Plant Nutrition Diagnostics

Plant Diagnostics

Plant diagnostics — determination of the supply of plants with chemical elements based on their chemical composition, taking into account the biological characteristics of the variety, growth rates, and duration of growing periods.

Plant nutrition diagnostics is carried out taking into account the field history, soil and agrochemical maps, experimental results, and zonal recommendations for the application of fertilizers for a specific crop. Plant nutrition diagnostics is carried out taking into account analysis of the chemical composition of leaves and roots. Normal plant nutrient supply is considered to be a state of internal saturation, accumulation of a reserve of chemical elements in reserve zones.

For different conditions of soil-climatic zones, general optimal parameters of NPK content in cereal crops have been developed for use with plant diagnostic methods.

Plant diagnostics includes:

• visual;
• chemical (tissue and leaf);
• functional, or physiological.

Visual Diagnostics

The method of visual diagnostics is based on changes in the morphological characteristics of plants caused by a deficiency or excess of nutrients in the soil.

The accuracy of the visual method is reduced because a sharp deficiency or excess of elements, causing characteristic symptoms, is rarely encountered, whereas a partial deficiency or excess may not be visually apparent. In addition, similar visual symptoms may be caused by deviations in temperature or water regimes, or by damage from pests or diseases.

Any disruption of plant vital processes is reflected in its appearance, which can be detected in various organs. However, for each disorder there are the most characteristic indicator organs, by which diagnosis is easier to perform.

Plant starvation is observed during a short-term shift in the optimal ratio of elements; it can manifest even against a high nutrient background when unfavorable external growth factors combine — illumination, humidity, temperature, and aeration.

In practice, excessive uptake of certain nutrients by plants is quite often observed, for example ammonium nitrogen, chlorine, and manganese.

The nutrient requirements of different crops are not the same; therefore, in the same field rye can produce a good yield without showing signs of potassium starvation, whereas potato cannot develop normally.

Indicator plants — plants whose appearance makes it easiest to determine a deficiency or excess of a particular mineral nutrition element.

Visual diagnostics of plant nutrition is one of the simplest methods, requiring no equipment, allowing a conclusion to be made in a relatively short time about plant nutrition disorders, their causes (soil and weather factors), and recommendations for changing cultivation technology.

In visual diagnostics, in addition to general principles, the following are taken into account:

• When assessing, the plant is considered simultaneously in three temporal aspects: in the past, reflecting the set of measures for its cultivation, including environmental factors; in the present, assessing growth rates and degree of development; and in the future, that is, forecasting the quantity and quality of the yield.
• Signs of starvation or excess of nutrient elements more often appear not over the entire area but in individual plots, which is associated with different soil fertility, relief features, fertilizer application, and treatments.
• It is better to study the appearance of the plant together with the root system, or its parts, for damage from pests, fungi, bacteria, viruses, pesticides, and growth substances.
• Often the signs of deficiency and excess are visually similar. Nevertheless, deficiency is characterized by clearer signs.
• Plant damage caused by viruses often has signs similar to mineral nutrition disorders, but differs by a clearer boundary of the affected area.
• The final conclusion about the causes of the disorder is made after elimination with appropriate treatments or fertilizers. External signs may persist on severely damaged leaves.

With a deficiency or excess of an element, external signs may differ depending on species and variety. However, there are general signs.

Many nutrient elements, such as nitrogen, phosphorus, potassium, and magnesium, have the ability to be reused, that is, remobilized. Deficiency of these elements manifests primarily on the lower, older leaves. The ability of calcium, sulfur, chlorine, boron, and others to be reused is weaker, so their deficiency manifests in growth points and on young leaves.

For visual diagnostics, the following are assessed:

• the general condition of plants in the plot or field;
• plant mass and height, and whether development corresponds to the growing period (phase);
• internode length (in young plants they are shorter);
• fullness, elasticity, and maturity of the stem; for example, with balanced nutrition the stem fits more fully into a circle; maturity is assessed by the color of the cut at the level of the 3rd internode from the top;
• leaf elasticity, leaf color by tiers, and the nature of disorders within a tier;
• root development, presence of root hairs, and root color.

Based on the assessment results, conclusions are drawn and recommendations are developed aimed at changing cultivation technology. It is possible to correct unbalanced nutrition only partially, since the appearance of external signs of a mineral nutrient deficiency indicates profound changes in plant metabolism that subsequently cannot be completely eliminated.

To confirm a suspected plant nutrition disorder during visual diagnostics, injection and spraying methods are used. By spraying a leaf or injecting into the stem (leaf pulvinus), the suspected deficient element is introduced. Then observations are carried out over several days. If the deficient element is correctly identified, the deficiency signs disappear, but not on the leaves where they were detected, rather on newly forming leaves.

To eliminate a nutrient deficiency, 0.5% solutions of potassium and calcium salts, 0.1% solutions of urea, sodium monophosphate, magnesium sulfate, and 0.02–0.1% solutions of trace element salts are used.

Chemical Diagnostics

The method of leaf or tissue diagnostics is based on the relationship between changes in the nutrition regime and the chemical composition of leaves or tissues, the most responsive organs. For various plants, optimal concentrations of nutrient elements have been determined at which crops show maximum productivity.

