Nutrient diagnosis of the soil and foliar at the beginning of the productive system

In order to identify the nutritional and fertility conditions of the soil of the avocado orchard under study, a nutritional analysis was performed. Table 1 shows the parameters obtained in the initial soil analysis. The results indicated that the soil had a slightly acidic pH, very high in P, K, Mg and Cu, a low level of N, Ca, Mn, Zn and B (^{Ankerman, 1977}).

While the CIC was medium and the density was low. This meant that it is necessary to restore deficient nutrients and reduce the application of elements that are in excess to level fertility and improve crop development. The foliar analysis interpreted by the Kenworthy balance index technique, indicated that the nutrients Ca, Cu, Mn and B were found in the foliage at a level below normal, while Zn, P and Mg, in the normal range and in excess Fe, N and K. Based on these results, correspondence was found in soil level and foliage for nutrients Ca and Mn.

On the other hand, while K was high on the ground and in the foliage. The mismatch between the nutrients of the soil and foliage can be attributed to the fact that the farmer fertilizes foliarly and some that are deficient in the soil were located in high concentration in the foliage. The high level of Cu in the soil was due to its contribution in the tree trunk and its accumulation in the soil, but which was not absorbed in the foliage.

It can be seen that the high concentrations of P, K and Mg in the soil reflected normal concentrations in the avocado foliar area, Ca was found in a medium concentration in the soil, but not in the foliar area, which could be explained as ^{Kass (2007)} mentions because this nutrient is considered as a stationary element within plants. There is very little calcium translocation in the phloem's conductive tissues and the ability to absorb calcium is limited, because it is absorbed only through the young apexes of the roots.

The foliar concentrations of Mn, Cu and B coincide with the low amount of these elements in the soil, as does Zn, which, although in a foliar form is in a just normal concentration in the soil, has a low concentration.

Superficial root development

In order to see if there were significant differences due to the application of compost as padding and promoter of superficial roots in avocado, the analysis of variance (Anova) was performed, which is shown in Table 3. It could be evidenced that a significance of 0.001 was obtained, which shows that there is high significance in the different treatments evaluated in terms of the result of the parameter of superficial radical development.

In the test of comparison of means by Tukey three homogeneous subgroups were obtained that allow to locate the treatments with significant differences. The treatments with the greatest amount of new roots were those that had medium to high levels of compost applied to the soil. It is worth mentioning that the analysis of variance results with significant differences in all the treatments that had organic fertilizer application with respect to the control group and the nutritional balance treatment that did not contemplate the organic fertilizer, so it is conclusively verified that there is an impact positive in the soil that allowed the development of new roots from the first application.

The effect of organic matter on radial development is attributable to its influence on the structure and aggregation of the soil, of factors such as (1) soil fauna; (2) microorganisms; (3) environmental variables; (4) inorganic cementing agents; and (5) the roots. In recent years ^{Six et al. (2004)}; ^{Bronick and Lal (2005)} have published the relationship of organic matter with radical systems and their relationship with soil structure, in ecosystems and agriculture. Also ^{Gregory et al. (2006)} and ^{Hinsinger et al. (2009)} have published the importance of the dynamics of the biochemical and biogeochemical properties of the rhizosphere and its relationship with soil aggregation.

As can be seen in Figure 1, it is clear that the application of compost increased the amount of new roots produced by avocado trees. The evaluated root development can directly impact the next cycle of flowering and fruit production, because there will be a greater amount of roots with the ability to absorb nutrients and better nourish the tree. The promotion of new roots could allow trees with root disease problems to minimize their effects and increase their yield gradually. The treatments without compost application and leachate of earthworm humus lagged far behind in the production of new roots.

Figure 1 Tukey means comparison test for root development. a, b and c = subsets with significant differences (Tukey ≤ 0.05). 

Avocado yield

With the data obtained, the test of equality of variances of Levene was performed and the results of the Analysis of variance (Anova) of the performance factor of the experiment in question, which are presented in Table 4.

