Posts Tagged ‘soil phosphorus’

How to sow new pasture and forage crops

Wednesday, September 10th, 2014

In our quest to become dairy farmers we have leased a few acres in Gembrook to grow pasture and forage crops for our small herd. The land is run down pasture and I have outlined soil test results in the last few entries.

To get this pasture productive again we need to raise the pH, correct nutrient deficiencies and increase the soil health mainly through increasing organic matter.

Connor Shea disc seeder at work in Gembrook. Discs slice the soil open and the seeder drops in a trickle of fertilizer and seed. The next crop can be sown without disrupting the existing crop to get a smooth succession. .

Connor Shea disc seeder and John Deere at work in Gembrook. Discs slice the soil open and the seeder drops in a trickle of fertilizer and seed. The next crop can be sown without disrupting the existing crop to get a smooth succession.

Strategy: Make sure some legumes are included in the planting. Balance short term production and removal with longer term growth of pasture (persistance). Horse pasture, cut hay if possible but allow for some pasture suitable for horses to become established in the longer term for grazing. Perennial ryegrass, subterranean clover and cocksfoot. Hay pasture, mainly for hay cutting with some persistence into the next year. Italian ryegrass, balansa clover and cocksfoot. Forages for cows. This will be cut with a forage harvester and fed to cows. Oats, vetch and field peas. Sowing rate for the pasture mixes will be 25 kg / ha.

How much fertilizer? The major trace element deficiencies were boron and copper. We assumed that molybdenum could be deficient given the type of soil and history and because we wanted to establish legumes again we opted to include molybdenum. The final mix had 0.02% B, 0.01% Cu and 0.003% Mo.

We had CaCO3 lime added to the pastures in the previous autumn at 1 tonne / ha.

Unfertilized pasture will produce around 2 tonne / ha (as dry matter). Fertilized pasture can be expected to produce up to 10 t/ha maybe even higher for some varieties. Figures for nutrient uptake by different crops are hard to find and interpret but there are a few guideline figures available. We based calculations for fertilizer requirement on 8 t/ha. A harvested ryegrass / clover pasture (8 t/ha) will typically remove N : 104 kg/ha, P : 30 kg.ha, K : 102 kg/ha, S : 15 kg/ha, Ca : 2 kg/ha and Mg : 9.2 kg/ha. Our soil test results show that around 100 kg/h DAP should supply enough P but not all the N required. Legumes in the pasture may help fill the gap. The DAP also contains sulphur so 100 kg/ha should supply all the S required. The soil is not short of calcium and magnesium for crop growth but we have limed the soil to reduce exchangeable acidity.

Ideally we would have preferred to apply phosphorus in a organic or organically coated form because this soil has the potential to lock up P. The decision to use DAP to supply nitrogen and phosphorus was a compromise but we figured that we had to balance fast short term growth against loss to the soil. However if things go well and organic matter increases in the soil some of that locked up P will be available again (see previous entries for a discussion on P in soils).

It is an expensive business to plant pasture especially to restore a pasture. To get a return we need to concentrate on quality as well as quantity of production. That’s why we opted to resow with productive varieties and to invest in fertilizer. Also there needs to be some carry over of growth so not all the pasture needs to be resown the next year. Our strategy is to keep something growing and includes allowing some production to return to the soil. Basically that means we are preserving and enhancing our capital.

Diversity is important. That’s why we opted to include at least Cocksfoot in the mix – maybe when we better understand the potential and problems with other varieties they can be included also.

In Spring 2014 we sprayed the existing pasture with a low strength glyphosate spray. This was to weaken the weeds and reduce competition without unduly affecting existing grasses.

Direct seeding pasture. The seeder is cutting into existing pasture that has been sprayed to weaken any weeds. The slots can be instected to make sure that seed and fertilizer is being fed in at the required rate.

Direct seeding pasture. The seeder is cutting into existing pasture that has been sprayed to weaken any weeds. The cuts can be inspected to make sure that seed and fertilizer is being fed in at the required rate.

Most small seeded pasture varieties can be sown along with fertilizer with a spreader but this needs to be followed by a pass with pasture harrows and maybe a roller to help bury the seed. A direct drill seeder with either discs or tines is designed to bury the seeds along with the fertilizer. The main advantages of this are more efficient sowing where the fertilizer is placed with the more desirable species, ability to sow larger seeded varieties in the soil away from pests and less disturbance of the soil – particularly important where exposed soil can dry out. Settings on the seeder regulate the flow of seed and fertilizer but every now and again it helps to jump off the tractor to check that the seed and fertilizer is being released at a suitable rate.

