Precise citrus fertilization – guiding mineral applications by high-throughput diagnoses

Or Sperling, Eran Raveh, Train Kagan, and Uri Yermiyahu

Scientific Background

Future farming relies on precise and sustainable use of resources that would optimize productivity and protect the environment. Nutrients are becoming pricy, while their residues wash away from the farms and risk the surrounding natural or urban habitats. It leads to profit losses, alarms communities, and forces regulators to act. Hence, mineral nutrient application, the hallmark of the ‘green revolution’ and industrialized farming, needs to adjust to new market demands.

Nevertheless, in industrialized farming regions, nutrient applications are often suboptimal. The challenge is to maximize crops while eliminating contaminating runoffs. To achieve it, the applications of water and nutrients should advance and fit the crops’ actual requirements. For that matter, the application of mineral nutrition through irrigation (i.e., fertigation) has great potential to minimize farming contamination and improve yields. Up-to-date, fertigation isn’t extremely efficient. In many farms, the concentration of nutrients in irrigation recurrently remains constant throughout the growing season because there is no high-resolution information on crops’ mineral uptake.

The technical challenge is that in fertigation minerals are readily available for the plants, or to wash away, at the time of application. Hence, to be precise, their composition needs to fit the temporal requirements of the crops. For instance, tomato plants require more nutrients in the afternoon, presumably to reserve osmolytes for early morning stomata apparatus. Otherwise, perennial trees accumulate potassium and microelements in late fall to support bloom. It all should incorporate into the fertigation regime, though such observations could also mislead. High phosphorus concentration in roots may infer that P fertilization is critical in late winter when only roots grow, or it could be the mineral’s last resort, and irrelevant to precise fertilization. Hence, we need to finely adjust differential fertigation (DF) by crop phenology through fundamental research in major crops.

The mineral composition in plant tissues, and the seasonal development of these tissues, determine the overall nutritional requirements of plants. Plants’ parts differ markedly in their mineral compositions. Generally, tissues have ~2% nitrogen (N; per dry mass, DM) constituting the nuclei material and functional proteins. Leaves N concentration is rigid, and they downregulate photosynthesis and cease growth in N deficiency. Roots, on the other hand, would extend to extract residual minerals at <0.5% N to mitigate the deficiency. P is prominent in nucleic acids, yet in low concentrations that accumulate to just ~0.5% of DM. Yet P esters make the available energy for cellular metabolism, and they regulate numerous enzymatic processes. Therefore, P levels are high in expanding tissues with fast metabolism and high turnover rates. Additionally, P can accumulate to 85% of cells’ vacuoles for storage and buffer the extreme demands of phosphorylated intermediates of photosynthesis. Finally, plants extract large doses of potassium (K) during the growing season, being the most abundant cation in the cytosol which facilitates osmoregulation and hydraulic imbalances inside plants. K is also critical in cellular carbohydrate management through enzyme synthesis and activation or sugar transport in phloem. All in all, K could accumulate to <3% in vegetative tissues or fruits, and its total concentration in trees fluctuate seasonally according to growth, yield, and senescence. Hence, during the intensive spring growth, plants require substantial measures of all minerals. Perennials cannot support such mineral allocation by roots extraction, and they utilize last year’s reserves too. In summer, fruits are the primary metabolic sinks, and trees require N. In arid climates, maintaining the hydraulic integrity of trees in summer also necessitates K for osmoregulation. Then, towards winter, trees accumulate P for energy storages, and K for osmotic protection from winter hazards. Nevertheless, to fully account for the productivity of crops throughout the year, it is critical to determine what functionalities are permitted by the abiotic conditions.

Research Contributions

The rise in market demands for citrus fruits promotes massive investments in plantations and technology. Citrus is a very large market (138 M tons in 2016), while China alone consumed 38 M tons in 2018 and is projected to consume 45 M tons by 2025. Considering the transport expenses, trade agreements, and growth requirements, China, which already grows 28% of global citrus production, has a great incentive to promote domestic citrus production. For that matter, a collaboration with Israeli agronomy researchers (considering Israel’s extended scientific and agro-technical knowledge in citrus cultivation) is highly promising. Precise fertilization should be an essential aspect of this process, especially as new farms apply continuous mineral fertigation in large scales, but there are no research-based guidelines for fertigation of citrus plantations. Essentially, there are seasonal recommendations with minimal references to environmental conditions, fruit-load, or soil-solution mineral composition. There is an exemption, as farmers cease fertilization 2-3 months before harvest to ensure color-break in the peal for marketing reasons. Otherwise, there is no emphasis on fixing fertilization to the phenological requirements of citrus trees, despite ample data, to ensure both efficiency and sustainability of the crops.

Research objectives

We intend to advance intensive farming to precise mineral applications that maximize cost-effectiveness and eliminate contaminating runoffs. Up-to-date, there are few resources to guide mineral applications because there are finite diagnostic tools. The limited datasets of seasonal changes in the mineral composition of crops are not sufficient for breakthroughs in computational approaches to fertilization. Hence, we are upgrading the experimental and analytical facilities in ARO-Gilat to:

• Treat citrus trees with deficient to excessive nutritional applications in a large lysimeter setup.
• Determine the nutritional composition of leaves with a wide range of mineral concentrations.
• Develop and calibrate chemometric tools for micro-elements and macro-elements leaf analyses.
• Construct a seasonal dataset of nutrients uptake and the corresponding leaf mineral composition.
• Associate between leaves’ mineral composition and their vegetative or reproductive physiological performances.
• Construct decision support tool to guide citrus fertilization by frequent leaf mineral diagnoses.