*Editorial* **Site-Specific Nutrient Management**

**Witold Grzebisz**

> Department of Agricultural Chemistry and Environmental Biogeochemistry, Poznan University of Life Sciences, Wojska Polskiego 28, 60-637 Poznan, Poland; witold.grzebisz@up.poznan.pl; Tel.: +48-618-487-788

> **Abstract:** The editorial introduces to a Special Issue entitled "Site-Specific Nutrient Management. The concept of the nitrogen gap (NG) is as a core challenge for an effective realization of the so called "twin objectives" in sustainable agriculture. This special issue stresses on some hot spots in crop production, being responsible in the yield gap development, that farmers have to take control. The yield gap cannot be ameliorated without the synchronization of the in-season requirements of the currently grown crop for N with its three-dimensional variability in a supply on a field (temporal, spatial and vertical). A recognition of soil fertility status in the rooted zone, which includes availability of both mineral N and nutrients decisive for its uptake, is the first step in the NG amelioration. The sustainability in soil fertility, as a prerequisite of N fertilizer application, requires a wise strategy of organic matter management, based on farmyard manure, and/or cultivation of legumes. The soil fertility status, irrespectively of the World region determines ways of the N rate optimization. The division of a particular field into homogenous productive units is the primary step in the NG cover. It can be delineated, using both data on soil physico-chemical properties of the soil rooted zone, and then validated by using satellite spectral images of the crop biomass in a well-defined stage of its growth, decisive for yield. The proposed set of diagnostic tools is a basis for elaboration an effective agronomic decision support system.

> **Keywords:** a field; crop production; sustainability; homogenous productivity units; soil fertility; nitrogen indicators: in-season; spatial; vertical variability of N demand and supply; spectral imagery; vegetation indices

**Citation:** Grzebisz, W. Site-Specific Nutrient Management. *Agronomy* **2021**, *11*, 752. https://doi.org/ 10.3390/agronomy11040752

Received: 17 March 2021 Accepted: 8 April 2021 Published: 13 April 2021

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#### **1. Introduction: On a Way to Reach Sustainability in Crop Production**

The effective realization of the 'twin objectives' of the concept of sustainable agriculture assumes efficient food production and the simultaneous protection of both the local and global ecosystem. This concept is based on the optimization of the applied production inputs [1]. In crop production, the efficient use of inputs is defined by the size of the yield gap (YG), which results from inefficiency in the use of N fertilizer (Nf) under well-defined soil and weather conditions [2,3]. The full recognition of the production factors allows the climatic-yield potential (CYP) of the currently grown plant and its seasonal variability to be determined [4]. The size of the existing YG, considered as the deviation from the CYP, is the basis for the elaboration of an agronomic decision support system oriented towards its cover. In rain-fed agriculture, the first step towards YG cover is to increase the resistance of a grown plant to in-season weather variability. This action, as a rule, focuses on improvement in water managemen<sup>t</sup> [5]. The key, and in fact the long-term strategy, of water managemen<sup>t</sup> control should be oriented to soil fertility improvement (organic matter content, soil pH). This is required in order to decrease the yield variability. The efficient use of both indigenous fertilizer and Nf requires efforts that concentrate on the optimization of both the Nf rate and the factors responsible for its uptake and utilization by the currently-grown plant. Solving these challenges requires well-elaborated diagnostic methods which take into account both classical chemometric and remote-sensing tools [2].

#### **2. Special Issue Overview: General Topics**

#### *2.1. In-Season Management of Nitrogen: A Challenge for the Present Generation*

The first chapter of the Special Issue comprises two papers which focus the reader's attention on N managemen<sup>t</sup> within a particular field, as a basic production unit [2,6]. Nitrogen, under conditions of ample water supply, is the main production factor. It affects growth and the exploitation of the yield potential of the currently-grown plant. This first temporal (in-season) variability is defined by a crop's requirement for N, and can be taken under control, provided there is a recognition of the crop's critical stages of yield formation. In the case of seed crops, the game for yield takes place during the linear period of a crop's biomass increase. For cereals, it covers the phase of stem elongation [7]. In spite of the in-season variability of the seed crop requirements for N, the yield for a particular field depends to a considerable degree on the factors which are responsible for its spatial variability. Hence, the main challenge for a farmer is to divide the whole field area into homogenous field productive units, HFPUs [8]. Spectral imagery is a useful tool to recognize the plants' nutritional status within a growing season. Its advantage over classical biometric methods is the quick determination and simultaneous discrimination of the difference in the rate of a crop biomass increase between HFPUs [9]. However, the accurate discrimination of a given HFPU boundary requires strict data on the soil's physical and chemometric characteristics which are decisive for water and nutrient content in the whole rooted soil zone [10].

The sound managemen<sup>t</sup> of N, both indigenous to the soil and applied as Nf, relies on simultaneously maximizing yield and minimizing the negative impact of the N present in the soil/plant continuum of the environment. A basic set of operationally-required data comprises: (i) the N productivity in a particular HFPU, (ii) the size of the pool of mineral nitrogen (Nmin) at the onset of the growing season or at a time of winter crop regrowth in spring, (iii) the total amount of applied Nf, and (iv) the in-season division of the whole Nf rate. The reliable indicators of the in-season N managemen<sup>t</sup> are strongly correlated with N released from its organic pool during the growing season. This assumption, resulting from the study by Łukowiak at al. [6], can be fulfilled by using the N-balance (Nb) as the N managemen<sup>t</sup> indicator. This only seemingly-simple index is based on the N input into the soil/crop system (Nin = Nmin + Nf) and the total N content in a crop plant at harvest (TN). It allows the discrimination of HFPUs differing significantly in productivity, and consequently defines the requirement of a crop for Nf. Its practical advantage is to calibrate the variability in the Nf rate between neighboring HFPUs.
