Page 26 - ALG Issue 3 2014

Expert guide to...
Phosphate and Potash
Selwyn Richardson is a retired Soil Scientist and Chairman of Hale Road A&GA at Swavesey, Cambridgeshire. He spent over 40 years with ADAS, the Government’s former advisory service for farmers and growers.
His philosophy on crop nutrition is “keep it simple”.
Nutrient supply
It doesn’t matter to a crop if it gets its nutrients
from a brightly decorated plastic bag or from
the back end of a horse – they are all the same nutrients. Therefore it makes good sense to use
a source of nutrients which are i) effective and ii) cheap.
Plants need adequate supplies of major nutrients such as nitrogen, phosphorus and potassium. Among other functions, phosphorus encourages vigorous root exploration while potassium regulates cell water content and, essentially, keeps a plant standing upright. As an aside, although crops use phosphorus (P) and potassium (K) as the elements, European Fertiliser Regulations require the nutrient content on the bag or bottle to be expressed as their oxides, phosphate (P2O5) and potash (K2O). Where both are declared the conversion factors of 2.291 (P to P2O5) and 1.205 (K to K2O) have been applied to confuse users – you can work it out for yourselves from the atomic weights of the elements.
Phosphate and potash removal by crops
When crops are harvested they remove nutrients from the land. However, even for major crop nutrients the offtake is surprisingly small. Assuming that you achieve a similar crop yield to that of a commercial grower (most plot holders don’t) the typical offtakes
of phosphate and potash (gms/sq metre) from a square metre of harvested crop for a range of vegetables are shown in Table 1 (note the relatively large potash offtake by potatoes).
Table 1. Offtake of phosphate and potash by vegetables, gms/sq metre. (1 oz/sq yard = 34 gms/sq metre)
Taken from Defra’s Fertiliser Manual (RB209, 2010)
Be informed - get your soil analysed
In truth, most long-established allotment sites do not need any annual phosphate or potash input because the amounts of P and
K in the soil have already accumulated to luxury levels from years
of bulky organic manure use. A possible exception is on very sandy land where it is difficult to achieve a high level of potassium because on very sandy soil winter rain causes it to “leach” down into the deeper subsoil.
There is no benefit in adding fertiliser such as growmore (typically 7% N, 7% P2O5 and 7% K2O) if your soil already contains a
luxury level of phosphorus or potassium or both. Conversely, if the level of a particular nutrient in the soil on your plot is low you may not be adding enough. The accurate way to find out how much phosphate or potash is needed, if any, is to get the soil analysed by a laboratory. Soil pH and magnesium content will be included in the suite of results. See ALG 3.13 page 20 for guidance on soil analysis, including where to send your soil sample.
Interpreting the lab results
All laboratories report results first as milligrams per litre of each nutrient then each value is assigned to an Index according to how low or high the measured value is. Table 2 gives the ranges for each Index so you can see where your results fit in the general scheme of things.
Table 2. Classification of soil P, K and Mg analysis results into Indices
Index
Phosphorus (P)
Potassium (K)
mg/litre
0
0-9
0-60
1
10-15
61-120
2
16-25
121-180 (2-) 181-240 (2+)
3
26-45
241-400
4
46-70
401-600
5
71-100
601-900
6
101-140
901-1500
7
141-200
1501-2400
8
201-280
2401-3600
9
over 280
over 3600
For an allotment plot growing a wide range of crops the target level should be Index 3, possibly Index 4, for both nutrients. Index 0 or 1 is deficient. There is no virtue in being above Index 4 although long- established plots which have received regular manure dressings may well be above 4. Where past manure use has been modest/none, sandy soils tend to be on the low side for potassium and clay soils can be low in phosphorus.
Which fertiliser to use?
Although there are lots of multi-nutrient fertilisers containing major nutrients, and often trace elements too, there is no profit in supplying a nutrient which is not needed. Such products are unnecessarily costly if you are paying for nutrients you don’t need. Furthermore, with some of them the nutrients are woefully slow in being released to plants, thereby restricting crop growth early on. The most straightforward and economical way to supply phosphorus and potassium is to use superphosphate (18 to 21% P2O5, sometimes labelled single superphosphate) and sulphate of potash (48% K2O).
In the absence of soil analysis results, general guidance to replace the nutrients removed by crops is a dressing of superphosphate applied at 70 gms/sq metre (2 ozs/sq yard) every 3 years and sulphate of potash at 100 gms/sq metre (3 ozs/sq yard), also every 3 years. By far the best place in the rotation is in spring on land where potatoes are about to be grown, firstly because potatoes respond
to fresh phosphate and secondly they remove the most potash. The fertiliser should be very thoroughly mixed into the top 20 cm (8 inches) of soil.
Having said all the above, frankly the simplest way to supply
the nutrients required by crops on established sites, plus valuable organic matter, is to use a dressing of 5 to 10 kg/sq metre (9 to 18 lbs/sq yard) of horse manure, cattle manure, pig manure or compost (soil improver) applied every 4 or 5 years in the rotation, preferably
in front of potatoes or brassicas. Then boost the nitrogen supply by using nitro chalk or sulphate of ammonia.
Crop
Phosphate (P2O5)
Potash (K2O)
Potatoes
6.5
37.7
Brussels sprouts
6.0
14.5
Carrots
4.2
21.4
Onions
4.2
10.8
Cabbages
3.6
14.4
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