1. Introduction
Nitrogen plays an important role in apple tree production [
1], and apple tree nitrogen nutrition depends on soil and climate conditions. Soil with high organic matter content, favourable weather conditions and proper soil management enhance the likelihood of adequate nitrogen nutrition in fruit trees [
2,
3,
4]. If fruit trees cannot satisfy their physiological needs with soil nitrogen, fertilisers are used.
Plants can easily utilise mineral forms of nitrogen. Ammonium or nitrate fertilisers dissolve in the soil solution, and plants subsequently absorb them. They are extremely useful for the rapid correction of nitrogen deficiency in the soil. Apple trees require more nitrogen during the first half of vegetation, so spring application of ammonium nitrate is the most common practice for enhancing the fruit trees’ supply of nitrogen in conventional orchards [
5]. Precise nitrogen management using organic fertilisers involves an additional challenge, as the organic fertilisers are usually not as well defined and predictable as mineral ones [
6]. Various organic materials are used for nitrogen content enhancement in the soil: compost, manure, manure-based fertilisers, green manures with legumes, etc. [
7,
8]. The nitrogen in most organic sources is in the form of amino acids or proteins. It becomes available to plants only during the process of mineralisation. Biotic and abiotic factors influence this process. In practice, it is difficult to predict the course of mineralisation and control the amount of mineral nitrogen released during the vegetation period [
9].
Cattle horn shavings, which contain about 15% of nitrogen (N), constitute a promising source of nitrogen fertiliser [
10]. In addition, they are biodegradable waste from the meat industry. The advantage of cattle horn shavings over other organic fertilisers is considerably higher nitrogen content. Protein hydrolysates from bovine horns and hooves also have the potential to be used for foliar fertilisation, whether alone or in a combination with other complementary hydrolysates [
11].
According to the requirements of EU regulations [
12], animal by-products can be used in the organic farming system. Products of animal origin are used to fertilize plants, including keratin-containing waste: horn chips, shredded feathers, bristles, horn core powder, etc. [
13,
14]. Keratin contains about 15–18% nitrogen, 1.5–2.0% phosphorus, sulphur and other elements that are in organic form. Keratin-containing wastes decompose more slowly than other organic animal wastes because of cysteine, sulphur-containing amino acids, that form strong intermolecular bonds and give to the protein a crystalline structure and strength. Cattle horns used to be a great raw material for haberdashery, but recently they have been replaced by more colourful and easier to recycle plastics.
More than half of the animal by-products are not suitable for normal consumption, because of their unusual physical and chemical characteristics [
15] Jayathilakan et al., 2012. Slaughterhouses generate between 0.5 and 2 kg of horns and other keratin-containing waste from each slaughtered bovine animal. As a result, thousands of tons of waste are accumulated each year that could be used in organic horticulture.
Cattle horn shavings could be beneficial for optimising nitrogen nutrition in fruit trees. However, data on the effects of these fertilisers remain limited.
Therefore, this research is aimed at evaluating the possibility of using cattle horn shavings as nitrogen fertiliser for apple trees.
2. Materials and Methods
The trial was conducted in the experimental orchard of the Institute of Horticulture (55°60′ N, 23°48′ E), at the Lithuanian Research Centre for Agriculture and Forestry from 2015 to 2018. The experiment was performed in 2–6-year-old apple tree orchard with 3.5 × 1.25 m planting scheme. Apple trees of the Ligol cultivar on P 60 rootstock were investigated. The experimental plots were fully randomised and had four replications. Each experimental plot contained five fruit trees, and guard trees were planted at the ends. Orchard floor management comprised frequently mown grass in the alleyways with 1.5 m wide herbicide strips along the tree rows. The orchard was not irrigated, and fruitlets were not thinned.
