The importance of K in crop yield and nutrient use efficiency

In the Northwest Region of China there are some soil tests that indicate higher available P, that is above the critical value of 12 mg/L as determined using the ASI Systematic Approach (Portch and Hunter, 2002). However, farmers usually apply more P than K in crop production, most probably because of an assumed lower availability of P in soils and a lack of demonstrated K deficiencies. It is our opinion that increased attention must be paid to nutrient management practices that focus on improving P use efficiency, and use of K in balance with P is one approach to addressing this. Below we have provided some examples that showed crop responses to potash in soils with higher available soil P levels.

In Inner Mongolia an experiment showed that application of K significantly increased the yield of sunflower (Table 1). It also improved partial factor productivity of P fertilizer (PFP_P), and agronomic efficiency of P (AE_P), a response that increased with increasing rates of applied K. Adding P in this study did increase crop yields (treatment 1 vs 4).

Table 1 Field experiment of P and K on sunflower in Inner Mongolia (2003)

Treatment	Yield  (kg/ha)	PFPN	PFPP	AEP
1. N135P0K120	4578 c	33.9	-	-
2. N135P90K0	4533 c	33.6	50.4	-0.5
3. N135P90K60	4889 bc	36.2	54.3	3.5
4. N135P90K120	5178 ab	38.4	57.5	6.7
5. N135P90K180	5500 a	40.7	61.1	10.2

Note: Soil testing results by ASI system of analysis: N= 11.3 mg/L, P = 22.2 mg/L, K = 58.5 mg/L;
Yield LSD_0.05 = 372 kg/ha
Partial Factor Productivity (PFP) (kg/kg) = crop yield per unit nutrient applied
Agronomic Efficiency (AE) (kg/kg) = crop yield increase per unit nutrient applied

In Qinghai another experiment with turnip showed that with the application of potash the PFP_P increased, with the highest PFP_P and turnip yield recorded when 90 kg K₂O was applied (Table 2). Addition of K also improved the PFP_N , with the 90 kg K₂O application rate providing the best response.

Table 2 Effect of potash on yield of turnip in Qinghai (2004)

Treat.	Yield  (kg/ha)	PFPN	PFPP
N75P103.5K0	56040 c	747.2	541.4
N75P103.5K45	61105 b	814.7	590.4
N75P103.5K90	74938 a	999.2	724.0
N75P103.5K135	71027 a	947.0	686.3
N75P103.5K180	64439 b	859.2	622.6

Note: Soil testing results by ASI system of analysis: N= 32.9 mg/L, P = 69.1 mg/L, K = 105.6 mg/L;
LSD_0.05 = 4976 kg/ha

In Xinjiang, the results from a trial with cotton demonstrated that when 207 kg/ha of N and 72 kg/ha of K₂O were applied, the lint yield increased, but the PFP_P decreased with increased use of P. When P was in the same rate of 138 kg/ha of P₂O₅ PFP_P and AE_P slightly increased with increasing rate of K applied up to a maximum with 72 kg/ha of K₂O (Table 3). These experiments suggest that K was important in improving crop yield and nutrient use efficiency, a clear indication that it was out of balance with the N and P nutrients being applied to this cotton crop.

Table 3 Effect of P and K on lint yield of cotton in Xinjiang (2002)

Treat.	Yield  (kg/ha)	PFPN	PFPP	AEP
N207P0K72	1331 e	6.4	-	-
N207P69K72	1478 b	7.1	21.4	-
N207P207K72	1539 a	7.4	7.4	-
N207P138K0	1393 d	6.7	10.1	0.4
N207P138K36	1459  bc	7.0	10.6	0.9
N207P138K72	1547 a	7.5	11.2	1.6
N207P138K108	1421 cd	6.9	10.3	0.7

Note: Soil testing results by ASI system of analysis: N= 15.8 mg/L, P = 31.7 mg/L, K = 277.6 mg/L;
LSD_0.05 = 44.8 kg/ha

These results clearly show the impact that a proper balance of macro-nutrients has on the uptake and efficiency of use for all nutrients being applied to a crop. The importance of this is obvious when we consider the recovery of nutrients by the crop grown, and the reduction of nutrients left in the soil and movement into the environment. Balancing the nutrients applied to crops will go a long way to improving crop production and nutrient use efficiency, but also in minimizing environmental impacts.

Reference:
Portch S. and Hunter, A. 2002. A Systematic Approach to Soil Fertility Evaluation and Improvement, Modern Agriculture and Fertilizers, China Program Spl. Publication No.5.