Fertilizer Efficiency

MAXIMIZING FERTILIZER USE EFFICIENCY
I Assessment of nutrient use efficiency

Three basic questions must be answered in all assessments of fertilizer use efficiency:

• How much of the nutrients applied are taken up by the crop?
• How much additional yield is obtained for each additional unit of nutrient uptake?
• To what extent can the crop benefit from the nutrients not recovered by the crop during the period of assessment?

There are five indices that can be used to assess nutrient use efficiency.

Partial factor productivity (PFP)

PFP answers the question: How much yield is produced for each kg of fertilizer nutrient (FN) applied?

PFP_FN = kg bunch kg^-1 fertilizer nutrient (FN) applied:
PFP_FN = BY+FN / FN (1)
where BY_+FN is the bunch yield (kg ha^-1) and FN is the amount of fertilizer nutrient applied (kg ha^-1).

Because BY at a given level of FN represents the sum of yield without fertilizer inputs (BY_0FN ) plus the increase in yield from applied fertilizer (ΔBY_+FN),

PFP_FN = (BY_{0 FN} + ΔBY_+FN) / FN (2)

or

PFP_FN = (BY_{0 FN} / FN) + (ΔBY_+FN / FN) (3)

and by substitution with equation (5):

PFP_FNFN = (BY_{0 FN} / FN) + AE_FN (4)
where AE_+FN is the agronomic efficiency of applied fertilizer nutrients (see below).

Equation 4 shows that PFP_FN can be increased by increasing the uptake and use of indigenous soil-N resources (measured as BY_0FN) and increasing the efficiency of applied fertilizer nutrient use (AE_FN).

Agronomic efficiency (AE)

AE answers the question: How much additional yield is produced for each kg of fertilizer nutrient (FN) applied?

AE_FN = kg bunch yield increase kg^-1 FN applied (often-used synonym: nutrient use efficiency):
AE_FN = (BY_+FN - BY_{0 FN}) / FN (5)
where BY_+FN is the bunch yield in a treatment with fertilizer nutrient application; BY_{0 FN} is the bunch yield in a treatment without fertilizer nutrient (FN) application; and FN is the amount of fertilizer nutrient applied, all in kg ha^-1.

AE_FN represents the product of the efficiency of nutrient recovery from applied nutrient sources (= recovery efficiency, RE_FN) and the efficiency with which the plant uses each unit of nutrient acquired (= physiological efficiency, PE_FN):

AE_FN = PE_FN x RE_FN (6)

Both RE_FN and PE_FN thus contribute to AE_FN, and each can be improved by crop and soil management practices, including general crop
management practices and those specific to nutrient management, e.g. a more balanced N:P:K ratio or improved splitting and timing of nutrient applications (see Table 2 and 3).

Because AE_FN = PE_FN x RE_FN, 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?

RE_FN = kg fertilizer nutrient taken up kg^-1 fertilizer nutrient applied:
RE_FN = (UN_+FN - UN_{0 FN}) / FN (7)
where UN_+FN is the total palm uptake of fertilizer nutrient measured in aboveground biomass in plots that receive applied fertilizer nutrient at the rate of FN (kg ha^-1); and UN_{0 FN} is the total nutrient uptake without the addition of fertilizer nutrient.

RE_FN 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
difference in the application rate are compared:

RE_FN = (UN_FN2 - UN_FN1) / (FN_FN2 - FN_FN1) (8)
where RE_FN is the recovery efficiency (kg nutrient uptake kg^-1 nutrient applied); UN is the total nutrient uptake in bunches, fronds and trunk (kg ha^-1); and FN is the amount of fertilizer nutrient added (kg ha^-1) in two different nutrient treatments (FN2 and FN1) e.g. FN2 receiving a larger nutrient rate than FN1.

RE_FN 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?

PE_FN = kg bunch yield increase kg^-1 fertilizer FN taken up:
PE_FN = (BY_+FN - BY_{0 FN}) / (UN_+FN - UN_{0 FN}) (9)
where BY_+FN is the bunch yield in a treatment with fertilizer nutrient (FN) application (kg ha^-1); BY_{0 FN} is the bunch yield in a treatment without fertilizer nutrient (FN) application; and UN is the total uptake of fertilizer nutrient (kg ha^-1) in the two treatments.

PE_FN 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?

IE_FN = kg bunch kg^-1 FN taken up:
IE_FN = BY / UN (10)
where BY is the bunch yield (kg ha^-1), and UN is the total uptake of fertilizer nutrient (kg ha^-1).

This definition of IE_FN includes FN taken up from indigenous and fertilizer sources. IE_FN 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
Sumatra, Indonesia (after Prabowo et al., 2002).

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
efficiency (RE) of fertilizer nutrients in oil palm.

Table 3. Effect of fertilizer placement on bunch yield in Malaysia.

Reference
Goh K.J., Rolf Härdter and Thomas F. (2003) Fertilizing for maximum return. In: Thomas Fairhurst and Rolf Hardter (eds). Oil palm: Management for large and sustainable yields. Potash & Phosphate Institute and International Potash Institute: 279-306

Note: The full list of references quoted in this article is available from the above paper.