Scheme of the experiment:
No fertilisers
50 kg/ha N equivalent applied in spring as NH4NO3
100 kg/ha N equivalent applied in spring as NH4NO3
50 kg/ha N equivalent applied in spring as horn shavings
100 kg/ha N equivalent applied in spring as horn shavings
50 + 50 N equivalent applied at equal parts in autumn and spring as horn shavings
50 kg/ha N equivalent applied in autumn as horn shavings
100 kg/ha N equivalent applied in autumn as horn shavings
In spring fertilizers were applied at 07–09 BBCH, in autumn—at 95 BBCH stage [
16]. The first autumn application of horn shavings was performed in 2014. The effects of the horn shavings were compared to the effects of ammonium nitrate (34.4% N) and unfertilised treatment. Both fertilisers were broadcasted throughout the entire area of the relevant experimental plots on the soil surface.
Cattle horn shavings from a haberdashery company were used. The waste generated during production is shredded into horn shavings with a special mill. A 2.5–3.0 mm chip fraction was used for the research, which is similar in size to the mineral fertilizer granules, containing 14.1% N (
Table 1).
The soil in the orchard was Epicalcari-Endohypogleyic Cambisol, a heavy clay loam containing 3.22% of humus, 244 mg/kg P2O5, 188 mg/kg K2O, 7412 mg/kg Ca, 1848 mg/kg Mg, with pH 7.0 (in 1 M KCl extract).
In Lithuania, the growing season lasts from April until November. This period has the greatest influence on processes in plants and the soil. The perennial average precipitation during the April–November period is 464.6 mm, and the average temperature is 11.1 °C (
Table 2). In 2015 and 2016, the total amount of period precipitation exceeded the perennial average, but in 2017 and 2018, it was less than normal. The average temperature of the period was higher than the perennial average in all research years.
Mineral nitrogen (Nmin) content was determined in April, June, August and October as a sum of ammonium (N-NH4) and nitrate (N-NO3) ones. Soil sampling was performed in each experimental plot close to the edge of canopy projection of each five fruit trees. Five soil subsamples were mixed, and a composite soil sample of 0.5 kg was used for analysis. Nmin was established in two layers of the soil (0–30 and 31–60 cm) and presented as the average content in the 0–60 cm layer. N-NO3 was ascertained via the colorimetric method using hydrazine sulphate and sulphanilamide, and N-NH4 was ascertained via the colorimetric method using natrium phenolate and natrium hypochlorite.
Sampling procedure for the leaf chemical analysis was done in the first half of August. A total of 1–2 fully developed leaves were taken from the middle of current-season terminal shoots located on different sides of tree canopy at the height approximately 1.5 m. Samples of 50 leaves from each experimental plot were collected. The leaf N content was measured via the Kjeldahl method using a DK 20 Tecator Digestion System DK 20 (VelP Scientifica, Usmate, Italy) and a UDK139 Semi-Automatic Distillation Unit (VelP Scientifica, Usmate, Italy).
All laboratory analyses were performed at the Agrochemical Research Laboratory of the Lithuanian Research Center for Agriculture and Forestry, which is accredited accord-ing to the standard LST EN ISO/IEC 17025: 2018. The results of laboratory tests are pre-sented with an average of at least two replicates, with estimating the repeatability limit of the method.
Yield in each experimental plot was recorded and recalculated as tons per hectare. Yield efficiency was calculated as a ratio of yield per tree (kg) to trunk cross-section area (TCSA, cm2, 20 cm above graft union) and expressed in kg cm2.
Random samples of fifty apples per plot were used to determine individual fruit weight. Laboratory measurements were conducted on random samples of ten apples from each experimental plot. Fruit flesh firmness (kg/cm2) was measured using a penetrometer (FT-327, TR Turoni, Forli, Italy) with an 11-mm diameter probe; soluble solids content (SSC) (% Brix) was measured using a digital refractometer (ATAGO 101, Atago Co., Ltd., Tokyo, Japan). The index of the absorption difference (IAD) between 670 and 720 nm was assessed in 2017 and 2018 using a DA-Meter (Bologna, Italy).
The data was analysed using the analysis of variance (ANOVA) procedure, and means were separated using Duncan’s multiple range tests with three probability levels (p = 0.20, 0.10, or 0.05).