Nutrient Management Research Database

This research is designed to determine the fate of fertilizer N applied to a drip irrigated almond orchard under acidified soil conditions. This information does not currently exist for finer textured soils nor for sandy soils. The use of stable isotopes will allow us to directly measure whole plant uptake (tops, roots and the crop). Knowing uptake, N efficiency in drip systems can then be calculated.

• Three replications of drip irrigated Nonpareil almonds on Lovell peach root were fertilized with ammonium sulfate or calcium nitrate to produce different pH levels in the wetted soil volume.
• Fertilizer application rate for two years prior to treatment was 800 lbs per acre in split monthly or bimonthly applications for 1989 and 1990 respectively.
• Beginning in April 1991 15N-depleted ammonium sulfate (at a rate of 200 lbs acre) was applied to the ammonium sulfate and calcium nitrate treatments.
• On January 21 and 22, 1992 six treated trees were extracted, chipped and weighed to determine the biomass in branches, stems, medium roots and coarse roots.
• Sub-samples of these components were analyzed for total and isotopic N ratio in order to determine total N uptake and recovery of the applied label.
• Core samples from treated trees were obtained in April to determine root distribution and fine root mass as well as nitrogen distribution around the treated trees.
• Vacuum soil solution samplers and platinum electrodes were installed to measure soil solution composition and aeration status of the drip zone.

• Extraction and measurement of whole trees found the majority of the biomass is associated with the tree stem and branches, which averaged 264 lbs per tree.
• None of the component biomasses or total nitrogen content were significantly different between the calcium nitrate and ammonium sulfate pre-treatments.
• The roots greater than 1/4 inch in diameter accounted for 16% of the total measured biomass but contained 36% of the nitrogen in these components.
• Average above ground biomass plus the coarse and medium roots was 33.9 tons of dry material per acre.
• About 360 lbs of N are needed to construct the above ground structures plus the coarse and medium roots in this orchard.
• Additional N is cycled into the foliage and fine roots.
• This additional N is retained in the orchard, while some N is removed in the crop.
• In the calcium nitrate plots a greater fraction of the total N for all components was derived from the 15N-depleted fertilizer source.
• Using the crop values for 1991 and the 1992 biomass data total uptake derived from the 15N-depleted material was 42 lbs per acre for the ammoniumsulfate plots and 63 lbs per acre for the calcium nitrate plots.
  • This accounts for 21 and 31% of the N applied (200 lbs per acre) during the 1991 season.
  • The reason for this greater utilization in thecalcium nitrate plots is not known.
  • Since there was less available N in the calcium nitrate plots prior to ammonium sulfate addition, the fertilizer would have been a greater fraction of the available N pool in the calcium plots.
• Analysis of extracted soil solutions shows that ammonium treated plots are more acidic, but have high levels of soluble N.

Before the 15N-depleted ammonium sulfate was applied the ammonium sulfate treatments were distinctly more acid than the calcium nitrate plots. In the surface two feet, pH measured in I:2.5 CaC12 averaged 5.59 for the calcium nitrate plots and 4.23 in the ammonium sulfate plots. In spite of the differences in pH, there were no differences in the total soil nitrogen. However, extractable ammonium and nitrate distribution was different between the treatments. With this pretreatment, soils in the calcium nitrate plots were less acidity and lower in extractable nitrogen than soils of the ammonium sulfate pre-treatments prior to application of the 15N-depleted material. A higher N status in the ammonium sulfate plots was associated with higher foliar N in previous years.