Farm soil testing – acidity and phosphorus

Thursday, April 24th, 2014

In a previous blog entry, March 11 2014 I described some of the physical factors in a pasture soil in Gembrook that I had tested. The soil was very acidic with a very low proportion of fresh organic matter.

Exchangeable acidity results from prolonged leaching of good nutrients from soils. The pool is considerable larger than that represented by pH but is in balance with pH. A high exchangeable acidity usually means low pH. In this soil exchangeable acidity is very high and is creating a low pH. A low level of exchangeable acidity is normal and is always present in soils but a high level indicates a problem. The exchangeable acidity has to be at least partly overcome to raise the pH. Therefore exchangeable acidity is a good measurement on which to base calculation of lime requirement.

Exchangeable acidity (calculated by our partner lab as Lime requirement) is 4.8 meq% which is high. The exchangeable acidity determined on the same soil by Apps Labs was 0.32 meq% which by comparison to other figures is relatively low. Around 0.5 to 1 meq% is normal and acceptable (the lower the better). It looks like our lab is including aluminium in the exchangeable acidity whereas our method specifically measures the H ions.

Gembrook pasture and soil. The photo was taken in Summer and shows exposed soil and weeds.

Gembrook pasture and soil. The photo was taken in Summer and shows exposed soil and weeds.

The M3-PSR is the Mehlich Phosphorus Saturation Ratio, an environmental and soil quality test designed to show if phosphorus is likely to be leached from the soil. Conversely it will show the tendency of the soil to fix phosphorus and to make it less available to plants. A M3-PSR < 0.062 in below the agronomic minimum and shows that P uptake by plants will be poor. The result for this soil is 0.003 which indicates a strong tendency of the soil to hold phosphorus in an unavailable form.

The red Kraznozem soils around Gembrook are highly oxidized soils and the red colour comes mainly from the oxidized iron. These are similar to many of the soils found in equatorial regions including those in Africa, Asia and south America. I already expected a problem with phosphorus lockup in this soil as phosphorus binds strongly with iron and aluminium minerals at low pH. The M3-PSR mostly confirmed this.

Soil phosphorus was extracted using Mehlich 3 extractant. Mehlich 3 extractable P has been found to correlate well with a number of other indicators for more readily ‘plant available’ or potentially available phosphorus (see my previous blog entry on phosphorus in dairy farm soil for more detail). The result for phosphorus was 6.2 ppm. The ideal range is 30 – 70 ppm.  Therefore not only will this soil tend to bind up phosphorus, the overall level of plant available phosphorus is very low.

If inorganic phosphorus fertilizers are added to this soil much could be potentially lost before being used by plants. To get around this some farmers add up to twice the calculated plant phosphorus requirement. The result is that some soils have high phosphorus levels (see my previous blog entry on phosphorus in dairy farm soil). Other solutions are to use a slowly soluble form of phosphorus like rock phosphate or to create a fertilizer made up of granules of inorganic phosphorus compounds coated in compost or organic matter.

How can I improve the pasture on my farm?

Tuesday, March 11th, 2014

A complete soil test is an important first step to help restore the productivity of pastures. Most soil tests provide a breakdown of major and minor plant nutrients but many also profile the soil’s overall health, potential and fertility.

I was recently asked to visit a property in Gembrook east of Melbourne that was used to run horses. This was in February and the soil was very dry, around 3% moisture. At this level most biological activity in the soil ceases (more about this later). Now no-one expects a farm to be at its most productive during summer but its a good time to check on how well the soil and pasture is standing up to the stress of summer. Good indicators are the amount / proportion of bare ground, if there is any useable pasture left standing and the presence of weeds. Unfortunately this property showed signs of stress with little standing feed for the horses.

Horse paddock in Gembrook. A complete soil test is the first step towards re-establishing a good feed base for horses.

Horse paddock in Gembrook. A complete soil test is the first step towards re-establishing a good feed base for horses.

This is a higher rainfall area and soils are highly oxidized hence the red iron colour common in the area. On the positive side these soils have a good structure and are well aerated but they tend to be acidic and this is difficult for most plants.

