The Effects of Different Organic Apple Production Systems on Seasonal Variation of Soil Properties and Foliar Nutrient Concentration

1. Nutrient contents of ground cover mulches and nutrient sources. The mulches varied in [N], C/N, and provided a range of nutrients to the system (Table 1). Likewise, the nutrient sources of the PL and CF provided a range of nutrition. For nutrient source treatments, PL and CF were applied to rates to equalize the amount of N applied per tree per year. Roughly, PL was applied at a volume rate of 5.8-times the CF to equalize the [N] applied. The SP mulch had very low [N] by but high [C] and a resulting very high C:N ratio. The GC treatment applied total N at a rate similar to PL.

2. Soil water status. Irrigation was applied from May to August as needed to maintain average soil tensiometer readings indicated dry soils (50 cbars or greater). The MB and GC plots generally had greater soil water tension (drier) during growing season, especially in the early season (Table 2). SP treatment plots were generally less than 30 centibars of soil water tension which indicated sufficient water status. Correspondingly, when sampled, soils from SP plots had greatest soil water content (data not presented). There were no significant differences among ground cover treatments for water infiltration rate (data not presented). However, the SP treatment had an approximately 50% increase in the rate of water infiltration.

3. Soil properties and nutrient status. The soil bulk density in the top 10 cm soil was lowest in the MB treated plots during October (Fig. 1), but SP treated plots had the highest soil bulk density. The soil pH in the top 10 cm soil was greatest in the SP plots throughout the season (Fig. 2A). Plots treated with GC tended to decrease in pH during the season compared to other treatments. Soil pH of all nutrient source treatments decreased from March until July (Fig. 2B). In May, plots treated with PL and CF had lower pH than the NF treatments, but pH was similar among all treatments in September. Correspondingly, soil EC, an indication of dissolved salt ions, increased early in the season with GC plots having significantly higher EC (Fig. 3A). The NF nutrient source treatments generally had lower soil EC than other treatments (Fig 3B). GC treated plots had higher soil OM than other treatments early in the season, but there was no difference among treatments as the season progressed (Fig. 4). At 10-30cm, the OM content tended to increase during the season, even though OM content was low, approximately 1%. There was no significant effect of nutrient treatment on soil OM (data not presented).
Soil [NO3-N] measured either lysimetrically (Fig. 5) or from a soil sample (Table 3 and 4) varied among treatments. Soil [NO3-N] was highest in the GC plots and in those plots it increased during the season (Fig. 5). However, in WC and SP, [NO3-N] tended to decrease between March (table 4) and May (table 5).
Soil solution [NO3-N] in 30 cm depth varied among the treatments (Fig. 5). For the main effects of ground cover, GC plots had the highest [NO3], and SP plots had the lowest [NO3] during both measurement dates. The main effects of nutrient source indicated that plots with NF had highest [NO3-N]. Although nonintutive, it is thought that the [NO3-N] in the PL and CF may have either been more rapidly leached from the soil solution, especially for the CF treatment or alternatively been tied up in the biological processes of the PL due to the relatively high C:N ratio. Thus, in the WC, and SP treatments with high C:N, the [NO3-N] would have been biologically mineralized, while the [NO3-N] from GC was released , because the relatively GC+NF plots had the highest [NO3-N] during July and September among all treatments. However, it is not clear why the GC+NF treatments had higher NO3-N concentration in soil solution, and this is an unexpected anomaly (data not presented).
GC treated trees had the highest [NO3-N], [P], [K], [Ca], [Mg], [S], and [B] at 0 to 10 cm and 10 to 30 cm soil depth during March and May (Table 3 and 4). For the majority of macro nutrients, MB and SP treated plots had lowest concentrations in 0-10 cm and 10-30 cm soil depths. The SP ground cover resulted in the highest concentrations of soil [Mn]. There were few significant differences for extracted micronutrients among the nutrient source treatments, especially at the May sample date. The PL nutrient source resulted in generally the highest concentrations of soil [P], [K], [Ca], and [Mg] at both 0-10 cm and 10-30 cm soil depths. The CF nutrient source treatments had intermediate P, K, and Mg concentrations but concentrations of Ca similar to the NF treatments. The soil [NH4-N], [Cu], and [B] decreased in concentration between the March (Table 3) and May (Table 4) sample dates at 0-10cm depth.

4. Tree Performance.
Foliar Nutrient Content Foliar nutrient content was sampled 100 days after budbreak following standard protocols. At that time, there were no differences in foliar nutrient content of N, K, Ca, Mg, S, and Fe among the ground cover treatments GC, MB, WC (Table 5). The SP ground cover resulted in significantly less foliar [N], [Mg], [Zn], and [Cu] but increased foliar [P], [K], [Ca] and [B] (Table 5). The CF and PL treated trees had lower foliar nutrient contents of [P], [K], and [Zn], but higher foliar concentrations of [Mg].
The foliar [N] of the GC, MB, and WC ground cover treatments were in the low portion of the sufficiency range (Garcia, 2007; Shear and Faust, 1980), and all of the nutrient source treatments would be regarded as deficient in [N]. All ground cover treatment and nutrient source treatments resulted in adequate ranges of foliar [P], [K], [Ca], [Mn], and [B] but in the low range or deficient for foliar [Mg], [S], and [Cu].

