Enhanced nutritional programs (ENPs) are slow- or controlled-release, liquid or dry soluble granular fertilizers that contain all or most essential macronutrients and micronutrients to provide the citrus trees with readily available nutrients throughout the production season to mitigate the debilitating impacts of HLB. There are three major criteria that qualify a mineral element to be considered an essential plant nutrient ^[1][2][46,47]. These include (1) a given plant must be unable to complete its life cycle in the absence of the mineral element, (2) the function of the element must not be replaceable by another mineral element, and (3) the element must be directly involved in plant metabolism, for example, as a cofactor of an enzyme ^[1][46]. This means that all essential mineral elements for a citrus tree are deemed important, and if one of them is deficient, it can limit the growth potential of the tree. Similar to that of many other higher plants, citrus trees require all essential nutrients in their right proportion ^[3][1].

The goals of optimal nutrient management are to (1) ensure that plants have optimal levels of essential nutrients for growth and development throughout all critical growth stages, (2) guarantee an adequate supply of all essential nutrients either through plant roots or leaves, and (3) ensure that soil physical and chemical properties favor nutrient absorption by plant roots ^[1][3][1,46]. It is well understood that a growing plant may have already lost its potential while deficiency symptoms are observed on the leaves. Therefore, it is the goal of any nutrient management program to test plant leaf tissue to ensure that the levels of all essential nutrients are optimized.

Manganese is an essential element for plants, intervening in several metabolic processes, mainly in photosynthesis and as an enzyme antioxidant cofactor ^[1][4][46,48]. Reduced Mn (Mn²⁺) form is the only available metal form for plants. It is taken up through an active transport system in epidermal root cells and transported as divalent cation Mn²⁺ into the plants ^[1][5][6][46,49,50]. According to past research, Mn has a profound influence on three physiological (metabolic) functions: (i) photosynthesis, particularly electron transport in photosystem II and chloroplast structure, (ii) N metabolism, especially the sequential reduction of nitrate, and (iii) aromatic ring compounds as precursors for aromatic amino acids, hormones (auxins), phenols, and lignin ^[6][7][50,51]. The concentration of Mn in the soil may be controlled by chemical complexes formed by Mn²⁺ due to low or high pH ^[8][52]. At higher soil pH (up to about pH 8), autooxidation of Mn²⁺ is over MnO₂, Mn₂O₃, and Mn₃O₄, which are not normally available to plants ^[5][8][9][49,52,53]. Manganese is an important oligo element involved in the regulation of many different physiological processes as well as part of the antioxidant enzyme Mn-SOD ^[10][54]. Manganese deficiency greatly affects photosynthesis; however, visual symptoms occur when plant growth is severely depressed ^[11][55]. Deficiency symptoms are observed in newly emerged leaves because of low phloem mobility of Mn that prevents remobilization of Mn from older to younger leaves ^[11][55]. In addition, Mn deficiency causes reductions in lignin concentrations in plant roots ^[8][11][52,55]. Research has revealed that Mn deficiency in citrus may significantly reduce yield and fruit color, and the fruit may become smaller and softer than normal ^[3][1].

Iron (Fe) is a transitional element that is characterized by the relative ease by which it may change its oxidation state and by its ability to form complexes with different ligands ^[1][46]. This variability expressed by Fe is essential in biological redox systems ^[1][46]. Iron as a micronutrient is required by most plants in small quantities. It is well known for its metabolic processes such as deoxyribonucleic acid (DNA) synthesis, photosynthesis, and respiration ^[12][56]. It is also a constituent of many electron carriers and enzymes, and therefore, important in plant metabolism ^[12][56]. The presence of Fe in iron-containing heme proteins makes its levels in the plant critical in the electron transfer chain e.g., cytochromes ^[13][57]. Cytochromes are found in the electron transfer systems in chloroplasts and mitochondria ^[1][13][46,57]. Other heme enzymes are catalase and peroxidases ^[1][46]. It is reported that, under conditions of Fe deficiency, the activity of both types of enzymes declines ^[1][46].

Although Fe is abundant in the soil, it is mostly in a complex form, and plants absorb Fe by an active process, thus, by giving out energy to reduce Fe³⁺ to Fe²⁺ to make it available for absorption in the rhizosphere ^[13][14][57,58]. Plant iron absorption is also dependent on soil pH and redox potential ^[14][58]. At lower pH, Fe is readily available to plants, however, in aerobic soil conditions and high pH soils, Fe is in the form of insoluble ferric oxides ^[1][14][46,58]. Since HLB weakens the tree’s immune system ^[3][15][1,3] and contributes to the loss of more than 40% of the fibrous root system ^[16][32], it is a concern that affected trees may not exert enough energy to absorb required Fe, hence, affecting the rate of Fe absorption ^{[1][14][16][17]}[7,32,46,58]. It is therefore critical to provide an adequate amount of Fe in the form that is readily available in the rhizosphere to increase their chances of being absorbed ^[3][18][1,24].

Iron deficiency is characterized by chlorosis in young leaves, which is not only associated with the decline of chlorophyll and ß-carotene, but also with changes in the expression and assembly of other components of the photosynthetic apparatus ^[1][14][46,58]. Due to the low solubility of the oxidized ferric form in an aerobic environment, Fe in the soil is mostly not available to plants ^[19][59]. When the plant is deficient in Fe, the ferredoxin content is decreased to a similar extent as the chlorophyll content, and the fall in ferredoxin level is associated with a lower nitrate reductase activity ^[1][20][46,60].

Low pH and moisture conditions could trigger Fe toxicity and may be a serious problem for the growth and development of citrus ^[1][3][14][1,46,58]. Even though this condition is predominantly observed in waterlogged soils and in the event of heavy rainfall or excess irrigation ^[1][46], other researchers have reported that the iron catalyzed formation of oxygen-free radicals in the chloroplasts can cause Fe toxicity under dryland conditions ^[21][61].