4R Nutrient Management Practices for Potato Production in China

China is the world’s largest potato producer. Sixty-four percent of potato production in China comes from six provinces, viz., Inner Mongolia, Gansu, Sichuan, Guizhou, Yunnan, and Chongqing (city). The remaining 36 % is produced in the southwest region where mountains and plateaus present a complex topography and production challenges. Most potato cultivars grown are round whites and oblong yellows with medium to late maturity, and are dominated by Chinese selections (Jansky et.al, 2009).
Low fertilizer use and imbalanced nutrient application are partially responsible for low tuber yields and quality of potato. This paper describes the nutrient requirements of potato and demonstrates nutrient management practices to help use nutrients in right form, in the right amount, with the right placement, and at the right time (Roberts, 2007).

Nutritional requirements
Before considering a nutrient management strategy for potato, it is important to know the nutrient requirements of potato. Westerman (2005) reported in a review of potato nutrition that data from the USA and Canada showed the nutrient uptake of potato to be on average 4.19 kg N/t, 1.26 kg P₂O₅/t and 7.20 kg K₂O/t. These values can vary considerably for all nutrients based on soil test levels, and in the case of phosphorus, the additional impact of the level of free lime in surface soils. Experiments in Inner Mongolia in Northwest China from 2002 to 2007 (Table 1) indicated that an average of 6.03 kg N, 1.30 kg P₂O₅, 6.18 kg K₂O were required for producing 1 tonne of potato tuber. Most values for N are considerably higher than those reported by Westerman (2005), while the P and K requirements were very similar. The difference in N may reflect on both the severely degraded soils in the Chinese locations, and overuse of N fertilizers. Differences between rainfed and irrigated conditions were inconsistent in the current project (Table 1), as were the ratio of nutrient uptake reported.

Table 1. Nutrient requirements of rainfed and irrigated potato

Year	Water regime	Yield (t/ha)	Nutrient uptake (kg/t)
			N	P2O5	K2O	N:P2O5: K2O
2002	Rainfed	11.8	7.36	1.65	6.37	1:0.22:0.87
2002	Irrigated	34.4	5.89	1.41	4.89	1:0.24:0.83
2003	Rainfed	9.6	4.23	1.37	5.45	1:0.32:1.29
2003	Irrigated	32.4	5.71	1.15	5.64	1:0.20:0.99
2004	Rainfed	14.4	6.87	1.13	5.02	1:0.16:0.73
2004	Irrigated	26.0	4.70	0.73	5.67	1:0.16:1.21
2005	Rainfed	19.3	4.79	1.23	4.18	1:0.26:0.87
2005	Irrigated	37.5	4.40	1.58	6.63	1:0.36:1.51
2006	Rainfed	14.2	5.84	1.42	6.36	1:0.24:1.09
2006	Irrigated	31.5	6.91	1.62	7.91	1:0.23:1.14
2007	Rainfed	10.3	8.04	1.09	6.55	1:0.14:0.81
2007	Irrigated	30.6	7.58	1.21	9.44	1:0.16:1.25
Average		22.7	6.03	1.30	6.18	1:0.22:1.02

The right source
There are several nutrient sources including chemical fertilizers and organic manures. For N, rapid soluble N sources such as urea and ammonium bio-carbonate are more commonly used in China. Also there are slow-/controlled- release N fertilizers, which are produced by adding nitrification inhibitor (DCD and/or NBPT) to the commonly-used N fertilizers or by coating N products by inorganic materials or organic polymer. For P, the more commonly used sources include diammonium phosphate (DAP), mono-ammonium phosphate (MAP), single super-phosphate (SSP), calcium-magnesium phosphate (CMP), and triple super-phosphate (TSP). Potassium chloride or muriate of potash (KCl), the main K source for potato production in China, is mainly imported (>70 %) from other countries. Other K sources include sulfate of potash (K₂SO₄) and potassium nitrate (KNO₃).
Slow/controlled-release N fertilizers regulate the release of fertilizer N over time, so can improve N use efficiency by synchronizing the supply of N to the crop with the crop demand. It can reduce the application rate and save labor cost. But slow/control release N fertilizer should be applied in irrigated potato systems, where N release can be regulated by soil moisture content. Experiments conducted in irrigated potato in Inner Mongolia of China by IPNI China program from 2009 to 2011 indicated that at the same N rate control-release urea (CRU) resulted in better yield and higher N use efficiency than regular (RU). At 75 % of the recommended N rate, CRU produced similar yield and higher N use efficiency compared with 100 % RU (Table 2).

