Three basic questions must be answered in all assessments of fertilizer use efficiency:
There are five indices that can be used to
assess nutrient use efficiency.
PFP answers the question: How much yield is produced for each kg of fertilizer nutrient (FN) applied?
Because BY at a given level of FN represents the sum of yield without fertilizer inputs (BY0FN ) plus the increase in yield from applied fertilizer (ΔBY+FN),
and by substitution with equation (5):
Equation 4 shows that PFPFN can be increased by increasing the uptake and use of indigenous soil-N resources (measured as BY0FN) and increasing the efficiency of applied fertilizer nutrient use (AEFN).
Agronomic efficiency (AE)
AE answers the question: How much additional yield is produced for each kg of fertilizer nutrient (FN) applied?
AEFN represents the product of the efficiency of nutrient recovery from applied nutrient sources (= recovery efficiency, REFN) and the efficiency with which the plant uses each unit of nutrient acquired (= physiological efficiency, PEFN):
Both REFN and PEFN thus contribute to AEFN,
and each can be improved by crop and soil
management practices, including general crop
Because AEFN = PEFN x REFN, it is necessary to quantify the relative contribution of each component to explain measured differences in agronomic efficiency that result from different nutrient or crop management strategies.
Recovery efficiency (RE)
RE answers the question: How much of the nutrient applied was recovered and taken up by the crop?
REFN is obtained by the ‘nutrient difference’
method based on measured differences in
plant nutrient uptake in treatment plots with and
without applied nutrient (Equation 7). Recovery
efficiency of applied nutrient is estimated more
accurately when two treatments with a small
REFN is affected by agronomic practises and rainfall (Table 2)
Physiological efficiency (PE)
PE answers the question: How much additional yield do I produce for each additional kg of nutrient uptake?
PEFN represents the ability of a plant to transform a given amount of acquired fertilizer nutrient into economic yield (oil or bunches) and largely depends on genotypic characteristics such as the bunch index and internal nutrient use efficiency, which is also affected by general crop and nutrient management (Table 2).
Internal efficiency (IE)
IE answers the question: How much yield is produced per kg fertilizer nutrient (FN) taken up from both fertilizer and indigenous (soil) nutrient sources?
This definition of IEFN includes FN taken up from indigenous and fertilizer sources. IEFN largely depends on genotype, harvest index, interactions with other nutrients and other factors that affect flowering and bunch formation.
II Implementation of nutrient use efficiency assessment in oil palm fertilizer experiments
In annual crops, destructive sampling methods can be used to measure nutrient uptake in fertilized and unfertilized plots in each crop season and fertilizer nutrient use efficiency can then be calculated by difference (Dobermann and Fairhurst, 2002). The relative ease with which this can be carried out explains why in grain crops, measurement of nutrient use efficiency is standard practice when analyzing data from field fertilizer experiments. Destructive sampling cannot be used in oil palm fertilizer experiments, however, because it is costly and precludes the possibility of further measurements in the experiment. For this reason, Fairhurst (1996) and Fairhurst (1999) devised a nondestructive approach to measure nutrient uptake, based on standard methods for estimating above ground biomass production in trunk, leaf, bunches (Corley et al., 1971, Appendix 6) combined with tissue analysis. Nutrient uptake is calculated from the nutrient concentration and the amount of biomass produced (kg ha-1 yr-1) respectively in the trunk, leaves, and bunches, and nutrient use efficiency is measured by comparing nutrient uptake in different treatments in fertilizer experiments.
Differences in nutrient use efficiency between plantations, blocks, single palms or fertilizer sources are explained by a range of factors (Table 2). The goal of a good field management is to maximize uptake by identifying possible limiting factors and implementing remidial measures.
These methods were used to assess nutrient use efficiency in six fertilizer trials at Bah Lias Research Station (BLRS) (Prabowo et al., 2002). Preliminary results from one year of measurements indicate recovery efficiencies of 19–36% (N), 7–29% (P), 29–70% (K) and 10–60% (Mg) (Table 1). Large differences in RE were measured for different fertilizer sources of P and Mg fertilizer and RE was much greater when these nutrients were supplied in soluble forms respectively as TSP and kieserite (Table 1).
Table 1. Recovery of nutrients from mineral fertilizers in five fertilizer experiments in North
In almost all cases, RE was greater for each nutrient when other nutrients were supplied in non-limiting amounts. RE was smaller in Trial 231 where high rainfall resulted in large fertilizer nutrient losses in surface water runoff and eroded soil (Prabowo et al., 2002). In Trial 231 RE was >100% for K where yield was less than 23 t ha-1. This suggests that palms were able to use soil indigenous K more efficiently after K deficiency had been corrected.
The separation of AE into its components of RE and PE provides the means to identify problems in fertilizer response experiments. For example it may be possible to achieve large values for RE but low values for PE result in low values for AE. Field management factors can be separated into those affecting RE and PE (Table 2). For example, RE may be large in a fertilizer treatment but a low value for PE is caused by inter palm competition and the genetic characteristics of the planting material.
Table 2. Examples of factors affecting and physiological efficiency (PE) and recovery
Table 3. Effect of fertilizer placement on bunch yield in Malaysia.