As can be seen in Figure 2, there are significant differences between treatments, and with the Tukey test the homogeneous subsets formed are observed. In Figure 2 it is denoted that the treatments without application of compost and leachate produced less kg of fruits per tree and therefore are located within the subgroup of lower production.

Figure 2 Test comparison of means by Tukey performance avocado cv. Hass. Data with different letters present significant differences (Tukey ≤ 0.05). 

With the Tukey test, two homogeneous subgroups were obtained, where significant differences were observed between treatment 1 (control) and treatment 6 and 4, of 100 and 150 kg of compost application respectively. It is noted that the highest performance was in treatments with average levels of compost application. The yield per treatment evaluated shows in Figure 2, that the behavior of this variable was superior where there was application of compost, but at medium levels, this is possibly due to high compost applications affecting the temporary retention of nutrients to the tree that influence in lower yields in the short term.

Correlations of nutritional parameters with performance means and superficial root development

The Pearson correlation matrix shown in Table 5, showed differences that report significance with respect to the means obtained of yield and root development of the experimental design evaluated and its relation to the amount of specific nutrients in the soil. The performance correlations with the nutritional data obtained in the final sampling of the experiment and that present some kind of correlation.

It can be seen that the yield showed a positive correlation with root development; that is to say, that to a greater amount of roots the plant absorbed better the necessary nutrients to increase the yield of the fruits. Similarly, the B show a positive correlation with performance considering a significance of α = 0.05. The K was very close to presenting significance with the yield parameter.

For its part, Figure 3 shows a positive trend with the increase in root development promoted by organic fertilization in combination with mineral nutrients. It has been determined that humic and fulvic substances favor plant growth directly through physiological and nutritional effects. Some of these substances function as natural plant hormones (auxins and gibberellins) and are capable of improving seed germination, radical initiation and can serve as a source of nitrogen, phosphorus, sulfur and micronutrients.

Figure 3 Correlation between yield and root development. 

With organic matter physical characteristics are improved such as porosity, water retention, permeability, etc. and the microbial flora is stimulated which in turn facilitates the transformation of soil compounds into available nutrients such as Zn, Cu and B for crops.

In Table 6 it can be seen that there was a positive correlation with C, Organic Matter, Ca, Mg, Fe, Zn, Cu and B. This result coincides with ^{Yamada (2000)} who mentions that Ca and B are essential nutrients importance for the development of apical sprouts and root tips. Without these nutrients, the growth of new shoots and that of new roots is stopped and for a good root system to develop, it is necessary that B be in adequate quantities at the same place where the roots grow since it acts simultaneously with Ca.

^{Liderman (1989)} mentions that with increasing the organic matter in the soil there is an increase in the concentration of nutrients and in microbial populations and that it offers the soil an aeration and drainage capacity that avoids excess water, which gives the conditions necessary for optimal root development. The composting and leaching applications of earthworm humus contained significant amounts of microelements, which in general raised the level of the latter in the soil of all the experimental units evaluated to which they were applied.

Figure 4 shows the high correlation that the superficial root development had with the concentrations of B in the different treatments. The pattern of the line of the graphs is very similar between both parameters. Similarly, Cu and Zn presented the same patterns of correlation with root development (Figure 5 and 6).

Figure 4 Correlation between root development and nutrient concentration of B in the soil after the fertilization treatments applied. 

Figure 5 Correlation between root development and nutritional concentration of Cu⁺² in the soil after the fertilization treatments applied. 

Figure 6 Correlation between root development and nutritional concentration of Zn⁺² in the soil after the fertilization treatments applied. 

The correlation between root production and nutrient absorption is associated with the low mobility of copper and Zn in the soil, as well as the contribution of dead roots in the supply of nutrients. For example, it has been determined that, when harvesting crops, the stored C derived from the roots reaches 50% of the total C, while only 13% corresponds to aerial biomass (^{Gale et al., 2000a}).

The roots are influenced by the soil in which they live, so when the soil is compacted or has low nutrient content or the water is limited or there are other problems, the plants will not grow well. The physical pressure of the roots growing through the soil helps form aggregates with neighboring particles. When the vegetative material is returned to the soil, it will become the primary food source for bacteria, fungi and the plants themselves.