Table 2. Effect of control-release urea (CRU) on tuber yield and N use efficiency compared with regular urea (RU)

Treatment†	Tuber yield (t/ha)	AEN (kg tuber /kg N)‡	REN (%)§
CK	30.2 d	-	-
100% CRU	38.6 a	33.3 ab	45.3 ab
100% RU	36.4 b	24.5 bc	32.1 c
75% CRU	37.0 ab	35.6 a	52.3 a
75% RU	34.6 c	22.4 c	40.6 bc

^†CK = without N; 100 % CRU = 100 % recommended N applied as CRU; RU 100 % = 100 % recommended N rate applied as RU; CRU 75 % = 75 % recommended N rate applied as CRU; RU 75 % = 75 % recommended N rate applied as RU. N, P and K fertilizers were basal application in all treatments.
^‡AE_N = Agronomic efficiency of N.
^§RE_N = Recovery efficiency of N.

Potato requires more potassium for high yield/quality, which can be affected by K sources. Kumar et al. (2007) observed that KCl produced similar processing-grade tuber yield, total tuber yield and biomass yield to K₂SO₄, but higher than KNO₃. Qin et al. (2008) showed that there was no significant effect on potato yield by application of KCl and K₂SO₄, but K₂SO₄ had a better effect on the percentage of commercial tubers than KCl. The content of starch and vitamin C in potato tuber increased and the content of reduced sugar decreased when KCl was applied.
Organic fertilizers such as animal manures and/or organic compost are alternative fertilizer sources. Several studies indicated that combined use of chemical fertilizer with manure could increase tuber yield and economic returns compared with fertilizer or manure alone (Gallandt et al., 1998; Parmar et al., 2007).

The right rate
Nitrogen is the first limiting factor in potato production in China. The average removal of N is about 6.03 kg N for producing one ton of tuber (Table 1). N application rate should be at least the amount of N removed by potato. A number of approaches have been developed to determine the amount of fertilizer nutrients applied. Generally, a fertilizer recommendation based on soil testing and target yield is commonly used in China. The ASI systematic approach (Hunter, 1980; Portch and Hunter, 2002) for soil testing and nutrient recommendation, improved by the IPNI China Program, was found to be an effective method in nutrient management and widely used in China (Jin et al., 2006). The more balanced “recommended” optimum treatment (OPT) by ASI procedure could significantly increase tuber yield by an average of 2981 kg/ha and farmer’s income by nearly 200 US$/ha compared with farmer’s practice (FP) in most cases (Table 3).

Table 3. Comparison of optimum fertilizer treatment (OPT) with farmer’s practice (FP) in selected potato field trials from China.

