Potash for Sugar Beet

Timing of applications

click to enlarge

For soils at K Index 2- there are several potential benefits to be gained by applying the necessary K, Na and Mg in the late autumn and early winter prior to ploughing the land destined for the beet crop. Sugar beet has active roots at depth, but the main advantage of such early spreading is that the absence of newly applied nutrients in the seedbed is helpful to germination and the establishment of a good plant population. The fact that there will be no wheel-marks in the seedbed from the spreading and incorporation of these nutrients is another significant benefit.

However growers on light-textured sandy soils with low clay content should be aware that it may be difficult to raise a low soil K Index even up to Index 2-. In this situation it is better to apply the appropriate amount K for the crop in the early spring after ploughing and for this K to be well incorporated by cultivation. It is not recommended to apply large 'rotational manuring' dressings of K to these soils.

Use of sodium (salt)

Sugar beet has a specific requirement for sodium (Na) in addition to K, for growth. The crop usually takes up about 50 kg Na/ha on a soil well supplied with K, an amount much less than that of K. Most of the Na is retained in the tops where it is mainly used to sustain the growth of the leaf canopy. On soils very low in K (Index 0-1), Na can replace K in its role of maintaining cell turgor. Sodium is usually applied as agricultural salt, Sylvinite or Kainit, the latter two supplying both K and Na and some Mg. Soils can contain Na deposited in rainfall.

It has been thought by some that adding salt to soil might damage soil structure, a view which stems from seeing the adverse effects on soil following prolonged flooding with seawater. There is no evidence that where salt is applied to soils at the appropriate recommended rate, it has any adverse effect, even on soils with little structural stability.

Sources of potassium, sodium and magnesium

These three plant nutrients are available in a number of sources from which they are readily available for uptake by plant roots, especially from the inorganic materials.

	Material		Nutrient
			K2O	Na2O	MgO
	Inorganic sources:		percent concentration (%)
		Muriate of potash (KCl)	60	-	-
		Sulphate of potash (K2SO4, 45% SO3)	50	-	-
		Nitrate of soda (NaNO3, 16% N)	-	36	-
		Sylvinite*	16-21	26-34	1.5
		Kainit* (10% SO3)	11-13	20-30	5-6
		Kieserite (MgSO4, 52% SO3)	-	-	25
		Calcined magnesite	-	-	80
	Organic sources:**		kg/t fresh weight (‰)
		Cattle FYM (25% DM)	8	-	0.7
		Pig FYM (25% DM)	5	-	0.7
		Layer manure (30% DM)	9	-	2.2
		Broiler/turkey litter (60% DM)	18	-	4.2
* The content varies slightly in these naturally occurring materials. ** For more details see RB 209. Where organic manures are applied to soils at K Index 2- and above, the amount of K applied in the manure should be deducted from the recommended amount of fertiliser K. For soils at K Index 0 and 1, the manure K will help to increase soil K reserves and thus improve soil fertility.

Approximate conversion factors:

	K2O to K	Multiply by 0.83
	Na2O to Na	Multiply by 0.74
	MgO to Mg	Multiply by 0.60