The accuracy of these methods for predicting fertilizer requirements is higher than that of soil analyses, because when determining the amount of nutrients in soil it is difficult to predict what portion of them will enter plants under changing plant life factors.

A disadvantage of chemical diagnostics, as with visual diagnostics, is the delay in obtaining information. Chemical diagnostics of plant nutrition under unfavorable meteorological conditions also gives distorted data on plant supply because of.

Integrated Diagnostic and Recommendation System (DRIS)

The Integrated Diagnostic and Recommendation System (DRIS) was developed in the USA. It is based on a probabilistic approach, based on the fact that the balance of nutrient elements in plant tissues and organs follows characteristic patterns. It is assumed that the ratio of elements has diagnostic informativeness and better reflects plant supply.

In our country, the first experiments using this system, called the Integrated System of Operational Diagnostics (ISOD), were conducted at the Soil Institute named after V. V. Dokuchaev.

The Integrated System of Operational Diagnostics is a set of methods used for diagnosing fertilizer requirements, forecasting crop productivity, and developing models of highly fertile soils. The effects of each factor on productivity indicators are expressed in indices. The basis for calculating the index is the optimal level of the factor under study.

The methodology of the system consists in determining the actual ratio of nutrient element amounts (N : P, N : K, K : P, N : Ca, N : Mg, etc.) in leaves and comparing the obtained data with standards that are constant for different soil types. The index shows the degree of deviation of the factor under study from the optimum. The magnitude and sign of the indices show the level of nutrient element deficiency.

Example.

Standards for the ratios N:P, N:K, K:P were obtained for Moscow Oblast by Elnikov et al., 1986.

Maize was grown on leached chernozem in years with different weather conditions. For analysis, all maize leaves were taken in the flowering phase. According to the method, in 1963 with a favorable moisture regime in the unfertilized variant, the soil was insufficiently supplied with nitrogen, with an index of −13.6; excessively supplied with potassium (index +11.2); and close to optimal in phosphorus (index +2.4).

Thus, a strong imbalance of nutrient elements developed — the sum of indices without regard to sign (13.6 + 2.4 + 11.2) = 27.2. Application of N₆₀ reduced nitrogen deficiency, but at the same time phosphorus deficiency increased (index −5.8), and the overall imbalance remained high (sum of indices 18.6).

Application of P₆₀ increased phosphorus supply and intensified nitrogen deficiency. Application of potassium balanced the deficiency in nitrogen and phosphorus, but the overall imbalance remained.

Table. Fertilizer doses and deficiency of N, P, K in maize leaves (by DRIS indices).

Experimental variant	Content in leaves			DRIS indices			Grain yield, t/ha
	N	P	K	N	P	K
Experiment 1963
Control (no fertilizers)	2.53	0.29	2.50	-13.6	+2.4	+11.2	3.40
N60P0K0	2.78	0.28	2.49	-3.5	-5.8	+9.3	3.41
N0P60K0	2.68	0.33	2.50	-16.0	+11.3	+4.7	3.88
N60P60K0	2.80	0.31	2.45	-7.9	+3.6	+4.3	4.53
N0P0K60	2.71	0.28	2.66	-8.0	-17.2	+15.2	3.68
N60P60K30	2.82	0.31	2.51	-8.1	+2.4	+5.7	4.56
N90P60K30	3.00	0.30	2.40	0	-2.2	+2.2	4.77
N120P60K30	3.22	0.29	2.32	+8.3	-7.7	-0.6	5.00
N60P90K30	2.85	0.31	2.40	-5.7	+3.4	+2.3	4.64
N60P120K30	2.80	0.31	2.41	-7.4	+4.2	+3.2	4.94
N120P120K120	3.05	0.33	2.50	-4.8	+3.8	+1.0	5.42
Experiment 1964
Control (no fertilizers)	2.60	0.30	2.74	-16.0	+0.5	+15.5	2.71
N60P0K0	2.70	0.28	2.74	-9.1	-8.3	+17.4	2.72
N0P60K0	2.78	0.38	2.82	-25.7	+18.8	+6.9	2.98
N60P60K0	2.87	0.29	3.15	-10.4	-14.4	+24.8	2.93
N0P0K60	2.91	0.28	3.07	-6.4	-17.3	+23.7	3.39
N60P60K30	2.90	0.36	3.15	-21.5	-6.5	+15.0	3.48
N90P60K30	3.12	0.36	2.82	-11.0	+6.2	+4.8	3.67
N120P60K30	3.48	0.33	2.57	+4.8	-4.0	-0.8	3.79
N60P90K30	2.86	0.39	2.90	-25.5	+18.5	+7.8	3.83
N60P120K30	2.89	0.40	2.99	-27.2	+19.4	+7.8	3.99
N120P120K120	3.11	0.37	2.83	-7.7	+5.3	+2.4	4.17

Note. Indices were calculated using the standards: N/P = 10.0, N/K = 1.26, K/P = 7.50 with coefficients of variation 10, 19, 15.