Location	Treat.	N kg/ha	P2O5 kg/ha	K2O kg/ha	Tuber yield † kg/ha	Cost ‡ US$/ha	GRF§ US$/ha
Jishishan, Gansu	OPT	120	120	150	35350 a	314	2479
	FP	60	30	0	29017 b	69	2223
Zhangjiachuan, Gansu	OPT	104	72	68	29583 a	193	2144
	FP	104	0	0	24236 b	74	1841
Wuchuan, IMAR	OPT	125	125	100	14200 a	281	841
	FP¶	60	18	0	13300 a	294	992
Wuchuan, IMAR	OPT	250	225	200	31500 a	540	1949
	FP#	141	51	0	29600 b	853	2193
Huzhu, Qinghai	OPT	158	75	135	17893 a	289	1125
	FP	240	52	90	17197 a	290	1069
Xining, Qinghai	OPT	158	75	135	17902 a	289	1125
	FP	240	52	90	17083 a	290	1060
Xining, Qinghai	OPT	158	75	135	30893 a	289	2152
	FP	240	52	90	27500 b	290	1883
Huaxian, Shaanxi	OPT	181	322	225	47916 a	596	3189
	FP	194	504	225	45833 b	766	2855
Mizhi, Shaanxi	OPT	307	322	225	26527 a	686	1410
	FP	358	0	0	22500 b	254	1523
Zhijin, Guizhou	OPT	105	30	66.5	14540 a	155	993
	FP	75	22.5	0	10230 b	73	735

^†: Means in the same location followed by the same letter are not significantly different at P<0.05.
^‡: The total cost of N, P, and K fertilizer (6.36 Y = 1 USD), N=$0.71/kg, P₂O₅=$0.88/kg, K₂O=$0.82/kg.
^§: GRF is the gross return to fertilizers. Potato tuber, $0.079/kg.
^¶: Organic manure 7500 kg/ha, $31.45/t
^#: Organic manure 22500 kg/ha, $31.45/t

Slow/controlled-release N fertilizers can regulate fertilizer N supply over time and should match plant demand, so N rate can be reduced by 15-20 % while maintaining the same yield target. Work in Minnesota on coarse-textured irrigated soils has shown improvements in potato yield and quality with polymer-coated urea compared to the same rates applied as regular urea (Zvomuya et al. 2001). Also, the optimum N rate was 30 to 40 % lower with coated urea compared to regular urea. Worthington et al. (2007) observed that potato in the controlled-release fertilizer treatment averaged 12 % higher marketable tuber yields with 13 % less N applied compared with the ammonium nitrate treatment.
Phosphate generally has maximum solubility in soils within a pH range of 5.5-6.5. This is a very narrow range in which P is not tired up in low solubility complexes with iron and aluminum (pH<5.5) or calcium (pH>6.5) (Davenport et al., 2005). The average removal of P is about 1.30 kg P₂O₅ for producing 1 ton of potato tuber in northwest China (Table 1). Due to its low solubility, a common practice is to add P fertilizers in excess of plant removal to increase the amount of plant available P.
The average removal of K is about 6.18 kg K₂O for producing 1 ton of potato tuber in northwest China (Table 1). Few soils could produce high potato yields for many seasons without replenishing removed K by potato tuber. In China, IPNI experiments showed that yield increases were as much as 22.2 t/ha in Qinghai with the application of 97.2 kg K₂O/ha, 16.7 t/ha in Gansu when 150 kg K₂O/ha was applied.
Other factors should be considered when determining nutrient application rate. Irrigated potato requires more nutrients and higher application rates than rainfed potato to meet the much higher yields produced. However, drip fertigation can reduce recommended N and K rate while obtaining higher total tuber yield as compared to application of recommended nutrients through furrow irrigation (Sasani et al., 2006).
After N and K, calcium (Ca) and magnesium (Mg) are removed the most by potato (Westermann, 2005). So, the secondary nutrients should be supplemented in Ca- and Mg- deficient soils, like the acidic red soil (Ferralsols) in south China.

