers allows not only for correcting plant nutrition, but also for significantly reducing the harm caused by common diseases. Microelements act as protectants, increasing the physiological resistance of crops to infections. They stimulate the thickening of the cuticle and epidermis, contribute to the formation of protective cortical layers and the synthesis of phytoalexins. In addition, they accelerate plant development during critical phases of ontogenesis, reduce the reproductive capacity of pathogens, and slow down their spread.

To reduce the harm caused by specific diseases, targeted top dressing is used in practice:

• Cereal crops: manganese (against helminthosporiosis);
• Cotton: boron and copper (against verticillium wilt), manganese (against root rot), zinc (against fusarium wilt);
• Sugar beet: zinc (against black leg);
• Potato: copper and manganese (against rhizoctoniosis, late blight, and black leg), molybdenum (against late blight), cobalt (against verticillium wilt);
• Cabbage: manganese and boron (against clubroot);
• Carrot: boron (against phoma rot);
• Apple: boron and manganese (against black cancer);
• Strawberry: manganese (against grey mould).

Monitoring microelement accumulation and accounting for incidental application

To prevent toxicity, it is necessary to control the biological absorption coefficient (BAC). It is calculated by the formula: BAC = Ca / Cs, where Ca is the content of an element in the plant ash (mg/kg), and Cs is the total content of the same element in the soil (mg/kg). This indicator helps to estimate how intensively a crop takes metals from the soil.

• Vigorous accumulation (BAC) — 10–100
• Strong accumulation (BAC) — 1–10
• Weak accumulation (BAC) — 0.1–1.0
• Share of mobile forms in mineral fertilizers — 70–75%
• Share of mobile forms in manure — up to 25%

When calculating dosages, take into account the volumes of microelements that enter the fields along with main fertilizers. Urea, ammonium nitrate, double superphosphate, and potassium chloride practically do not contain microelements. On the contrary, simple superphosphate and phosphorite meal contain them in significant quantities.

It is also important to pay attention to the form of the compounds. In mineral fertilizers, up to 70–75% of microelements are in a mobile form, whereas in manure, this figure is no more than 25%. At the same time, regular application of organic fertilizers can fully meet the crops' demand for microelements.