Some results:

Test measured preferred comments
pH in water 5.83 6 – 7 too acidic
pH in CaCl2 5.04 5.4 – 6.4 too acidic
Bulk density 0.89 ‘light’ soil
Soil water 3.1% around 20% very dry

The soil is acidic as expected.  The bulk density value shows that the soil is lightly textured and because the soil felt soft this suggests that there may be organic materials in the soil and the soil may be well aerated. By contrast some soils in West Gippsland have a heavy consistency like butter and when dried lose their structure and become powdery.

How much organic matter was in the soil?

Test measured preferred comments
Total organic C 37184 ppm 29000 – 52000 moderate
Fresh organic C 466.5 ppm 860 – 2100 very low
Proportion fresh org C 1.23% 2.9 – 3.9% low

The soil had moderate levels of organic matter but most is ‘older’ humic type organic matter. There is not much fresh organic matter present. Overall the picture is of little recent return of plant material to the soil. Is this important? Yes, because organic matter has a significant role in making nutrients such as phosphorus and sulphur available to plants – a very important role in these types of soils. Check my previous blog entries for some results and comments for dairy farm soils.

Complete soil tests are a cost effective management tool. Tests that cover all nutrients, many of the main physical factors and organic matter levels cost around AU$150. In future blog entries I will cover more of the test results from this Gembrook soil.

How much phosphorus is in dairy farm soil?

Tuesday, November 26th, 2013

How much phosphorus is there in farm soils?

I’m in the process of carrying out a quick assessment of soils on a dairy farm in West Gippsland. Phosphorus use is an increasingly important topic from a $ cost as well as environmental perspective.

In this study samples were taken from the same sites previously tested for soil organic matter. One sample was taken at each site in a core between the surface and 10 cm. One subsample was prepared after mixing the soil for each core. Phosphorus was extracted using Mehlich 3 extractant.

Mehlich 3 is a widely used extractant for several nutrients suitable for alkaline as well as neutral to acidic soils. It will extract a proportion of the inorganic forms of phosphorus. Mehlich 3 extractable P has been found to correlate well with a number of other indicators for more readily ‘plant available’ or potentially available phosphorus (Moody et al 2013).

Results:

Site Colour Organic matter pH Phosphate ppm
1 Grey low 5.5 27
2 Red-brown medium 6 36
3 Red-brown high 6 632

Phosphate measurements at other locations taken by Apps Laboratories (using Mehlich 3) have ranged from 23 ppm for Gembrook pasture through to 385 ppm for a well composted garden soil. Generally phosphate levels around 30 ppm are considered low and levels around 150 ppm high. The phosphate levels at sites 1 and 2 are low but the phosphate level at site 3 is very high. Some further tests might be useful to find if this applies to the whole paddock. Information on fertilizer history could also be helpful.

Dairy cows grazing on mixed species pasture in West Gippsland. Levels of more readily available phosphorus can vary widely between paddocks.
Dairy cows grazing on mixed species pasture in West Gippsland. Levels of more readily available phosphorus can vary widely between paddocks.

In soils, phosphorus is thought to be present in around 4 ‘pools’. Readily available P is dissolved ready for plant uptake. This amount can last between 1/2 to 3 days for average crops. Some P is temporarily attracted to and held by soil minerals (called adsorbed P). Some P is held in organic forms in the soil organic matter. Adsorbed and organic P form moderately available P and phosphorus moves slowly between these pools and soluble P. Up to 70% of the available P can be held in organic form. However some P finds its way into more permanent ‘bound up’ or ‘occluded’ pools in the soil minerals. This phosphorus is only released again very slowly so is in effect ‘lost’. The total amount of P in soils may be much larger than the amount recovered by extractants like Mehlich 3. Much of this is in the ‘occluded’ pool.

Challenge problem: A pasture contains 30 ppm Mehlich 3 extractable phosphate – a low value. This is close to 9.8 ppm phosphorus (P). This part is done – each hectare contains approximtely 9.8 Kg of P (down to 10 cm). Is this amount of P adequate for the milk produced in a year assuming that 1000 L of milk contains approx 1 Kg of P? For the non dairy farmers some approximate figures that will help are stocking rate 2 cows / ha, production 6000 L / yr / cow. What assumptions have to be made and what factors are missing in this calculation?

References:

Moody, P.W. et al. 2013. Soil phosphorus tests I: What soil phosphorus pools and processes do they measure? Crop and Pasture Science 64(5) 461-468.

Phosphorus fertility. Mississippi Agricultural and Forestry Experiment Station. Downloaded from http://msucares.com/crops/soils/phosphorus.html Nov, 2013.

Phosphorus fractions in soil diagram.