Some aspects of soil management practice and nutrient cycling in apple orchards

Effects of three contrasting orchard soil management practices on the soil physical properties, levels of available nutrients in the soil and leaf nutrient content of 11 y old Golden Delicious apple trees were examined over a three-year period. The treatments consisted of (a) grass sward mown at either 9 cm height or 18 cm height, or controlled with the use of a grass suppressant maleic hydrazide, (b) total herbicide coverage (zero tillage) using simazine/paraquat or terbacil, and (c) clean cultivation. In comparison with grass, herbicide treatments resulted in a natural compaction of the surface soil with increased bulk density, penetrometer resistance, aggregate stability and root density, but reduced total porosity, macroporosity, infiltration capacity and earthworm density. The poor infiltration capacity led to an accumulation of NaHCO₃-extractable phosphorus (P) and total P in the surface soil of the herbicide treatments. A P fractionation indicated the accumulation of P in the Al- and Fe-PO₄ fractions. To a lesser extent exchangeable K also accumulated at the surface of the untilled soil. Cultivation led to a breaking down of the well structured previously grassed surface soil with decreased bulk density, penetrometer resistance, tree root density and markedly reduced earthworm density, but increased total porosity, macroporosity, infiltration rate and oxygen diffusion rate near the soil surface. The porous nature of the soi1 surface under cultivation apparently led to downward movement of applied P and K, since there was no measureable accumulation of these elements in the surface soil under cultivation in comparison with herbicide-treated plots. Total N and S and oxidizable C were highest in the surface soil of grassed plots, intermediate in zero tilled plots and lowest in cultivated plots. The seasonal pattern in levels of NO₃⁻-N and SO₄²⁻-S showed similar trends in both the surface soil and down the profile. Levels of both NO₃⁻- N and SO₄²⁻-S in the top 5 cm of soil remained high for a longer period into autumn under zero tillage than under cultivation. During the summer and autumn periods, concentrations of NO₃⁻-N in the surface soil from non-grassed treatments were clearly higher than those from grassed plots. Levels of exchangeable Ca and Mg in the surface soil under the non-grassed treatments were markedly lower than those from the grassed treatments. There was a parallel drop in the pH of the surface soil of non-grassed treatments, whether measured in distilled water or 1N KCl in comparison with plots under continued grass sod. This drop in pH was accompanied by a decrease in base saturation and a consequent increase in exchange acidity. These effects were attributed to the leaching of Ca and Mg from the surface-soil of the non-grassed treatments. In a subsequent laboratory leaching study, using cubic soil blocks (16 cm³), it was found that over a 24-week period the cumulative quantity of cations leached (in meq) per soil block from the herbicided and cultivated treatments was approximately 3.5 times that leached from the grassed treatment. Over the three years of the field trial, soil management practice had no measureable effect on tree growth or crop yield. However, in the second and third seasons, trees from grassed plots tended to have lower leaf total N contents than those from non-grassed treatments, while trees from cultivated plots tended to have lower leaf K levels than trees from other plots. A separate 3-year field trial on a separate site, using newly planted Golden Delicious/MM 106 apple trees, showed that with no fertilizer applications grassing down, in comparison with cultivation, had a detrimental effect on tree growth as measured by shoot extension and trunk girth increment in all three seasons arc by total dry weight of both tops and roots of trees at the end of the three-year period. Application of fertilizer N to cultivated plots had no significant effect on tree growth. However, N fertilization significantly increased tree growth and total leaf area per tree under grassed-down conditions. In the third season there was a large positive interaction between N fertilization and irrigation on tree growth under grassing-down. Grassing-down generally resulted in decreased leaf total N contents and increased leaf concentrations of P, K and Ca. As part of a nutrient cycling study, leaves and fruit of Golden Delicious and Granny Smith apple trees, pruned on the Hawkes Bay tree training method on the 11 y old site, were sampled at two weekly intervals from bloom time till leaf fall, during the 1976-77 and 1977-78 growing seasons from the bulk of the top, middle and bottom thirds of the tree crown. With decreasing height in the crown, tree leaves had a markedly lower dry weight per unit area and lower contents of N, P, K, Ca, Mg, chlorophyll and soluble carbohydrates per unit leaf area, while there was a concomitant decrease in the size, dry matter content and soluble carbohydrate concentration of fruits. The largest dry weight and nutrient accumulation of both leaves and fruit was in the middle crown position followed by the lower and then the upper regions. Data on the dry matter distribution and nutrient reserves (N, P, S, Cl, K, Ca, Mg and Na) in the standing biomass of the grassed-down 14 year-old Golden Delicious apple orchard were also collected along with the mean estimates of nutrient inputs, returns and losses over a 2-year period (1977-78 and 1978-79 seasons). Total nutrient inputs by precipitation plus irrigation plus fertilizer in kg ha⁻¹ yr⁻¹ were: N, 81; P, 20; S, 42; Cl, 58; K, 64; Ca, 35; Mg, 10 and Na, 33. Nutrient returns to the orchard floor were dominated by those from returns of herbage clippings originating from the mowing of the orchard sward. Total nutrient returns to the orchard floor through petal fall, fruit drop, leaf fall, foliar leaching (includes leaf washing) and sward clippings in kg ha⁻¹ yr⁻¹ were: N, 545; P, 33; S, 41; Cl, 107; K, 442; Ca, 147; Mg, 35 and Na, 16. The major loss of Na, Mg, Ca, Cl and S was through leaching. In contrast, the major loss of P and K was in the harvested fruit crop, while for N, losses were about equally divided between the fruit crop and leaching. Total nutrient losses were estimated to be, in kg ha⁻¹ yr⁻¹: N, 58; P, 5; S, 28; Cl, 81; K, 124; Ca, 55; Mg, 39 and Na, 80. Inputs minus losses in kg ha⁻¹ yr⁻¹ were positive for N, P and S (+ 23, + 16 and + 14 respectively), and negative for Cl, K, Ca, Mg and Na (-24, -60, -19, -30 and -47 respectively).