In the dry year, eliminating potassium excess or reducing its content to a minimum in the absence of nitrogen deficiency (variants N₁₂₀P₆₀K₃₀ and N₁₂₀P₁₂₀K₁₂₀) led to the highest maize yield, which fully corresponds to the soil fertility characteristic in the unfertilized variant. In the remaining variants, a higher nitrogen deficiency was noted, which agrees with the position on the low efficiency of nitrogen fertilizers in years unfavorable for moisture. In the experiments, maize yield was determined by the balance of nutrient elements, which manifested under conditions of relatively favorable moisture.

Functional Diagnostics of Plant Nutrition

The uptake of nutrient elements is not always a consequence of their necessity for the plant. This determines the main disadvantage of chemical diagnostic methods. In addition, a deficiency or excess of elements can disrupt plant uptake of other nutrient elements. For example, phosphorus deficiency leads to nitrate accumulation, and boron deficiency leads to a deficiency. Moreover, this is in no way related to nitrogen nutrition.

Functional diagnostic methods allow assessing plant requirements for nutrient elements by determining the intensity of physiological and biochemical processes. For example, the level of nitrogen supply and requirement is determined by the ability of tissues to reduce nitrates to nitrites, that is, by the activity of the enzyme nitrate reductase (Muravin, Slipchik, Pleshkov, 1978).

A. S. Pleshkov and B. A. Yagodin (1982) developed a diagnostic method for determining the photochemical activity of chloroplasts, which is based on measuring the photochemical activity of a chloroplast suspension from an average sample of leaves of the diagnosed plants, followed by analysis with the addition of nutrient elements. If the photochemical activity of the suspension increases upon addition of a nutrient element compared with the control, a conclusion is made about a deficiency of this element; if it decreases, about an excess; and if the activity is the same, about optimal content.

The proposed method allows determining the plant requirement for 12–15 macro- and microelements within 40–50 min and making recommendations for root and foliar top dressing. The method was introduced in 80 greenhouse farms, including in Moscow Oblast (“Belaya Dacha”) and Ivanovo Oblast (“Teplichny”).

Soil Diagnostics

The particle-size composition, content of organic matter in soil, total content of nutrients, and absorption capacity (CEC) change slowly and serve as characteristics of a specific soil difference.

The content of mobile forms of nutrients, soil reaction, composition of absorbed cations, and degree of base saturation change faster, especially under the influence of ameliorants and fertilizers. Therefore, agrochemical soil surveys for these indicators are carried out at certain intervals, as a rule, once every two years, or more often, depending on the amount of fertilizers and ameliorants applied. The results of such surveys are presented in the form of agrochemical maps, field passports, or cartograms, and are used to determine optimal rates, forms, timing, and methods of fertilizer application, as well as the need for and doses of ameliorants.

Soil diagnostics of plant nutrition is the periodic determination of relatively rapidly changing agrochemical indicators of soils. Soil diagnostics allows fertilizers and ameliorants to be applied most rationally, maximizing their agronomic and economic efficiency and environmental safety.

The most dynamic indicator determined in soil diagnostics, capable of changing within several days, is the content of mineral forms of nitrogen. For this reason, this indicator is not used for compiling agrochemical maps, cartograms, and field passports. However, for economical and environmentally safe use of nitrogen fertilizers, annual information on reserves of mineral forms of nitrogen is necessary.

For regions of sufficient and excessive moisture, soil diagnostics of nitrogen forms is carried out before applying nitrogen fertilizers; in zones of insufficient moisture and arid climate, before autumn application. Depending on the depth of penetration of crop root systems and the water and air regimes of soils during the growing season, the content of mineral forms of nitrogen is determined in soil layers up to 100, 150, and 180 cm. For most agricultural regions of the country, it has been experimentally established that 60–80% of the mineral nitrogen in the 0–180 cm soil layer is concentrated in the 0–60 cm layer. For individual regions, conversion coefficients have been established for mineral nitrogen reserves for layers 0–40 and 0–60 cm to layers 0–100, 0–150 cm, etc.

There are several variants for correcting or calculating nitrogen fertilizer doses based on the results of mineral nitrogen nutrition diagnostics. In all variants, reserves of ammonium and nitrate nitrogen in a specific layer are converted to kg/ha, and taking into account possible nitrogen utilization coefficients by specific crops, the resulting value is subtracted from the total crop requirement.

An employee of the Department of Agrochemistry of the MAA, Yu. P. Zhukov, proposed a simplified modification for correcting nitrogen fertilizer doses. Mineral nitrogen is determined before applying nitrogen fertilizers in the arable soil layer, that is, 0–20 or 0–30 cm; the obtained result is converted to kg/ha and subtracted from the established dose or the total crop nitrogen requirement. This approach reduces the time required for studying deep soil layers; moreover, the uncertainty of weather conditions does not allow an accurate forecast regarding the stability of nitrates and ammonium forms in deep layers, especially since during the first month after sowing the roots do not have time to penetrate to a sufficiently great depth.

References

1. Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. (2002). Agrochemistry. Edited by B. A. Yagodin. Moscow: Kolos.