The right time
Understanding characteristics of nutrient uptake and accumulation by the potato plant during the growing season is important to nutrient management. Knowing the total season demand and the daily nutrient uptake provides guidance for determination of the time for nutrient applications. Figure 1 is an example of nutrient uptake and accumulation by rainfed and irrigated potato in Inner Mongolia and showed that nutrient was accumulated rapidly during tuber bulking stage. The highest daily nutrient uptake by irrigated potato appeared about 2 weeks earlier than rainfed potato, suggesting nutrients need to be applied sometime early for irrigated potato to match nutrient supply to demand.
One of the major challenges in potato production is the efficient management of nitrogen (N) fertilizer. Excessive N fertilizer applied at or before tuber setting can extend the vegetative growth period and delay tuber development, resulting in a lower tuber yield. However, too much N applied later in the season can delay maturity of the tubers, reducing yield and adversely affecting tuber quality. Split application of N can meet the demand of plant uptake, improve nutrient use efficiency, and provide increased flexibility in fertilizer N management, allowing the grower to modify N management based on crop growth stage and climate conditions. Chadha et al. (2006) found that application of N in four splits and K in two splits, along with farmyard manure at 25 t/ha, resulted in better productivity and economic seed potato in India. In some irrigated area or high rainfall regions, N can be applied in three or four splits to improve yield and nutrient use efficiency. In irrigated production on sandy soils, split N application is very effective in reducing environmental N losses (Errebhi et al., 1998). However, there is little or no benefit to split N application in situations where the risk of nitrate leaching is low. In rainfed potato production, split N application may reduce yield potential under dry conditions (Porter and Sisson, 1993; Zebarth et al., 2004).
P and K fertilizers are generally applied pre-plant and mixed with soil before planting. Micronutrients such as Zn, Mn, and Fe applied pre-plant may oxidize or precipitate to unavailable forms before plant uptake, particularly on calcareous soils with high pH. Elemental S should be applied in advance of planting, allowing S oxidization to plant available sulfate, especially in cold areas and on soils with low S oxidation capability.

The right place
Nutrients can be applied in various ways to meet the requirements for potato production. Westermann (2005) summarized the nutrient application methods commonly used with potato. Most nutrients including N can be applied pre-plant if tilled into the rooting zone before planting. Both Manganese (Mn) and Iron (Fe) applied pre-plant may oxidize to unavailable forms before plant uptake, particularly on the high pH calcareous soils. Nutrient source also influences application method and rate. Fertilizer applications after planting are usually done before row closure. When topdressing fertilizer materials are broadcast on the soil surface and should be followed by tillage operation such as ridging. Side-dressed materials are usually physically injected into the soil a few centimeters away from the potato seed.

Figure 1. Characteristics of total and daily rates of N, P, and K accumulation by rainfed (left) and irrigated (right) potato (cv. Zihuabai) in Inner Mongolia.

Fertigation can be an alternative practice for nutrient application, particularly if the nutrient is mobile in the soil, such as NO₃-N. Fertigation application of nitrate can be more efficient than a pre-plant application when the nutrient is not leached out of the plant’s root zone during the process (Westermann et al. 1988). When nutrients are easily fixed by the soil, e.g., P in calcareous soil or acidic red soil, they should not be applied by fertigation. In Northern China, where a single crop of potato is grown each year, consolidated farms with up to 100 ha of potato fields are becoming more common. Potato is irrigated by sprinkler irrigation systems which can provide flexibility and efficient water application. Nitrogen and potassium fertilizer can be applied by fertigation through the sprinkler irrigation system.
Fertilizer banding can also improve efficiency of fertilizer N and P use. Banding fertilizer in the ridges would also be expected to reduce the risk of nitrate leaching because of greater water infiltration in the furrow compared with the ridges (Zebarth and Rosen, 2007). Because potato has a low P use efficiency and limited ability to take up P at low soil P levels (Dechassa et al., 2003), P should be band applied to increase P concentration in the root zone.

Summary
Results from this research indicate that there is considerable opportunity to modify fertilizer rates for potato production in China. While degraded soils can influence the nutrient rates applied, the negative impact from the overuse of nutrients must be addressed. Fertilizer rates not only depend on potato requirements but on fertilizer source, water regime and soil conditions. The best nutrient management practice for potato is to apply nutrients using right source, at right rate, at right time and at right place for producing high tuber yield and nutrient use efficiency. Determining these “rights” is location- or site-specific.


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