Leaf development and gas exchange Leaves from trees with GC and WC were the largest, and had the greatest specific leaf weight (SLW) while leaves from the SP ground cover management treatments had significantly smaller, less dense leaves, and had less chlorophyll either measured directly or colormetrically (Table 6). Correspondingly, trees with SP ground cover management had lower assimilation rates. Late in the day, leaves from the SP treatments had higher evapotranspiration, greater stomatal conductance, but higher internal CO2 concentration reflecting the low CO2 assimilation (Fig. 6). There were no differences in leaf size among nutrient source treatments but, NF treated trees had significantly lower SLW. Foliar assimilation rate was significantly curvelinearly related to SLW with an asymptotic maximum for assimilation occurring at a tangent of approximately 0.25g/cm2 SLW (Fig. 7) as has been reported by Hagidimitriou et al. (2004). There were strong correlatiosn between foliar [N] or [Mg] and SPAD meter chlorophyll estimates with R2 of 0.630 and 0.712, respectively (Fig. 8). Neilsen et al. (1995) showed that there were significant relationships between leaf N concentration and SPAD readings for all apple cultivars until mid-July (R2 = 0.44 to 0.89), but not later in the growing season.
Tree height and monthly height increment varied with both ground cover and nutrient source (Fig. 9). Trees with GC and WC had larger growth early in the season (Apr-May), but the MB treated trees had the largest growth increment in late season (Aug-Sep) (Fig. 9. A). The SP mulch resulted in significantly reduced tree height at the end of the second season compared to other treatments (data not presented). Similarly, the CF and PL trees had the greatest increase in height during the early season (Apr – May) while the NF had the largest growth increment at mid-season (Jun-July) (Fig. 9B). There were significant interaction effects for tree height increment, and SP with NF treated trees had the smallest (Fig. 10).
Trunk cross-sectional area (TCSA), an indication of the total vegetative growth of a tree, varied with treatments. The GC and WC ground cover systems had the largest TCSA in year 2 of the study (Fig. 11). There were no significant differences in TCSA due to nutrient treatments but the NF trees were observed to be approximately 30% smaller than PL treated trees.
Total current season shoot extension was longest in the trees treated with GC and WC; almost 100% greater than the MB and SP treatments (Fig. 12). Trees that received the NF nutrient source treatment had significantly less total shoot length development in the second season. Total shoot length was highly correlated to TCSA increase in the second season (R2 = 0.84) (data not presented) as has been reported for many woody species (Jacyna, 2004).
The low C:N ratio of mulch biomass such as compost will significantly affect nutrient availability in the soil. Gale et al. (2006) mentioned that high C/N typically reduces the decomposed ability of residue, and the C/N ratio is good indicator of plant available nitrogen released for fresh crop residues or manures. Under SP treated plots, residues with a high C/N ratio (164:1) decomposed slowly due to a lack of nitrogen source for microorganisms which decompose surface mulch.
Summary: The ground cover management treatments and nutrient source treatments provided a range of nutrients to the trees. With irrigation, the soil moisture was maintained in an adequate range although the MB treatment tended to be drier on average. Because the soil moisture was maintained during the season, it is not thought that the mulch effects on soil moisture contributed to the observed differences in tree growth. After two years of organic management although some differences in soil OM were minimal. Therefore, it is assumed that the differences in tree growth were likely not due to moisture or OM but rather due to differences in either in soil chemistry and nutrient availability, or other soil factors (environmental characteristics such as temperature, soil gas exchange, etc.) and soil microbiological activity. This is at least partially evidenced in the fact that ground cover treatments affected soil bulk density with SP resulting the densest soil and both GC and MB resulting in significantly less dense soil.
Soil pH at both 0-10cm and 10-30cm, although varying during the season were generally within an adequacy range of ph 6.2- 7.2. The soil pH of SP ground cover treatments were in the upper reaches of that range (7.1 – 7.25). The EC of the GC ground covers and the CF and PL nutrient sources tended to be the highest and may be an indication of total available nitrate concentrations in the soil solution.
Differences in soil nutrient content were observed, particularly early in the season shortly after the application of the ground cover and the nutrient source treatments. Foliar [N] was least in the SP treated trees and appears to correlate to the lowest soil NO3-N. The NF trees had highest concentrations of foliar [N] and several other nutrients which could in part be due to the late season increase in soil content of nutrients that was observed in the NF plots, the low shoot growth and likely low total leaf area of the tree, and the small leaf size which may result in a concentration effect in those trees and a dilution effect in trees growing vigorously with large shoot growth, large leaves, and perhaps larger total leaf area. The differences in the C:N ratio of the mulch treatments may have affected availability of nitrate concentrations early in the season by microbial decomposition when apple trees tend to take up the majority of nutrients.
When all growth effects were evaluated, it appeared that ground cover management had a greater overall effect on growth than did nutrient source. The nutrient source treatments did have some manifestation in the foliar nutrient contents. There were significant interactions of ground cover management treatment and nutrient sources for tree growth, where in some cases the NF treatment in combination with the ground cover treatment was larger than with additional nutrient source. We observed (data not presented) that there were differences in weed competition which may have been exacerbated by nutrient treatments and thereby resulting in less growth due to competition.
A practical target for an orchard is to fill its allocated space as quickly as possible so that trees can become productive early in their life (Elfving, 2005). In this trial, trees treated with GC and WC all exceeded 3.1m total height at the end of the second season, the target tree height, and a spread of 2 m of the 3 m tree width allocated. Trees treated with SP did not achieve growth targets and fill their allocated space within the two seasons of this study. Therefore, it would be assumed that SP trees should not be allowed to crop in the third season. However, those trees treated with GC and WC appear to be large enough to carry an economic crop in the third season. The ability to carry a crop in the 3rd growing season would enhance orchard sustainability.